Energy and Buildings

Energy and Buildings 59 (2013) 62 72 Contents lists available at SciVerse ScienceDirect Energy and Buildings j our na l ho me p age: www.elsevier.com/locate/enbuild Experimental thermal characterization of radiant barriers for building insulation C. Escudero a,,k. Martin b,a. Erkoreka b,i. Flores b, J.M. Sala b a Laboratory for the Quality Control in Buildings, Basque Government, C/ Agirrelanda n. 10, 01013 Vitoria-Gasteiz, Spain b Department of Thermal Engineering, University of the Basque Country (UPV/EHU), Alameda Urquijo s/n, 48013 Bilbao, Spain a r t i c l e i n f o Article history: Received 18 July 2012 Received in revised form 21 December 2012 Accepted 30 December 2012 Keywords: Radiant barriers Thermal resistance Laboratory testing Simulation Standards a b s t r a c t To obtain traceable thermal resistance of radiant barriers for building insulation laboratory tests have been carried out. Heat flow meter apparatus and guarded hot box methods have been used. The heat flow meter method has been evaluated as a method to characterize the insulating layer itself, while the guarded hot box method has been used as a tool to determine the total thermal resistance of a building component including a radiant barrier. A material with both surfaces of reflective material has been used in the study. The measurements have been carried out using two configurations in the heat flow meter apparatus: simple air chamber and double air chamber. The results have been compared with a simple analytical model of heat transfer according to ISO 6946 standard and with a Computational Fluid Dynamic (CFD) model. The CFD model enables to assess the relationship between experimental and analytically estimated results. The main conclusion is that the laboratory tests are valid for the thermal characterization of radiant barriers. It also follows that thermal resistance of radiant barriers can be estimated with precision using the simplified ISO 6946 methodology, as long as values of the thermal properties of the reflective material such as emissivity and conductivity are reliable. 2013 Elsevier B.V. All rights reserved. 1. Introduction It can be stated that an effective insulation of the building envelope is needed to achieve high energy savings. One possibility for thermal insulation is the use of systems based on radiant barriers. These systems have commonly been used in roofing insulation, although they are also used in faç ades and floors. There are a number of studies [1 3] that determine the effectiveness of these systems by experimental tests in roofs. Nevertheless, there are not many studies on radiant barriers as insulating systems in faç ades and floors [4,5]. As stated above there are several studies to characterize the thermal performance of insulation solutions based on radiant barriers, but the conditions and test methodologies applied do not follow any standard. In most studies real prototypes are tested [6 8] or self-made test systems are used [9,10]. These studies provide wide information for evaluation, modelling and development of radiant barriers, but do not have the necessary features for result traceability. In order to obtain comparable results, the use of standardized measurements under controlled conditions is required [11]. This is why the most common test methods to characterize thermally insulating materials in buildings are to be used in this study: heat Corresponding author. Tel.: +34 945 268 933; fax: +34 945 289 921. E-mail address: cesar.escudero@ehu.es (C. Escudero). flow meter apparatus (HFM apparatus) and guarded hot box test facility. These equipments can be used to determine thermal resistance in steady conditions, both for the insulating layer and for the whole solution of faç ade or floor, respectively. It is important to note that radiant barriers are an effective thermal insulation when installed as boundary surfaces of an air chamber, either in one or two sides, Fig. 1. The low emissivity of these materials reduces the component of heat transfer by radiation, thus increasing the value of the equivalent thermal resistance of the air chamber. Therefore the effective thermal resistance depends on the emissivity of the radiant barrier and the thickness of the air chambers. The usual configuration is based on the use of double air chamber separated by the low emissivity layer which acts as a radiant barrier. For a defined insulating layer thickness, this is the solution that optimizes the effect of insulation and gets the highest values of thermal resistance (R t ). Double air chamber solution is usually applied to faç ades. But in those cases in which the insulating layer thickness of the constructive solution is limited or complex to install, it is often built up with a simple air chamber. This is the case of floors. Anyway, in both cases, the radiant barrier and associated air chambers are located inside the constructive solution, as an intermediate layer. This configuration is different from the usual application of radiant barriers on roofs, where the low emissivity material is placed as finishing surface below the roof, which further 0378-7788/$ see front matter 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2012.12.043

