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Thermal bridging analysis on cladding systems for building facades
Accepted Manuscript Title: THERMAL BRIDGING ANALYSIS ON CLADDING SYSTEMS FOR BUILDING FACADES Author: Theodoros G. Theodosiou Aikaterini G. Tsikaloudaki Karolos J. Kontoleon Dimitrios K. Bikas PII: DOI: Reference:
S0378-7788(15)30351-0 http://dx.doi.org/doi:10.1016/j.enbuild.2015.10.037 ENB 6231
To appear in:
ENB
Received date: Revised date: Accepted date:
9-6-2015 31-8-2015 15-10-2015
Please cite this article as: T.G. Theodosiou, THERMAL BRIDGING ANALYSIS ON CLADDING SYSTEMS FOR BUILDING FACADES, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.10.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
THERMAL BRIDGING ANALYSIS ON CLADDING SYSTEMS FOR BUILDING FACADES Theodoros G. Theodosiou1, Aikaterini G. Tsikaloudaki2, Karolos J. Kontoleon3, Dimitrios K. Bikas4,
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Keywords: point thermal bridge, external cladding systems, heat transfer
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Dep. of Civil Engineering, Aristotle University of Thessaloniki, PO BOX 429, 541 24 Thessaloniki, Greece 1 2 3 e-mail: [email protected], [email protected], [email protected], [email protected] ; web page: http://lbcp.civil.auth.gr
INTRODUCTION In contemporary architecture, cladding systems are widely used as an outer skin of new and retrofitted buildings. Among their most important advantages one could mention the flexibility in developing interesting aesthetic effects due to the numerous alternatives of cladding materials, as well as their potential to outline efficient multi-layered building elements, thus providing the necessary protection against the climatic elements and contributing to the formation of comfortable indoor conditions. Within the frame of improving the energy performance of new and existing buildings in all European countries, requirements regarding the thermal insulation of the building envelope have become more demanding over the last decades, and are expected to become even more exigent in the future as an important step towards minimisation of thermal losses [1]. Lightweight façade cladding systems are a worldwide established method for constructing new buildings and renovating existing ones. In the case of façade renovations, these systems have become a very attractive alternative to external thermal insulation composing systems (ETICS). Condensation issues in most varieties of such systems are very limited since an air cavity which allows “breathing” separates the insulation material from the external cover (usually an aluminium panel). The ability to use any type of insulation material, without condensation risks on a lightweight construction, combined with the superiority of external insulation, has made such systems very attractive in the effort to upgrade buildings’ energy performance with a view to fulfil recently introduced energy performance requirements. The most prevalent types of cladding systems are lightweight aluminium curtain wall panels, applied mainly to provide a good external appearance with limited maintenance needs, to achieve a reduction in construction time or to adopt important sustainable characteristics in the design process. Such systems can be recycled by almost 100%, can host photovoltaic elements for electricity production and can also increase the energy performance of the building by permitting the application of any type of thermal insulation material on the external side of the interior wall. Beyond these characteristics, with proper design and control, such systems can have a significant contribution to the overall energy performance of a building, with the use of phase change materials, airflow channelling or optimal control of the ventilation within the façade [2, 3].
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ABSTRACT Cladding systems have become an attractive solution in energy renovation of existing buildings since they combine multiple benefits. In such systems, the use of a metallic frame and brackets, which penetrates the thermal insulation layer, creates point thermal bridges that are usually neglected by thermal insulation regulations due to their calculation complexity and their supposed small magnitude in overall heat losses. The objective of this study is to perform a parametric analysis to quantify the magnitude of this thermal bridging effect. All necessary calculations are performed using a detailed 3-dimensional numerical analysis approach in order to overcome oversimplifications found in most thermal bridge estimation methods. Results of the analysis are applied in a refurbishment study of an existing office building in order to underline the significance of the problem under investigation, in practice. As it is shown, point thermal bridge effects in cladding systems can constitute a significant part of buildings’ thermal balance. Neglecting their presence can lead to significant underestimation of actual heat flows which can account for 5% to almost 20% of total heat flows through the building envelope, depending mostly on the thermal transmittance of the load bearing wall and the ventilation characteristics of the air cavity.
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Despite the very detailed nature of recent published studies on the effects of ventilation of the air cavity, the extent of heat flow due to thermal bridges is usually not examined as this matter could complicate further the already complex simulation models. In most such studies, the presence of the anchorage system, despite its dense geometry, is usually not taken into account [4-6]. On the other hand, common solutions to ensure the energy consciousness of buildings (insulation placed externally, internally or within the cavity) can take into account of the varying thermal bridging effect. For example, as it is well known, external insulation systems can result to a significant reduction in thermal bridging compared to cavity and internal insulation in brick-wall constructions. The ever demanding legislation requirements for improved thermal insulation protection has resulted in thicker insulation layers than can provide better thermal protection. Unfortunately, since heat flow, due to thermal bridging, remains relatively unaffected by insulation thickness, its effect to the overall heat loss from the building envelope has increased over time [7, 8]. In most national legislations concerning thermal insulation and energy performance of buildings, only linear thermal bridges are considered; accordingly, the heat loss calculation methodology is covered by methodologies found in international standards like ISO 10211[9] which neglect the calculation of point or 3D thermal bridges (complicated approach requiring modern computational tools and proper engineering background). This simplification is acceptable for the usual design process, although still some criticism suggests further accuracy improvements [10, 11]. This assumption is feasible since in conventional envelopes these types of thermal bridges have a minor and localised contribution compared to the prevailing linear thermal bridges (diminutive error). Thus, linear thermal bridges are responsible for the greatest part of such thermal losses. Moreover, in many countries, not only is there no methodology provided for taking into account these heat flows, but even worse, in practice, such considerations are not permitted by official regulations. As it is clear for lightweight cladding systems, where the cladding is usually connected to the building envelope through metallic wall fasteners that penetrate thermal insulation, neglecting the point thermal bridges might be a source of significant estimation error. Even very thin metallic frames within the envelope can lead to annual energy consumption by 5% or more [12]. In the case of new buildings, a modern metallic frame that can support the load due to the cladding can be treated in a variety of ways so as to minimise thermal insulation penetration. Fewer anchorage points would result in larger point loads that can be taken into account during the design process and, in most cases, the effect of point thermal bridges can be reduced, although it might still constitute a considerable portion of the overall heat loss [13, 14]. In existing buildings, however, it is safer to distribute the load to a dense network of anchors due to the uncertainty of actual bearing capabilities of the existing wall which forms the substrate of the cladding system. Although this construction method provides significant safety concerning the load bearing capabilities of the envelope, is has a negative effect due to additional point thermal bridges caused by the wall-fastening unit. Such thermal bridges are usually neglected in many national building codes. The result is that, although such a renovation is thought to improve the energy performance of the building according to existing methodologies, in reality it might fail to do so, if treated as a traditional solid wall construction. On the other hand, the alternative of the extended aluminium supporting frames attached to the building can better distribute the bearing load, but unfortunately, modify the nature of thermal bridges from point to linear ones, causing excessive thermal losses and almost abandoning the purpose of thermal insulation of the envelope. During the last two decades, the design of such systems has concentrated on replacing the use of linear metal parts in contact with the substrate with brackets where the framework will be mounted on, in front of the insulation layer to minimise linear thermal bridges [13]. This has drastically altered the nature of heat flows by conduction. The purpose of this study is to investigate and quantify the influence of the most common design factors of cladding systems on the actual thermal loss through the building envelope. METHODOLOGY Heat flows through point thermal bridges can only be accessed through 3D finite element modelling, since their complex nature does not permit simpler approaches as are usually assumed with linear thermal bridges [1517]. For the purpose of this study, a representative lightweight cladding system is selected and examined under steady-state thermal conditions using finite element analysis Abaqus [18] . The most important parameters that can be determined during the design process of lightweight cladding systems are investigated in order to quantify the effect of point thermal bridges on the unwanted heat flow. Afterwards, the results of this investigation are utilised in order to evaluate the magnitude of point thermal bridge effect in a refurbishment study of an office building situated in the university campus of the Aristotle University of Thessaloniki, in Greece (40.63° N, 22.96° E). 2.1
Description of a lightweight cladding system
The examined model represents the most common arrangement in the case of envelope renovations. It consists of a solid substrate wall, on which the cladding system is anchored through steel wall-fasteners. The examined distance among them is determined by the standard dimensions of external aluminium cladding
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elements on the market, having a width of 80 cm, although the distance among wall-fasteners does not alter the thermal bridge effect. Wall-fasteners are distributed equally in the vertical as well as the horizontal direction. An aluminium mullion connects the external edges of the adjacent brackets. The external cover of the system consists of aluminium panels connected to the mullion. Insulation (mineral-wool) of thermal conductivity λ = 0.03 m²•K/W is in contact with the substrate. The width of the remaining air gap between the insulation layer and the external cladding (which is also analysed) acts as an additional insulation layer, while additional benefits related to the ventilation of the air gap can be considered. The contribution of the air cavity width and ventilation type is a significant factor of the examined element’s thermal balance that cannot be neglected. Since the actual air velocity and flow within the cavity is a highly dynamic, multi-factor related phenomenon affected by the wind, solar gains, temperature variations, stack effect and the actual geometry, no general results could be obtained by a transient study in order to relate the outputs of the modelling to the steady-state approach applied in all thermal insulation methodologies and national codes [15, 19-21]. In this study, the analysis is carried out in accordance with most national codes; hence, the air cavity is treated as an additional thermal resistance, as described in ISO 6946 [22]. In all examined scenarios, two types of steel brackets having a thickness of 4 mm are examined, both having characteristics in accordance with the relevant ETAG (European Technical Approvals Guidelines) [23]. Type “L” has a surface 50 mm x 100 mm in contact with the solid wall. A single steel anchor, having a diameter of 8 mm and a length of 80 mm (placed in the centre), ensures a tight contact with the solid wall element. The length of the anchor that penetrates the solid wall is 66 mm. Type “T” has a surface of 100 mm x 100 mm in contact with the solid wall construction and two steel anchors penetrating the substrate element (Fig. 1). Anchors in type “T” bracket have the same characteristics as in the case of type “L”. Both bracket shapes are tested with and without the existence of a 6 mm thermal break layer made of FPVC with density of 800 kg/m³ that is placed between the bracket and the substrate construction to decrease heat flow by preventing the direct contact between the steel bracket and the solid wall element (heat flow by conduction).
