Hygrothermal performance of ventilated prefabricated sandwich wall panel from recycled construction and demolition waste – A case study

Energy & Buildings 0 0 0 (2020) 109573

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Hygrothermal performance of ventilated prefabricated sandwich wall panel from recycled construction and demolition waste – A case study Marina Bagaric´ ∗, Ivana Banjad Pecˇ ur, Bojan Milovanovic´ Faculty of Civil Engineering, University of Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 27 May 2019 Revised 27 September 2019 Accepted 2 November 2019 Available online xxx Keywords: Heavyweight building envelope Ventilated façade Recycled aggregate concrete Field measurements Transient hygrothermal performance EN ISO 52016 Thermal mass

a b s t r a c t Construction and demolition waste has been used for production of recycled aggregate concrete (RAC) with high replacement ratio (50%) of natural coarse aggregate. Upscaling RAC from material laboratory testing to new product implementation – ventilated prefabricated sandwich wall panel, which at the same time tackles the problem of natural resources conservation and extensive energy consumption in buildings, requires a proof-of-concept. This paper presents an experimental investigation of dynamic hygrothermal performance of developed building envelope system under real outdoor climate conditions and with real occupants living in the house for a one-year period in 2017/2018. Temperature and relative humidity distributions through all characteristic layers of panel showed that the positive effect of air ventilation on hygric (no highly moist conditions) and thermal (damping and time shifting of peak temperatures) performance was most pronounced for south panel adjacent to conditioned living room, while the less pronounced effect was for north panel subjected to unconditioned staircase. Furthermore, transient hygrothermal simulations using measured boundary and initial conditions showed that ventilated RAC wall panel has significantly outperformed both wall panel with closed air cavity and wall with applied ETICS system during summer, which indicates that naturally ventilated air has positive effect on wall’s thermal performance in summer period. Additional simulations according to new EN ISO 520161:2017 Standard confirmed that high thermal mass of ventilated RAC wall panel has significant influence on dynamic building energy performance. For intermittent occupancy, it is beneficial when cooling energy is observed, while contrary to that it can require more heating energy compared to building with no heat storage capacity. However, when switching to continuous operating mode, high thermal mass of building envelope is beneficial from both of these energy aspects. This pattern has been confirmed in two different Croatian climates – continental and littoral. © 2019 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Background Building envelopes are, during their lifetime, exposed to heat, air and moisture (HAM) loads from the indoor and outdoor environment, respectively. The performance of building envelope under the HAM loads, so-called hygrothermal performance, is dependent on the planning, materials used and their assembly, as well as the detailing of the building [1]. Building envelope’s hygrothermal performance influences the service life of building, energy needs, indoor comfort and indoor air quality which is directly related to the health of occupants [2–4]. Building failures due to moisture problems are frequently observed. When considering only the building envelope it is estimated that about 75 – 90% of all damages are ∗

Corresponding author. ´ E-mail address: [email protected] (M. Bagaric).

https://doi.org/10.1016/j.enbuild.2019.109573 0378-7788/© 2019 Elsevier B.V. All rights reserved.

caused by moisture [5]. Most of these damages are preventable by appropriate design calculations in the field of building physics. Hygrothermal properties of envelope on a material level determine how the envelope will perform under given boundary conditions in terms of heat and moisture transport. This is crucial especially in case of new materials being developed. On the other hand, experimental testing on a material level is not always capable of capturing all effects that can occur in real environment in full-scale, i.e. hygrothermal performance of building element or building as a whole cannot be described solely on the basis of hygrothermal behaviour of individual materials. It is desirable to assess hygrothermal performance of building element or building as a whole on a large-scale level, particularly when high-performing and resilient buildings are considered. Their high-performance and resilience is reflected through the capability of adapting to climate changes in terms of energy consumption, moisture safety and indoor comfort.

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Construction industry is being revolutionized by rapid development of new construction materials and products, as well as improvement of existing “traditional” ones, which offers solutions for increasing demands of versatile construction sector. Building sector is undergoing a significant paradigm shift, where reducing energy consumption and sustainable use of materials are set as main drivers. The fact that buildings are responsible for more than 40% of total energy consumption is well-known. On the other hand, sustainable construction is obviously of great importance, since only in Europe over 30% and up to 50% of national waste production comes from the construction sector [6]. Enormous amount of waste being generated during manufacturing of construction materials and products, during the construction phase, maintenance and refurbishment of buildings as well as their demolition, has triggered comprehensive research activities worldwide with one main goal – how to use construction and demolition waste (CDW) in a sustainable way for production of new construction materials and minimizing waste landfilling? Construction and demolition waste (CDW) can be sorted, crushed and distributed by the size of particles, resulting with material called recycled aggregate. Some European countries like The Netherlands, Belgium and Denmark are recycling more than 80% of their CDW, thus at the same time they are reducing the exploitation of natural resources [6,7]. Depending on its origin, there are different types of recycled aggregate – brick, concrete, glass, asphalt, and others. For so-called recycled aggregate concrete (RAC), recycled aggregate can be used as partial or 100% replacement of conventional natural fine or/and coarse aggregate. Having in mind that aggregate accounts for 60 – 80% of the volume and 70 – 85% of the weight of concrete, it is clear that there is a tremendous potential for incorporating CDW into concrete and for buildings to serve as a material, or better to say, CDW banks. Since the early 1980s, European countries have endeavored to use recycled aggregates for new concrete structures. Despite the fact that the potential of recycled aggregate has been widely addressed, its generally inferior properties and the lack of proper specification inhibits their massive use in concrete [8]. Specific structure, for example the presence of different interfacial transition zones between the new and old matrix in RAC produced with recycled aggregate from old concrete, is responsible for generally inferior mechanical and durability properties compared to natural aggregate concrete [8]. Due to their limited properties, more attention regarding the overall performance of RAC is needed. In practice, the properties of recycled aggregate obtained from various sources, i.e. different CDW, also vary from structure to structure. However, depending on requirements, RAC can unquestionably find its application, as for example in structures for which the environmental influence on their durability and load-bearing demands are lower than for other applicants. Numerous comprehensive researches have been conducted over the past years for understanding the behaviour of concrete made with different types of recycled aggregate. While the vast majority of these studies have been focused primarily on its physical, mechanical and durability properties [8–15], there is a scarcity of research findings in the literature regarding its hygrothermal performance. Thermal properties of concrete with different types of recycled aggregate in different replacement ratio were investigated and reported by authors of following References [10,12,15–17]. Generally, it can be concluded that suitable replacement ratio of recycled aggregate can improve thermal behaviour of RAC, but the thermal conductivity and the recycled aggregate replacement ratio do not show clear trend. The quantity of pores and type of aggregate, as well as its size (fine or coarse aggregate), are highlighted as the main factors affecting the thermal conductivity of RAC in the conditions of certain temperature and humidity [17]. Regarding the hygric properties of RAC, with the main focus on water vapour per-