C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 63 Fig. 1. Common configuration radiant barriers in faç ades and floors. improves the roof thermal performance by increasing the internal surface thermal resistance. This effect does not occur when applied as insulation for faç ades and floors. Several tests have been carried out to determine the possibility of characterizing the insulation solution itself with the air chambers, and also inside a full faç ade solution. The first case needs working with samples of small size, 0.6 m 0.6 m, while the second case works with real building component samples with dimensions of 2 m 2 m. 2. Methodologies to be evaluated The aim of this paper is to present the results of the thermal resistance obtained for different configurations of radiant barriers using different methodologies. Firstly an analytical model is presented according to the standard ISO 6946 [12]. This simplified model is validated comparing the model results with those obtained from experimental tests. Tests are carried out both in the reflective material element level and in a faç ade with the reflective material as a component using heat flow meter and guarded hot box facilities respectively. In order to improve the reliability of the results, a numerical model based on Computational Fluid Dynamics is also used. 2.1. Analytical model The most common configuration of radiant barriers has an air space on both sides of the reflective material where a convective/conductive/convective heat flow through the air and reflective material happens. There is also a direct radiant exchange between surfaces that form the cavity. Fig. 2 shows an equivalent circuit diagram using the electrical analogy for the heat transfer process. The heat flux between two surfaces separated by air chambers can be determined, according to the expression Eq. (1), as the sum of a radiative component and a convective component. Both components are function of the temperature difference between the involved surfaces. q ij = h ij r (T si T sj ) + h ij c (T si T sj ) (1) where q ij is the heat flux between surfaces i and j (W/m 2 ); h ij r is the radiation heat transfer coefficient between surfaces i and j (W/m 2 K); h ij c is the convection heat transfer coefficient between surfaces i and j (W/m 2 K); T si and T sj are the temperature of surface i and j respectively (K) Natural convection inside air chambers has been extensively studied for many years and many correlations have been developed. ISO 6946 proposes the following approximations for the convection coefficient h ij c according to the heat flux direction: Horizontal flow: h ij c is the greatest of 1.25 (W/m 2 K) and 0.025/d (W/m 2 K). Vertical flow up: h ij c is the greatest of 1.95 (W/m 2 K) and 0.025/d (W/m 2 K). Vertical flow down: h ij c is the greatest of 0.12 d 0.44 (W/m 2 K) and 0.025/d (W/m 2 K). where d is the thickness of the air chamber in (m) and temperature difference between surfaces must be smaller than 5 K. If temperature difference between surfaces is greater, these other approximations should be used: Horizontal flow: h ij c is the greatest of 0.73 ( T ij ) 1/3 (W/m 2 K) and 0.025/d (W/m 2 K). Vertical flow up: h ij c is the greatest of 1.14 ( T ij ) 1/3 (W/m 2 K) and 0.025/d (W/m 2 K). Vertical flow down: h ij c is the greatest of 0.09 ( T ij ) 0.187 d 0.44 (W/m 2 K) and 0.025/d (W/m 2 K). In the case of radiant heat exchange, the infinite parallel flat surfaces hypothesis is acceptable for air chamber thickness ten times lower than other dimensions. Under this hypothesis the Fig. 2. Equivalent thermo-electrical circuit of the heat transfer for radiant barriers.

64 C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 Fig. 3. Heat flux meter apparatus. coefficient of heat transfer by radiation, h ij r, is given by expression Eq. (2). h ij 4Tm 3 r = (2) (1/ε i ) + (1/ε j ) 1 where, is the Stefan Boltzman constant (W/m 2 K 4 ); T m is the average temperature between T si and T sj (K); ε i and ε j are the emissivity of surface i and j respectively. The simplified model described in the standard ISO 6946 makes another hypothesis to enable the heat flux calculation be proportional to a linear temperature difference. This way the temperature difference between layers is expressed as Eq. (3). T 4 si T 4 sj = 4 T 3 m (T si T sj ) (3) The relative error (e) of this approximation can be estimated by the expression Eq. (4). For usual temperature values in building physics and testing, it is estimated to be less than 0.02%. e = 2 (T si T sj ) Tm 2 (4) The heat transfer model proposed by ISO 6946 determines the flow of heat transfer through the air chamber by using and equivalent convection radiation thermal resistance (Ra), ij see expressions Eqs. (5) and (6). Therefore the air chamber can be considered as another opaque homogenous layer for heat transfer calculations. q ij = T si T sj (5) R ij a R ij a = 1 h ij c + h ij r Finally, for the configuration of Fig. 2, the total thermal resistance would be the one obtained from the sum of the thermal resistances of the two air chambers (R a 12 and R a 34 ) and the conductive resistance of the reflective material (R RF ). The results obtained by this mathematical approximation will be evaluated by comparing the thermal resistance results for different configurations of radiant barriers with those obtained from tests. 2.2. Experimental methodology Two standardized tests have been carried out to characterize thermally radiant barriers under controlled conditions. The high (6) precision of the facilities which are presented below is necessary in order to obtain traceability in the results. 