Investigation of the thermal resistance of timber attic spaces with reflective foil and bulk insulation, heat flow up

Applied Energy 88 (2011) 127–137

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Investigation of the thermal resistance of timber attic spaces with reflective foil and bulk insulation, heat flow up M. Belusko *, F. Bruno, W. Saman Institute for Sustainable Systems and Technologies, University of South Australia, Mawson Lakes Boulevard, SA 5095, Australia

a r t i c l e

i n f o

Article history: Received 11 December 2009 Received in revised form 23 June 2010 Accepted 16 July 2010 Available online 14 August 2010 Keywords: Heat transfer Buildings Thermal resistance Timber roof Bulk insulation Reflective foil

a b s t r a c t An experimental investigation was undertaken in which the thermal resistance for the heat flow through a typical timber framed pitched roofing system was measured under outdoor conditions for heat flow up. The measured thermal resistance of low resistance systems such as an uninsulated attic space and a reflective attic space compared well with published data. However, with higher thermal resistance systems containing bulk insulation within the timber frame, the measured result for a typical installation was as low as 50% of the thermal resistance determined considering two dimensional thermal bridging using the parallel path method. This result was attributed to three dimensional heat flow and insulation installation defects, resulting from the design and construction method used. Translating these results to a typical house with a 200 m2 floor area, the overall thermal resistance of the roof was at least 23% lower than the overall calculated thermal resistance including two dimensional thermal bridging. When a continuous layer of bulk insulation was applied to the roofing system, the measured values were in agreement with calculated resistances representing a more reliable solution. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction With the trend towards zero emission buildings, increasing the levels of insulation is a critical factor in achieving this goal [1]. The evaluation of the impact of increased insulation is often determined through building modelling such as TRNSYS, EnergyPlus or, as in the case of Australia, AccuRate [2]. As a result regulators mandate the minimum thermal resistance or R value (m2 K/W) in a wall or roof, to achieve improved building energy efficiency. In a number of locations, including Australia, it is general engineering practice to determine this R value using a one dimensional analysis, which ignores thermal bridging and assumes the R value of the building element to equate to the sum of the rated value of the bulk insulation [3], a constant value for any air space, and a constant value of other materials within the construction [4]. Building thermal models determine the transient heat flows through the building based on the thermal capacitance of building materials as well as assumed thermal resistances of bulk insulators, which resist conduction, and air layers, which resist radiation and convection. These models may consider thermal bridging through the insulation applying a two dimensional method if relevant building elements are specified [5]. In addition, the thermal resistance of reflective and nonreflective air layers may be as-

* Corresponding author. Tel.: +61 8 8302 3767; fax: +61 8 83023380. E-mail address: [email protected] (M. Belusko). 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.07.017

sumed constant by some models such as TRNSYS [6] and variable as specified in EnergyPlus [7]. In contrast the ESP-r model considers thermal bridging in three dimensions [8], which has been shown to be a significant cause of heat transfer [9]. In Australia, which utilises the building model AccuRate, no consideration is given to thermal bridging, whereas the effect of temperature on the thermal resistance of air spaces is considered [2]. Furthermore, all building models assume insulation is installed without defects. Many regions account for thermal bridging in calculation methods [10]. Thermal bridging has been shown to be a significant factor in reducing the thermal resistance of installed insulation [11,12]. To address this factor the apparent thermal resistance is regulated and is determined using two dimensional calculation methods [13,14]. An alternative approach to addressing thermal bridging has been the application of continuous layers of insulation with minimum thermal bridging as recommended in [1], and as required for many steel structures in the US [14], which also eliminates any three dimensional thermal bridging. The application of insulation is related to design and construction methods which vary considerably across jurisdictions. This variation results in different degrees of thermal bridging across the insulation path, as well as varying levels of the quality of the installation, which can significantly reduce the thermal resistance of the insulated assembly [15]. Consequently, the accuracy of building thermal models, and R value calculation methods will depend on these design and construction methods. Therefore, with increasing insulation levels in buildings, differences between the

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Nomenclature A (m2) area Q (W) heat flow R (m2 K/W) thermal resistance T (°C) temperature t (s) time a timber area fraction b bulk insulation area fraction Subscripts a attic air space b bulk insulation c cavity ceiling