Figure 1. The selected cladding element system.
The examined cladding element is free of linear thermal bridges and the point thermal transmittance (χ) can be estimated by the following simplified equation [9]:
χ = L3D −
Ni
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where:L3D is the thermal coupling coefficient obtained from a 3-D calculation of the 3-D component separating the two environments being considered; Ui is the thermal transmittance of the 1-D component i separating the two environments being considered; Ai is the area over which the value Ui applies; Ni is the number of 1-D components. 2.2
Investigated scenarios
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Heat transfer within a lightweight cladding system is a relatively complex phenomenon that is mainly affected by the following factors: The thickness and thermal characteristics of the main wall element (substrate).
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The thickness and thermal characteristics of the insulation layer placed on the external surface of the main wall element.
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The characteristics of the air cavity between the external cover and the insulation layer in contact with the substrate (air movement, air temperature, dimensions of air gap).
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The existence of a thermal break, to minimise the contact between the steel bracket/anchor and the substrate .
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The material and characteristics of the brackets (geometry and thermophysical properties of anchoring elements) that thermally connect the exterior cladding with the interior façade, by penetrating the thermal insulation protection. With the exception of the characteristics of the air cavity, which can be sufficiently treated only in the case of a whole building dynamic energy simulation, all the other factors are investigated in this work. Table 1 specifies the values and variations of the studied parameters (thickness and thermal conductivity) for each section of the lightweight cladding system. Although some values have no rational applicability, like insulation layer thicknesses of less than 5 cm, such values are considered in this study to assist presenting the physics beneath the examined phenomenon as it will be shown in the following sections.
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Table 1. Values and variations of studied parameters used in every parametric scenario. Section of the cladding system
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Thermal break (FPVC) Bracket base
Steel bracket/anchor
(2.40*1), 1.80, 0.50, 0.27, 0.20
dins = 10, 15, 20, (50*1), 60, 80, 100, 150, 200, 250, 280 ventilated scenario: dair gap = 20 unventilated scenario: dair gap = 20 no air cavity: dair gap = 0
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Aluminium profile (mullion)
200
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Insulation Air cavity
Thermal conductivity λ [W/m·K]
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Substrate
Thickness d [mm]
0.03
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50
160 [24]
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L: 50 x 100 T: 100 x 100
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db = dins + dair cavity + 15 *2
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Remarks: *1 within parentheses the default values are provided. *2 the length of the steel bracket/anchor equals the insulation thickness plus the air cavity thickness plus 15 mm to account for the mullion connection. The examined element’s boundary conditions are determined by a temperature difference of 20°C (0°C for the external air and 20°C for the interior, “room” air). This temperature difference does not affect the magnitude of the thermal bridge effect, since all the related values are independent of temperature; it affects only the presented isothermals within the element. The equal distribution of wall-fasteners in the examined façade permits the modelling of only one representative square wall element of 0.70 m x 0.70 m. Boundary conditions of the four sides of this element can thus be described by thermal symmetry, leading to significant reduction of computation time. Preliminary calculations have shown that for the examined wall element, dimensions larger than 0.60 m x 0.60 m do not have a significant effect on the magnitude of the calculated point thermal transmittance. Each scenario requires rebuilding the calculation mesh which consists of 100000 to 140000 triangular domain elements, depending on the examined parameter (Fig2). This discretization is in compliance with ISO 10211 requirements in order to ensure calculation errors due to discretization, below 1%.
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Figure 2. Calculation mesh (part of the external layers are missing for presentation purposes)
3 3.1
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With respect to the earlier decisions, it becomes evident that the actual heat flow in a lightweight cladding system is a multi-parametric problem that involves several factors. In order to simplify the analysis, some factors were selected in such a way so as to represent the most common cases found in real constructions. These cases can be expressed in the following scenarios. Firstly, the effect of the thermal resistance of the substrate was initially modelled for an unventilated air cavity. The scope of this modelling scenario is to examine a range of thermal conductivities for typical building materials, ranging from a highly conductive one, like reinforced concrete (with thermal conductivity as high as λ = 2.40 W/m·K), to a material used for walls with low thermal conductivity, such as lightweight concrete blocks (λ = 0.20 W/m·K). Since the steel anchors penetrate the wall, it is important to know how heat will propagate from the interior to the exterior environment depending on the wall type (concrete or lightweight brick wall). Secondly, the overall thermal protection of a lightweight cladding system is essentially influenced by the proper selection of the insulation material (thermal resistance of insulation layer based on its thickness and thermal conductivity), as well as the air cavity existence and characteristics. Thus, if the air cavity is naturally ventilated or not, influences the heat flows, according to existing standards and national codes. In this work, three scenarios that take into account the effect of the insulation thickness and the air cavity state on the magnitude of thermal bridges are considered and analysed. In every case, the length of the bracket, from which large heat flows are expected, is adjusted to the varying geometry in order to be extended by 15 mm beyond the air cavity (or the insulation in the absence of air cavity), so as to permit the connection to the mullion. Additionally, the effect of the bracket shape (“L” or “T” type) as well as the thermal break is taken into account in every scenario. RESULTS & DISCUSSION Bracket type
The geometry of the bracket type is a significant factor of the thermal bridge effect. Heat is transmitted by direct contact among the bracket and the substrate and through the anchors penetrating the substrate Fig. 3(a)(b) shows the temperature distribution at the junction among the two elements (steel bracket and solid wall) for all bracket types examined. The presented isothermal graphs correspond to the “L” and “T” bracket shapes: (a) no thermal break and (b) with thermal break. It is obvious that the use of a thermal break can decrease direct heat flow due to the low thermal conductivity of FPVC placed at the junction among the bracket and the substrate. Although the bracket area affected by direct contact is minimised, the thermal break cannot provide a fully effective measure against increased heat flow concentrated around the fasteners penetrating the load bearing wall. It becomes clear that the fasteners are the weakest part of the anchorage system, since they does not allow a further heat flow decrease due to load transfer requirements. As it can be seen in Fig. 3, the direct effect of using a thermal break is to limit the surface area affected by heat flow through the anchor, while an almost analogous increase in the intensity of heat flow around the fasteners, within the body of the substrate, is exposed. In this area, heat flow is fundamentally affected by the construction material of the substrate. Its influence is defined in the following section.