meability and moisture sorption isotherm curves, there are even more limited research findings compared to RAC’s thermal properties [15,18]. Despite the importance of understanding dynamic hygrothermal performance of RAC building elements under real variable climate conditions and occupants’ behaviour on a large scale, to the best knowledge of authors, it remains an under researched area by comparison with their mechanical and durability behaviour. The lack of confirmation of RAC’s suitability for constructing energy high-performing, moisture safe, durable and sustainable building envelopes is a definite drawback in this regard. 1.2. The goal of the study This paper aims to enlarge understanding of RAC’s suitability for constructing energy high-performing, moisture safe, durable and sustainable building envelopes. Innovative ventilated prefabricated RAC sandwich wall panel was conceived, developed and used for constructing the envelope of Nearly-zero energy building occupied by real tenants. The scope of the research is the analysis of panel’s dynamic hygrothermal performance on a large scale under real outdoor and indoor environment conditions, whereas the main contribution of the research is the quantification of hygrothermal performance metrics for various orientations and indoor environment conditions. To the best of the authors’ knowledge, there is no available research considering hygrothermal performance of ventilated heavyweight RAC building envelope at large scale. Therefore, one of the main objectives of this research is to investigate if RAC façade cladding and ventilated air cavity impact moisture and thermal performance of external walls and if yes – to which extent? Furthermore, another objective of this study is to analyse the influence of RAC panel’s high thermal mass on building’s heating and cooling energy needs. For this purpose, exemplary building was simulated using a new method for calculating energy performance of buildings that was adopted in Europe in 2017. 2. Ventilated prefabricated sandwich wall panel from CDW Setting a research problem, how to tackle the CDW issue in a way that would provide added value to it, while at the same time addressing the need for reducing energy consumption in buildings, resulted with development of novel prefabricated sandwich wall panel. Innovation of panel lies in utilising recycled aggregate from CDW for production of concrete layers (sustainability aspect) and in naturally ventilated air layer (to prevent possibility of water vapour condensation). This ventilated prefabricated RAC wall panel, Fig. 1, consists of four characteristic layers: (i) inner self loadbearing RAC layer (12 cm), (ii) formaldehyde-free mineral wool thermal insulation layer (20 cm), (iii) air cavity (4 cm) and (iv) outer RAC façade layer (6 cm); combining the role of both external wall and façade. Comprehensive research activities aimed at mechanical and durability properties were carried out on a material level to optimize RAC mix design [15]. Two different types of recycled aggregate were used: recycled concrete for inner RAC layer (RAC–Concrete) and recycled brick for outer RAC façade layer (RACBrick). Replacement ratio of recycled aggregate was varied, where replacement of 50% natural coarse aggregate with recycled one deemed as the most favourable (from the mechanical and durability aspects [15]) for both outer and inner concrete layer. Waterrepellent coating was applied to external surface of outer RAC facade layer, to protect it from direct environmental and climatic loads (e.g. wind-driven rain) and thus, improve durability. The internal surface of outer RAC façade layer (the one facing air cavity) was not treated. Formaldehyde-free mineral wool thermal insula-

M. Bagari´c, I. Banjad Pecˇ ur and B. Milovanovi´c / Energy & Buildings 000 (2020) 109573

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Fig. 1. Ventilated prefabricated RAC wall panel: (a) model, (b) technical detail.

Table 1 Basic hygrothermal and physical properties of RACs and thermal insulation. Dry density [kg/m3 ]

Open porosity [%]

Thermal conductivity λdry,+10 °C [W/(mK)]

Vapour diffusion resistance factor μ [-]

Water absorption coefficient W [kg/(m2 h0.5 )]

RAC–Concrete RAC-Brick

2204.96 1948.22

16.67 19.27

0.944 0.745

Glass wool thermal insulation

25

/

0.034

41 28 26,196 (WR∗ ) 1

1.362 1.089 0.039 (RAC brick+WR∗ ) /

∗

Adsorption isotherm Similar sorption capacity in hygroscopic range [18] / /

WR = only water-repellent coating.

tion is a glass wool faced with a black glass veil, providing a wind barrier. Table 1 summarizes basic hygrothermal properties at material level, for both RAC types, such as thermal conductivity, water vapour diffusion resistance factor and sorption isotherms, that were investigated and partially presented in previous study [18]. Basic hygrothermal properties of thermal insulation, also shown in Table 1, are taken directly from the manufacturer. To evaluate the environmental impact of panel, i.e. to quantify its sustainability, Life Cycle Assessment was performed, where all activities throughout the life cycle of panel are included in the assessment, from raw material extraction throughout manufacturing, distribution, use of the panels and the disposal at the end of life [19]. Embodied energy of ventilated RAC sandwich wall panel is lower than that of lightweight composite insulation panels such as those made of aluminium or galvanized sheets and polyurethane insulation. The same applies for conventional concrete sandwich wall panels with expanded polystyrene (EPS) thermal insulation. The conventional concrete sandwich wall panels are usually composed of two concrete layers separated by a layer of rigid foam plastic insulation like EPS or rarely mineral wool (MW) insulation [20,21]. Air layer is omitted from these conventional concrete sandwich wall panels. Prefabricated construction is a well-known construction technology with number of advantages such as, for example, speed of construction and production in factory in controlled environment. However, prefabricated sandwich wall panels have experienced certain stagnation in their development in contrast to other building products, such as for example glass facades, insulation materials, windows and bricks. Transitions of building sector towards sustainability and energy efficiency has triggered a transformation of prefabricated concrete industry. Ventilated RAC wall panel, originally conceived in 2012, is one of the first steps forward by incorporating 50% of recycled aggregate from CDW for concrete production and formaldehyde-free mineral wool thermal insulation, as well as introducing naturally ventilated air layer with main

aim to prevent possibility of water vapour condensation. Incorporation of recycled material provides opportunity for effective waste disposal, saving raw materials and mitigating the overall negative impact of concrete industry on environment. The latest development in prefabricated concrete industry are carbon concrete façade elements with high-performance aerogel-based insulating materials, presented in 2019 [22]. Replacing standard steel reinforcement with carbon reduces needed amount of concrete, which saves raw materials and reduces CO2 emissions [23]. Consequently, these prefabricated sandwich elements are slender and weight less; thus, their thermal mass is lower. However, the used concrete is conventional concrete and there is no contribution to CDW issue, neither they have air cavity. Moreover, their influence on dynamic energy performance of buildings, as well as overall dynamic hygrothermal performance on a large scale remains unknown. It can be concluded that cavity with naturally ventilated air remains specificity of prefabricated RAC wall panel, and thus it can be referred to as ventilated façade panel with open joints. Furthermore, ventilated panel made with CDW, as presented in this paper, has surface density over 400 kg/m2 , which classifies it as a heavyweight building element. The following section outlines the methodology used for this research, including the description of both experimental setup for real time hygrothermal performance monitoring of constructed building and calculation procedure for predicting and assessing the impact of panel massiveness on building energy performance. 3. Methodology description 3.1. A full-scale experimental setup 3.1.1. Building description Building envelope of a 3-storey family house was constructed with prefabricated ventilated RAC panels (Fig. 2). House is located in city of Koprivnica, Croatia, and it is designed and built within

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Fig. 2. Construction of family house using ventilated RAC wall panels.