actual performance and the evaluated performance, warrant investigation. A popular roof construction used around the world, particularly in Australia, involves a lightweight timber framed attic structure with bulk insulation on the ceiling sandwiched between horizontal ceiling supports, and the use of reflective foils under the roofing material. To account for thermal bridging through the timber frame, the thermal resistance can be approximated by applying either of the two dimension methods, the isothermal planes method or parallel path method [3]. For timber structures it can be shown that the difference between these methods is negligible [3], however, as specified in [14], the parallel path method is more appropriate for determining the total thermal resistance of timber framed structures. Modelling and experimental work has shown that applying the parallel path method, gives an accurate representation of the total surface to surface R value. Work conducted at the Oak Ridge National Laboratory in the United States, applied CFD modelling to a wall with 20% area weighted timber framing and showed less than 2% variation from the parallel path method [5]. Hot box tests on a timber framed insulated wall with 24% timber frame and fibreglass bulk insulation rated at R2.3 has been conducted, and showed the difference to be 3.5% [16]. Considerable research has been conducted in developing and validating mathematical models of insulated roof spaces which have been incorporated into the building models described. A number of studies have investigated radiant barriers with and without bulk insulation and developed experimentally validated models [17–19]. The most comprehensive model was an amalgamation of much of this work, as presented in [20], which has been now upgraded and incorporated into ASTM C1340-04. These attic models which include factors such as the transient effects of the roofing structure, latent energy of the moisture in the timber, varying convection heat transfer and three dimensional radiation, focus on calculating the heat flow through the ceiling into the internal space. These models consider thermal bridging in two dimensions, and have been validated, suggesting that the consideration of three dimensional conduction is negligible. The most applicable research presented in [18], measured the ceiling heat flow in pitched roofs in a typically insulated cabin, with both a radiant barrier and bulk insulation within the attic timber frame. This research investigated the heat flow through the ceiling with wall elements of a lower thermal resistance than the ceiling/roof system. Therefore three dimensional heat flow was limited and therefore most likely not observed. Significant three dimensional modelling of walls has been conducted, applying a validated model, to determine an equivalent whole wall R value [21]. This R value includes two dimensional thermal bridging through the wall as well as three dimensional heat flow at the roof edge, floor edge, corners and window and

f g h w r R1 R2 t T

cavity floor gypsum board heat input cavity wall roof roof, cabin 1 roof, cabin 2 timber frame total

door interfaces. For a timber framed wall with R1.9 bulk insulation and cladding on both sides, the whole wall surface to surface R value was 90% of that measured through the wall only considering two dimensional heat flow. The whole of wall surface to surface R value was found to be 68% of the centre of cavity R value which only considers the cladding and bulk insulation materials, which assumes one dimensional heat flow. For a wall with R3.3 bulk insulation, the corresponding values were 83% and 61% for two and one dimensional heat flow, respectively. These values have been determined from the data presented in [21], by deducting the measured air film resistance of 0.194 m2 K/W. These results show that three dimensional effects can be significant and increase with higher bulk insulation R values. This research is yet to be applied to roof structures. Installation of insulation is commonly assumed to be carried out exactly in accordance with building regulations. For reflective insulation, the reflective foil must be continuous and form a sealed air space. Air infiltration through gaps and tears can significantly deteriorate the thermal resistance of reflective air spaces [3]. For compressible bulk insulation, the insulation is to be uncompressed and must be installed without gaps between it and the roofing frame [3]. Compression directly reduces the thickness and subsequently the thermal resistance. The introduction of gaps induces natural convection loops, and even a 4% air void can reduce the effective thermal resistance of bulk insulation rated at R3.4 in a ceiling application by 50% [22]. Under controlled indoor testing, these effects are prevented, however, minor defects in installation may occur, depending on the design and construction method, and can only be accounted for by direct measurement. To overcome these issues continuous insulation in which bulk insulation is installed end to end or layered in brick arrangement is recommended [3]. An additional installation issue which requires observation, is that compressible bulk insulation is supplied compressed and the installed thickness is lower than the rated thickness. It is expected that the bulk insulation expands to its rated thickness over several minutes. The impact of these potential defects has not been investigated in insulated roofing systems considered in this study. Other assumptions which apply to building analysis include how roof spaces are exposed to varying degrees of ventilation depending on construction method and roofing material which can affect the thermal resistance [4]. Furthermore, thermal properties of materials vary with temperature [23], and sometimes with direction [24]. In order to test the impact of these assumptions and their application to both engineering R value calculations and building thermal models, two outdoor cabins were constructed with different insulation systems, as typically installed. The research program undertaken was to measure the total surface to surface thermal resistance of the timber framed roofing systems under outdoor

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conditions. This measurement would encompass all the potential variables, including the impact of minor installation defects. This work will identify the likely thermal resistance which can be achieved in different roofing insulation systems in a typical house and provide an indication of the critical parameters which affect the heat transferred from the ceiling to the environment. The study will also investigate the performance of continuous insulation, as an alternative design and construction method for bulk insulation. Ultimately, a comparison can be made between calculated thermal resistances, applying either the one or two dimensional method, and measured thermal resistances of insulated roofing systems. This paper investigates the heat flow up case.