2.2.1. Heat flow meter A heat flow meter (HFM) manufactured according to the standard EN 12667 [13] (equivalent to ISO 8301 [14]) is used to determine the thermal resistance experimentally. The HFM apparatus consists of two plates where the test sample is placed as in Fig. 3. The dimensions of the samples are of 0.6 m 0.6 m, with variable thickness. Each plate has its own heat flow sensor, 0.25 m 0.25 m in size, located in the central area. Working with a smaller measuring area than the dimensions of the sample reduces the measurement error due to lateral heat losses. The average sample temperature can be set from 263 K to 323 K being the temperature differences between hot and cold plates from 10 K to 30 K, with a precision of ±0.25 K. The top plate acts as a hot plate and the bottom one as a refrigerated plate, so steady conditions can be obtained by placing the sample between these two plates. There are three T type thermocouples in each of the plates, recording the central temperature and two perimeter temperature values of the metering area. The thermal resistance of the tested sample is calculated from Eq. (5) using the measured average temperatures of each plate and the heat flux obtained averaging the heat flux measurements of both plates. The heat flow sensors are calibrated using as a reference an internationally traceable calibration standard sample. The standard material is NIST 1450c and the heat flux for each plate is determined by the expression Eq. (7). q p = f e p (7) The value of heat flux q p is obtained from the temperature difference between both faces. The thermal conductivity of this sample is known since it is a reference standard of NIST. The e p is the voltage signal measured for both plate heat flow sensors average and f is the calculated calibration value for the heat flow sensors. The required uncertainty of the measured thermal resistance using this method must be under 5%. It is determined by the accuracy of the measurement of heat flux and the measurement of the temperature difference, as can be seen in Eq. (5). Since the hypothesis of uncorrelated measurements can be applied, the uncertainty can be determined by applying the law of error propagation using the uncertainty of each of the measured magnitudes. The error propagation theory for the most unfavourable test conditions, which corresponds to the measurement of the thermal

C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 65 Fig. 4. Wall to be tested in the guarded hot box equipment (a) base wall and (b) base wall insulated with the radiant barrier. resistance of a 0.005 m air chamber, is 3.5%. This value is used as the uncertainty of all the measurements carried out in the HFM apparatus. 2.2.2. Guarded hot box The last step for characterizing the thermal behaviour of the radiant barrier is based on analysing its response inside a real faç ade. This characterization is carried out by the guarded hot box method, according to ISO 8990 [15] standard, which uses a real wall specimen with dimensions 2 m 2 m (Fig. 4). This method is required to determine the thermal resistance of real constructive solutions under steady state conditions. The HFM apparatus can be used to characterize the insulating solution itself, but the sample does not consider the possible non-homogeneities of the real walls such as the ones formed by masonries locked with mortar joints or possible thermal bridges. Two steps are required to obtain the thermal resistance of the radiant barrier by means of the hot box method. Firstly a masonry wall (concrete block) is tested (Fig. 4a) and then the radiant barrier with its air chamber and plasterboard layer are added and the new thermal resistance value is tested (Fig. 4b). The difference of both tested thermal resistances is used to get the thermal resistance of the radiant barrier with the air chambers. In Fig. 5 the final building component is detailed. Note that initially a 0.01 m air chamber was designed using 0.01 m thick crossbars, but since the blanket is compressible, once the crossbars where fixed to the blanket, the contact zone was compressed and the measured real air chamber has been of 8.5 10 3 m. Before the test is carried out, the specimen is conditioned to equilibrium with an atmosphere at 296 K and 50% rh. Once the base wall weight is stabilized, the sample is introduced to the guarded hot box equipment. The testing goal is to get steady state and onedimensional heat flow conditions using two climatic chambers: a hot chamber at 293 K and a cold chamber at 273 K. Within the hot chamber a 1 m 2 section metering box can measure with a 0.2% uncertainty the power introduced by a group of resistance heaters and a fan to keep an homogeneous temperature of 293 K (Fig. 6). Surface to surface temperature difference of the wall is measured by 8 T type thermocouples in each surface. Then, Eq. (5) can be used to determine the thermal resistance of the tested sample. To reduce the two-dimensional heat transfer effects, the central measurement area is surrounded by a perimeter guarded zone of 0.5 m, which is maintained at the same conditions as the central measuring area. The temperature difference between the metering box inner air and the hot chamber air is maintained below 0.1 K, being the metering box insulated with 0.05 m of polyurethane to minimize the heat transfer between these environments. Finally, the perimeter of the sample is protected by a tempered ring that keeps the average temperature of the test minimizing perimeter losses. The later conditions ensure that the contribution of above described effects to the measurement of heat flow through the sample is under 0.5%. Adding the uncertainty of the thermocouples, the final testing uncertainty increases up to 5.4%. The possibility to maintain test temperatures under the described highly controlled settings is what gives the quality and validity to the experimental characterization of the thermal resistance of real constructive solutions by means of the guarded hot box. During the experiment the data is acquired with a 1 min frequency. The determination of thermal resistance starts once the wall reaches steady state conditions. This condition is fulfilled once the surface temperatures of the hot face do not have gradients Fig. 5. Tested building solution configuration.