Fig. 3. Panels selected for monitoring: (a) south-oriented M1, (b) east-oriented M2, (c) north-oriented M3.

socially-supported programme as an A+ building, where both specific annual energy need for heating Q”H,nd and cooling Q”C,nd are below 15 kWh/(m2 a), respectively. In accordance with current Croatian building code and for Croatian continental climate conditions, this family house could be classified as Nearly-zero energy building (primary energy below 45 kWh/(m2 a)). Large transparent openings are located at south façade to allow solar gains in winter period and maximize natural daylight, while minimum number of openings is foreseen on the north side. A great amount of effort has been put in solving construction details to minimize the thermal bridging effects [24]. Panel joints were properly sealed to ensure required air-tightness and fire resistance. Building has mechanical ventilation system with heat recovery. Ground floor apartment (net surface area 95.69 m2 ), as the first apartment occupied by tenants, was selected for monitoring. Three differently oriented panels were chosen as measuring locations: south-oriented panel M1 (Fig. 3a), east-oriented panel M2 (Fig. 3b) and north-oriented panel M3 (Fig. 3c). Except for orientation, panels differ by indoor boundary conditions. Panels M1 and M2 separate conditioned spaces (living room and bedroom, respectively) from outdoor environment, while panel M3 is adjacent to unconditioned staircase. With main aim to monitor heat and moisture transfer through prefabricated ventilated RAC panels on a real-time basis, when exposed to real outdoor and indoor environment conditions, temperature and relative humidity were defined as key monitoring parameters. 3.1.2. Instrumentation and field monitoring system description Temperature (T) and relative humidity (RH) sensors (Fig. 4a) were installed in all characteristic layers of RAC panels during their production in precast factory (Fig. 4b–d). In each panel, T and RH sensors (RDL Client-THR system [25] from Caption Data Limited) were installed at seven positions in total, marked as S1 up to S7 in Fig. 5(e).

As demonstrated in Fig. 5(a) presented monitoring system is based on Machine-to-machine (M2M) technology, where communication between sensors and central measuring unit is established. Sensors (S1-S7) at each measuring location (M1-M3) are wired to the central measurement unit in building’s entrance hall (Fig. 5b), where sensors readings are captured on hourly basis, communicated by ModbusBrightNode open protocol and transmitted wirelessly by Internet on a dedicated server at Faculty of Civil Engineering in city of Zagreb. The measured data can be accessed via customized Overwatch HMI (Human Machine Interface). These system components are part of BrightCore solutions [26]. Further analysis of measured data was performed using programming language R and Microsoft Excel software. Boundary conditions in terms of indoor and outdoor environment were also monitored. Indoor climate is dependent on the occupants’ behaviour – young couple with a small child. TFA Klimalogg Pro Thermo-Hygro-Station data logger was used for monitoring and recording the air temperature and relative humidity in living room, bedroom and staircase every 15 min. Basic meteorological data, such as air temperature, relative humidity, wind velocity and direction, air pressure and precipitation, were measured at 5 min-basis at nearest available meteorological station Herešin (3 km from the location of the building). The global solar irradiance on a horizontal surface was measured at the same meteorological station at 30min basis. These data are taken as representative for the climate of building location (city of Koprivnica, continental part of Croatia), based on which the climate of Koprivnica can be classified as Cfa climate according to Köppen climate classification (Cfa - temperate climate characterized by hot summer and without dry season, with the coldest month averaging above 0 °C, at least one month’s average temperature above 22 °C, and at least four months averaging above 10 °C.). Hourly data for ambient temperature and relative humidity are shown in Fig. 6(a) and (b), while global solar irradiance and wind-driven rain are presented by Fig. 6(c) and (d), respectively.

M. Bagari´c, I. Banjad Pecˇ ur and B. Milovanovi´c / Energy & Buildings 000 (2020) 109573

Fig. 4. T-RH sensors installation process and positions in RAC wall panels.

Fig. 5. Monitoring system: (a) operational workflow, (b) T-RH sensors connecting scheme.

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Fig. 6. Measured climatic data: (a) ambient air temperature, (b) ambient relative humidity, (c) global solar irradiance on horizontal surface, (d) driving rain sum [mm/s]. Table 2 Overview of sensors activity for all measuring locations and positions (red indicating defective sensors).

Monitoring system was commissioned on March 09, 2017 and after that it was detected that some of the sensors are not measuring, i.e. no information was received from certain positions. In Table 2, red colour indicates panels and positions for which no temperature and/or relative humidity information was received. For the purpose of this study, experimentally measured data during monitoring period of approx. one year (March 9, 2017 6:00 PM – February 28, 2018 11:00 PM) are analysed. 3.2. Impact of panel massiveness on dynamic energy performance – calculation procedure In most of the European countries, energy performance of buildings is calculated according to methods required by Energy Performance of Building Directive (EPBD) [27] which has intro-

duced new standard EN ISO 52016-1:2017 [28]. There is a lack of studies evaluating accuracy and capabilities of new Standard compared to dynamic simulations [29]. To help bridge this gap, new Standard and its new simplified hourly calculation method has been used in this paper for predicting and assessing the impact of panel high mass on dynamic building energy performance. One of the advantages of new Standard is the capability of specifying thermal mass per building element instead of lumping multiple building elements into the overall thermal capacity of a building or a building zone, as it is case with previous Standard [30]. For the purpose of another study [29], using general programming language MATLAB an in-house Algorithm was developed in accordance with Standard EN ISO 52016-1:2017. In that study, Algorithm was used to assess EN ISO 52016 Standard accuracy for a wide range of building types and heating/cooling energy needs in

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Table 3 Exemplary building characteristics and model input parameters. U-value of prefabricated ventilated RAC walls [W/(m2 K)] Heat capacity of heavy walls [J/(m2 K)] (Table B.14, Standard EN 52016-1) – dynamic energy performance analysis Heat capacity of walls [J/(m2 K)] – static energy performance analysis Heat capacity of indoor environment (Table B.17 Standard EN 52016-1)

0.16 175 000 0 10 000

cupancy hours, temperature was allowed to float freely in response to the outside temperature. Twelve cases were simulated in total (Table 4). 4. Results and discussion 4.1. Dynamic hygrothermal response of prefabricated ventilated RAC sandwich wall panels

Fig. 7. Exemplary building model used for the energy performance assessment.

comparison to dynamic simulation model in TRNSYS. The validation of Algorithm itself was done based on the BESTEST 600, 640, 900 and 940 cases as described in ANSI/ASHRAE standard 140– 2004 [31]. As reported in [29], the maximum differences between the Algorithm and the ANSI/ASHRAE mean values are 14% for heating and 10% for cooling, which is both within the reported range in ANSI/ASHRAE standard 140–2004. The same Algorithm was used for analysis in present study. For more detailed description of the Standard EN ISO 52016-1:2017, developed in-house Algorithm and results of Algorithm verification procedure, eager reader is encouraged to consult Reference [29]. Exemplary building consisting of a single zone was chosen as a case study with external dimension (Fig. 7) corresponding to geometry of test room from Standard 52016-1. Wall thickness of 42 cm was assumed (thickness of ventilated RAC panel), which results with useful floor area A = 36.9456 m2 and net air volume V = 92.364 m3 . Simulations of static and dynamic energy performance of exemplary building were carried out for two different climates and three operating modes (setpoint temperatures). Selected locations were Zagreb Maksimir as representative of continental climate in Croatia (HDD=2939.5) and Split Marjan as representative of littoral climate in Croatia (HDD=1636.4). According to Croatian legislation [32], those are Croatian reference climates. Static energy performance implies energy required for building envelope without heat storage capacity, i.e. walls without thermal capacity, while calculations of dynamic energy performance take into account high thermal mass of simulated envelope. Three different operating modes (working hours of technical systems) were chosen in order to be able to distinguish their influence on building energy performance. To analyse the impact of panel thermal mass on building energy performance, building (Fig. 7) with only opaque elements was modelled (Table 3). Heat capacity of ground floor and flat roof was set to 0 [J/(m2 K)] and adiabatic boundary conditions were applied to them, so that transmission only through wall panels could be captured. All other parameters, such as windows, infiltration losses and internal heat gains have been also excluded from the model. For the sake of simplicity, thermal bridges were not considered in simulations. Mass placement can have large impact on heat accumulation, so it was defined in model according to Table B.13 in Standard EN 15206-1 as Class I – mass concentrated at internal side, since observed panels consist of 12 cm thick concrete layer in direct contact with internal air and thermal insulation is placed externally. Set-point temperature for heating was defined as 20 °C, while cooling during occupied hours was set to 22 °C. Outside oc-