2. Experimental arrangement Two identical cabins were constructed with an internal floor area of 3.8 by 3.8 m along a north–south axis (Fig. 1). The walls of the cabins are constructed from 200 mm thick expanded polystyrene (EPS) with a thermal resistance of 5.26 m2 K/W. The roof of each cabin is a 18° pitched timber framed roof constructed using galvanised corrugated steel roof sheeting with a 200 mm eave. The ceiling is 10 mm gypsum board. The bottom end of the corrugated roof is sealed with foam to prevent infiltration occurring at the wall/roof edge. The hip joints, where each roof face meet, is covered with capping which is open allowing infiltration to occur. The cabins were built with the rafters which support the roof extending beyond the edge of the wall (Fig. 2). To minimise edge effects this section was insulated with edge insulation. This insulation consisted of fibreglass bulk insulation within a 200  200 mm box section made from polyurethane sheeting with an R value of 0.8 m2 K/W (Fig. 1). Cabin 1 has no bulk insulation and contains a double sided reflective bubble foil under the timber purlins which support the corrugated roof such that the foil is not in contact with the roof (Fig. 3). Cabin 2 contains fibreglass bulk insulation installed within the timber frame supporting the ceiling and has no reflective insulation (Figs. 4 and 5). The bulk insulation and reflective foil was installed in a typical fashion with reasonable care taken to minimise installation defects. The thermal resistance was defined by the average measured heat flow and temperature difference across the roofing system (Eq. (1)), under steady state conditions.

RT ¼

Tc  Tr Qr

Fig. 2. Cabin 1 without edge insulation, showing protruding rafters and the bubble foil.

Fig. 3. Attic space within cabin 1 showing reflective bubble foil insulation.

ð1Þ

To accurately measure the heat flow a cavity with a height of 300 mm was created under the ceiling with a floor of 200 mm thick EPS insulation (Fig. 6). Within this cavity a 500 W heater was installed with a 60 W continuously running circulation fan. The

Cabin 1 Cabin 2

Fig. 4. Cabin 2 attic space showing R2 fibreglass bulk insulation. Edge insulation

Fig. 1. Monitored cabins with different roof insulation systems. Cabins have edge insulation.

heater was controlled, using a PID controller, to maintain the ceiling temperature of both cabins at 25 °C to within 0.1 °C. One of the tests in cabin 2 involved no bulk insulation, and in this case, the ceilings of both cabins were maintained at 20 °C. Forced convection

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Concentric ceiling temperature measurement zones

Timber supports External wall Internal wall

Fig. 5. Plan view of the timber framed structure which supports the ceiling, equating to 7.7% of the plan area. Ceiling temperature measurement zones are also shown.

Roof Edge insulation 35 mm high timber capping

Timber frame

ATTIC

CAVITY

fan

Ceiling

300

Electric heaters

R 5.26 EPS

Cavity internal wall Cavity external wall Cavity floor

Air conditioner

3800 Fig. 6. Arrangement of testing equipment showing cavity below ceiling.

was applied to the ceiling to generate a uniform temperature to ensure data can be compared between insulation arrangements. This process changes the boundary condition at the ceiling relative to what can be expected in a typical ceiling subject to natural convec-

tion. However, given that the thermal resistance of internal air films are considerably lower than the insulation systems being tested, the impact of this difference was assumed negligible. By providing for a cavity of low height, the heat flow through the wall

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Qr ¼ Qh  Qw  Qf

ð2Þ

All heat flows except for the power measurement are based on a temperature difference which has a measurement error of ±0.1 °C. Based on Eqs. (1) and (2), an error analysis reveals that a measurement error in the R value of 3% is expected. To validate the exper-

imental arrangement a continuous sheet of 50 mm thick expanded polystyrene (EPS), was added under the gypsum board ceiling in cabin 2 within the cavity. Temperature measurements were made above and below this sheet, which enabled the measured R value to be compared to the known R value of the material. This configuration was also used to measure the thermal resistance of continuous insulation. Overall the configurations tested include bubble foil only in cabin 1, and in cabin 2, fibreglass bulk insulation with a nominal rating of R3 and R2, no insulation and the 50 mm sheet of EPS. The fibreglass insulation was independently tested according to ASTM C518-04, and the measured R value was 2.77 m2 K/W for R3 and 1.83 m2 K/W for R2, both within 10% of the rated values. The measured densities for the R2 and R3 were 10.1 and 7.3 kg/m3, respectively. These values are based on a mean temperature of 23 °C. The rated thicknesses of the bulk insulation were 90 mm and 165 mm for R2 and R3, respectively, and the measured thicknesses were 84 mm and 145 mm for R2 and R3, respectively. The rated temperature coefficient for fibreglass insulation is 0.65%/°C [4]. The foil has a rated emissivity on both sides of 0.03. The thermal resistance of the EPS, based on the most likely value as per the manufacturer’s specifications, was 1.19 m2 K/W, at a mean temperature of 20 °C. 3. Results Measurements were taken over a period of 24 months. Data was logged every minute which represented an average over the minute sampled every 15 s. Figs. 7–10 show the measurements for a typical day. Fig. 7 shows how the ceiling temperatures are constant and the same in both cabins. Figs. 9 and 10 show how the temperatures of the ceiling, air in the attic, timber frame in the roof space and steel roof increase during the day and become steady during the night. Fig. 8 shows the heat flow through the ceiling varying during the day and being relatively constant during the night. The variation in the heat flow reflects the impact of the heater controller which applies a variable output. This data shows that the period from midnight to 0600 is essentially steady state. Therefore the steady state period for each day was taken over this time span, from which the average heat flow through the roof and temperature difference across the roofing system could be determined. 1.5