66 C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 Fig. 6. Guarded hot box facility and its scheme. greater than 1.5 K/m and the heat flux does not present a standard deviation over 1% during a sampling period of 24 h. Once steady state-state is reached the value of thermal resistance is determined as the ratio between the integration of temperature difference and integration of heat flux measured during a 24 h period. 2.3. Numerical model An additional method to compare thermal resistance results in radiant barriers is the numerical calculation, although some of the information needed to carry out the simulations is taken from tests. The purpose of these simulations is to analyze the differences between the standard simplified method and tests results. In simulation there are no geometric constraints as there are in the test facilities. To make a correct comparison, some of the data used during the simulations is taken from measured values during the testing, e.g. geometry, emissivities and thermal conductivity of the radiant barrier. A schematic of the boundary conditions and the mesh for 2D steady-state simulation is detailed in Fig. 7. The simplicity of the geometry allows a high quality mesh with regular elements of 0.001 m size. To achieve more precision in convective effects near surfaces the boundary layer is also defined. For calculations only the central metering zone of the HFM apparatus is considered. The numerical simulations are performed by means of the finite volume simulation tool FLUENT 6.2 [16], which solves the energy equation in each node defined by the mesh. As the air movement in the chambers and the radiant heat transfer is important in the thermal characterization of radiant barriers, these aspects are also taken into account. For natural convection, the simplified Boussinesq model can be used due to the small temperature difference inside the enclosure. The discrete ordinates (DO) radiation model is used to solve the radiative transfer equation for 2 discrete solid angles, which returns good results. 3. Emissivity measurement methodology To compare the different methodologies for the determination of the thermal resistance of radiant barriers, there is the need of using the same data. Therefore the first step is to determine the surface hemispherical emissivity of the hot and cold plates of the HFM apparatus. This value will be used in calculations for the analytical and numerical methods. In EN 1946-3 [17], Annex A, there is a method for the determination of the emissivity of the device, under the hypothesis that heat transfer between hot and cold plates can be considered as steady state conditions in a conducting medium bounded by two infinite flat and parallel surfaces. By this hypothesis, the total heat flux can be determined by Eq. (8), where the radiation heat flux exchange can be approximated by Eq. (9), see also Eq. (2). ( ) q TOT = q r + d T q TOT T = q r T + d where, q TOT is the total heat flow (W/m 2 ); q r is the radiant heat flow (W/m 2 ); is the thermal conductivity of the air (W/mK); d is the (8)

C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 67 Fig. 7. Geometry and mesh for a radiant barrier with two air chambers of 0.04 m thickness. Fig. 8. Composition of the radiant barrier used for the study. distance between the hot and cold plate (m); T is the temperature difference between the hot and cold plate (K) q r = 4 T 3 m T (1/ε 1 ) + (1/ε 2 ) 1 ε = 2 ((4 T 3 m T)/(q r )) + 1 where ε is the emissivity of the HFM apparatus hot and cold plates, being ε 1 = ε 2. Eq. (8) can be applied to measure the air chamber thermal resistance in the absence of natural convection between hot and cold plates. This condition is accomplished when the heat flow is vertical being the hot surface located at the top, with small temperature differences and small air chamber thicknesses. Moreover, because the small distance between plates compared to the plates dimensions, Eq. (9) becomes a convenient approximation for heat transfer by radiation. The inverse of the total thermal resistance for different air chamber thicknesses (q TOT / T) can be plotted against the inverse of the distance between hot and cold plates (1/d) for different tests. A straight line will be obtained where the slope is the conductivity of the air and intersection to axis of ordinates is q r / T. Using Eq. (9) this value can be used to determine the total hemispherical emissivity of both plate surfaces, assuming that emissivity for all surfaces participating in the radiative heat exchange are equal. The EN 1946-3 requires calculating the emissivity value in a second way to be able to compare the previously calculated value. The q r / T value for different 1/d measurements can be obtained using Eq. (8) by subtracting to q TOT / T the corresponding /d value, but this time the value is the one obtained by Eq. (10). (9) After the emissivity of the plates of the HFM apparatus is calculated, the emissivity of the tested radiant barrier must also be defined to allow analytical and numerical comparison. The methodology described above has been applied to air chambers of different thicknesses with the cold plate covered by a low emissivity material. The outer reflective foil of the radiant barrier insulation is placed over the cold plate. The radiant barrier is composed of five layers, two outer sheets of reflective material and two layers of fibrous filler material that provide the handling resistance, separated by a fifth layer of reflective material (Fig. 8). The only difference with the described methodology is the application of Eq. (9). In this case ε 1 /= ε 2, but one of the emissivities is already known, because it is the hot plate emissivity. 4. Results 4.1. Emissivity calculation To determine the plates and the radiant barrier surface emissivity, air chamber thickness ranging from 0.005 m to 0.02 m with 0.005 m steps have been tested (Fig. 9). The displacement sensor air = 0.0242396(1 + 0.003052 1.282 10 6 2 ) (10) where is the mean temperature of the air chamber in C. Air conductivity ( air ) calculated from Eq. (10) deviates from the real values up to 0.6% in the range of 10 C to 70 C. Then, using Eq. (9) emissivity can be calculated for the different measurement values and the average of these values will be the emissivity value for comparison. For the determination of the plate s emissivity, the mentioned standard EN 1946-3 requires that both emissivity values deviate less than 0.02. Fig. 9. Testing scheme for the emissivity calculation (a) without reflective coating and (b) with reflective coating (RC).