Outdoor temperature data recorded during the monitoring period at the nearest meteorological station Herešin, Fig. 8, are compared with ambient temperatures in living room (measuring location M1), bedroom (measuring location M2) and staircase (measuring location M3), respectively. Amongst three different measuring location, M1 and M2 represent conditioned spaces with relatively similar temperature profiles during whole year, whereas M3 represent unconditioned space with substantially lower temperatures in winter months compared to M1 and M2, respectively. As Fig. 8a) illustrates, during winter months outside temperature was occasionally below 0° At the end of February 2018 five days in a row was the longest continuous period of temperature below 0 °C with mean value of −6.6 °C and peak of −17.3 °C. Max value was 38.3 °C in August 2017 and its average monthly value was 22.5 °C. Relative humidity in living room (M1) and bedroom (M2) follow the same pattern of behaviour, whereas RH of M2 location exhibits somewhat higher values compared to M1 (Fig. 8b), which is in line with their thermal behaviour. Significantly higher RH values occur in unheated staircase (M3) and they strongly follow the pattern of outdoor RH behaviour. External and internal boundary conditions presented above, together with material characteristics and thicknesses of wall panel assembly, have resulted with specific temperature and relative humidity distributions (hygrothermal response) through the ventilated wall panels, Fig. 9–11, where measurement positions correspond to those described in Section 3.1.2. For all three measuring locations, the exterior surface temperature (S7) varies in a wide range following the pattern of outdoor climate. In case of M1 those variations are in range from 55.4 °C to −10.81 °C, for M2 from 53.0 °C to −9.38 °C, while at M3 sensor did not send any information. When comparing Figs. 9(a) and 10(b), temperature profile of exterior surface (S7) of M1 exhibits more pronounced fluctuations, which can be explained by M1’s south orientation and more intense solar radiation impact. Temperature fluctuations through MW and inner RAC layer are buffered and at interior surface (S1) they are quite stable and limited from 29.96 °C to 17.45 °C in case of M1, 30.89 °C to 17.85 °C in case of M2 and 28.60 °C to 7.75 °C when unconditioned staircase M3 is observed. The effect of panel’s high thermal mass and good thermal insulation on attenuating outdoor temperature was analysed for the coldest winter day (February 28, 2018). The highest difference between exterior surface temperature (S7) and interior surface temperature (S1) was recorded for panels adjacent to actively heated environments (M1, M2) reaching up to 28.3 °C. However, this temperature difference is not result solely of panel’s characteristics, but it is also artificially induced by active heating systems. In case of panel M3 adjacent to unheated staircase, there was no information on S7 temperatures, and thus difference between positions S6 and S1 was taken into account. The S6-S1

M. Bagari´c, I. Banjad Pecˇ ur and B. Milovanovi´c / Energy & Buildings 000 (2020) 109573

8 Table 4 Description of simulation cases. Case ID

Climate

Thermal mass (TM) of RAC panels included

Operating mode

ZG_TM_1 ZG_1 ST_TM_1 ST_1 ZG_TM_2 ZG_2 ST_TM_2 ST_2 ZG_TM_3 ZG_3 ST_TM_3 ST_3

Zagreb (ZG) Maksimir Zagreb Maksimir Split (ST) Marjan Split Marjan Zagreb Maksimir Zagreb Maksimir Split Marjan Split Marjan Zagreb Maksimir Zagreb Maksimir Split Marjan Split Marjan

YES NO YES NO YES NO YES NO YES NO YES NO

1 1 1 1 2 2 2 2 3 3 3 3

temperature difference of approx. 18.5 °C for panel in environment that is not actively heated is mostly induced by panel’s characteristics, and therefore it provides the closest representation of contribution that panel’s high thermal mass and good thermal insulation have on outdoor temperature attenuation. In summer period, during the day with the highest developed S6 temperature of panel M3 (Fig. 12a), the fluctuations of interior surface temperature (S1) remain within the range of less than 1 °C. By attenuating peak temperature (T) for 30.48% and with time delay (t) of 4 h, panel proved its capability of maintaining stable indoor temperature conditions. For completely opposite orientation, panel M1 developed maximum temperature at external surface (S7) of 55.37 °C, while the interior surface temperature had its peak 6 h later and it was attenuated for 47.11% (Fig. 12b). Three key factors are expected to have attributed to presented wall performance: 20 cm thick thermal insulation (decreased thermal transmittance, U-value), air ventilation in cavity and thermal mass. Low U-value (0.16 W/m2 K) unquestionably reduces transmission heat losses and position of thermal insulation on the external side of the wall has twofold role: to help maintain stable internal surface temperature and to enable thermal inertia effect. Real buildings perform dynamically, which imply that they respond to their environment. In that context, steady-state and dynamic behaviour are distinguished by thermal mass, which attenuates and delays temperature peaks (as presented above). Naturally ventilated air in cavity potentially can, depending on the air flow rate, impact the surface temperature of thermal insulation layer, and thus the thermal performance of the whole wall. The impact of ventilation in air cavity and thermal mass on RAC panel performance are analysed in following subsections. 4.2. The impact of ventilation on hygric performance Mineral wool (MW) is of special interest due to its thermal insulation role, while RAC outer façade layer is expected to be the most humid area of panel by being directly submitted to winddriven rain. MW exhibits larger RH fluctuations than inner and outer concrete layers. This pattern is present for all three measuring locations. For the sake of brevity and clarity of the tabular presentation, Table 5 shows only the measuring positions S7 (external surface of outer RAC façade layer), S5 (external surface of MW insulation), S4 (middle of MW) and S3 (contact between MW and inner RAC layer) for two different periods of year – summer months and winter months. Relative humidity values are shown as range of developed minimum and maximum values, as well as average monthly value. The results imply that the façade orientation has relatively large impact on hygric performance of ventilated wall panel. These findings are somewhat expected since solar radiation and wind-driven rain, differing from orientation to orientation, influence appearance

(intermittent heating/cooling – 05:00–08:00 & (intermittent heating/cooling – 05:00–08:00 & (intermittent heating/cooling – 05:00–08:00 & (intermittent heating/cooling – 05:00–08:00 & (intermittent heating/cooling – 06:00–23:00 h, (intermittent heating/cooling – 06:00–23:00 h, (intermittent heating/cooling – 06:00–23:00 h, (intermittent heating/cooling – 06:00–23:00 h, (continuous heating/cooling, 7 days) (continuous heating/cooling, 7 days) (continuous heating/cooling, 7 days) (continuous heating/cooling, 7 days)

17:00–23:00 h, 17:00–23:00 h, 17:00–23:00 h, 17:00–23:00 h, 7 days) 7 days) 7 days) 7 days)