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Figure 3. The effect of bracket type in the temperature distribution at the junction among the bracket with the substrate, in the case of (a) no thermal break and (b) thermal break existence.
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The material of the substrate supporting the cladding has a significant contribution to the magnitude of point thermal transmittance (χ), as the results demonstrate in Fig. 4. Since heat flow bypasses the thermal insulation layer through the bracket, the thermal resistance of the wall is highly related to thermal losses. In the examined range of thermal resistances that represent most common wall materials, reinforced concrete is the weakest one, since its thermal transmittance (U-value, shown in the right axis) can be almost double than in clay bricks, in all bracket arrangements. The examined U-value range represents the most common cases since materials with lower thermal transmittance are usually not adequate for supporting the weight of a cladding system. By examining a unit area (1m²) of a lightweight façade system, the additional heat flows due to the thermal bridge effect are significant. Expressing these additional heat flows as a percentage of the heat flows calculated in one dimentional methodologies, or otherwise expressing point thermal transmittance (χ) per unit area as a percentage of U-value in the case of high-conductive substrate like a concrete wall, its magnitude reaches 9.4% and 9.9% of the U-value for the “L” and “T” bracket respectively, without a thermal break. On the other hand, the protection offered by the thermal break decreases these values to 7.2% and 7.1% for the“L.TB” and “T.TB” bracket respectively. In the case of a wall material with lower thermal resistance, these values range between 2.0%-2.8%. These results clearly indicate the underestimation of thermal loss in many national thermal insulation methodologies pertaining to lightweight cladding. In order to fulfill the thermal insulation requirements, one should enhance by 7%-10% the insulation protection only to cover the excess losses due to the point thermal bridges. In addition, it is obvious that the importance of the themal break element reduces as the substrate becomes less conductive; as it is shown, in the case of lightweight concrete blocks, the difference between a bracket with and without a thermal break is hardly noticeable (for both investigated bracket shapes).
Figure 4. The effect of the substrate thermal resistance on point thermal transmittance for an unventilated façade element.
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3.3
Insulation layer thickness
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Although the substrate material plays an important role in heat flow through the wall-fastener, the thermal insulation protection of any building envelope relies mainly on the proper selection of the thermal insulation material. In the case of linear thermal bridges, linear thermal transmittance (Ψ) is inversely proportional to the overal thermal transmittance (U-value) [17]. In the extreme case that no thermal protection exists, then the thermal bridge effect is insignificant, since the thermal transmitance values of the materials composing the wall element are rather similar. On the contrary, for well insulated building envelopes, the heat exchange due to the thermal bridging effect is important. The question is wheter the same is valid for point thermal bridges. According to the results for the case of the investigated cladding system with an unventilated air cavity, this trent is not shown. Obviously, the significantly higher thermal transmitance of the steel anchor penetrating the substrate does not permit heat flow of the same magnitute in the area of a typical wall as well as the area close to the anchors, even when no thermal insulation material is present (Fig. 5). With better insulation protection, point thermal tranmittance increases up to a point close to 0.06 W/m²·K in the case of insulation layer thickness of about 0.06 m. For better insulated walls, point thermal bridge decreases. The results of this scenario clearly explain the absence of simple methodologies to account for point thermal bridges, since the magnitute of the phenomenon is simultaneously affected by many factors. Since heat flow within the bracket is related not only to the conductivity of steel, but also to the length of the bracket (which is surrounded by the insulation material), there is a specific point in the examined geometry, where the thermal resistance (defined as the length of the bracket normal to heat flow to the thermal conductance of it material) of the bracket, alters the relation between the heat flow within the bracket and the heat flow through the insulated wall, resulting in an inverse analogy among the presented parameters. For medium thermal insulation requirements, like those found in southern Europe (required U-value of wall around 0.50 W/m²·K), point thermal transmittance reaches its highest values, while for more demanding national requirements, like those found in northen Europe (U-value less than 0.30 W/m²·K), the effect of point thermal bridge effect decreases. On the other hand, despite the insulation requirements, with the exception of “L.TB” bracket shape (with thermal break), χ-values per unit area as a percentage of U-value for the “L”, “T” and T.TB” types demonstrate a small variation between 7.5%-9.8%. In the case of the “L.TB” bracket shape with thermal break, this range is 6%-7.5%. From the above, it can be easily concluded that although thermal insulation has a contribution to the magnitude of the thermal bridge, this is less significant than the substrate material, in contrast to the linear thermal bridging effect.
Figure 5. The effect of insulation thickness on point thermal transmittance for an unventilated air cavity. In the case of a ventilated air cavity, the modelling assumptions are that the influence of the air cavity is ignored and the surface resistance of the insulation layer and the exposed bracket decreases from 25 m²·K/W to 7.70 m²·K/W, according to ISO 6946. This assumption decreases and amplifies both the U-value and the χ-value of the examined scenarios, respectively (Fig. 6). Heat flow increases due to the thermal bridge effect accounting for a value between 5.5%-7% of the element thermal transmittance for all scenarios, apart from the “L.TB” shape bracket for which the difference ranges between 4.5%-6%.
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Figure 6. The effect of air cavity thickness on point thermal transmittance for a ventilated air cavity.
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In the absence of air cavity (Fig. 7), when the insulation layer is in contact with the external panel, the overall heat transfer of the wall increases due to lower thermal resistance. In modelling, this is not only related to the thermal exposure of the bracket directly to ambient conditions, but also to the shorter length of the bracket and its consequent smaller thermal resistance. In weak thermal protection levels, the point thermal bridge is maximised, despite the fact that the overall wall element is poorly protected, since the anchor penetrating the wall is in almost ambient temperature and leads to significant heat flow. Even in the other cases, the effect is significant and χ-values form a large proportion of the U-value, within the range of 8%-20% for every bracket type, except of “L.TB” shape bracket where the range is 6.5%-17%.
Figure 7. The effect of insulation thickness on point thermal transmittance for a wall without an air cavity. 3.4
Magnitude of point thermal bridges on the overall building thermal transmittance
The magnitude of the examined thermal bridge effect is examined in a refurbishment study of an office building located in the Aristotle University Campus in Thessaloniki, Greece (Fig. 8). The examined building has a compact shape. Each one of its storeys covers an area of 508 m², while its main facades are occupied by windows (60%) and concrete walls. Side facades consist of concrete walls with no windows. The ground floor hosts unheated spaces and is not considered part of the renovation study. In a study related to the refurbishment of the building envelope in order to meet the current national thermal insulation requirements, an aluminium cladding system is examined. Since current national requirements are not considered to be demanding, as is the case in most southern European countries, an additional case is examined with requirements resembling north European ones. Regarding the permitted allowable thermal transmittance of the external walls, U-values for these two sets of requirements are 0.40 W/m²·K and 0.20 W/m²·K, respectively. All possible air cavity ventilation types (unventilated, ventilated and no air cavity), and both bracket types “L” and “T” types with thermal break are examined. Table 2 presents the most important characteristics of the examined cases.
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Figure 8. Plan and axonometric view of the examined office building.
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For the secure installation of the façade system, the maximum distance between the brackets is 0.70 m, which is considered a normal size. In order for the aluminium cladding to cover all the facades of the building, a total of almost 9,000 brackets are required.