68 C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 Fig. 10. Linear regressions for the emissivity determination with and without reflective coating (RC). Fig. 11. Experimental and calculated results of thermal resistance of the air chamber without reflective coat. for measuring the distance between the plates has a measurement uncertainty of 0.5%. For each thickness ten trials have been performed and it has been statistically evaluated after verifying that the distribution was normal, so as to avoid using erroneous points during the calculation. This procedure also serves to verify the stability and validity of the test method. Table 1 shows the measured values of the air chamber mean temperature, temperature difference between plate surfaces and total heat flux through the chamber for an air chamber of 0.02 m thick (Fig. 9a). To avoid convection, and subsequently achieve more accurate results for the emissivity calculation, in these tests lower mean temperatures and temperature difference are fixed. The total thermal resistance of the air chamber is calculated with data from Table 1. The valid thermal resistance is the average value of those points which fits with the 95% normal probability distribution plot of the ten trials. This means that the test does not present a systematic bias in the measurements, and thus the set of values obtained is suitable for the determination of a representative mean value of the measurements. This analysis has been performed for all measurements of thermal resistance. The final thermal resistance value is used to calculate one of the points of Fig. 10. The same procedure is repeated for different air chamber thicknesses obtaining different points. In this way a linear regression equation can be derived which gives the information to calculate the emissivity of the hot and cold plates surfaces according to Eqs. (8) and (9). As can be seen in Eq. (8), the slope of the upper regressive line in Fig. 10 is the value for the air conductivity, air = 0.025 (W/mK), which differs less than 0.7% from the value determined by the Eq. (10) as required by EN 1946-3. q r / T value is obtained in the intersection to the axis of ordinates and evaluating Eq. (9) the plates emissivity value can be determined, ε = 0.869. If the emissivity of the plates is calculated using the second method: obtaining the thermal conductivity of the air from Eq. (10), then an emissivity of ε = 0.872 is obtained which deviates less than 0.02. These results fulfil the requirements of calculation specified in the standard. From now on the working value for the plates surfaces emissivity will be ε = 0.87. Once the emissivity of the plates is calculated, the methodology is repeated to obtain the emissivity of the radiant barrier. This time the tests are carried out covering the cold plate with a reflective coat (RC) as shown in Fig. 9b. The thermal resistance of the foil is negligible and knowing that the emissivity of the hot plate is ε 1 = 0.87, an emissivity of the reflective coating of ε 2 = 0.14 is obtained according to Eq. (9). 4.2. Air chamber characterization Once the emissivities of the HFM equipment plates and the radiant barrier have been measured, thermal resistances of air chambers that range from a minimum thickness of 0.005 m to a maximum of 0.09 m have been characterized. In residential buildings thicker air chambers are rarely used. The values obtained are compared with those determined by the analytical method described in ISO 6946, stated in Section 2.1, and the numerical model carried out by FLUENT according to Section 2.3. Results are shown for the air chambers without reflective coat, where ε 1 = ε 2 = 0.87, and for the cold plate emissivity changed due to covering it with the reflective coat, being ε 1 = 0.87 and ε 2 = 0.14. 4.2.1. Without reflective coating The results between tested and calculated values are presented graphically in Fig. 11. Simulation results hardly present differences compared to the test. The maximum deviation obtained between analytically calculated thermal resistances against the tested values is below 0.005 (m 2 K/W) and always within the uncertainty of the HFM apparatus. Note that thermal resistance of the air chamber increases rapidly at small air chamber thicknesses, but due to the convective effect from thicknesses greater than about 0.05 m the thermal resistance increase becomes negligible. Maximum values of thermal resistance that can be obtained by an air chamber between surfaces with emissivity ε = 0.9 (usual values in building materials), do not exceed 0.22 (m 2 K/W). The results of Fig. 11 validate the results of the HFM apparatus test and CFD simulation compared to the ISO 6946 values. The CFD approach verifies the accuracy of measurement of the HFM apparatus for the determination of the thermal resistance of radiant barriers. The simulation also evaluates the temperature field within the measurement zone and the convective motion of air. Different CFD simulations of the measurement conditions have been carried out to check the homogeneity of the temperature field within the test volume (Fig. 12). Fig. 11 shows that the values of thermal resistances of the CFD model fit better the experimental results. This is because in the CFD models the real effects of the air chamber such as the border effects. The standard method disregards the border effects when considering infinite parallel flat surfaces. Simulation temperature contours (Fig. 12a) confirm that heat flow in the metering zone is one-dimensional. The maximum temperature deviation between the central temperature profile and the edge of the heat flow meters