7 7 7 7

days) days) days) days)

of relative humidity within the building envelope and the resulting moisture transfer through the ventilated wall panel. For all three measuring locations, RH at surface and within MW layer (S5 and S4, respectively) follow the RH behaviour of external air. This pattern is triggered by external air entering cavity and circulating. In the middle of MW layer (S4) the lowest RH values are present for south-orientated M1 (average summer RH 48.33%, average winter value 37.53%), then increasing for east-orientated M2 (average summer RH 49.7%, average winter RH 48.1%) and achieving highest values for north-oriented M3 (average summer RH 55.1%, average winter RH 65.47% with values exceeding 70%). The warm side of insulation (position S3) performed satisfactory in M2 and M3 during both summer and winter, there was no problem with the highly most conditions, i.e. exceeding 80% (average RH values under 60%, for M3 max value reached 76.9%). This moisture-safe performance most likely can be attributed to the air ventilating in 4 cm thick cavity. External surface of outer RAC façade layer (S7) had relatively limited RH fluctuations despite high outdoor RH values (above 90%); which can be attributed to concrete’s inertia as well as applied water-repellent coating, thus resulting with generally lower RH values and its smoother development compared to position S6 (Figs. 9(b) and 10(b)) which is in direct contact with air ventilating in cavity. Recycled aggregate has increased water absorption properties compared to natural aggregate, therefore proper water-repellent protection is of great importance when durability of RAC elements is observed. The RH at external surface of outer RAC façade layer, i.e. position S7, is increasing for east orientation during winter period compared to south. The same trend of increasing RH can be assumed for north orientation. Façade orientation influences ventilation in cavity, which is expected to be in correlation with increasing RH values within eastand north-oriented panels. Lower ventilation rate will result with higher RH values through the panel, and vice versa. In that context, it can be stated that ventilation has positive impact on RH levels in recycled aggregate concrete wall panels and they are functioning physically correct. 4.3. Impact of ventilation on thermal performance In-situ experimental data, acquired in this study, confirmed presence of temperature gap between external surface of outer façade layer (S7 position) and external surface of thermal insulation (S5 position) for both summer and winter period, respectively. For the sake of brevity, results are shown only for measuring location M1 in Fig. 13. Experimental data show that temperature at external surface of thermal insulation is generally higher than temperature at external surface of outer RAC façade layer in winter and lower in summer period, respectively. In summer period, for M1 panel, S7 temperatures and S5 temperatures differ up to 10.79 °C, while in winter

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Fig. 8. Outdoor and indoor profiles: (a) temperature, (b) relative humidity.

period differences reach up to 8.08 °C. For M2 those differences are somewhat smaller, while for M3 due to technical problems no information was received from sensors at S7 position. Since mineral wool is protected with outer façade cladding and thereby not directly exposed to external environment, it was expected that certain temperature differences will occur. However, in the following subsection we have investigated whether air ventilation in cavity has also contributed to lowering thermal insulation temperatures in summer as well as raising them during winter period? Additional numerical analyses have been carried out using a transient heat and moisture transfer simulation tool WUFI Pro

5.2 [33] which has proven its capability of simulating hygrothermal performance of ventilated assemblies with high accuracy, as presented in different research studies such as for example [1,34,35]. 4.3.1. Transient numerical simulations To investigate whether, and to what extent, air ventilation in cavity impacts thermal performance, three different envelope types were modelled – (i) studied ventilated RAC wall panel (Fig. 5e), (ii) wall panel with the same geometry and materials used as ventilated RAC wall panel but with closed air cavity (hereinafter

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Fig. 9. Panel M1: (a) temperature distribution (b) relative humidity distribution.

referred to as non-ventilated wall panel), (iii) ETICS system applied to inner RAC layer with the same thickness of mineral wool layer (hereinafter referred to as ETICS, Fig. 14). ETICS system has the same number and locations of monitoring positions as ventilated and non-ventilated wall panels have. This monitoring positions are shown in Fig. 14 and Fig. 4e), respectively. The material parameters for ventilated and non-ventilated wall panels were taken from the laboratory measurements, as listed in Table 1 and in [18]. The ETICS model assumes that the substrate is made of the same inner RAC layer as the one in ventilated and non-ventilated

wall panels. Properties of MW were taken from the WUFI database “Mineral Wool (heat cond.: 0.04 W/mK)” but the following parameters were adjusted to correspond to commercially available stone wool product used in ETICS systems and to provide the same level of thermal protection as glass wool in ventilated and nonventilated walls: bulk density 95 kg/m3 ; specific heat capacity (dry) 1030 J/kgK; thermal conductivity (dry, 10 °C) 0.034 W/mK; water vapour diffusion resistance factor 1.1 [-]. For exterior finishing plaster all material properties were taken as defined in WUFI database, “Mineral Plaster (stucco, A-value: 0.1 kg/m2 h0.5 )”.

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Fig. 10. Panel M2: (a) temperature distribution, (b) relative humidity distribution. Table 5 Measured RH at each measuring location (M1, M2, M3) in summer and winter. RH range (min-max) and monthly average value [%] Jun 2017 Outside air M1 S7 S5 S4 S3 M2 S7 S5 S4 S3 M3 S7 S5 S4 S3

(33.7–96.3) (44.3–55.3) (21.9–67.9) (33.4–65.5) No data (40.5–54.9) (21.3–78.6) (34.8–69.3) (52.6–61.4) No data (36.3–76.4) (39.9–69.8) (41.9–70.2)

Jul 2017 70.9 48.5 46.9 49.6 46.1 48.2 52.1 57.6 54.1 55.9 58.0

(31.2–96.0) (40.9–55.2) (19.1–70.4) (31.6–61.4) No data (35.2–50.9) (16.3–75.6) (27.7–65.5) (48.6–59.2) No data (33.8–71.6) (37.4–68.3) (37.8–68.2)

Aug 2017 68.1 46.5 44.1 47.7 41.2 44.1 48.2 55.3 51.5 53.4 55.1

(27.7–96.2) (36.5–53.3) (18.5–65.7) (31.6–65.4) No data (33.5–49.9) (20.7–72.2) (32.9–64.7) (46.8–60.3) No data (37.0–77.7) (39.1–78.0) (36.9–76.9)

Dec 2017 68.3 42.8 43.4 47.7 39.4 46.3 48.8 54.2 55.7 56.0 55.9

(53.3–96.9) (50.0–64.5) (19.7–67.4) (25.9–53.4) No data (67.4–73.3) (57.5–76.3) (38.4–59.6) (39.1–45.5) No data (67.4–82.8) (54.8–73.9) (39.2–68.8)

Jan 2018 84.5 55.2 48.1 37.2 70.9 67.0 48.1 41.5 77.3 63.8 49.8

(53.1–97.0) (53.4–66.7) (22.2–63.8) (28.2–50.3) No data (70.9–76.3) (55.8–76.2) (39.8–59.3) (39.3–46.4) No data (73.9–84.6) (56.9–75.6) (43.2–65.6)

Feb 2018 87.5 59.2 52.2 40.0 73.4 68.7 49.8 42.1 80.7 67.2 52.8

(57.0–97.0) (55.2–66.1) (16.4–63.9) (19.6–47.1) No data (71.4–75.8) (45.3–72.2) (33.5–53.1) (34.7–41.5) No data (57.5–83.4) (45.7–75.1) (29.9–62.2)

88.1 58.1 49.3 35.4 73.4 66.3 46.1 38.9 78.0 65.4 48.4

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Fig. 11. Panel M3: (a) temperature distribution, (b) relative humidity distribution.

