Thermal transmittance of historical stone masonries: A comparison among standard, calculated and measured data

Energy and Buildings 151 (2017) 393–405

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Thermal transmittance of historical stone masonries: A comparison among standard, calculated and measured data Elena Lucchi ∗ Politecnico di Milano, Via Bonardi 9, 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 6 December 2016 Received in revised form 27 May 2017 Accepted 3 July 2017 Available online 4 July 2017 Keywords: Thermal performances U-value R-value Tabulated data Analytical calculation In situ measurement Heat flux meter measurement

a b s t r a c t The paper presents the results of an on-site campaign on several historic stone masonries, characterized by different heritage values, historical ages and intended use. Experimental data has been compared with the standard procedures normally used in the Italian legislation framework for assessing the thermal performance of existing masonries. Normally, the standard procedures tend to overestimate their thermal performance for security reasons. Similarly, wrong estimations or excessive simplifications have serious impact on the thermal assessment. For this reason, the paper presents an interdisciplinary assessment methodology for the thermal performance evaluation of the traditional stone walls located in Lombardy Region. The most important challenges are related to the correct definition of the wall morphology and thickness, the thermal properties of stone and the material proportions. The study shows a correspondence between the geology of the territory, the historical ages, the thermal performances and the heritage values of stone masonries. Furthermore, the proportion of materials, as well as the presence of internal air affect greatly the thermal performances. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The building envelope is the principal responsible for the energy losses through the building. Its amount in existing buildings is commonly quoted in the range from 10% to 45% [74], related to climatic conditions, wall surface area, degree of material degradation and construction technologies. The accurate identification of the thermal performance of a building component is a key requirement for ensuring an appropriate energy assessment [65,66]. Two parameters described this performance: the thermal transmittance (U-value) and the thermal resistance (R-value) that respectively outline the thermal insulation and the thermal resistance of the building element. It depends on: (1) global layout; (2) stratigraphy; (3) characteristics of each material; (4) presence of internal moisture; (5) variation of the climatic parameters; (6) presence of damage and conservative problems; and (7) application techniques. Its evaluation is challenging particularly for historic masonries, where a range of hypotheses is possible for assessing different wall thicknesses, thermal conductivities (␭-value), moisture contents, surface heat transfer coefficients, presence of mixed materials and air cavities. Wrong estimations, simplifications and

∗ Present address: EURAC Research, Viale Druso 1, 39100, Bolzano, Italy. E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2017.07.002 0378-7788/© 2017 Elsevier B.V. All rights reserved.

overcomes can have several consequences on the overall energy balance of the building, producing on overestimation of the energy consumption [1,25,64–66]. Misguided assessments affect also the energy retrofit, promoting substitution or energy improvement of components [1,15,64] without any real benefits for energy-savings and sustainability. The problem is more serious for inhomogeneous masonries, as stone and mixed walls, where the gap in knowledge is related to the identification of the following elements [25,48,52]: (1) traditional morphologies (stratigraphy, layers composition, mortar proportion, . . .); (2) inhomogeneity or geometric discontinuities (thickness variability, air cavities, cracks, materials decay, . . .); (3) physical properties of the materials used along the time (composition, density, thermal performances, . . .); (4) ageing and damage problems; and (5) moisture contents. Furthermore, traditional materials (particularly the range of available ␭-value of stones) are hardly represented in available tools, standards and databases [1,65]. These approximations and simplifications cause an overestimation of the real R-value of a historic masonry of (10 ÷ 30%) compared to the in situ measurements in 50 ÷ 77% of cases [1,15,34,52,65]. On the contrary, the use of accurate data inputs can improve the agreement with the on-site results [1,64]. To overcome this problem, the thermo-physical characterization of traditional masonries is worthy of continuous research efforts. A detailed review on the methodologies for assessing the thermal performance of historic walls is reported in a previous

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Nomenclature Nomenclature and abbreviations R Thermal resistance (or R-value) (m2 K/W) U Thermal transmittance (or U-value) (W/m2 K) Ut Tabulated U-value (W/m2 K) U-value from abacus of masonry structure (W/m2 K) Ua Uc Calculated U-value (W/m2 K) Um Measured U-value (W/m2 K) Thermal conductance (or C-value) (W/m2 K) C ␭ Thermal conductivity (or ␭-value) (W/mK) Cp Specific heat (MJ/m3 K) ␣ Thermal diffusivity (mm2 /s) Density (kg/m3 ) ␳ ␧ Emissivity  Heat flow rate in steady-state conditions (W) q Density of heat flow rate (W/m2 ) A Area (m2 ) Surface temperature (K or ◦ C) Ts Te Environmental (ambient) temperature (K or ◦ C) Temperature difference (K or ◦ C) T d Wall thickness (m) Number of hours – monitoring period (h) n MC Water content (%) Water mass (%) mW mO Weight of dray sample (g) Index j m e i

Individual measurements Total measurements Exterior Interior

paper on brick masonries [55]. Here, the literature review focuses only the on the on-site campaigns of historic masonries. Extensive studies attempted to measure the in situ U-values of historic walls in order to study the potential impact of the insulation materials for improving their thermal performance [13,14,19,35,65]. The surveys focused on stone [13,14,65] and brick [19,35] masonries, respectively in Scotland and England. The practice in these countries can give some guidance for the present study. They defined specific procedures for thick solid walls based on the increasing of the standard monitoring period to allow the impact of the thermal mass, the surface temperature and the heat flow fluctuations. They demonstrated that the thermal properties of materials are constant over the range of temperature fluctuations; and the changes of the internal energy are negligible if compared to the amount of heat going through the element. Thus, historical masonries are considered sufficiently homogeneous for using the heat flux meter (HFM) measurements and for applying the standardized surface heat-transfer coefficients [14,19,35,65]. In general, they found measured U-values significantly lower than the standard ones, evidencing the implications of these discrepancies in energy audit and modelling. The problem is more serious for stone masonries. Particularly, Rye and Scott [65] noted that the calculation for traditional stone walls is particularly problematic for the following reasons: (1) lack of knowledge of vernacular materials and construction methods; (2) absence of data on traditional features and potential ambiguity of historic stone walls in the standard procedures; (3) paucity of ␭-values data for individual stone types; (4) greater geological diversity of rocks; and (5) use of default assumption. In order to better model the wall build-ups, the following information are required [14,19]: (1) thickness of layers, (2) status of cavities; (3) ratio and types of stone and mortar; and (4) thermal