C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 69 Table 1 Measures and results for a 0.02 m thick air chamber. Trial num. T mean ( C) T ( C) q TOT (W/m 2 ) q TOT/ T (W/m 2 K) R TOT (m 2 K/W) 1 9.76 10.00 51.96 5.20 0.1925 2 9.98 10.00 52.10 5.21 0.1919 3 10.02 9.98 52.57 5.27 0.1898 4 9.99 10.03 52.46 5.23 0.1912 5 9.96 9.98 52.31 5.24 0.1908 6 10.00 9.98 52.55 5.27 0.1899 7 10.00 10.02 52.52 5.24 0.1908 8 9.96 9.97 52.52 5.27 0.1898 9 10.01 9.99 52.16 5.22 0.1915 10 10.00 9.99 52.28 5.23 0.1911 Average 9.97 9.99 52.34 5.24 0.191 Deviation 0.08 0.02 0.21 0.02 0.001 measurement area, located at a distance of 0.175 m, does not exceed 0.1 K for the extreme case of a 0.09 m air chamber. On the other hand, velocity vectors (Fig. 12b) give low velocity values even for the tested thickest air chamber. Maximum values are obtained near the edge where the heat flow is not measured. 4.2.2. With reflective coating A similar analysis has been performed to analyze the influence of covering the cold plate with a low emissivity surface (ε 2 = 0.14). It can be seen (Fig. 13) that reducing the emissivity of one of the faces of the air chamber the thermal resistance increases from 0.21 (m 2 K/W) to nearly 0.90 (m 2 K/W) for a 0.09 m thickness. On average, the use of low emissivity surfaces can multiply by three the thermal resistance of the air chamber. Calculated and measured thermal resistances line up for the different air chambers thicknesses. The average residual value between test and standard value is 0.006 (m 2 K/W), not exceeding a maximum of 0.016 (m 2 K/W). This fit between measured and experimental results validates the emissivity parameter determination for the reflective material surface according to Section 3. Fig. 13. Experimental and calculated results of thermal resistance of the air chamber with reflective coat. Therefore the possibility of characterizing the emissivity of a reflective material surface by means of the HFM apparatus has also been demonstrated. 4.3. Reflective film characterization Once the emissivity value is known and the testing procedure has been proved, it is possible to evaluate experimentally the common construction solutions used in building envelopes with radiant barriers. Usual configurations involve the formation of one or two air chambers, with one or both low emissivity surfaces facing the air chambers (Fig. 14). The reflective film without air chambers has been tested in the HFM apparatus and a conduction thermal resistance of R RF = 0.310 (m 2 K/W) has been obtained. Fig. 12. Profiles of temperature (a) and velocity (b) from the left adiabatic edge of the 0.09 m thick air chamber. Fig. 14. Testing scheme for the reflexive film characterization (a) with one air chamber and (b) with two air chambers.

70 C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 Fig. 15. Experimental and calculated results of thermal resistance of the reflexive film with one air chamber. 4.3.1. Single air chamber One of the current insulation solutions for floors is done by radiant barriers. Because of thickness or constructive limitations, often only one air chamber is possible with one of the faces covered by the reflective material (Fig. 14a). This solution can also be applied to faç ade solutions. For the experimental characterization the radiant barrier insulation, which has a nominal thickness of 0.01 m, it has been placed on the cold plate of the HFM apparatus. The experimental thermal resistance values obtained for different air chamber thicknesses fit to those determined by calculation, with average deviations below 0.03 (m 2 K/W) (Fig. 15). This thermal resistance includes the radiant barrier conduction thermal resistance. In all tests and calculations the heat flow direction is vertical being the top plate the hot one. If heat flow direction would be different, the thermal resistances of the air chambers would vary due to changes in the convection coefficients. Nevertheless, once the thermal resistance of heat flow in one direction is measured accurately, the calculation of the thermal resistance for any other configuration could be simply and accurately achieved by means of the widely known suitable convection correlations. 4.3.2. Double air chamber Once the thermal behaviour of the simple air chamber solution and the conduction thermal resistance of the reflective blanket have been studied, the double air chamber configuration is tested. This solution is mainly applied in double-leaf walls and is the best solution regarding the use of low emissive surfaces. These surfaces multiply around three times the thermal resistance of the air chambers, so doubling the air chambers improves the thermal efficiency of the reflective blanket, although constructively entails more effort because crossbars are needed on both sides of the reflective blanket. The total thickness of the solution consists of two symmetrical air chambers plus the reflective blanket thickness (Fig. 14b), which as already mentioned has a nominal thickness of 0.01 m with a conductive thermal resistance of 0.310 (m 2 K/W). Once more the CFD simulations fit to the tested results with differences below 0.07 (m 2 K/W) (Fig. 16). However thermal resistance values obtained by the analytical model differ when the thickness of the air chambers increase. This deviation is due to the hypothesis of infinite parallel surfaces used by the standardized method. As the air chamber thickness increases, the analytical model is less accurate because the influence of the edge effect on the heat flow cannot be neglected. Fig. 16. Experimental and calculated results of thermal resistance of the reflexive film with two air chambers. 