25

1.0

20

0.5

15

0.0

Temperature (°C)

o

30

Ceiling temperature difference, C

was minimised. Under the cavity an air conditioner was installed to maintain the space at 25 °C, minimising heat from flowing through the floor of the cavity. A whole series of surface temperatures were measured using T type thermocouples. Each surface was measured by applying a number of thermocouples evenly distributed and the average of these values was recorded every minute. Each of the four triangular faces of the roof were measured with four thermocouples for each face, each of the internal and external walls of the cavity was measured with four thermocouples, nine thermocouples were used to measure the top and bottom surface of the cavity floor. The ceiling temperature was measured by six measurements arranged in three concentric zones so that a measurement of the outer, middle and centre section of the ceiling was taken (Fig. 5). In each zone, the average temperature under a timber support and the average between the timber support, which would be located directly under the bulk insulation as in the case of cabin 2, was recorded. The outer zone had a total of 18 thermocouples, the middle and inner zone had 10 and 4 sensors, respectively. Half of the thermocouples in each zone were under timber supports. To determine the heat flow lost through the ceiling/roof system, the total heat into the cavity was measured over a steady state period using an AMPY P1 single phase power meter accurate to within 1%. The total measured heat into the cavity consisted of the heaters and the fan. From this value the calculated heat flow through the walls and floor of the cavity were deducted giving the heat flow through the roof (Eq. (2)). The heat flow through each wall and floor were calculated based on the measured steady state temperature difference across each wall and floor and the specified thermal resistance. This calculated heat flow through the walls and floor represented less than 5% of the total heat flow into the cavity. The average heat flow through the roof and the average roof/ceiling temperature difference were then able to be determined for each steady state period.

10

-0.5

Ambient temperature Cabin 1 ceiling temperature

5

Cabin 2 ceiling temperature

-1.0

Ceiling temperature difference

0 9:00

-1.5 11:00 13:00 15:00 17:00 19:00 21:00 23:00 1:00

3:00

5:00

7:00

Time Fig. 7. Ceiling temperatures of both cabins and the ceiling temperature difference between cabins, 24 May 2008. Cabin 2 has R3 bulk fibreglass insulation, cabin 1 has bubble foil insulation. Both cabins have edge insulation.

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25

250

20

200 175

15

150 125

10

100 75 50

5

Roof heat flow cabin 1

Temperature (°C, averaged per hour)

Heat Flow (W, averaged per hour)

225

Roof heat flow cabin 2

25

Ambient temperature

0

0

9:00

11:00

13:00

15:00

17:00

19:00

21:00

23:00

1:00

3:00

5:00

7:00

Fig. 8. Measured heat flow through the ceiling of each cabin, 24 May 2008. Cabin 2 has R3 bulk insulation, cabin 1 has bubble foil insulation. Both cabins have edge insulation.

35

30

Temperature (°C)

25

20

15

Ambient temperature

10

Attic air temperature Attic timber temperature

5

Roof temperature

0 9:00

11:00

13:00

15:00

17:00

19:00

21:00

23:00

1:00

3:00

5:00

7:00

Time Fig. 9. Temperatures throughout the roof in cabin 1, 24 May 2008. Cabin 1 has bubble foil insulation and edge insulation.

35 Ambient temperature

30 Attic air temperature Attic timber temperature

Temperature (°C)

25

Roof temperature

20

15

10

5

0 9:00

11:00

13:00

15:00

17:00

19:00

21:00

23:00

1:00

3:00

5:00

7:00

Time Fig. 10. Temperatures throughout the roof in cabin 2, 24 May 2008. Cabin 2 has R3 bulk insulation and cabins have edge insulation.