properties of materials used in traditional construction. In addition, Baker [14] established that further research should be carried out to establish a better understanding of the thermal properties of traditional building materials and construction components. Particularly, the standard databases should include more data on the traditional solid stonewalls to allow easier and user-friendly modelling of traditional buildings. Finally, a standardized methodology for the in situ measurements of the U-values should be established to ensure that future measurement results are comparable [14]. Mainly due to the extreme complexity of historic stone masonries, the comparison among standard assessment methodologies and databases on thermal performances of traditional masonries lacks. On the contrary, the literature provides several criteria and operative procedures that can be used for the definition of a specific evaluation method for this type of masonries.

2. Research aims The paper presents a comparative analysis of different standard procedures normally used in Italy for assessing the thermo-physical properties of traditional stone masonries. This research carries on the work on brick masonries, following a similar approach [55] and suggesting specific techniques for stone characterization. The standard procedures defined by the Italian legislation framework [70] (“tabulated design method”, “abacus of masonry structures” and “analytical calculation”) has been compared with the experimental data collected during an on-site campaign on ten historic buildings in Lombardy Region (30 survey points). These buildings are characterized by different heritage values, ages and intended use to have a wide range of masonries. On purpose, we compared masonries with similar wall morphologies (rubble stone masonries) and types of rock (sedimentary rock), to verify the match between historical ages, heritage values, thicknesses and material percentages. Similarly, we excluded the masonries with damage and moisture problems in order to avoid their influence on the thermal performance. This work permits to: (1) compare different standard procedures for assessing the energy performance of historic stone masonries; (2) characterize better the traditional stone masonries in the northern Italy, (3) enhance the knowledge on traditional materials and construction methods; (4) define a procedure that consider traditional features and potential ambiguity of historic stone walls; and (5) enlarge the existing databases on Italian constructive technologies. The research neither means to be exhaustive or definitive, but simply aims to serve as a guidance for energy auditors and simulators that require simplified data and clear procedures to operate.

3. Research methodology The research methodology is based on the following steps: • Selection of traditional stone masonries; • Masonry characterization using an interdisciplinary assessment method based on preliminary historical researches, geometrical reliefs, visual inspections (VI), infrared thermography (IRT) surveys and gravimetric tests [54,56]; • Thermal performance evaluation of walls, using different procedures suggested by the Italian legislation framework; • Comparison among the results; • Final performances assessment.

This methodology has been illustrated below (Fig. 1).

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Fig. 1. Research methodology.

3.1. Selection of traditional stone masonries The case studies focused on the construction techniques of Lombardy Region that are representatives of the northern Italy, thanks to the diversity of its geography and territory. The Region lies in a large alluvial plain (named Pianura Padana), bounded on the north by hills (Prealpi) and mountains (Alpi Lepontine). Its geomorphological variety allowed to have different building techniques, which include: brick, stone and mixed masonries. As explicated above, this paper carries on the work on brick masonry [55], presenting the results on stone masonries. Stone masonries were built in the areas near hills and mountains (Provinces of Como, Lecco, Sondrio, Bergamo and Brescia) [57]. The type of rock depended on the geomorphology of the territory. Metamorphic (marble, gneiss, granulite) and igneous (granite, diorite, porphyry) rocks were found mainly in historic buildings with high representative values, while sedimentary rocks (conglomerates, sandstones, breccia, limestone, dolomites) and rubbles (moraines) were used in traditional buildings especially near the Pre-alpine area (Provinces of Como, Lecco, Bergamo) [20,57]. We decided to compare only rubble stone masonries built with sedimentary rocks, to characterize better the thermal performance of this type of walls. These masonries are located in the provinces of Como, Lecco and Bergamo. Thus, we selected ten buildings in these neighboring provinces. Three measurement points have been taken on each wall (30 survey points). Next the buildings and the characteristics of stone masonries are presented (Table 1). To have a wide range of traditional constructive techniques, the masonries have different historical ages (ranging from XII to XVIII Centuries) and intended use (residential buildings, libraries, museums, offices and churches). Historic buildings are mainly monumental (80%) rather than traditional (20%). 3.2. Interdisciplinary assessment of stone masonries The qualitative assessment of the thermal performance of masonries considers the application of different nondestructive techniques (NDT) [5,15,39,41,54,56]. The present methodology departs from the work on brick masonries [15,55], setting out a specific procedure for stone walls [4,18,27,63]. It involves six steps: (1) preliminary historical research; (2) geometric survey; (3) visual

inspections (4); (iv) IRT survey; (5) gravimetric tests on mortar and plaster; and (6) cross-referencing of the previous data. It has been illustrated below (Fig. 2). The historical research covered the studies on architecture and petrography, in order to obtain data on: (1) age of buildings; (2) traditional wall apparatus of the area; (3) geometric factors (shapes, dimensions, thicknesses, percentages of stone and mortar); and (4) materials habitually used (characterization, type of stone and mortar, internal and external finishing). In the first case, we checked previous studies, handbooks, repositories, conference papers, and degree theses, both on traditional Italian masonries and specific case studies. Repository and general literature provided information mainly on the typical wall morphologies of the pre-alpine area and the percentages of stone and mortar [31,40,73]. Specific books gave information on the chronological stages and the materials habitually used [20,27,63]. Books on petrography and geology permitted to gather data on the distribution of rocks and quarries in Lombardy Region. Also, they gave data on mechanical and chemical characteristic of stones. Unfortunately, we didn’t find data on their thermal properties. The historical research allowed to do accurate hypotheses about internal morphology, constructive techniques and materials used. Heritage buildings in this area were essentially constructed as solid masonries, made from natural stone bedded in lime mortar. The choice of the stonework was influenced by its location: more formality was required in monumental fac¸ades than in traditional buildings. The typical stones were sedimentary rocks formed by sediments of pre-existing rocks. The type of stone was different in these three provinces because was related to the local geology. Como’s masonries, both traditional and monumental ones, were built with “Moltrasio stone” [20,57], a limestone formed mainly by calcite and small percentages of quartz and orthoclase. It has got a dark gray color, fine grain and good compactness. The quarries were located near the town of Moltrasio, near to the lakeshore. The poor quality, especially for uneven layering and strong discoloration, reduced its use only in this area. Lecco’s masonries were built with “Varenna stone” [20,57], a limestone composed by calcite. It has got black color, fine grain and good compactness. The quarries were located near the town of Varenna and it was widely used for civil and monumental buildings in the province of Lecco. Finally, the Bergamo’s masonries were built with “Sarnico stone”