4.4. Hot box results The major limitation of the HFM apparatus is that only downward vertical heat flow can be tested, although it is an accurate method to measure the surface emissivity of radiant barriers and its conductive thermal resistance. In order to evaluate different heat flow directions and the behaviour of a radiant barrier within a real building element component a hot box test has been carried out. In this case the heat flow through the specimen is horizontal, from the hot chamber (293 K) to the cold chamber (273 K). The thermal resistance of the radiant barrier plus the air chambers (RF) is calculated by difference of thermal resistances between a base wall composed by a mortar layer and concrete block masonry, wall type 1, and the second specimen, wall type 2 (Fig. 5). The value of the thermal resistance of the sample is obtained as the average of 5 test values. Table 2 shows the average air temperatures on both chambers of the hot box facility during the tests, as well as the average values of surface temperatures and heat flux. The recorded maximum and minimum deviations during the measurement period are also presented. Therefore the thermal resistance of the radiant barrier composed by both air chambers and the reflective blanket can be calculated by Eq. (11) using hot box measured values and knowing that the thermal resistance of the plasterboard measured in the HFM apparatus is 0.05 (m 2 K/W). R RF = R wall 2 R wall 1 R plaster board R RF = 0.834 (m 2 K/W) (11) The same configuration of the radiant barrier with the air chambers calculated by means of ISO 6946 leads to a thermal resistance of 0.862 (m 2 K/W). The difference between calculated and measured values is 3.25% and this is within the measurement error of the guarded hot box equipment itself, a 5%. The values obtained in the HFM apparatus taking into account that the heat flow is vertical and downwards instead horizontal, results in a 0.908 (m 2 K/W) thermal resistance. In this case the air chamber is not very thick, so the convection effects due to the heat flow direction are not significant. 5. Discussion After comparing three methodologies for the calculation of the thermal resistance of radiant barriers, with either simple or

C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 71 Table 2 Test conditions and measured thermal resistance for the base wall (type 1) and the base wall with a radiant barrier and plasterboard (type 2). Wall type Hot chamber Cold chamber T surface ( C) q TOT (W/m 2 ) R TOT (m 2 K/W) T air ( C) T surface ( C) T air ( C) T surface ( C) 1 19.7 ± 0.2 17.3 ± 0.2 2.3 ± 0.3 0.1 ± 0.6 15.0 ± 0.3 55.96 ± 0.32 0.276 ± 0.01 2 20.2 ± 0.4 19.3 ± 0.2 1.0 ± 0.4 0.1 ± 0.7 18.3 ± 0.4 15.73 ± 0.42 1.160 ± 0.03 Fig. 17. Thermal resistance of the radiant barrier depending on the emissivity of the reflective surface (a) simple air chamber and (b) double air chamber. double air chamber, results indicate great agreement between them. Nevertheless, the standard analytical method differs when the air chamber thickness is over 0.07 m for the double air chamber configuration. These results demonstrate that the calculation method proposed by ISO 6946 is suitable for determining the thermal resistance of insulation solutions based on reflective materials. The main drawback is that an accurate surface emissivity value for the reflective foil is needed. It also requires knowing the conductive thermal resistance of the reflective blanket for a correct thermal characterization. The measured thermal properties by means of the HFM apparatus during this research can differ considerably from the values found in literature. During this research it has been found that the uncertainty in the thermal properties can lead to great differences in the results of thermal resistance. The emissivity of reflective materials may range from ε = 0.03 to values that can reach ε = 0.3. Fig. 17 shows how this variability in the actual emissivity of the surfaces can change the total thermal resistance of radiant barriers in the single and double air chamber configurations. On the other hand, the determination of the thermal resistance of the air chamber by means of the ISO 6946 standard requires the determination of the average air temperature for each air chamber. But this average temperature depends on the total heat flow which is a function of the total thermal resistance and this total thermal resistance depends also on the conduction thermal resistance of the reflective material itself. It would be therefore necessary to perform an iterative calculation. Although this iterative methodology has been applied during this study, it is demonstrated that for low temperature gradient like the tested one, the difference that implies the mean temperature of the air chambers can be neglected. Note that the ISO 8301 method for thermal resistance measurement using the HFM apparatus is a suitable method for the accurate determination of the insulation solutions without the need of the reflective material emissivity value, although just downward vertical heat flow direction can be tested. For other heat flow directions the combination between the test and the analytical or numerical method would be suitable. Anyway, for horizontal heat flow it is possible to test the radiant barrier in the hot box facility although it entails a more complex specimen construction. 