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To prevent any unsteady effects, the data collection process was repeated for all nights where the standard deviation across the data set was less than 1 °C for the roof temperature, timber frame temperature and ambient temperature and less than 0.1 °C for the ceiling temperature. These conditions applied to all data except for cabin 2 with no insulation in which the specified maximum standard deviation was 0.2 °C for the ceiling temperature. The thermal capacitance of the steel roof, the timber, R3 bulk insulation, the reflective foil and the gypsum board were 71 kJ/K, 137 kJ/K, 12.6 kJ/K, 14.9 kJ/K and 133 kJ/K, respectively. Assuming a 1 K temperature variation in the steel roof, timber, the bulk and reflective insulation and a 0.1 K variation in the gypsum board, over 6 h, represents a maximum heat flow of 10 W for a roofing system with either R3 bulk insulation or reflective foil. This heat flow represents the unsteady heat flow and is small relative to the transmitted heat flow through the roof, as shown in Fig. 8. This figure presents the test with the lowest heat flow, consequently, the data collected for all tests over which the thermal resistance is measured is demonstrated to be steady state. As presented in Figs. 9 and 10, the attic air temperature in cabin 1 approaches the ceiling temperature and the attic air temperature in cabin 2 is similar to the ambient temperature. The roof temperature in cabin 1 is also higher than the roof temperature in cabin 2, reflecting the increased heat flow. Both roof temperatures approach the ambient temperature with the roof temperature in cabin 2 identical to the ambient temperature. Such a small temperature difference between the roof and the ambient temperature does not allow for a significant amount of convection. Therefore, it is likely that the majority of the heat is escaping to the night sky by radiation. Temperature measurements were also taken of the insulation. Fig. 11 shows the temperature of the north side of the foil, the middle of the bulk insulation, at the centre and the south edge of the ceiling with R3 bulk insulation. As expected the foil temperature approaches the ceiling temperature due to the convection in the roof space. The temperature of the bulk insulation is around the midpoint between the ceiling and roof temperature. However, given that the ambient temperature was approximately 14 °C, the large temperature difference between the south edge and centre measurement indicates considerable heat flow in the horizontal direction. The temperature variation across the ceiling was small and less than 0.5 °C for cabin 1 and less than 0.3 °C for cabin 2 with bulk insulation. This variation highlights that heat transfer was uniform from the cavity into the ceiling.

Moisture levels were also recorded in the roof space of both cabins. Since there was no moisture load from within the cavity, the moisture level remained constant with the relative humidity in the roof space of cabin 1 around 40% and the maximum relative humidity in cabin 2 reaching 90%. 4. Testing validation The test cabins were subjected to varying outdoor conditions and the cavity is not truly adiabatic at the walls and floor. As a result a validation of the testing process was conducted. This test involved, adding a 50 mm sheet of EPS directly under the gypsum board ceiling, and removing the bulk insulation in the attic. The thermocouples on the gypsum board provided a measurement of top surface temperature of the EPS sheet. An additional nine thermocouples were attached to the underside of this sheet, the average of which, represented the new ceiling temperature measurement of the cavity. Steady state tests were conducted, in which the average temperatures above and below the layer of EPS were measured along with the average heat flow through the roof. Measurements of the roof and underside of the EPS sheet also enabled the total thermal resistance to be determined for the arrangement with continuous bulk insulation. Fig. 12 presents the results for each steady state test with the EPS insulation in cabin 2 with edge insulation. The R value of the polystyrene was 1.19 m2 K/W at 20 °C, the average temperature of the polystyrene insulation. This R value was selected as it was anticipated to give similar thermal resistances of the roof as that measured with R3 bulk insulation, which was the most insulated case. The testing shows that the measured R value was 1.05 m2 K/ W ± 6.6%, 12% lower than the rated R value of the polystyrene. This difference highlights an increase in heat flow due to edge effects, defects in installation, measurement error and the transient effects estimated at 10 W. At lower levels of insulation, these effects have a relatively reduced impact and the accuracy of measurements are higher. Therefore, the measured error represents the maximum error of the tests conducted, and validates the measurement process in determining the total thermal resistance for all tests. 5. Thermal resistance measurements A series of tests were conducted with different configurations in cabin 2 while cabin 1 remained the same with only the bubble foil

30

Temperature (°C)

25

20

15

Cabin 1 foil insulation temperature

10

Cabin 2 bulk insulation south edge temperature Cabin 2 bulk insulation centre temperature

5

0 9:00

11:00

13:00

15:00

17:00

19:00

21:00

23:00

1:00

3:00

5:00

7:00

Time Fig. 11. Temperature of insulation in both cabins, 24 May 2008. Cabin 2 has R3 bulk insulation and cabins have edge insulation.