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Table 1 Characteristics of the selected buildings and masonries. Type of masonry

Type of stone

Moltrasio stone (sedimetary rock)

Location

Como

Stone Varenna stone (sedimetary rock) Sarnico stone (sandstone)

Lecco Bergamo

Building name

Building use

Palazzo Giovio Palazzo Erba Odescalchi Palazzo Natta Palazzo Volpi Chiesa di San Francesco Palazzo Cernezzi Traditional building Traditional building Palazzo della Ragione Torre

Office Library Office Museum Church Office Residential Residential Office Library

Wall stratigraphy (Outside-Inside)

plaster/stone/plaster

stone/plaster plaster/stone/plaster stone

Points measured

Thickness (m)

Age (Century)

3 3 3 3 3 3 3 3 3 3

0.60 0.68–0.70 0.70–0.72 0.70–0.72 0.80 1.00 0.46 0.54 1.00 1.10

XVI XVI XVIII XVII XIV XVI XIX XIX XII XII

(Source: Elaboration of the Author).

Fig. 2. Interdisciplinary assessment method for stone characterization.

[20,57], a sandstone (greywacke) composed by quartz, muscovite with lime cement and clay matrix with calcareous cement. It has gray or yellowish colors, fine grain and good compactness. The most important quarries were located in the mountain that overlooks the town of Sarnico and in the hills between Paratico and Capriolo. The use was remarkable in the city of Bergamo, especially in the middle Ages [20]. Then, a geometrical characterization has been executed to obtain: (1) wall thickness (d); (2) geometry of masonry surfaces; (3) percentage and general characteristics of the wall constituents (stone, mortar and voids); and (4) shape and geometry of stones. Particularly, wall thickness ranged from 0.46 m to 1.10 m. The more detailed characterization of masonry and stone was possible only in three walls not completely plastered. Two masonries are located in Lecco (cases 1 and 2) and one in Bergamo (case 3). In this case we used a procedure developed by Almeida et al. [4] that includes walls’ photographic record and geometrical reliefs. First, the photos (A = 1.0 m2 ) are made using a ruler as scaling factor, in order to facilitate their rescaling. The pictures are straightened using the software RDF and then redesigned. The material percentage is computed by dividing the areas of the different constituents by the total area [4]. This survey shows that the masonries are very similar. The walls consist of medium size stones (0.20 ÷ 0.40 m diagonally measured) arranged on regular alignments with mortar joints. The stones have prismatic shapes, very regular in Lecco and less formal in Bergamo. The mortar joints have grey color with a thickness in the range 0.020 ÷ 0.035 m. The percentages of stone, mortar and void calculated on the elevation pictures was respectively: 83.5/15.3/1.4 (case 1); 85.0/13.4/1.6 (case 2); and 86.5/8.8/4.7 (case 3 outside) and 83.6/13.3/3.1 (case 3 inside). Unfortunately, this calculation was not feasible in the

cross-sections because we hadn’t access to the walls. These results may lead to significant errors due to: (1) estimation of the final layers of the walls, not of the nucleus; (2) significant differences in the results for the inner and outer parts of the Bergamo’s survey; and (3) difficulty on measuring the real shape of stone, the joints’ thickness and the void volume. VI and IRT survey have been performed to investigate deeper the following aspects: (1) wall stratigraphy; (2) surface finishing; (3) geometry of the masonry; (4) presence of decay and damage; and (5) presence of internal moisture and water absorption. Particularly, VI shows the materials used and the characteristics of the surface finishing, as well as the absence of failures, decay and moisture in the external layers [54]. Generally, internal and external finishing are painted with lime or lime-gypsum plaster while the internal surfaces are plastered with gypsum. The walls in Lecco hadn’t the external plaster while the wall of the city tower of Bergamo (3 survey points) hadn’t any plaster. IRT survey gives several information on the wall morphology, revealing the distribution of the radiant thermal energy emitted from a target surface and visualizing the presence of thermal anomalies in the layers just below the plaster [43]. Several studies used this techniques for the qualitative evaluation of existing walls [2,3,12,41,51,56,58,60] in order to discover the presence of: (1) non-homogeneities (thermal bridges, different thicknesses, different materials); (2) energy inefficiencies (excessive heat loss areas, air leakages); and (3) damage (decay, cracks, moisture, water percolation). We executed this technique for detecting the geometry of the Como’s masonries that were covered by plasters, both inside and outside. We applied a similar procedure of the geometrical relief, using a ruler as scaling factor. The straightening of the image was not necessary because each thermogram was taken perfectly