Enhanced insulation Insulation U AxU thickness [m] [W/m²·K] [W/K] 0.198 426.08 0.16 0.201 433.80 0.205 441.81 0.16 0.197 99.96 0.16 0.197 99.99 1.400 1,511.44 224.62
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Table 2. Characteristics of the examined building envelope. Area Minimum insulation Insulation U AxU Element Type of thickness air gap ventilation [m²] [m] [W/m²·K] [W/K] Walls Unventilated 0.403 867.34 0.07 Ventilated 2,153.50 0.418 899.96 No cavity 0.434 935.13 Roof 508.30 0.07 0.398 202.21 Floor 508.30 0.07 0.398 202.32 Windows 1,079.60 2.300 2,483.08 Linear thermal bridges 224.62
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Under steady state conditions, the weakest parts of the building envelope, by means of thermal losses, are the windows on the main facades of the building; more specifically, according to the data presented in Table 2, 62% to 64% of heat flows through the envelope are attributed to windows, 20% to 18% to walls and 6% to 10% to linear thermal bridges, for the cases of present and more demanding thermal insulation requirements respectively. These values refer to a calculation methodology that neglects the presence of point thermal bridges. Utilising the results from the previous study to incorporate the magnitude of heat flows due to point thermal bridge effect, significant additional thermal flows have to be taken into account. For every examined scenario of air cavity ventilation type, bracket shape, under both thermal insulation requirement levels, these additional heat flows are presented in Table 3. In all presented scenarios the best practice of using a thermal brake in every bracket has been taken into account. Table 3. Heat flows through thermal bridges for the examined building. Percentage on Point thermal Heat flow total heat flows transmittance χ Air cavity type [W/K] [%] [W/K ] Bracket type Thermal insulation Minimum Enhanced Minimum Enhanced Minimum Enhanced requirements insulation insulation insulation insulation insulation insulation L 0.047 0.059 424.69 528.61 10% 18% Unventilated T 0.059 0.050 528.61 451.80 12% 16% L 0.037 0.043 334.33 388.55 89% 1413% Ventilated T 0.036 0.040 320.78 361.44 78% 1312% L 0.064 0.050 578.30 447.28 13% 16% No cavity T 0.079 0.060 709.33 537.64 15% 18%
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As it can be seen, the magnitude of point thermal bridge can be significant in the case of the examined office building ranging from almost 320 W/K to 710 W/K for the case of a ventilated façade and a façade with no air cavity respectively, both for a T-shape bracket. One important conclusion is that the magnitude of point thermal bridges –which are not taken into account in most national methodologies- can be even by 200% higher than the effect of linear thermal bridges –which are almost always taken into account. Among the thermal insulation requirements, better insulated elements (adequate insulation) result in a higher percentage of heat flow as it has already been analysed in the parametric analysis. Especially in the case of the ventilated air cavity, the upgrade of thermal insulation thickness, makes the problem of point thermal bridges more intense. In fact, the ventilated cavity seems to be the most sensitive to the insulation thickness, as it can be seen from the wider distribution of the results, compared to the other two examined cavity types. On the other hand, maximum values are found in the other two air cavity types. CONCLUSIONS
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The results of this study have shown that, neglecting the point thermal bridge effect in lightweight cladding systems can lead to a considerable underestimation of actual heat flows. The magnitude of this error is comparable (if not higher in some cases) to that of linear thermal bridging. Compared to the linear ones, point thermal bridges present a more complicated nature and are not strongly related to the overall insulation level of the wall. As it is shown, the most important factors of the examined phenomena are the thermal resistance of the load bearing wall where the cladding system in anchored and the air cavity existence. The thermal resistance of the inner wall controls the heat flow through the highly conductive wall-fastener unit that penetrates the insulation layer. The air cavity acts as an additional insulation layer that decreases heat flow through the point thermal bridge. Although thermal breaks are supposed to thermally separate the bracket and the substrate, the existence of fasteners directly inserted within the substrate cannot allow for an efficient heat flow decrease. Consequently, thermal breaking is not among the factors with the greatest impact. Since aluminium cladding systems have become an appealing method for the upgrade of the thermal insulation of existing buildings but current national methodologies are not always well adapted to the particularities of such systems, the magnitude of heat flow underestimation can range from 5% up to even 20%, which is definitely a significant part of the thermal budget of a building. In such cases, special attention should be paid during the design process to minimise these heat flows in order to fully exploit their numerous advantages, as it has been presented in this study. Finally, this study proves that compared to other façade retrofitting techniques like ETICS, aluminium cladding systems require different treatment and simplifications for the estimation of thermal bridging effects in order to avoid underestimations that could significantly affect the effectiveness of the retrofit action. REFERENCES [1] E. Parliament, Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings., in, 2010. [2] A. de Gracia, L. Navarro, A. Castell, L.F. Cabeza, Energy performance of a ventilated double skin facade with PCM under different climates, Energy and Buildings, 91 (0) (2015) 37-42. [3] M.I. Nizovtsev, V.T. Belyi, A.N. Sterlygov, The facade system with ventilated channels for thermal insulation of newly constructed and renovated buildings, Energy and Buildings, 75 (0) (2014) 60-69. [4] A.S. Anđelković, B. Gvozdenac-Urošević, M. Kljajić, M.G. Ignjatović, Experimental research of the thermal characteristics of a multi-storey naturally ventilated double skin façade, Energy and Buildings, 86 (0) (2015) 766-781. [5] F. Marques da Silva, M.G. Gomes, A.M. Rodrigues, Measuring and estimating airflow in naturally ventilated double skin facades, Building and Environment, 87 (0) (2015) 292-301. [6] A.L.S. Chan, T.T. Chow, Calculation of overall thermal transfer value (OTTV) for commercial buildings constructed with naturally ventilated double skin façade in subtropical Hong Kong, Energy and Buildings, 69 (0) (2014) 14-21. [7] T.G. Theodosiou, A.M. Papadopoulos, The impact of thermal bridges on the energy demand of buildings with double brick wall constructions, Energy and Buildings, 40 (11) (2008) 2083-2089. [8] B. Berggren, M. Wall, Calculation of thermal bridges in (Nordic) building envelopes – Risk of performance failure due to inconsistent use of methodology, Energy and Buildings, 65 (0) (2013) 331-339. [9] E.C.f. Standardization, EN ISO 10211, Thermal bridges in building construction - heat flows and surface temperatures - Detailed calculations, in, European Commitee for Standardization, 2007.
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[10] F. Ascione, N. Bianco, F. De Rossi, G. Turni, G.P. Vanoli, Different methods for the modelling of thermal bridges into energy simulation programs: Comparisons of accuracy for flat heterogeneous roofs in Italian climates, Applied Energy, 97 (2012) 405-418. [11] F. Aguilar, J.P. Solano, P.G. Vicente, Transient modeling of high-inertial thermal bridges in buildings using the equivalent thermal wall method, Applied Thermal Engineering, 67 (1–2) (2014) 370-377. [12] A.P. Gomes, H.A. de Souza, A. Tribess, Impact of thermal bridging on the performance of buildings using Light Steel Framing in Brazil, Applied Thermal Engineering, 52 (1) (2013) 84-89. [13] M. Gorgolewski, Developing a simplified method of calculating U-values in light steel framing, Building and Environment, 42 (1) (2007) 230-236. [14] S.-Y. Song, J.-S. Yi, B.-K. Koo, Insulation plan of aluminum curtain wall-fastening unit for high-rise residential complex, Building and Environment, 43 (7) (2008) 1310-1317. [15] C. Sanjuan, M.J. Suárez, M. González, J. Pistono, E. Blanco, Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade, Solar Energy, 85 (9) (2011) 1851-1863. [16] F. Ascione, N. Bianco, R.F. De Masi, F. de' Rossi, G.P. Vanoli, Simplified state space representation for evaluating thermal bridges in building: Modelling, application and validation of a methodology, Applied Thermal Engineering, 61 (2) (2013) 344-354. [17] A.E. Ben-Nakhi, Minimizing thermal bridging through window systems in buildings of hot regions, Applied Thermal Engineering, 22 (9) (2002) 989-998. [18] I. Abaqus, Pawtucket,RI,USA, Abaqus, in, http://www.3ds.com/. [19] H.Poirazis, Double-skin facades for office buildings-literature review, inQ Report ECBCS Annex 43,, in, Lund Institute of Technology, Sweden, 2006. [20] M.J. Suárez, C. Sanjuan, A.J. Gutiérrez, J. Pistono, E. Blanco, Energy evaluation of an horizontal open joint ventilated faade, Applied Thermal Engineering, 37 (2012) 302-313. [21] C. Marinosci, P.A. Strachan, G. Semprini, G.L. Morini, Empirical validation and modelling of a naturally ventilated rainscreen façade building, Energy and Buildings, 43 (4) (2011) 853-863. [22] EN, ISO 6946, Building components and building elements - Thermal resistance and thermal transmittance - Calculation method, in, 1996. [23] ETAG, ETAG 034. Guideline for European technical approval of kits for external wall claddings. Part 1: Ventilated cladding kits comproising cladding components and associated fixings, in, 2012. [24] E.C.f. Standardization, EN ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values and procedures for determining declared and design thermal values, in, European Commitee for Standardization, 2007.