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Measured heat flow through EPS, W

240 220 y = 13.7545x

200 180 160 140 120 100 80 60 40 20 0 0

2

4

6

8

10

12

14

16

18

20

22

24

Temperature difference across EPS, oC Fig. 12. Measured heat flow as a function of temperature difference for cabin 2 with EPS.

insulation. The arrangements in cabin 2 included having no bulk insulation, R3 and R2 bulk insulation within the timber frame, and continuous EPS bulk insulation rated at R1.19 m2 K/W under the ceiling. This group of measurements were carried out with the edge insulation on the cabins. Tests without edge insulation were done with R3 bulk insulation in cabin 2 and bubble foil only in cabin 1. Each group of tests involved a number of days with each data from each day delivering a single value of average heat flow and temperature difference, under steady state conditions. Fig. 13 presents the heat flow through the roof as a function of the ceiling to roof surface temperature difference for all configurations. All relationships presented are linear and are forced through the origin. The variability of the data is a function of the impact of thermal mass and the variation of convection, radiation, infiltration into the attic and thermal properties of the bulk insulation with temperature. Overall, this variability is less than ±9%. The results highlights that the nonlinearity of the convection and radiation, even for the foil insulation, for the heat flow up case is small and within the error range of the R value. Furthermore, the variation in the thermal properties of the bulk insulation cannot be observed. This highlights that the R value can be defined by a single value, without considering these effects to an accuracy of less than

±9% for the heat flow up case for a temperature difference of 20 °C. For the heat flow down case where temperature differences and variations are larger, this assumption may not be valid. Table 1 presents the surface to surface R value measurements as derived from Figs. 13 and 14. The table also presents the calculated R value using the simple one dimensional method, as defined by Eq. (3) [3]. For the attic space of both cabins, since the roof consist of sheets of corrugated steel, it is classed as unvented compared to a tiled roof [4]. The thermal resistance of an unvented nonreflective attic and a reflective attic are specified as 0.18 and 0.56 m2 K/W, respectively [4]. The average temperature of the fibreglass was 17 °C, and based on the measured thermal resistances of the fibreglass insulation and the rated temperature coefficient, the values used for R3 and R2 were 2.88 and 1.90 m2 K/W, respectively. To consider thermal bridging, Table 1 presents calculations for cabin 2 in which the thermal bridging in the timber frame through the bulk insulation is accounted for applying the parallel path method, as shown in Eq. (4) and detailed in [3]. The timber on the cabin ceiling has an area faction of 7.7%, has an assumed conductivity of 0.1 W/mK for radiata pine [4], and a measured height of 86 mm. The thermal resistance of the gypsum board was 0.06 m2 K/W, and the thermal resistance of the steel roof was ignored.

Measured heat flow through roof, W

600

Cabin 2, R3 bulk insulation

550

Cabin 2, R2 bulk insulation

500

Cabin 1, bubble foil insulation

y = 51.8183x

450

Cabin 2, no insulation

400

Cabin 2, R1.19 EPS insulation

350 y = 18.1938x

300

y = 12.1139x y = 10.5524x

250 200

y = 10.25x

150 100 50 0 0

2

4

6

8

10

12

14

16

18

20

22

Ceiling to roof temperature difference, oC Fig. 13. Measured thermal resistance for configurations with edge insulation on cabins.

24

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M. Belusko et al. / Applied Energy 88 (2011) 127–137 Table 1 Measured and calculated total surface to surface R values (m2 K/W). Arrangement

Measured total R value (m2 k/w)

With edge insulation Cabin 2, 0.28 ± 7.0% uninsulated 0.79 ± 9.0% Cabin 1, bubble foil only 1.19 ± 6.6% Cabin 2, R2 bulk insulation 1.37 ± 7.1% Cabin 2, R3 bulk insulation 1.41 ± 5% Cabin 2, R1.19 EPS continuous insulation Without edge insulation 0.67 ± 8.7% Cabin 1, bubble foil only 1.20 ± 8.1% Cabin 2, R3 bulk insulation

Calculated total R value, 1-D method, ignoring thermal bridging

Calculated total R value, 2-D method, including thermal bridging

0.24

0.24

0.62

0.62

2.14

1.99

3.12

2.73

1.43

1.43

0.62

2.07

3.12

2.73

RT ¼ Rg þ Rb þ Ra þ Rr RT ¼

1 þ Rg þR Rg þRt þRa þRr a

ð3Þ ð4Þ

b b þRa þRr

For the case with edge insulation, the measured R value with foil only is 27% higher than the rated value and without edge insulation the measured value is 8% more than the rated value. For the uninsulated case, with edge insulation, the measured R value was 17% higher than the rated value. This difference is most likely due to the amount of infiltration which can occur through the ridge capping of the roof sheets, and construction details. However, it shows that the calculated value does provide an indicative resistance for this arrangement. The increased values for the bubble foil may be due to the surface to surface R value of the bubble foil itself which has a reported R value of 0.14 m2 K/W, resulting in a calcu-