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orthogonal to avoid any possible distortions. Stone dimensions and material percentages are determined using computational tools on the images. These operations confirmed the results obtained with the geometrical characterization. The walls in Como consist of medium size prismatic stones (0.20 ÷ 0.35 m diagonally measured), arranged on regular alignments with mortar joints with a thickness in the range 0.020 ÷ 0.035 m. We saw the presence of voids, but was not possible to calculate their percentages due to the noise of the IRT images. Despite IRT surveys didn’t show water absorption or moisture migrations, we did an accurate assessment of the moisture content of the walls not listed by the heritage Authority (the residential buildings in Lecco). A gravimetric test was made [71] on 8 samples of mortar (depth = 0.10 m). The samples were dried for 24 h to constant weight in an oven (T = 105 ◦ C), cooled to ambient temperature in a desiccator (T = 20 ◦ C) and finally weighed with an analytical balance [24,71]. The water content (MC) is calculated as a proportion between the water mass (Mw ) and the weight of the dry sample (MO ) [24]. At the end of the test, MC was low and constant (MC < 0.02%). Unfortunately, Authorities didn’t allow the extraction of wall samples for their characterization using the hot-disk technique. Thus, it was not possible to measure the thermal properties of material in terms of ␭-value, thermal diffusivity (␣) and specific heat (Cp ). 3.3. Standard methods for assessing the thermal building performance The Italian legislation framework introduces four methods for assessing the U-value of existing masonries [70]: (1) “tabulated design method” that offers “standard” values for typical masonries; (2) “abacus of masonry structures” with indications and data for calculating R and U values of traditional masonries; (3) “analytical calculation” that provides a finite element analysis procedure for computing the U-value of thermally homogeneous components; and (4) “heat flow meter (HFM) measurement” for measuring directly in situ the thermal performance of building masonries. The “tabulated design method” [70] classifies five typologies of masonries by compositions and thicknesses (d): (1) brick solid walls plastered on both surfaces; (2) stone walls plastered on both surfaces; (3) semi-solid bricks or tuff walls; (4) concrete walls without insulation systems; and (5) cavity brick walls. The applicability of the standard U-values for historic stone masonries is very low (only in the category “stone walls plastered on both surfaces”). First, the standard thickness ranges from 0.30 ÷ 0.60 m while, on the contrary, historic walls have high thickness to ensure thermal mass and insulation (e.g. 0.60 ÷ 1.20 m). Second, the standard database doesn’t specify the type of rocks. In contrast, the rocks used in the traditional masonries have very different thermal properties based on the geomorphology of the territory. Furthermore, tabulated walls are plastered on both surfaces with lime, so the data are applicable only on this type of walls. Finally, the standard doesn’t consider the influence of conservative problems and water content on the final thermal performances. The “abacus of masonry structures” [70] offers a guidance for assessing the thermal performance of typical Italian walls made by: (1) brick; (2) stone; (3) brick and pebbles mixed; (4) tuff blocks; (5) stone edged with bricks; and (6) mixed walls. It provides the procedure for calculating the U-values, giving the typical ␭-values. Contrasting with the previous case, no thickness limits are introduced. Unfortunately, this method does not give any information on the thermal performance of rocks due to their complexity and variety. Thus, it is not applicable to stone walls. The “analytical calculation” [46] for homogeneous and multilayer masonries is defined by the International standard ISO 6946 [46] as a simple sum of the R-value of each homogeneous layer. The calculation for non-homogeneous elements is based on complex

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computer models (e.g. Delphin, Wärme Und Feuchte Instationär – Wufi, or fluid dynamics models). Both the procedures require detailed and accurate input data on stratigraphy, thickness (d), position and thermal properties of each building material, in terms of ␭-values, density (␳), thermal mass (cp ), vapor pressure resistance (␮) and emissivity (␧). Consequently, it can be used with a deep knowledge of the wall layouts and the material properties. The most important challenges for the thermal assessment of stone masonries are related to the correct definition of [1,14,65]: (1) wall morphology and random nature of its amalgamation; (2) thermal properties of stones; (3) proportions of materials involved in its construction (stone, mortar, voids and other non-visible characteristics); (4) water content and moisture; and (5) physical and structural damage. Otherwise, the information on the material properties comes from pre-defined [47,67,68] or manufacturer databases that contain mainly industrial materials [1] and use a series of precautionary hypotheses on the water content [29]. Furthermore, the “standard” surface resistances [46] use high ␧values, normally applicable to the buildings without decay, dirt, or superficial injury. On the contrary, these values are related to the conservative conditions of the wall. Finally, HFM measurements can avoid many inaccuracies related to the other methods. They test the thermal properties of a building element directly in situ under actual thermal conditions. R-value and conductance (C-value) determination is based on the simultaneous measurement of the mean “density of heat flow rate” (q = /A) and “surface temperature difference” (Tsi -Tse ) of different measurements (j ) [45] while U-value is based on the simultaneous measurement of the mean q and “environmental (ambient) temperature difference” (Ti -Te ) of different measurements (j ) [45]. The measuring apparatus is composed by a data-logger equipped with one plate (a thin resistive plastic layer) and two temperature sensors (thermocouples and resistance thermometers) for measuring and registering the internal and external temperature and the heat flows through the walls [9,45]. The data-logger gathers the thermal data (q; T; Ts ) for a specific interval during the monitoring period. International and national standards defined the procedures for measuring the thermal transmission properties of homogenous building components directly in situ [8,9,45,49]. Otherwise, some experimental studies evidenced many meteorological and practical issues on measurement accuracies, errors, and uncertainties [25,26,36]. The researches concern mainly new walls, but the results are useful also for old masonries. The main uncertainties are related to: (1) measurement location [25,26,59]; (2) non-homogeneity of the materials [25,36]; (3) heat flux perturbation generated by the HFM itself [59]; (4) thermal inertia of the wall [25,59]; (5) conservation state and moisture content of the wall [25,26,36,52]; (6) data processing techniques [26]; and (7) influence of boundary conditions, particularly temperature fluctuations [25,26,36], outside weather conditions [11], wall orientation [11] and ambient conditions [36]. The most critical factors are temperature differences (T) [36], heat flow gradients [7,36], boundary [7,11,25,26,32,36] and moisture conditions variations [36]. On the contrary, the role of measurement location produced different estimations [25,26,36]. Standards [45,49] and literature [25,26,32,36] suggest several advices for reducing these influences, especially on massive and historic masonries. They are related to the proper installation of sensors for having a representative part of the wall and reducing the influence of boundary conditions and singularities (e.g. thermal bridges, different thicknesses, windows, plants, natural and artificial ventilation, internal humidity or other defects that could bring to incorrect results). Also, the presence of large T (up to 20%) improves the accuracy of the results [25,26,32]. Furthermore, the transient effects of energy absorption and thermal inertia can be reduced with long monitoring periods (from 3 to more than 7 days), according to the component feature and the