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S0378-7788(15)30351-0 http://dx.doi.org/doi:10.1016/j.enbuild.2015.10.037 ENB 6231
To appear in:
ENB
Received date: Revised date: Accepted date:
9-6-2015 31-8-2015 15-10-2015
Please cite this article as: T.G. Theodosiou, THERMAL BRIDGING ANALYSIS ON CLADDING SYSTEMS FOR BUILDING FACADES, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.10.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
THERMAL BRIDGING ANALYSIS ON CLADDING SYSTEMS FOR BUILDING FACADES Theodoros G. Theodosiou1, Aikaterini G. Tsikaloudaki2, Karolos J. Kontoleon3, Dimitrios K. Bikas4,
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Keywords: point thermal bridge, external cladding systems, heat transfer
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Dep. of Civil Engineering, Aristotle University of Thessaloniki, PO BOX 429, 541 24 Thessaloniki, Greece 1 2 3 e-mail: [email protected], [email protected], [email protected], [email protected] ; web page: http://lbcp.civil.auth.gr
INTRODUCTION In contemporary architecture, cladding systems are widely used as an outer skin of new and retrofitted buildings. Among their most important advantages one could mention the flexibility in developing interesting aesthetic effects due to the numerous alternatives of cladding materials, as well as their potential to outline efficient multi-layered building elements, thus providing the necessary protection against the climatic elements and contributing to the formation of comfortable indoor conditions. Within the frame of improving the energy performance of new and existing buildings in all European countries, requirements regarding the thermal insulation of the building envelope have become more demanding over the last decades, and are expected to become even more exigent in the future as an important step towards minimisation of thermal losses [1]. Lightweight façade cladding systems are a worldwide established method for constructing new buildings and renovating existing ones. In the case of façade renovations, these systems have become a very attractive alternative to external thermal insulation composing systems (ETICS). Condensation issues in most varieties of such systems are very limited since an air cavity which allows “breathing” separates the insulation material from the external cover (usually an aluminium panel). The ability to use any type of insulation material, without condensation risks on a lightweight construction, combined with the superiority of external insulation, has made such systems very attractive in the effort to upgrade buildings’ energy performance with a view to fulfil recently introduced energy performance requirements. The most prevalent types of cladding systems are lightweight aluminium curtain wall panels, applied mainly to provide a good external appearance with limited maintenance needs, to achieve a reduction in construction time or to adopt important sustainable characteristics in the design process. Such systems can be recycled by almost 100%, can host photovoltaic elements for electricity production and can also increase the energy performance of the building by permitting the application of any type of thermal insulation material on the external side of the interior wall. Beyond these characteristics, with proper design and control, such systems can have a significant contribution to the overall energy performance of a building, with the use of phase change materials, airflow channelling or optimal control of the ventilation within the façade [2, 3].
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ABSTRACT Cladding systems have become an attractive solution in energy renovation of existing buildings since they combine multiple benefits. In such systems, the use of a metallic frame and brackets, which penetrates the thermal insulation layer, creates point thermal bridges that are usually neglected by thermal insulation regulations due to their calculation complexity and their supposed small magnitude in overall heat losses. The objective of this study is to perform a parametric analysis to quantify the magnitude of this thermal bridging effect. All necessary calculations are performed using a detailed 3-dimensional numerical analysis approach in order to overcome oversimplifications found in most thermal bridge estimation methods. Results of the analysis are applied in a refurbishment study of an existing office building in order to underline the significance of the problem under investigation, in practice. As it is shown, point thermal bridge effects in cladding systems can constitute a significant part of buildings’ thermal balance. Neglecting their presence can lead to significant underestimation of actual heat flows which can account for 5% to almost 20% of total heat flows through the building envelope, depending mostly on the thermal transmittance of the load bearing wall and the ventilation characteristics of the air cavity.
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Despite the very detailed nature of recent published studies on the effects of ventilation of the air cavity, the extent of heat flow due to thermal bridges is usually not examined as this matter could complicate further the already complex simulation models. In most such studies, the presence of the anchorage system, despite its dense geometry, is usually not taken into account [4-6]. On the other hand, common solutions to ensure the energy consciousness of buildings (insulation placed externally, internally or within the cavity) can take into account of the varying thermal bridging effect. For example, as it is well known, external insulation systems can result to a significant reduction in thermal bridging compared to cavity and internal insulation in brick-wall constructions. The ever demanding legislation requirements for improved thermal insulation protection has resulted in thicker insulation layers than can provide better thermal protection. Unfortunately, since heat flow, due to thermal bridging, remains relatively unaffected by insulation thickness, its effect to the overall heat loss from the building envelope has increased over time [7, 8]. In most national legislations concerning thermal insulation and energy performance of buildings, only linear thermal bridges are considered; accordingly, the heat loss calculation methodology is covered by methodologies found in international standards like ISO 10211[9] which neglect the calculation of point or 3D thermal bridges (complicated approach requiring modern computational tools and proper engineering background). This simplification is acceptable for the usual design process, although still some criticism suggests further accuracy improvements [10, 11]. This assumption is feasible since in conventional envelopes these types of thermal bridges have a minor and localised contribution compared to the prevailing linear thermal bridges (diminutive error). Thus, linear thermal bridges are responsible for the greatest part of such thermal losses. Moreover, in many countries, not only is there no methodology provided for taking into account these heat flows, but even worse, in practice, such considerations are not permitted by official regulations. As it is clear for lightweight cladding systems, where the cladding is usually connected to the building envelope through metallic wall fasteners that penetrate thermal insulation, neglecting the point thermal bridges might be a source of significant estimation error. Even very thin metallic frames within the envelope can lead to annual energy consumption by 5% or more [12]. In the case of new buildings, a modern metallic frame that can support the load due to the cladding can be treated in a variety of ways so as to minimise thermal insulation penetration. Fewer anchorage points would result in larger point loads that can be taken into account during the design process and, in most cases, the effect of point thermal bridges can be reduced, although it might still constitute a considerable portion of the overall heat loss [13, 14]. In existing buildings, however, it is safer to distribute the load to a dense network of anchors due to the uncertainty of actual bearing capabilities of the existing wall which forms the substrate of the cladding system. Although this construction method provides significant safety concerning the load bearing capabilities of the envelope, is has a negative effect due to additional point thermal bridges caused by the wall-fastening unit. Such thermal bridges are usually neglected in many national building codes. The result is that, although such a renovation is thought to improve the energy performance of the building according to existing methodologies, in reality it might fail to do so, if treated as a traditional solid wall construction. On the other hand, the alternative of the extended aluminium supporting frames attached to the building can better distribute the bearing load, but unfortunately, modify the nature of thermal bridges from point to linear ones, causing excessive thermal losses and almost abandoning the purpose of thermal insulation of the envelope. During the last two decades, the design of such systems has concentrated on replacing the use of linear metal parts in contact with the substrate with brackets where the framework will be mounted on, in front of the insulation layer to minimise linear thermal bridges [13]. This has drastically altered the nature of heat flows by conduction. The purpose of this study is to investigate and quantify the influence of the most common design factors of cladding systems on the actual thermal loss through the building envelope. METHODOLOGY Heat flows through point thermal bridges can only be accessed through 3D finite element modelling, since their complex nature does not permit simpler approaches as are usually assumed with linear thermal bridges [1517]. For the purpose of this study, a representative lightweight cladding system is selected and examined under steady-state thermal conditions using finite element analysis Abaqus [18] . The most important parameters that can be determined during the design process of lightweight cladding systems are investigated in order to quantify the effect of point thermal bridges on the unwanted heat flow. Afterwards, the results of this investigation are utilised in order to evaluate the magnitude of point thermal bridge effect in a refurbishment study of an office building situated in the university campus of the Aristotle University of Thessaloniki, in Greece (40.63° N, 22.96° E). 2.1
Description of a lightweight cladding system
The examined model represents the most common arrangement in the case of envelope renovations. It consists of a solid substrate wall, on which the cladding system is anchored through steel wall-fasteners. The examined distance among them is determined by the standard dimensions of external aluminium cladding
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elements on the market, having a width of 80 cm, although the distance among wall-fasteners does not alter the thermal bridge effect. Wall-fasteners are distributed equally in the vertical as well as the horizontal direction. An aluminium mullion connects the external edges of the adjacent brackets. The external cover of the system consists of aluminium panels connected to the mullion. Insulation (mineral-wool) of thermal conductivity λ = 0.03 m²•K/W is in contact with the substrate. The width of the remaining air gap between the insulation layer and the external cladding (which is also analysed) acts as an additional insulation layer, while additional benefits related to the ventilation of the air gap can be considered. The contribution of the air cavity width and ventilation type is a significant factor of the examined element’s thermal balance that cannot be neglected. Since the actual air velocity and flow within the cavity is a highly dynamic, multi-factor related phenomenon affected by the wind, solar gains, temperature variations, stack effect and the actual geometry, no general results could be obtained by a transient study in order to relate the outputs of the modelling to the steady-state approach applied in all thermal insulation methodologies and national codes [15, 19-21]. In this study, the analysis is carried out in accordance with most national codes; hence, the air cavity is treated as an additional thermal resistance, as described in ISO 6946 [22]. In all examined scenarios, two types of steel brackets having a thickness of 4 mm are examined, both having characteristics in accordance with the relevant ETAG (European Technical Approvals Guidelines) [23]. Type “L” has a surface 50 mm x 100 mm in contact with the solid wall. A single steel anchor, having a diameter of 8 mm and a length of 80 mm (placed in the centre), ensures a tight contact with the solid wall element. The length of the anchor that penetrates the solid wall is 66 mm. Type “T” has a surface of 100 mm x 100 mm in contact with the solid wall construction and two steel anchors penetrating the substrate element (Fig. 1). Anchors in type “T” bracket have the same characteristics as in the case of type “L”. Both bracket shapes are tested with and without the existence of a 6 mm thermal break layer made of FPVC with density of 800 kg/m³ that is placed between the bracket and the substrate construction to decrease heat flow by preventing the direct contact between the steel bracket and the solid wall element (heat flow by conduction).