For each envelope type calculations were performed for three different orientations – north, east, south. Simulations covered the period from January 01, 2017 at 0 0:0 0 to February 28, 2018 at 23:00 with a time step of one hour. For both outdoor and indoor boundary conditions of all simulated models, the measured data (Fig. 6) were applied. The initial moisture (60%) and temperature (15 °C) conditions are set as constant across component, assuming the average conditions in January. Values of heat resistance for exterior and interior surface corresponding to conventional values for external wall, 0.0588 m2 K/W and 0.125 m2 K/W, respectively are taken. Water-repellent layer was simulated only for ventilated and non-ventilated model by applying Sd-value at exterior surface as 0.25 m (taken from the laboratory measurements of water-

repellent coating) and setting no absorption for adhering fraction of rain. Calibration of ventilated wall panel model with experimentally measured data resulted with quantification of ventilation rate in terms of air change rate per hour (ACH). Simulation results (temperatures) are following the same pattern as measured temperatures for all three ventilated panels (M1, M2, M3), respectively. Therefore, for the sake of brevity, the results only from southoriented panel M1 are introduced here, both for summer period (August 2017 Fig. 15) and winter period (February 2018 Fig. 16), respectively. Regarding the comparison of experimentally measured and simulated temperatures for panel M1, although certain discrepancies are present, the tendency achieved a high accuracy for ACH=50 in

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Fig. 12. Thermal response of panels on summer day with highest developed external temperature: (a) M3 (S6 temperature), (b) M1 (S7 temperature).

Fig. 13. Temperature gap at measuring location M1: (a) summer period, (b) winter period.

Fig. 14. 1D model of simulated ETICS system and monitoring positions.

summer period and ACH=30 in winter, respectively. Temperature differences are more pronounced during the night time and particularly at the surface of MW insulation (S5 position), with average difference during winter period (Fig. 16b) of 4.46 °C. Detected temperature gap is most likely the consequence of sensors installation and panel’s production process. As it can be seen in Fig. 4) sensors are attached to the metal girder and there is a possibility that they got into direct contact with metal girder during the panel production. By being in direct contact, or even in nearby area, sensors are influenced by point thermal bridges. The top of the girder is protected with plastic cap (Fig. 4c), and thus sensors at position S7 are less influenced by this point thermal bridge. Simulation tool WUFI Pro is not capable of taking into account the influence of thermal bridges, therefore to investigate this assumption additional thermal

bridge assessment was performed using program package AnTherm v.9.135 [36]. Ventilated wall panel of dimensions 60 × 60 × 42 cm with four metal girders (Fig. 17a) was modelled. The diameter of metal girders was set to 8 mm and they are penetrating through all layers of panel. Ventilated air in cavity was approximated with effective thermal conductivity of air in accordance with [37,38]. Four different boundary conditions (BC1–BC4) were simulated for winter period, whereas two sets of boundary conditions are representative for period where S5 temperature peaks (measured and simulated) overlap and other two sets are representative for periods where measured and simulated S5 temperatures diverge from one another (Fig. 16b). Metal girders are stainless steel girders; however, the exact type of stainless steel is not known as well as its thermal conductivity value. Therefore, analyses have been performed for two different thermal conductivities (λ = 15 W/mK and λ = 50 W/mK) to cover its possible influence on thermal bridge intensity. Simulation results presented in Table 6 and in Fig. 17(b) and (c), show that temperature at metal girders and at MW surface (outside the area of thermal bridge influence) differ up to approx. 4.32 °C (λ = 50 W/mK) and 2.9 °C (λ = 15 W/mK) for the highest analysed S7-S1 temperature difference (BC4). This confirms that sensors in ventilated RAC wall panel, especially those located in MW layer are influenced by point thermal bridges. The greater the difference between external and internal surface temperature of wall panel, the stronger the effect of point thermal bridge.

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Fig. 15. The comparison of experimentally measured and simulated temperatures for south-oriented panel M1 in summer period: (a) position S7, (b) position S5.

Fig. 16. The comparison of experimentally measured and simulated temperatures for south-oriented panel M1 in winter period: (a) position S7, (b) position S5. Table 6 Influence of point thermal bridge. Date and time

External surface temperature [°C] – position S7

Internal surface temperature [°C] – position S1

Temperature [°C] for λ = 15 W/mK girder

19.02.2018 13:00 h (BC1) 19.02.2018 14:00 h (BC2) 19.02.2018 23:00 h (BC3) 28.02.2018 07:00 h (BC4)

20.24

20.33

15.82

20.39

3.99

20.58

−10.81

17.48

Girder 20.25 MW 20.24 Girder 16.47 MW 16.00 Girder 6.34 MW 4.63 Girder −6.81 MW −9.71

The influence of air ventilation on thermal performance was further investigated by comparing ventilated RAC wall panel with non-ventilated wall panel and ETICS system. It needs to be noted, that for comparison simulated data were used instead of measured, to remove the influence of thermal bridges which were not covered by hygrothermal simulations. For the sake of brevity, results are presented only for south orientation. For the clarity of the graphic presentations a mean daily temperature profile at the MW surface (position S5) has been calculated, and in Fig. 18 they are representative of August 2017 (summer) and February 2018 (winter). Time shift of mean daily temperature peak occurs in cases of ventilated and non-ventilated wall panels compared to ETICS; and this shift is present during both summer (2 h) and winter period (1 h), respectively. In case of ETICS; MW is in contact with finish-

T=0.01 °C T=0.47 °C T=1.71 °C T=2.9 °C

Temperature [°C] for

λ = 50 W/mK girder Girder 20.26 MW 20.24 Girder 16.70 MW 16.00 Girder 7.19 MW 4.65 Girder −5.36 MW −9.68

T=0.02 °C T=0.70 °C T=2.54 °C T=4.32 °C

ing plaster which is directly exposed to outdoor conditions, while in ventilated and non-ventilated wall panels MW is protected with outer RAC façade layer and air layer. This protection reduces temperature peaks and offsets them (Fig. 18). During summer period, it is more favourable for MW to have lower surface temperatures which would lead to lower temperature difference between interior and exterior, whereas heat inflow to the inside of the building (heat gains) would be reduced. The opposite situation is preferable during winter period – higher surface temperature of MW would result with lower temperature difference between interior and exterior, hence lower heat outflow from the inside of the building (heat losses) would be realised. Ventilated RAC wall panel outperformed non-ventilated panel during summer by having lower MW surface temperatures the whole day, except in period between 7 - 9 AM when their temperatures practically overlapped. This pat-

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Fig. 17. Analysis of point thermal bridge influence: (a) 3D model of the ventilated RAC wall panel with four metal girders, (b) BC4 conditions and λ=15 W/mK girders, (c) BC4 conditions and λ=50 W/mK girders, (d) BC2 conditions and λ=15 W/mK girders, (e) BC2 conditions and λ=50 W/mK girders.

Fig. 18. Mean daily temperature profile at MW surface for south orientated walls: (a) August 2017, (b) February 2018.

tern of behaviour indicates the positive influence that air ventilation has in summer period on thermal performance of wall panels with cavity. However, when comparing with ETICS in summer conditions, during the daytime ventilated RAC wall panel developed lower temperatures at MW surface while other half of the day (morning, late afternoon, night time) MW surface temperatures were higher compared to ETICS temperatures. When considering the winter conditions, naturally ventilated air in cavity has reduced MW surface temperatures during daytime (12 - 19 PM) in ventilated RAC wall panel compared to non-ventilated panel. Nevertheless, the rest of the day MW surface temperatures were higher in

case of ventilated RAC wall panel. ETICS exhibited strongest temperature development, reaching up to 3.31 °C higher MW surface temperature in period between 09 AM to 15 PM compared to ventilated wall panel, while out of that period MW surface in ETICS has cooled down quickly and temperature was up to 4.16 °C lower than in ventilated wall panel. Heat transfer between external surface of MW and internal surface of inner RAC layer has been quantified for each modelled envelope system in transient conditions capturing the effect of thermal inertia of the inner RAC layer (Fig. 19 and Table 7), whereas heat flux defined from outside to inside to be positive.