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Table 2 Characteristics of the HFM apparatuses used for the on-site campaign. Characteristics Name of the instrument

Traditional Instrument Ahlborn Almemo 2590-3S

Wireless Instrument Carlesi Optivelox Thermozig Standard

Surface temperature probes

Number of probes Typology Position Range of the measurement Accuracy of the measurement

2 Thermocouples 1 internal and 1 external 0–400 ◦ C ±0.5%

3 Thermocouples 1 internal and 2 external 0–400 ◦ C ±0.5%

Heat flux plate

Shape of plate Dimension Thickness Type of the substrate Range of the measurement Accuracy of the measurement Sensitivity of the instrument

Square 250 × 250 mm 1.5 mm Resin ±2000 W/m2 ±5% Not specify

Round diameter 100 mm 5 mm Resin ±2000 W/m2 ±5% 0.01 W/m2

Air temperature probes

Number of probes Typology Position Range of the measurement Accuracy of the measurement

2 Thermocouple Internal −5/+95 ◦ C ±0.5%

2 Thermocouple Internal −5/+95 ◦ C ±0.5%

Software used

For data transfer For data elaboration

AMR Control 5.14 SUBB

Dataget Ucalc

Source: Elaboration of the Author from the manufactured manuals.

temperature variations [45]. However, it is common practice in historic and massive walls to extend the monitoring campaigns to two weeks or more (n = 336 ÷ 662 h) to achieve satisfactory results and stable conditions [14,19,35,65]. Finally, two methods may be used for the data post-processing [45]: (1) the “daily average method” to be used with standard boundary conditions; and (2) the “dynamic methods” to be used with large variations in temperature and heat flow rates. The first method can be used when the hypothesis of steady-state conditions is met and thus there are negligible effects of the thermal mass. In this case, the R-value is defined as the ratio of the mean temperature difference measured between the two sides of the element and measured heat flow passing through the element. The second methods can be used with heat flux perturbation generated by the presence of drift in outdoor temperatures and random noise [26]. In this case, the post processing data requires the use of complex dynamic methods not normally adopted by the energy auditors for their higher complications compared with traditional methods.

inserted on non-damaged masonries, applying a thermally conducting paste to the back of the plate. The IRT survey helped for the proper installation of the apparatus, reducing the influence of thermal singularities, moisture content and damage [14,19,65]. To reduce the impact of massive masonries, the monitoring period was established according to standard [45], experimental studies [13,14,19,35,65] and first monitoring of a stone wall that revealed a good thermal stability after 145 h. It lasted 7 days (n = 168 h) for walls with a thickness <0.90 m; and 14 days (n = 336 h) for walls with a thickness from 0.90 m to 1.10 m. During this time, internal and external temperatures averaged out respectively 20 ◦ C and 5 ◦ C (T = 15 ◦ C). In all case, a T of 10 ◦ C was reached for guarantying reliable measurements [1]. Finally, data were processed mainly with the “average method” [45], thanks to the thermal homogeneity of the walls and the stability of internal and external Ts [35].

3.4. HFM measurement procedure used

Following, a discussion on the results of the Italian on site campaign have been presented.

The HFM measurements reported in the present paper have been realized according to the international standard ISO 9869 [45], the prEN 12494 [17] and the specific literature on historic walls [13,14,19,35,65]. The procedure was already detailed described [55]. Next, just a summary has been reported. The monitoring has been conducted during two winter seasons, using a traditional and a wireless instrumentation. In addition, indoor and outdoor temperature and relative humidity has been recorded with a dual channel Testo loggers, to verify the environmental stability for data verification purposes. The characteristics of the HFM apparatuses are illustrated below (Table 2). To avoid the influence of measurement location and boundary conditions (i.e. solar radiation, rain, snow), we located the apparatuses in a representative part of the north-facing walls. Similarly, to minimize the influence on the inner surface of heat sources, users and vertical stratification of temperature, the apparatuses were located in the central part of the wall [25,26,36], sited away from potential sources (i.e. radiator, computers and lamps) [26]. Also, the internal sensors for temperature and relative humidity were mounted in the center of the room. To reduce the thermal break between the wall and the plate, the apparatuses were also

4. Results and discussion

4.1. Comparison between standard and measured data Tabulated U-values (Ut ) are taken directly from the Italian standard UNI TS 11300-1 [70] for stone walls plastered on both surfaces. The standard ␭-values vary from 0.89 W/mK to 1.20 W/mK, with an average value = 1.06 W/mK [70]. These values correspond to a sedimentary rock with a ␳ of 1600 ÷ 1790 kg/m3 in the European standard UNI EN 1745 [69] while no correspondence in the Italian standard UNI 10351 has been found [67]. The data have been matched with measured U-values (Um ). This comparison is possible only for the masonries with a thickness from 0.45 m to 0.60 m (42% of the survey points). Their thermal performance varies in the range 38 ÷ 47%, but always Um are better of about 40% than Ut (Um < Ut ). As a matter of fact, the standard UNI TS 11300-1 [67] contains restrictive data for security reasons. Below, the results of this comparison are reported (Fig. 3). As cited above, the comparison among the “abacus of masonry structure” (Ua ) and Um was not possible for the absence of information on the properties of rocks.

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Fig. 3. Comparison between Ut and Um (d = 0.30 ÷ 0.60 m). Table 3 Thermal properties of different stones in the Italian and the European standards. Standard UNI 10351

Standard UNI EN 1745

Material

Density [kg/m3 ]

␭-value [W/mK]

Limestone

1900 2100 2700 2800 –

1.50 1.60 2.90 3.50 – 2.37

Average value Final average value

Material

Density [kg/m3 ]

␭-value [W/mK]

Sedimentary rock

<1590 1600–1790 1800–1990 2000–2190 2200–2590

0.85 1.10 1.40 1.70 2.30 1.47

Average value 1.92

Sources: Standards UNI 10351 [70] and UNI EN 1745 [69].