Figure 1. The selected cladding element system.
The examined cladding element is free of linear thermal bridges and the point thermal transmittance (χ) can be estimated by the following simplified equation [9]:
χ = L3D −
Ni
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(1)
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where:L3D is the thermal coupling coefficient obtained from a 3-D calculation of the 3-D component separating the two environments being considered; Ui is the thermal transmittance of the 1-D component i separating the two environments being considered; Ai is the area over which the value Ui applies; Ni is the number of 1-D components. 2.2
Investigated scenarios
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Heat transfer within a lightweight cladding system is a relatively complex phenomenon that is mainly affected by the following factors: The thickness and thermal characteristics of the main wall element (substrate).
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The thickness and thermal characteristics of the insulation layer placed on the external surface of the main wall element.
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The characteristics of the air cavity between the external cover and the insulation layer in contact with the substrate (air movement, air temperature, dimensions of air gap).
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The existence of a thermal break, to minimise the contact between the steel bracket/anchor and the substrate .
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The material and characteristics of the brackets (geometry and thermophysical properties of anchoring elements) that thermally connect the exterior cladding with the interior façade, by penetrating the thermal insulation protection. With the exception of the characteristics of the air cavity, which can be sufficiently treated only in the case of a whole building dynamic energy simulation, all the other factors are investigated in this work. Table 1 specifies the values and variations of the studied parameters (thickness and thermal conductivity) for each section of the lightweight cladding system. Although some values have no rational applicability, like insulation layer thicknesses of less than 5 cm, such values are considered in this study to assist presenting the physics beneath the examined phenomenon as it will be shown in the following sections.
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Table 1. Values and variations of studied parameters used in every parametric scenario. Section of the cladding system
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Steel bracket/anchor
(2.40*1), 1.80, 0.50, 0.27, 0.20
dins = 10, 15, 20, (50*1), 60, 80, 100, 150, 200, 250, 280 ventilated scenario: dair gap = 20 unventilated scenario: dair gap = 20 no air cavity: dair gap = 0
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200
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Thermal conductivity λ [W/m·K]
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Substrate
Thickness d [mm]
0.03
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50
160 [24]
6
0.09
L: 50 x 100 T: 100 x 100
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db = dins + dair cavity + 15 *2
60
Remarks: *1 within parentheses the default values are provided. *2 the length of the steel bracket/anchor equals the insulation thickness plus the air cavity thickness plus 15 mm to account for the mullion connection. The examined element’s boundary conditions are determined by a temperature difference of 20°C (0°C for the external air and 20°C for the interior, “room” air). This temperature difference does not affect the magnitude of the thermal bridge effect, since all the related values are independent of temperature; it affects only the presented isothermals within the element. The equal distribution of wall-fasteners in the examined façade permits the modelling of only one representative square wall element of 0.70 m x 0.70 m. Boundary conditions of the four sides of this element can thus be described by thermal symmetry, leading to significant reduction of computation time. Preliminary calculations have shown that for the examined wall element, dimensions larger than 0.60 m x 0.60 m do not have a significant effect on the magnitude of the calculated point thermal transmittance. Each scenario requires rebuilding the calculation mesh which consists of 100000 to 140000 triangular domain elements, depending on the examined parameter (Fig2). This discretization is in compliance with ISO 10211 requirements in order to ensure calculation errors due to discretization, below 1%.
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Figure 2. Calculation mesh (part of the external layers are missing for presentation purposes)
3 3.1
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With respect to the earlier decisions, it becomes evident that the actual heat flow in a lightweight cladding system is a multi-parametric problem that involves several factors. In order to simplify the analysis, some factors were selected in such a way so as to represent the most common cases found in real constructions. These cases can be expressed in the following scenarios. Firstly, the effect of the thermal resistance of the substrate was initially modelled for an unventilated air cavity. The scope of this modelling scenario is to examine a range of thermal conductivities for typical building materials, ranging from a highly conductive one, like reinforced concrete (with thermal conductivity as high as λ = 2.40 W/m·K), to a material used for walls with low thermal conductivity, such as lightweight concrete blocks (λ = 0.20 W/m·K). Since the steel anchors penetrate the wall, it is important to know how heat will propagate from the interior to the exterior environment depending on the wall type (concrete or lightweight brick wall). Secondly, the overall thermal protection of a lightweight cladding system is essentially influenced by the proper selection of the insulation material (thermal resistance of insulation layer based on its thickness and thermal conductivity), as well as the air cavity existence and characteristics. Thus, if the air cavity is naturally ventilated or not, influences the heat flows, according to existing standards and national codes. In this work, three scenarios that take into account the effect of the insulation thickness and the air cavity state on the magnitude of thermal bridges are considered and analysed. In every case, the length of the bracket, from which large heat flows are expected, is adjusted to the varying geometry in order to be extended by 15 mm beyond the air cavity (or the insulation in the absence of air cavity), so as to permit the connection to the mullion. Additionally, the effect of the bracket shape (“L” or “T” type) as well as the thermal break is taken into account in every scenario. RESULTS & DISCUSSION Bracket type
The geometry of the bracket type is a significant factor of the thermal bridge effect. Heat is transmitted by direct contact among the bracket and the substrate and through the anchors penetrating the substrate Fig. 3(a)(b) shows the temperature distribution at the junction among the two elements (steel bracket and solid wall) for all bracket types examined. The presented isothermal graphs correspond to the “L” and “T” bracket shapes: (a) no thermal break and (b) with thermal break. It is obvious that the use of a thermal break can decrease direct heat flow due to the low thermal conductivity of FPVC placed at the junction among the bracket and the substrate. Although the bracket area affected by direct contact is minimised, the thermal break cannot provide a fully effective measure against increased heat flow concentrated around the fasteners penetrating the load bearing wall. It becomes clear that the fasteners are the weakest part of the anchorage system, since they does not allow a further heat flow decrease due to load transfer requirements. As it can be seen in Fig. 3, the direct effect of using a thermal break is to limit the surface area affected by heat flow through the anchor, while an almost analogous increase in the intensity of heat flow around the fasteners, within the body of the substrate, is exposed. In this area, heat flow is fundamentally affected by the construction material of the substrate. Its influence is defined in the following section.