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Fig. 19. Heat flow rates through MW surface for south oriented walls: (a) August 2017, (b) February 2018. Table 7 Total energy flow between inner RAC layer and MW insulation.

Ventilated RAC wall panel Non-ventilated wall panel ETICS wall

August 2017 (summer)

February 2018 (winter)

Monthly [Wh/m2 ]

Mean daily [Wh/m2 ]

Monthly [Wh/m2 ]

Mean daily [Wh/m2 ]

283.8155 446.1579 499.8399

9.1553 14.3922 16.1239

−2129.0661 −2152.8479 −2307.1102

−76.0381 −76.8874 −82.3968

Differences between heat transfer of modelled wall systems are also expressed as percentage, calculated according to Eq. (1).

Di f f erencecooling =

Di f f erenceheat trans f er =

|Ventil ated RAC wal l panel| − |Non − ventil ated panel (or ET ICS )| (1) |Ventil ated RAC wal l panel|

Total monthly and mean daily energy flow from Table 6 indicate that for ventilated RAC wall panel heat inflow in summer is 57.19% lower compared to non-ventilated panel and 76.11% lower compared to ETICS wall. For opposite conditions, i.e. winter period, heat outflow through ventilated RAC wall panel is slightly lower compared to non-ventilated panel (1.12%) and 8.36% lower compared to ETICS wall. Based on heat flow analysis for south orientation, it can be concluded that ventilated RAC wall panel has significantly outperformed both non-ventilated and ETICS wall in summer period, while in winter period it has practically the same thermal behaviour as non-ventilated panel and somewhat thermally more favourable behaviour than ETICS wall. 4.4. The impact of thermal mass on building energy performance As mentioned previously in Section 2., ventilated RAC panel can be classified as heavyweight due to its surface mass over 400 kg/m2 . In this subsection, the impact of thermal mass on energy performance of exemplary building will be analysed based on comparison between dynamic and static energy consumption. The values of the heating and cooling energy needs resulting from simulations are reported in Figs. 20 and 21. The impact of opaque RAC wall panel massiveness on building annual energy need is further shown as percentage variations, which are calculated with reference to the model with high thermal mass. Those percentage variations are expressed as differences, separately for heating and cooling needs, as follows:

Di f f erenceheating =

H eating needwith T M − H eating needwithout T M Heating needwith T M (2)

Cooling needwith T M − Cooling needwithoutT M Cooling needwith T M

(3)

Observations from simulation results (Figs. 20 and 21) support following conclusions: • When thermal mass is taken into account, significant difference between dynamic and static energy consumption is present. • Influence of thermal mass on building energy needs cannot be analysed independently of the system operating mode and climate conditions. • In case of intermittent occupancy, building with high thermal mass will require generally more heating but less cooling energy in comparison to static scenario without heat storage capacity. This pattern applies for both Croatian climates – continental and littoral. For heating energy those differences are of the same order of magnitude in both climates (differences up to 8.34% in continental and up to 8.84% in littoral climate, respectively). On the other hand, for simulated cases with very low cooling energy needs (continental climate, Fig. 16a) those differences are up to 115.25%, while for simulated cases with higher cooling energy needs (littoral climate, Fig. 16b) differences are one order of magnitude smaller and reaching up to 12.06%. • When switching to continuous operating mode, high thermal mass is beneficial from both energy aspects – lower heating and cooling energy need, respectively. Building with high thermal mass behaves like a thermal sink and after storing heat energy, it will require less additional energy to maintain needed indoor conditions. This pattern of behaviour is confirmed in both Croatian climates. It should be highlighted that differences in heating energy needs are again of the same order of magnitude in both climates (8.57% in continental, 6.18% in littoral), while differences in cooling energy needs differ for one order of magnitude (−115% continental climate, 12.87% littoral climate). For the intermittent occupancy case, high thermal mass increases heating energy needs and this result is in line with

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Fig. 20. Heating and cooling energy needs for: (a) continental climate, (b) littoral climate.

Fig. 21. Impact of opaque RAC wall panel high thermal mass on building annual energy needs for: (a) continental climate, (b) littoral climate.

research [39]. However, by prolonging working hours of heating/cooling system, i.e. for continuous occupancy, high thermal mass exhibits positive effects and reduces heating and cooling energy needs. Continuous operating mode will result with lower fluctuations of indoor air temperature and thus higher indoor thermal comfort for occupants. 5. Conclusion and future work The main contribution of the research is upscaling recycled aggregate concrete (RAC), containing high amount of construction and demolition waste, from laboratory material experiments to full scale construction product implementation (ventilated prefabricated sandwich wall panel) and confirmation of RAC’s suitability for constructing energy high-performing, moisture safe, durable and sustainable building envelopes in real environmental conditions. After constructing Nearly-zero energy family house in city of Koprivnica (Croatia), the hygrothermal performance of the building envelope was measured in real outdoor climate conditions and with real occupants living in the house for a one-year period in 2017/2018. In-situ monitoring of transient hygrothermal performance was supported with numerical calculations, which were used to compare studied ventilated wall panel with two other envelope systems – prefabricated wall panel with closed air cavity (non-ventilated wall) and wall with applied ETICS system. The impact of panels’ high massiveness on energy performance of an exemplary building was assessed according to the new EN ISO 52016-1:2017 standard for different climates and different operating modes of technical systems. Experimental monitoring of hygrothermal performance was done for three differently oriented wall panels (south, east, north) adjacent to different indoor conditions (conditioned living room,

conditioned bedroom, unconditioned staircase). Temperature and relative humidity (RH) distributions through all four characteristic layers of panel were analysed and compared to simulation results from one dimensional transient heat and moisture transfer model, which resulted with quantification of air flow, in terms of air change rate (ACH), in cavity. It was observed that simulation results are following the same pattern as measured for all three ventilated panels with high accuracy. For the sake of brevity, additional analysis to investigate the influence of air ventilation in cavity on thermal performance of wall panel were performed only for south orientation. Experimental monitoring has shown that: • There was no problem with the highly moist conditions at the warm side of insulation (contact with inner self-loadbearing RAC layer). RH values never exceeded 80%, whereas the highest values were present for north panel reaching up to 76.9% in summer period and 65.5% during winter, respectively. • RH at surface and within mineral wool layer followed the RH behaviour of external air, which was triggered by external air entering cavity and circulating. In the middle of mineral wool layer, the lowest RH values were present for south panel then increasing for east panel and achieving highest values for north panel. • External surface of outer RAC cladding exhibited relatively limited RH fluctuations despite high outdoor RH values (above 90%) and their high fluctuations. This behaviour can be attributed to concrete’s inertia as well as applied water-repellent coating. Recycled aggregate has increased water absorption properties compared to natural aggregate, therefore proper water-repellent protection is of great importance when durability of RAC elements is observed.