4.2. Comparison between calculated and measured data The U-values are calculated (Uc ) according to the standard ISO 6946 [46]. We decided to use only the standard calculation procedure to simulate the work of the energy auditors. As a matter of fact, dynamic calculation models are considered complex, expansive and time consuming for their daily work [74]. The comparison between Uc and Um involves all the walls (100% of the survey points). The thermal performance of each type of stone come from national [67] and European databases [69] in order to compare the final performance of the masonries and to create an easy procedure for energy auditors and simulators. Databases, due to the variety of rocks, give information on the “general” type of stone that do not coincide properly with the sedimentary rock. Its properties are considered similar to the limestone for UNI 10351 [67] and the sedimentary rock for UNI EN 1745 [69]. The differences are in the range 10 ÷ 30% for UNI 10351 [67] and 8 ÷ 45% for UNI EN 1745 [69]. Moreover, there are significant differences between the standards linked to different ␳ considered. As an example, the difference between their average values (␭limestone = 2.37 W/mK; ␭sedimentaryrock = 1.47 W/mK) is 32%. The thermal properties of different stones reported in the standards are shown below (Table 3). To simulate the typical scheme used by the energy auditors, we decided to use all the standard ␭-values. The lowest values of the standard UNI EN 1745 [69] are closer to Um . They correspond to sedimentary rocks with low ␳ (1590 ÷ 1790 kg/m3 ). The results of this comparison are reported below (Fig. 4). The standard procedure considers monolithic stone walls, but actually the historical stone elements are jointed with lime mor-

tar and we can observe air gaps and pores in the binder and between the elements. Their presence has an impact on the thermal performance. Thus, the definition of these percentages has been obtained cross-referencing the information from literature [27,18], experimental surveys [53] and geometrical reliefs. First, the architectural handbooks estimated a proportion of 80 ÷ 90% of stone and 20 ÷ 10% of mortar, related to geographic areas, locations and heritage values. Second, coring and laboratory tests on several rural buildings near Como [53] discovered the presence of air due to manufacture and installation. They measured an average percentage of 86.6% for stone, 8.7% for mortar and 4.7% for air. Surveys and geometric reliefs on the selected masonries reveled a general material percentage of 80 ÷ 85% for stone, 10 ÷ 15% for mortar and 1.5 ÷ 4.5% for air. The data confirm the previous studies [20,57], despite their accuracy is low for the difficulty on measuring the real shape of stone, the joints’ thickness and the void volumes. Thus, we used all the literature estimations, verifying their influence on the performance assessment. The thermal properties of lime mortar, air and plaster have been taken from the national standard [67]. They are reported below (Table 4). For the presence of air, we preferred to consider a general ␭value of the material, than the value of air cavities [67]. Considering the presence of stone and mortar, Uc is closer to Um . Obviously, this proportion influences the results. Uc estimated with the two percentages (90/10 and 80/20) is in the range 8 ÷ 9%. On the contrary, the presence of internal air greatly affects the final performance. The difference between Uc calculated considering the presence of air and the other situations is 67 ÷ 73% (90/10 stone to mortar) and 65 ÷ 70% (80/20 stone to mortar). Moreover, the standards simplify

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Fig. 4. Comparison between Uc calculated using different ␭-values from Italian and European standards [67,69].

Fig. 5. Influence of the percentage of stone and mortar for the stone masonries in the analytical calculation of the U-value.

Table 4 Thermal performance of plaster, mortar and air in the Italian standard UNI 10351 [67]. Standard UNI 10351 Material

Type

Density [kg/m3 ]

␭-value [W/mK]

Plaster

Gypsum Lime gypsum Lime

1200 1400 1400 1800 1.3

0.35 0.70 0.70 0.90 0.026

Lime mortar Air Source: Standard UNI 10351 [70].

the actual behavior of walls, considering an “immobile air” instead of convective motions that decrease the final results. The difference between Uc and the average Um varies in the following ranges: (1) 29 ÷ 34% with 90/10 stone to mortar; (2) 24 ÷ 29% with 80/20 stone to mortar; and (3) 54 ÷ 58% with 86.6/8.7/4.7 stone to mortar and air. In general, the proportion 80/20 seems the more suitable. Also, the estimation is more correct for thin (s < 0.52 m) and thick

(s > 0.80 m) masonries. The results of this comparison are reported below (Fig. 5). Finally, we consider an average percentage of stone (85%), mortar (13.5%) and air (1.5%) from on-site surveys and geometric reliefs. In this case, Uc are very close to Um : 7% for ␭-values from UNI 10351 [67]; 20% for ␭-values from UNI EN 1745 [69]; and 12% for average ␭-values. The results are reported below (Fig. 6). Finally, we decided to compare Ut , Um and Uc . We considered three different Uc : (1) monolithic wall with an average ␭-value from UNI 10351 [67] (␭ = 2.37 W/mK); (2) monolithic wall with an average ␭ from UNI EN 1745 [69] (␭ = 1.47 W/mK); and (3) non monolithic wall using an average ␭ from the previous standards (␭ = 1.92 W/mK), typical values for mortar (␭ = 0.90 W/mK) and air (␭ = 0.026 W/mK) [67], and the most correct percentage of materials (stone 85%; mortar 13.5%; and air 1.5%). The difference between Uc and Um varies considerably but, generally, Um have lower value than Uc . For monolithic walls, the differences are in the range 34 ÷ 54% and 13 ÷ 38%, calculated respectively considering the cases (1) and (2). First, in the case (1) Uc are very far from Um for Como (44 ÷ 54%) than Bergamo (42 ÷ 46%) and Lecco (34 ÷ 36%).

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Fig. 6. Analytical calculation of the U-values using the following material percentages: stone 85%, mortar 13.5% and air 1.5%.