lated R value of 0.76 m2 K/W. Based on this calculated value the measured R value with and without edge insulation is 4% higher and 9% lower, respectively. These values are within the error range of the measurements. With the bulk insulation tests with fibreglass within the frame, the measured R values are considerably lower than the calculated values using both the one dimensional or two dimensional methods. The measured values with edge insulation, represent a decrease relative to the calculated values using Eq. (3) of 44% and 56% for cases with R2 and R3 bulk insulation, respectively. Considering thermal bridging, these values are 40% and 50% for cases with R2 and R3 bulk insulation, respectively. The measured R value for the case with EPS is within 2% of the calculated R value which involved Eq. (3). Given that transient effects are small, the testing procedure was validated, the thermal resistance of the bulk insulation has been separately confirmed, two dimensional thermal bridging has been accounted for, it is deduced that the low thermal resistance of the R2 and R3 cases are due to the impact of the frame. Reasonable care was taken to install the bulk insulation in accordance with building regulations. However upon a close visual inspection, it was noticed that small gaps in a few locations existed between pieces of the bulk insulation and the timber frame. These gaps were estimated at less than 1% of the attic floor area. As described in [22], small air gaps generate convection loops with significantly lower thermal resistances than the bulk insulation, allowing heat to bridge the insulation, increasing heat transfer. An inspection of the test with EPS identified insignificant gaps. The impact of convection loops was exacerbated for the R3 case where the height of the insulation is twice that of the timber frame supporting the ceiling. In this case, a convection loop is generated above a piece of timber which is 33 mm thick and the fibreglass insulation on either side. Two dimensional heat flow within the bulk insulation is enhanced, in which, heat flowing through the bulk insulation can escape laterally into this convection loop. It was also noted that small areas of the fibreglass insulation were slightly compressed near the timber frame, as a result of installation. Compression reduces the overall thickness of the insulation layer. Compression also increases fibre density which reduces thermal conductivity, however, a 10% reduction in thickness of fibreglass with a density of 7.3 kg/m3, still achieves a decrease in total thermal resistance of 6% [25]. Compression near the timber frame was greater for the R3 case relative to the R2 case where the height of the insulation was greater than the timber

Measured heat flow through roof, W

600 550

Cabin 2, R3 bulk with edge insulation

500

Cabin 2, R3 bulk with no edge insulation

450

Cabin 1, bubble foil with edge insulation

400

y = 21.4201x

Cabin 1, buble foil with no edge insulation

350

y = 18.1938x

300 y = 12.0468x

250 200 y = 10.5524x

150 100 50 0 0

2

4

6

8

10

12

14

16

18

20

22

Ceiling to roof temperature difference, oC Fig. 14. Measured thermal resistance of roofing systems with and without edge insulation.

24

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Table 2 Adjusted ceiling to roof R values (m2 K/W) for an attic with various bulk insulation R values in a 200 m2 house. Bulk insulation R value

Total calculated R value, 1-D

Total calculated R value with thermal bridging, 2-D

Effective total R value

Reduction, calculated (1-D) to effective R value (%)

Reduction, bulk to effective R value (%)

Reduction, calculated (2-D) to effective R value (%)

2 3 4 5 6

2.24 3.24 4.24 5.24 6.24

2.07 2.82 3.48 4.06 4.59

1.71 2.16 2.53 2.83 3.08

24 33 40 46 51

14 28 37 43 49

17 23 27 30 33

frame. In the middle section of the fibreglass the height was measured for both cases and corresponded to the measured height of the externally tested samples. It was also noted that the pitched roof compressed the R3 fibreglass insulation over the wall. This insulation was under compression from half way through the wall, reducing in thickness from around 145 mm to 90 mm at the external face of the wall. This arrangement is typical in roofing systems. Cumulatively, these effects may contribute to the difference between the calculated and measured results being greater for the R3 case than the R2 case. Overall, the general impact of installation defects increase the vertical heat loss through the roof. However, the difference in the bulk insulation temperatures between the centre and south edge of the cabin (Fig. 11) indicates that additional heat loss, through horizontal heat flow through the edge, is a contributing factor. Fig. 14 presents the measured heat flow and temperature difference before and after the edge insulation was added, and Table 1 provide the R value measurement. For both cabins the R value increased, with the R value for cabin 1 increasing by 18% and 14% for cabin 2 with R3 bulk insulation. These values confirm that the thermal resistance to heat flow through the edges is low, and may be exacerbated by the presence of the timber wall capping which is typical in Australian building practice. The application of edge insulation reduced the heat loss, however the temperature gradient measured within the insulation in cabin 2 indicates that heat is still flowing through the edges. This heat flow could be due to the edge insulation not performing as expected. In addition, compression of the fibreglass insulation at the wall/roof interface will also increase heat flow, and may explain why the edge insulation had reduced heat flow in cabin 1 more than in cabin 2. The presence of edge heat flow with edge insulation may further be exacerbated by the fact that the thermal conductivity for timber can up to three times along its length, parallel to the timber fibres, than across its width [24], generating three dimensional thermal bridging, throughout the timber frame. Overall, these results highlight that the framing system significantly deteriorated the thermal performance of the assembly through three dimensional effects, as well as increasing installation defects. 6. Heat flow analysis The results from Table 1 highlight that with increased levels of insulation preventing upward heat flow through the bulk insulation, the impact of installation defects is higher and the thermal resistance in the horizontal direction becomes relatively lower, allowing heat to flow parallel to the ceiling in three dimensions through the bulk insulation and timber frame. For horizontal heat flow, it can be argued that this effect is magnified by the small size of the cabin. If the measured results for the bulk insulated systems with edge insulation are principally caused by the installation defects, then the measured R values are directly transferable to expected R values within a typical installation, as shown in Table 1. This result represents a maximum reduction in R value of the