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M. Bagari´c, I. Banjad Pecˇ ur and B. Milovanovi´c / Energy & Buildings 000 (2020) 109573

• Temperatures at exterior surface of thermal insulation are higher compared to external surface temperatures of outer RAC cladding in winter and lower in summer period, respectively (for south panel up to 10.79 °C in summer and up to 8.08 °C in winter). This behaviour was induced by outer RAC cladding protecting thermal insulation from exterior conditions but also by naturally ventilated air in cavity, which was confirmed with numerical simulations. The main findings from transient numerical simulations are as follows: • Façade orientation, together with boundary conditions, influence the efficiency of ventilation in air cavity. • Metal girders for outer RAC cladding act as point thermal bridges and their influence on thermal performance should be covered by calculations. • Ventilated RAC wall panel outperformed non-ventilated wall during summer by having lower temperatures at exterior surface of thermal insulation practically the whole day. This pattern of behaviour indicates the positive influence of air ventilation in summer period on thermal performance of wall panels with cavity – passive cooling mechanism. • ETICS wall, by being directly exposed to exterior conditions, has the highest hygrothermal load affecting thermal insulation, and thereby reducing its long-term durability compared to wall panels with air cavity protected by cladding. • Total monthly and mean daily energy flow indicated that ventilated RAC wall panel has significantly outperformed both nonventilated and ETICS wall in summer period (up to 57.19% and 76.11% lower heat inflow, respectively), while in winter period it has practically the same thermal behaviour as non-ventilated panel and somewhat thermally more favourable behaviour than ETICS wall (up to 1.12% and 8.36% lower heat outflow, respectively). Regarding the influence of panel’s high thermal mass: • Panel was able to dampen temperature change between inner and outer layer for more than 18.5 °C in case of north panel (adjacent to unconditioned staircase) when the coldest winter day is considered. • On summer days with temperature attenuation of 30.48% and time delay of 4 h for north, and 47.11% and 6 h for south panel, respectively, panel proved its capability of maintaining stable indoor temperature conditions. • The hourly calculation procedure according to new Standard EN ISO 52016-1:2017 showed that the thermal mass of envelope system has significant influence on building dynamic energy performance, whereas it cannot be analysed independently of the system operating mode and climate conditions. • In case of intermittent occupancy, heavyweight building will require more heating energy but less cooling energy in comparison to static scenario without heat storage capacity. When prolonging working hours of systems (continuous mode), high thermal mass will be beneficial from both aspects – heating energy needs and cooling energy needs. In conclusion, the presented study has shown that heavyweight envelope constructed with ventilated prefabricated wall panels from CDW can perform satisfactorily, from both hygric and thermal aspect, in real outdoor climate conditions and conditions of use by real occupants. This means that presented envelope system could foster the large-scale deployment of buildings constructed utilizing CDW in high percentage (at least 50%). However, applicability of this envelope system for other climates and conditions of use should be verified by numerical simulations with a dynamic air flow model and/or conducting additional field large-scale monitoring. Hence, the future work will be focused on such analysis,

especially related to efficiency of ventilation (defining the effective thickness of air cavity) for different conditions (climate and building use). Monitoring of indoor comfort (in terms of temperature, relative humidity, CO2 ) and energy consumption in ground floor apartment has been recently commissioned and it will be thoroughly investigated. Declaration of Competing Interest None. Acknowledgment The experimental research was financially supported by the “ECO-SANDWICH” project funded within the frame of EU CIPECO Innovation programme (Grant no. ECO/11/304438/SI2.626301). The authors would like to especially thank Assistant Professor Tea Žakula from University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, who developed an in-house code in accordance with Standard EN 52016-1:2017 and made it available for the purpose of this paper. The authors express their deepest gratitude also to Professor Emeritus David W. Yarbrough from Tennessee Technological University for his valuable comments and suggestions. References [1] K. Nore, Hygrothermal performance of ventilated wooden cladding Doctoral thesis, , Norwegian University of Science and Technology, Trondheim, Norway, 2009. [2] X. Yang, Investigation on moisture buffering of hygroscopic materials by full-scale experiments and HAM simulations Doctoral thesis, , Concordia University, Montreal, Quebec, Canada, 2010. [3] F. Collet, J. Chamoin, S. Pretot, C. Lanos, Comparison of the hygric behaviour of three hemp concretes, Energy Build. 62 (2013) 294–303, doi:10.1016/j.enbuild. 2013.03.010. [4] C. Feng, H. Janssen, Hygric properties of porous building materials (II): analysis of temperature influence, Build. Environ. 99 (2016) 107–118, doi:10.1016/j. buildenv.2016.01.016. [5] B. Milovanovic´ , D. Mikulic´ , Assessment method for combined heat, air and moisture transfer in building components, in: V. Zanki (Ed.), Proceedings of the International Conference on Energy Management in Cultural Heritage, Dubrovnik, Croatia„ 2011. [6] J. Vyncke, J. Vrijders, Recycling of construction and demolition waste - An Overview of RILEM achievements and state of the art in the EU, in: J.C. Galvez, A. Aguado de Cea, D. Fernandez-Ordonez, K. Sakai, E. Reyes, M.J. Casati, A. Enfedaque, M.G. Alberti, A. De la Fuente (Eds.), Proceedings of the Second International Conference on Concrete Sustainability - ICCS16„ Madrid, Spain, , 2016. [7] D. Bjegovic´ , N. Štirmer, Theory and Technology of Concrete (In Croatian Teorija i Tehnologija Betona), 1st ed., University of Zagreb, Faculty of Civil Engineering, Zagreb, Croatia, 2015. [8] M. Behera, S.K. Bhattacharyya, A.K. Minocha, R. Deoliya, S. Maiti, Recycled aggregate from C&D waste & its use in concrete - A breakthrough towards sustainability in construction sector: a review, Constr. Build. Mater. 68 (2014) 501–516, doi:10.1016/j.conbuildmat.2014.07.003. [9] D. Bjegovic´ , N. Štirmer, M. Serdar, Ecological aspects of concrete production, in: J. Zachar, P. Claisse, R.T. Naik, E. Ganjian (Eds.), Proceedings of the Second International Conference on Sustainable. Construction Materials and Technologies, Ancona , Italy, , 2010. ´ N. Štirmer, I.B. Pecˇ ur, Residual mechanical properties of concrete [10] I. Milicˇ evic, made with crushed clay bricks and roof tiles aggregate after exposure to high temperatures, Materials (Basel) 9 (2016), doi:10.3390/ma9040295. [11] P. Marco, A conceptual model for designing recycled aggregate concrete for structural applications, Doctoral thesis, University of Salerno, Italy, Springer Theses, Springer, 2014, doi:10.1007/978- 3- 319- 26473- 8. ´ D. Bjegovic, ´ R. Siddique, Experimental research of concrete floor [12] I. Milicˇ evic, blocks with crushed bricks and tiles aggregate, Constr. Build. Mater. 94 (2015) 775–783, doi:10.1016/j.conbuildmat.2015.07.163. [13] D. Pickel, Recycled Concrete Aggregate:: Influence of Aggregate Pre-Saturation and Curing Conditions on the Hardened Properties of Concrete Thesis for the degree of Master of Applied Science, , University of Waterloo, Waterloo, Ontario, Canada, 2014. [14] E. Fraile-Garcia, J. Ferreiro-Cabello, L.M. López-Ochoa, L.M. López-González, Study of the technical feasibility of increasing the amount of recycled concrete waste used in ready-mix concrete production, Materials (Basel) 10 (2017) 817, doi:10.3390/ma10070817. ´ Recycled aggregate concrete for [15] I. Banjad Pecˇ ur, N. Štirmer, B. Milovanovic, nearly zero-energy buildings, Mag. Concr. Res. 67 (11) (2015) 575–584, doi:10. 1680/macr.14.00220.

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