Fig. 7. Comparison among Ut , Uc and Um for the selected stone masonries.

On the contrary, in the case (2) Uc are very close to Um for Lecco (13 ÷ 15%) than Bergamo (17 ÷ 23%) and Como (19 ÷ 38%). Finally, the presence of air improves the final Uc that results better than Um . In the case (3) the difference between Uc and Um is in the range 3 ÷ 20%. We obtained very close results for the Como’s masonries built from XIV to XVIII Centuries (3 ÷ 7%). Below, this comparison is reported (Fig. 7). Also for stone masonries, standard and calculated methods tend to overestimate the thermal performance compared with the HFM measurements. Finally, the plaster has a very low effect on the final performance (±2 ÷ 8%). 4.2.1. Measured data analysis The analysis shows a correspondence among the type of stone and the thermal performances of masonries. We found the best performance in Como (average ␭ = 0.92 ÷ 0.99 W/mK) and the worst both in Bergamo (average ␭ = 1.15 − 1.23 W/mK) and Lecco (average ␭ = 1.17 ÷ 1.35 W/mK). The results are particular interesting in Como, where we considered only monumental buildings from XIV to XVIII Centuries. The thermal performance of masonries increases during the Centuries, with average ␭-values from 1.08 ÷ 1.17 W/mK

(XVI Century) to 0.83 ÷ 0.89 W/mK (XVIII Century). The results are reported below (Fig. 8). The comparison among Uc and Um shows that the standard UNI 10351 [67] overestimates the ␭-values of stone masonries. On the contrary, the standard UNI EN 1745 [69] presents similar ␭-values for the sedimentary rocks with a ␳ in the range 1590 ÷ 1990 kg/m3 . Particularly: (1) masonries of XIII–XVI Centuries have medium ␭ and ␳ values (␭ = 1.10 W/mK; ␳ = 1600 ÷ 1790 kg/m3 ); (2) masonries of XVII–XVIII Centuries have higher performances (␭ = 0.85 W/mK; ␳ < 1590 kg/m3 ); and (3) masonries of XIX Century have lower performances (␭ = 1.40 W/mK; ␳ = 1800 ÷ 1990 kg/m3 ). No correspondence has been found between XII Century’s masonries and standard data. The results are reported below (Fig. 9). The analysis of the correlation between heritage values and thermal performances shows that heritage buildings have better performances than traditional constructions. Unfortunately, these results are not generalizable, because traditional buildings are built only with “Varenna stone” while heritage buildings with “Sarnico” and “Moltrasio” stones. Thus, masonries performance is related mainly to the type of stone than their heritage value. Obviously, a bigger sample of walls is required for having more reliable per-

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Fig. 8. ␭-Values of stone masonries related to building age.

Fig. 9. Comparison among standard and measured ␭-values of stone masonries.

Fig. 10. Measured ␭-values of stone masonries: comparison among types of stone.

formance ranges for these materials. The results related to the type of stone (Fig. 10) are reported below. In general, medium masonries (d = 0.60–0.79 m) have better average ␭-values than thick masonries (s = 0.80–1.10 m). This situ-

ation is probably due to the presence of other materials in thick walls, such as small pieces of bricks, different rocks, sand, and row materials, as confirmed also by architecture manuals [57] and on site surveys [4]. Lastly, the type of plaster has a very low influence on the final thermal performance. This is due to the low thicknesses (d = 0.020 ÷ 0.025 m) and to the similar properties among different historical plasters (␭limeandgypsum = 0.70 W/m2 K; ␭lime = 0.70 W/m2 K). Thus, for standard calculations it is not important to have a detailed knowledge of the thickness and the type of plaster to obtain accurate results. Similar studies have been conducted in Scotland and England, comparing on site results and standard data. Baker [13,14] analyzed twenty traditional Scottish masonry constructions and internal finishing. The majority of the walls used for the measurements were built pre-1919. They are mainly solid wall constructions with different type of stone, bedded in lime mortar [14]. For stonewalls, he stated that a ratio of 60/40 stone to mortar would most likely represent the correct proportion of the walls. After intrusive exploration

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in similar wall types a proportion of 70/30 has been experienced. We noted the difference with the present study, based both on Italian literature and on site survey. As a matter of fact, this proportion is strongly influenced by the traditional constructive technologies. For this reason, it is important to check the correctness of the energy audit. The measurements of the 0.60 m thick walls ranged from 0.80 to 1.60 W/m2 K; whereas the measurement of a 0.30 m thick wall was 2.30 W/m2 K. The average Um resulted in the present study for similar thickness is 1.50 W/m2 K, very close to Baker’s survey. Generally Uc tend to overestimate the Um , particularly when no account of the proportion of mortar is considered. Better agreement was achieved, if the wall was considered as a stone/mortar mix. Rye and Scott [65] adopted the same procedure to test several traditional Scottish masonries, composed by solid cob and stone walls, both ashlar block and rubble constructions with a variety of rocks (flint, granite, gritstone, malmstone and limestone) and air gaps. The thickness ranged from 0.20 m to 0.60 m. The typologies were intentionally varied from the previous analysis [14] to have a wide range of results. Um has been compared with Uc using a traditional software program (BuildDesk U 3.4), standard procedure (BR 443 calculating method) and performances (␭flint = 3.50 W/mK; ␭granite = 2.80 W/mK; ␭grifstone = 2.30 W/mK; ␭malmstone = 2.30 W/mK; ␭limestone = 1.10 W/mK). Standard data in 77% of cases overestimated the U-value of stone walls in relation to the in situ measurements. They established an average Um of 1.42 W/m2 K, independently by the type of rock. The average Um discovered with the present study is 1.14 W/m2 K. The results are comparable with standard data [67,69], considering the difference between sandstone and sedimentary rocks. Furthermore, they recognized the diversity of sandstone related to the range of densities, which makes problematic establishing a range of thermal performances. Also, they outlined that the variation of the air gaps has a significant influence on the R-value of the wall. Furthermore, Bros-Williamson [19] took into account pre-1919 stone masonries constructed by lime-bonded rubble, ashlar solid walls, or with both rubble and ashlar frontage. The type of stone was sandstone and the wall elements were 0.40 m to 0.60 m thick. Measured data are compared with calculated methods using standard procedures and input data (␭ = 0.44 ÷ 0.77 W/mK). The analytical calculation changed for: (1) monolithic sandstone walls (Uc = 2.1 W/m2 K); (2) 60/40 stone to mortar (Uc = 1.7 W/m2 K); (3) previous walls with 0.03 m plaster (Uc = 1.6 W/m2 K); and (4) previous walls with slightly ventilated air layer behind a lath and plaster lining (Uc = 1.20 W/m2 K). Um were in the ranges: 1.30–1.50 W/m2 K for walls with lath and plaster lining; and 1.20–1.60 W/m2 K for walls with plaster on-the-hard. These values fall within the Uvalue range previously reported for similar construction elements [14,65]. Thus, also in this case, standard calculation tends to overestimate the U-value of historic stone walls. For all the studies the factors that affect the U-value include [13,14,19,65]: (1) the thickness of the wall, (2) the type of stone and (3) the material proportion. Unfortunately no comparisons with the age of the masonry have been reported in the literature [13,14,19,65]. The results of these studies are perfectly comparable with the present Italian survey. Particularly, in all cases they showed that: (1) analytic calculation generally tends to overestimate the U-value of historic stone walls compared with the in situ surveys; (2) traditional and local constructive technologies strongly influenced the proportion of materials; (3) the diversity of stones makes problematic establishing a range for their thermal performance; and (4) thickness and proportion of air voids or gaps have an important role for estimating the thermal performance of building masonries. Finally, English studies are mainly dedicated to brick walls. The English Heritage [35] measured several solid brick walls dating from 18th and 19th Centuries; and the Building Research Establishment (BRE) [42,17] carried out HFM measurements, boroscopic