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Figure 3. The effect of bracket type in the temperature distribution at the junction among the bracket with the substrate, in the case of (a) no thermal break and (b) thermal break existence.
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The material of the substrate supporting the cladding has a significant contribution to the magnitude of point thermal transmittance (χ), as the results demonstrate in Fig. 4. Since heat flow bypasses the thermal insulation layer through the bracket, the thermal resistance of the wall is highly related to thermal losses. In the examined range of thermal resistances that represent most common wall materials, reinforced concrete is the weakest one, since its thermal transmittance (U-value, shown in the right axis) can be almost double than in clay bricks, in all bracket arrangements. The examined U-value range represents the most common cases since materials with lower thermal transmittance are usually not adequate for supporting the weight of a cladding system. By examining a unit area (1m²) of a lightweight façade system, the additional heat flows due to the thermal bridge effect are significant. Expressing these additional heat flows as a percentage of the heat flows calculated in one dimentional methodologies, or otherwise expressing point thermal transmittance (χ) per unit area as a percentage of U-value in the case of high-conductive substrate like a concrete wall, its magnitude reaches 9.4% and 9.9% of the U-value for the “L” and “T” bracket respectively, without a thermal break. On the other hand, the protection offered by the thermal break decreases these values to 7.2% and 7.1% for the“L.TB” and “T.TB” bracket respectively. In the case of a wall material with lower thermal resistance, these values range between 2.0%-2.8%. These results clearly indicate the underestimation of thermal loss in many national thermal insulation methodologies pertaining to lightweight cladding. In order to fulfill the thermal insulation requirements, one should enhance by 7%-10% the insulation protection only to cover the excess losses due to the point thermal bridges. In addition, it is obvious that the importance of the themal break element reduces as the substrate becomes less conductive; as it is shown, in the case of lightweight concrete blocks, the difference between a bracket with and without a thermal break is hardly noticeable (for both investigated bracket shapes).
Figure 4. The effect of the substrate thermal resistance on point thermal transmittance for an unventilated façade element.
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3.3
Insulation layer thickness
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Although the substrate material plays an important role in heat flow through the wall-fastener, the thermal insulation protection of any building envelope relies mainly on the proper selection of the thermal insulation material. In the case of linear thermal bridges, linear thermal transmittance (Ψ) is inversely proportional to the overal thermal transmittance (U-value) [17]. In the extreme case that no thermal protection exists, then the thermal bridge effect is insignificant, since the thermal transmitance values of the materials composing the wall element are rather similar. On the contrary, for well insulated building envelopes, the heat exchange due to the thermal bridging effect is important. The question is wheter the same is valid for point thermal bridges. According to the results for the case of the investigated cladding system with an unventilated air cavity, this trent is not shown. Obviously, the significantly higher thermal transmitance of the steel anchor penetrating the substrate does not permit heat flow of the same magnitute in the area of a typical wall as well as the area close to the anchors, even when no thermal insulation material is present (Fig. 5). With better insulation protection, point thermal tranmittance increases up to a point close to 0.06 W/m²·K in the case of insulation layer thickness of about 0.06 m. For better insulated walls, point thermal bridge decreases. The results of this scenario clearly explain the absence of simple methodologies to account for point thermal bridges, since the magnitute of the phenomenon is simultaneously affected by many factors. Since heat flow within the bracket is related not only to the conductivity of steel, but also to the length of the bracket (which is surrounded by the insulation material), there is a specific point in the examined geometry, where the thermal resistance (defined as the length of the bracket normal to heat flow to the thermal conductance of it material) of the bracket, alters the relation between the heat flow within the bracket and the heat flow through the insulated wall, resulting in an inverse analogy among the presented parameters. For medium thermal insulation requirements, like those found in southern Europe (required U-value of wall around 0.50 W/m²·K), point thermal transmittance reaches its highest values, while for more demanding national requirements, like those found in northen Europe (U-value less than 0.30 W/m²·K), the effect of point thermal bridge effect decreases. On the other hand, despite the insulation requirements, with the exception of “L.TB” bracket shape (with thermal break), χ-values per unit area as a percentage of U-value for the “L”, “T” and T.TB” types demonstrate a small variation between 7.5%-9.8%. In the case of the “L.TB” bracket shape with thermal break, this range is 6%-7.5%. From the above, it can be easily concluded that although thermal insulation has a contribution to the magnitude of the thermal bridge, this is less significant than the substrate material, in contrast to the linear thermal bridging effect.
Figure 5. The effect of insulation thickness on point thermal transmittance for an unventilated air cavity. In the case of a ventilated air cavity, the modelling assumptions are that the influence of the air cavity is ignored and the surface resistance of the insulation layer and the exposed bracket decreases from 25 m²·K/W to 7.70 m²·K/W, according to ISO 6946. This assumption decreases and amplifies both the U-value and the χ-value of the examined scenarios, respectively (Fig. 6). Heat flow increases due to the thermal bridge effect accounting for a value between 5.5%-7% of the element thermal transmittance for all scenarios, apart from the “L.TB” shape bracket for which the difference ranges between 4.5%-6%.
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Figure 6. The effect of air cavity thickness on point thermal transmittance for a ventilated air cavity.
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In the absence of air cavity (Fig. 7), when the insulation layer is in contact with the external panel, the overall heat transfer of the wall increases due to lower thermal resistance. In modelling, this is not only related to the thermal exposure of the bracket directly to ambient conditions, but also to the shorter length of the bracket and its consequent smaller thermal resistance. In weak thermal protection levels, the point thermal bridge is maximised, despite the fact that the overall wall element is poorly protected, since the anchor penetrating the wall is in almost ambient temperature and leads to significant heat flow. Even in the other cases, the effect is significant and χ-values form a large proportion of the U-value, within the range of 8%-20% for every bracket type, except of “L.TB” shape bracket where the range is 6.5%-17%.
Figure 7. The effect of insulation thickness on point thermal transmittance for a wall without an air cavity. 3.4
Magnitude of point thermal bridges on the overall building thermal transmittance
The magnitude of the examined thermal bridge effect is examined in a refurbishment study of an office building located in the Aristotle University Campus in Thessaloniki, Greece (Fig. 8). The examined building has a compact shape. Each one of its storeys covers an area of 508 m², while its main facades are occupied by windows (60%) and concrete walls. Side facades consist of concrete walls with no windows. The ground floor hosts unheated spaces and is not considered part of the renovation study. In a study related to the refurbishment of the building envelope in order to meet the current national thermal insulation requirements, an aluminium cladding system is examined. Since current national requirements are not considered to be demanding, as is the case in most southern European countries, an additional case is examined with requirements resembling north European ones. Regarding the permitted allowable thermal transmittance of the external walls, U-values for these two sets of requirements are 0.40 W/m²·K and 0.20 W/m²·K, respectively. All possible air cavity ventilation types (unventilated, ventilated and no air cavity), and both bracket types “L” and “T” types with thermal break are examined. Table 2 presents the most important characteristics of the examined cases.
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Figure 8. Plan and axonometric view of the examined office building.
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For the secure installation of the façade system, the maximum distance between the brackets is 0.70 m, which is considered a normal size. In order for the aluminium cladding to cover all the facades of the building, a total of almost 9,000 brackets are required.