6. Conclusions A deep analysis has been carried out to thermally characterize radiant barriers surrounded by different air chamber configurations. For this purpose results obtained by analytical, numerical and test methodologies have been compared. The analytical method is a simplified method defined by standard ISO 6946. The results indicate that there would be no inconvenience to quantify the thermal resistance of constructive solutions which use reflective films to form radiant barriers by this standard if the reflective film emissivity is accurately known and provided that edge effect are negligible. However, the wide variety of configurations that exist among these materials on the market makes it hard to extend these results to the whole family of films. The value of the conductive thermal resistance of the reflective material is also a possible error source. Thus the ISO 6946 can only be applied to solutions that accomplish the calculation method requirements. This situation can be avoided if the thermal resistance of the whole constructive solution is characterized experimentally. The HFM apparatus gives with an uncertainty of 3.5% the value of the thermal resistance for whatever configuration of radiant barriers, even if the emissivity or the conductive thermal resistance of the reflective film is unknown. The limitation is that only downward vertical heat flow direction can be tested. A methodology for the calculation of the surface emissivity has been developed with the HFM apparatus and the conductive thermal resistance can also be measured. In case other heat flow direction should be tested, measuring the emissivity of the reflective foil and the conductive thermal resistance would enable to achieve accurate results with the analytical or numerical model. Another choice to characterize experimentally radiant barrier solutions is the hot box facility. A real size sample should be built and tested. In these cases, it is not possible to obtain the intrinsic

72 C. Escudero et al. / Energy and Buildings 59 (2013) 62 72 value of reflective material although it is possible to estimate an approximate value and therefore the result according to standard ISO 8990 is only valid for the whole building element solution and test conditions. Finally the CFD simulation method has been evaluated. It has the same limitations as the simplified standard method. Moreover, simulation experience is needed for meshing and using natural convection and radiation models. This method has just been employed to validate the results of the experimental tests. Acknowledgments Special thanks to the Laboratory for the Quality Control in Buildings (LCCE) of the Basque Government for allowing us to undertake all the measurements of this research in its installations. References [1] C. Michels, R. Lamberts, S. Güths, Evaluation of heat flux reduction provided by the use of radiant barriers in clay tile roofs, Energy and Buildings 40 (4) (2008) 445 451. [2] S. Roels, M. Deurinck, The effect of a reflective underlay on the global thermal behaviour of pitched roofs, Building and Environment 46 (2011) 134 143. [3] M. Medina, On the performance of radiant barriers in combination with different attic insulation levels, Energy and Buildings 33 (1) (2000) 31 40. [4] Z. Pasztory, P. Peralta, I. Peszlen, Multi-layer heat insulation system for frame construction buildings, Energy and Buildings 43 (2 3) (2011) 713 717. [5] H. Saber, W. Maref, M. Swinton, C. St-Onge, Thermal analysis of above-grade wall assembly with low emissivity materials and furred airspace, Building and Environment 46 (7) (2011) 1403 1414. [6] M. Belusko, F. Bruno, W. Saman, Investigation of the thermal resistance of timber attic spaces with reflective foil and bulk insulation, heat flow up, Applied Energy 88 (1) (2011) 127 137. [7] F. Miranville, H. Boyer, P. Laurent, F. Lucas, A combined approach for determining the thermal performance of radiant barriers under field conditions, Solar Energy 82 (5) (2008) 399 410. [8] T. Soubhan, T. Feuillard, F. Bade, Experimental evaluation of insulation material in roofing system under tropical climate, Solar Energy 79 (3) (2005) 311 320. [9] P. Chang, C. Chiang, C. Lai, Development and preliminary evaluation of double roof prototypes incorporating RBS (radiant barrier system), Energy and Buildings 40 (2) (2008) 140 147. [10] C. Michels, R. Lamberts, S. Güths, Theoretical/experimental comparison of heat flux reduction in roofs achieved through the use of reflective thermal insulators, Energy and Buildings 40 (4) (2008) 438 444. [11] G. Baldinelli, A methodology for experimental evaluations of low-e barriers thermal properties: field test and comparison with theoretical models, Building and Environment 45 (4) (2010) 1016 1024. [12] ISO 6946, Building components and building elements thermal resistance and thermal transmittance calculation method. International Standard, 2007. [13] EN 12667, Thermal performance of building materials and products determination of thermal resistance by means of guarded plate and heat flow meter methods products of high and medium thermal resistance, European Standard, 2001. [14] ISO 8301, Thermal insulation determination of steady-state thermal resistance and related properties heat flow meter apparatus, 1991. [15] ISO 8990, Thermal insulation determination of steady-state thermal transmission properties calibrated and guarded hot box, International Standard. [16] Fluent Version 6.2-User s Guide, USA, 2005. [17] EN 1946-3, Thermal performance of building products and components specific criteria for the assessment of laboratories measuring heat transfer properties part 3: measurements by heat flow meter method, 1999.