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inspections, coring, dust sampling and laboratory tests in 300 dwellings with brick walls. The data are not comparable with the present study, but in all the cases standard calculations overestimated the thermal performance of walls by approximately one third. 5. Conclusions The paper presents the results of an on-site campaign on several historic stone masonries, characterized by different heritage values, historical ages and intended use. Experimental data (Um ) have been compared with the standard procedures normally used in the Italian legislation framework for assessing the thermal performance of existing masonries [70]. Deliberately, we don’t consider damaged and wet masonries for excluding these influences on the final thermal performances. In general, the most important challenges for the thermal assessment of stone masonries are related to the correct definition of the wall morphology and thickness, the thermal properties of stone and the proportion of material, particularly of air voids or gaps. On the contrary, the type and the thermal properties of plaster and mortar haven’t a high impact on the thermal performances. The traditional building elements have a better thermal performance than the one expected from the standard calculations. As a matter of fact, the tabulated design method and the analytical calculation tend to overestimate the U-values compared with the in situ measurements because, normally, the standard database gives conservative data to consider safety margins. The standard calculations consider monolithic walls, but the percentage of stone, mortar and voids greatly affects the final results. In fact, better agreement was achieved, if the wall was considered as a stone/mortar mix [13,14,19,65]. Similarly, the variation of the air gaps has a significant influence on the R-value [65]. Moreover, the standard data simplify the actual thermal performance of walls, considering an “immobile air” instead of convective motions that decrease the results. The study presents a correspondence between the historical ages and the thermal performances of stone masonries. First, the thermal performance of stone masonries increases during the Centuries, from XIII to XVIII Centuries. XIII–XVI Centuries’ masonries have medium thermal performances (␭ = 1.10 W/mK; ␳ = 1600 ÷ 1790 kg/m3 ) while XVII–XVIII Centuries’ masonries have high thermal performances (␭ = 0.85 W/mK; ␳ < 1590 kg/m3 ). On the contrary, XIX Century’s masonries have low thermal performances (␭ = 1.40 W/mK; ␳ = 1800 ÷ 1990 kg/m3 ). Finally, no correspondence has been found between XII Century’s masonries and standard data. In general, in the different periods, heritage masonries have better performances than traditional walls. The average ␭-values measured for the selected masonries are: 1.02 W/mK for “Moltrasio stone”, 1.35 W/mK for “Varenna stone” and 1.23 W/mK for “Sarnico stone”. As mentioned above, the proportion of materials greatly affects the thermal performances. A correct estimation of the proportion for rubble stone walls in this geographic area could be: 80 ÷ 85% for stone; 13.5 ÷ 16% for mortar; and 1.5 ÷ 4% for air (as discover crossing the data from historical manuals, geometric relief and on site surveys). The study shows that the percentage of voids is higher in traditional buildings (3 ÷ 4%) than heritage buildings (1.5 ÷ 3%). Finally, traditional plasters (gypsum, lime and gypsum and lime) have got lower effects than cement plasters. For standard calculations it is not important to have a detailed knowledge of the thickness and the type of plaster to obtain accurate results. Their influence on the thermal performance is higher for thin walls than for massive masonries. These results may be useful for improving the correctness of simplified thermal evaluation of historical stone walls. The results obtained considering monolithic walls, as suggested by the tabu-

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lated method and the analytical calculation, are not reliable. It is always necessary to consider the percentage of mortar and air to adjust the U-value for regulatory purposes. Only the HFM measurements permit to obtain data close to the reality, both for standard calculations (i.e. energy labelling) and accurate evaluations (e.g. energy audit or simulation) of stone walls. Although the focus of the current research is based in the northern of Italy, the general results are applicable to other European countries with similar masonries. Further work should regard the influence of moisture on the thermal performance of traditional masonries [52].

Acknowledgements The early stage of this study has been founded by Fondo di Ateneo per la Ricerca di Base (FARB) for the research “Efficienza energetica ed edilizia storica. Misurare le caratteristiche termofisiche delle murature antiche” conducted at Politecnico di Milano. The data analysis has been conducted independently in a second step, without founding. We are thankful to all professors, students and building managers who provided data, tools and instruments in the first step.

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