Enhanced insulation Insulation U AxU thickness [m] [W/m²·K] [W/K] 0.198 426.08 0.16 0.201 433.80 0.205 441.81 0.16 0.197 99.96 0.16 0.197 99.99 1.400 1,511.44 224.62
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Table 2. Characteristics of the examined building envelope. Area Minimum insulation Insulation U AxU Element Type of thickness air gap ventilation [m²] [m] [W/m²·K] [W/K] Walls Unventilated 0.403 867.34 0.07 Ventilated 2,153.50 0.418 899.96 No cavity 0.434 935.13 Roof 508.30 0.07 0.398 202.21 Floor 508.30 0.07 0.398 202.32 Windows 1,079.60 2.300 2,483.08 Linear thermal bridges 224.62
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Under steady state conditions, the weakest parts of the building envelope, by means of thermal losses, are the windows on the main facades of the building; more specifically, according to the data presented in Table 2, 62% to 64% of heat flows through the envelope are attributed to windows, 20% to 18% to walls and 6% to 10% to linear thermal bridges, for the cases of present and more demanding thermal insulation requirements respectively. These values refer to a calculation methodology that neglects the presence of point thermal bridges. Utilising the results from the previous study to incorporate the magnitude of heat flows due to point thermal bridge effect, significant additional thermal flows have to be taken into account. For every examined scenario of air cavity ventilation type, bracket shape, under both thermal insulation requirement levels, these additional heat flows are presented in Table 3. In all presented scenarios the best practice of using a thermal brake in every bracket has been taken into account. Table 3. Heat flows through thermal bridges for the examined building. Percentage on Point thermal Heat flow total heat flows transmittance χ Air cavity type [W/K] [%] [W/K ] Bracket type Thermal insulation Minimum Enhanced Minimum Enhanced Minimum Enhanced requirements insulation insulation insulation insulation insulation insulation L 0.047 0.059 424.69 528.61 10% 18% Unventilated T 0.059 0.050 528.61 451.80 12% 16% L 0.037 0.043 334.33 388.55 89% 1413% Ventilated T 0.036 0.040 320.78 361.44 78% 1312% L 0.064 0.050 578.30 447.28 13% 16% No cavity T 0.079 0.060 709.33 537.64 15% 18%
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As it can be seen, the magnitude of point thermal bridge can be significant in the case of the examined office building ranging from almost 320 W/K to 710 W/K for the case of a ventilated façade and a façade with no air cavity respectively, both for a T-shape bracket. One important conclusion is that the magnitude of point thermal bridges –which are not taken into account in most national methodologies- can be even by 200% higher than the effect of linear thermal bridges –which are almost always taken into account. Among the thermal insulation requirements, better insulated elements (adequate insulation) result in a higher percentage of heat flow as it has already been analysed in the parametric analysis. Especially in the case of the ventilated air cavity, the upgrade of thermal insulation thickness, makes the problem of point thermal bridges more intense. In fact, the ventilated cavity seems to be the most sensitive to the insulation thickness, as it can be seen from the wider distribution of the results, compared to the other two examined cavity types. On the other hand, maximum values are found in the other two air cavity types. CONCLUSIONS
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The results of this study have shown that, neglecting the point thermal bridge effect in lightweight cladding systems can lead to a considerable underestimation of actual heat flows. The magnitude of this error is comparable (if not higher in some cases) to that of linear thermal bridging. Compared to the linear ones, point thermal bridges present a more complicated nature and are not strongly related to the overall insulation level of the wall. As it is shown, the most important factors of the examined phenomena are the thermal resistance of the load bearing wall where the cladding system in anchored and the air cavity existence. The thermal resistance of the inner wall controls the heat flow through the highly conductive wall-fastener unit that penetrates the insulation layer. The air cavity acts as an additional insulation layer that decreases heat flow through the point thermal bridge. Although thermal breaks are supposed to thermally separate the bracket and the substrate, the existence of fasteners directly inserted within the substrate cannot allow for an efficient heat flow decrease. Consequently, thermal breaking is not among the factors with the greatest impact. Since aluminium cladding systems have become an appealing method for the upgrade of the thermal insulation of existing buildings but current national methodologies are not always well adapted to the particularities of such systems, the magnitude of heat flow underestimation can range from 5% up to even 20%, which is definitely a significant part of the thermal budget of a building. In such cases, special attention should be paid during the design process to minimise these heat flows in order to fully exploit their numerous advantages, as it has been presented in this study. Finally, this study proves that compared to other façade retrofitting techniques like ETICS, aluminium cladding systems require different treatment and simplifications for the estimation of thermal bridging effects in order to avoid underestimations that could significantly affect the effectiveness of the retrofit action. REFERENCES [1] E. Parliament, Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings., in, 2010. [2] A. de Gracia, L. Navarro, A. Castell, L.F. Cabeza, Energy performance of a ventilated double skin facade with PCM under different climates, Energy and Buildings, 91 (0) (2015) 37-42. [3] M.I. Nizovtsev, V.T. Belyi, A.N. Sterlygov, The facade system with ventilated channels for thermal insulation of newly constructed and renovated buildings, Energy and Buildings, 75 (0) (2014) 60-69. [4] A.S. Anđelković, B. Gvozdenac-Urošević, M. Kljajić, M.G. Ignjatović, Experimental research of the thermal characteristics of a multi-storey naturally ventilated double skin façade, Energy and Buildings, 86 (0) (2015) 766-781. [5] F. Marques da Silva, M.G. Gomes, A.M. Rodrigues, Measuring and estimating airflow in naturally ventilated double skin facades, Building and Environment, 87 (0) (2015) 292-301. [6] A.L.S. Chan, T.T. Chow, Calculation of overall thermal transfer value (OTTV) for commercial buildings constructed with naturally ventilated double skin façade in subtropical Hong Kong, Energy and Buildings, 69 (0) (2014) 14-21. [7] T.G. Theodosiou, A.M. Papadopoulos, The impact of thermal bridges on the energy demand of buildings with double brick wall constructions, Energy and Buildings, 40 (11) (2008) 2083-2089. [8] B. Berggren, M. Wall, Calculation of thermal bridges in (Nordic) building envelopes – Risk of performance failure due to inconsistent use of methodology, Energy and Buildings, 65 (0) (2013) 331-339. [9] E.C.f. Standardization, EN ISO 10211, Thermal bridges in building construction - heat flows and surface temperatures - Detailed calculations, in, European Commitee for Standardization, 2007.
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[10] F. Ascione, N. Bianco, F. De Rossi, G. Turni, G.P. Vanoli, Different methods for the modelling of thermal bridges into energy simulation programs: Comparisons of accuracy for flat heterogeneous roofs in Italian climates, Applied Energy, 97 (2012) 405-418. [11] F. Aguilar, J.P. Solano, P.G. Vicente, Transient modeling of high-inertial thermal bridges in buildings using the equivalent thermal wall method, Applied Thermal Engineering, 67 (1–2) (2014) 370-377. [12] A.P. Gomes, H.A. de Souza, A. Tribess, Impact of thermal bridging on the performance of buildings using Light Steel Framing in Brazil, Applied Thermal Engineering, 52 (1) (2013) 84-89. [13] M. Gorgolewski, Developing a simplified method of calculating U-values in light steel framing, Building and Environment, 42 (1) (2007) 230-236. [14] S.-Y. Song, J.-S. Yi, B.-K. Koo, Insulation plan of aluminum curtain wall-fastening unit for high-rise residential complex, Building and Environment, 43 (7) (2008) 1310-1317. [15] C. Sanjuan, M.J. Suárez, M. González, J. Pistono, E. Blanco, Energy performance of an open-joint ventilated façade compared with a conventional sealed cavity façade, Solar Energy, 85 (9) (2011) 1851-1863. [16] F. Ascione, N. Bianco, R.F. De Masi, F. de' Rossi, G.P. Vanoli, Simplified state space representation for evaluating thermal bridges in building: Modelling, application and validation of a methodology, Applied Thermal Engineering, 61 (2) (2013) 344-354. [17] A.E. Ben-Nakhi, Minimizing thermal bridging through window systems in buildings of hot regions, Applied Thermal Engineering, 22 (9) (2002) 989-998. [18] I. Abaqus, Pawtucket,RI,USA, Abaqus, in, http://www.3ds.com/. [19] H.Poirazis, Double-skin facades for office buildings-literature review, inQ Report ECBCS Annex 43,, in, Lund Institute of Technology, Sweden, 2006. [20] M.J. Suárez, C. Sanjuan, A.J. Gutiérrez, J. Pistono, E. Blanco, Energy evaluation of an horizontal open joint ventilated faade, Applied Thermal Engineering, 37 (2012) 302-313. [21] C. Marinosci, P.A. Strachan, G. Semprini, G.L. Morini, Empirical validation and modelling of a naturally ventilated rainscreen façade building, Energy and Buildings, 43 (4) (2011) 853-863. [22] EN, ISO 6946, Building components and building elements - Thermal resistance and thermal transmittance - Calculation method, in, 1996. [23] ETAG, ETAG 034. Guideline for European technical approval of kits for external wall claddings. Part 1: Ventilated cladding kits comproising cladding components and associated fixings, in, 2012. [24] E.C.f. Standardization, EN ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values and procedures for determining declared and design thermal values, in, European Commitee for Standardization, 2007.
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