roof/attic assembly caused by the timber frame. If the results are principally due to horizontal heat flow through three dimensional thermal bridging and edge effects, it is possible to calculate the edge loss from the testing and to apply this loss to a typical house. This calculation would represent the minimum reduction in R value that can be expected. Table 2 presents the effective thermal resistance of the roof which includes the edge losses for a typical 20  10 m single storey house. Calculated values considering one and two dimensional thermal bridging are also presented applying the rated thermal resistance of the bulk insulation. The edge heat flow was calculated as the additional heat flow relative to the heat flow defined by two dimensional thermal bridging, determined by the parallel path method, exposed to a 15 °C surface to surface temperature difference. This heat flow was determined as 5 W/m of edge. Table 2 show a significant reduction in the effective R value of the roof/attic system. Relative to the two dimensional calculation, the effective R value based on the measured results is 23% lower for the typical R3 case. Relative to the one dimensional calculation and the material R value of the bulk insulation the effective R value is 33% and 28% lower, respectively. At higher levels of bulk insulation the reduction is more dramatic. This result is similar to that presented in [21], in which the whole of wall surface to surface R value was determined to be more than 30% lower than that defined by the R value of the bulk insulation. 7. Conclusions With the drive towards more energy efficient buildings, the performance of insulated systems cannot be automatically assumed to equate to the evaluated performance, as determined by building models or calculation methods. The one dimensional calculation of the total surface to surface thermal resistance are 44% and 56% greater than that measured results for systems with R3 and R2 bulk insulation respectively. This difference clearly demonstrates that this method of calculation cannot be applied to determine resistances. Consideration given in current standards and regulations for the impact of thermal bridging does reduce the difference between measured and expected thermal resistance, however this only considers two dimensional effects. This study demonstrated that three dimensional effects can be important. Furthermore, current practice assumes fully compliant installation and ignores that gaps between structural members and bulk insulation can readily occur during the installation process. Collectively the effects of these details become considerably more significant in highly insulated roof assemblies. It was shown that systems of low thermal resistance such as foiled and uninsulated roof spaces correspond to current published thermal resistances. However with bulk insulation of high thermal resistance, the impact of these details resulted in a considerable impact on the total thermal resistance. It was not possible to fully quantify the reduction in R value associated with an increase in vertical heat loss through installation defects or horizontal heat loss through three dimensional effects. Overall the reduction in the thermal resistance of the total

M. Belusko et al. / Applied Energy 88 (2011) 127–137

roof assembly in a typical house construction relative to expected resistances as defined by the two dimensional method, assuming all increases in losses were through defects, increasing vertical heat flow, was determined as 50% for the R3 case and 40% for the R2 case. Assuming that the total effect was due to edge heat loss these values were determined as 23% and 17%, respectively. At higher values of bulk insulation this reduction is more dramatic. In contrast, the application of continuous insulation resulted in thermal resistances similar to calculated values using one dimensional methods. This system minimised thermal bridging, compression, gaps and edge effects. It was demonstrated that this method of insulation can effectively and reliably deliver roofing systems with higher thermal resistances. This work has highlighted the need to experimentally verify the performance of insulated systems, considering specific design and construction methods. In addition, more attention to the effects of three dimensional heat flow and installation practices in building thermal modelling and calculation methods is required. Acknowledgements The authors wish to express their appreciation for the support from the Australian Research Council and the Australian foil insulation industry. References [1] Schnieders J, Hermelink A. CEPHEUS results: measurements and occupants’ satisfaction provide evidence for Passive Houses being an option for sustainable building. Energy Policy 2006;34:151–71. [2] Delsante A. Is the new generation of building energy rating software up to the task? – a review of AccuRate. In: Proc ABCB conference ‘Building Australia’s Future 2005’, Surfers Paradise, Australia. [accessed December 2009]. [3] ASHRAE. ASHRAE handbook – fundamentals. Atlanta, USA: American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc.; 2005. [4] AIRAH. AIRAH handbook. Melbourne, Australia: Australian Institute of Refrigeration, Air-Conditioning and Heating Inc.; 2007. [5] Kosny J, Kossecka E. Multi-dimensional heat transfer through complex building envelope assemblies in hourly energy simulation programs. Energy Build 2002;34:445–54.

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