Thermal conductivity of wool and wool-hemp insulation

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2006; 30:37–49 Published online 23 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/er.1123

Thermal conductivity of wool and wool–hemp insulation Z. Ye1,z, C. M. Wells1, C. G. Carrington1,n,y and N. J. Hewitt2 1

2

Department of Physics, University of Otago, P.O. Box 56, Dunedin, New Zealand Centre for Sustainable Technologies, University of Ulster at Jordanstown, Northern Ireland BT37 0QB, U.K.

SUMMARY Measurements have been obtained for the thermal resistance of sheep-wool insulation and wool–hemp mixtures, both in the form of bonded insulation batts, using a calibrated guarded hot-box. The density was 9.6–25.9 kg m3 for the wool and 9.9–18.1 kg m3 for the wool–hemp mixtures. The measurements were made at a mean sample temperature of 13.38C using a calibrated guarded hot-box. The estimated uncertainly in the resistance measurements was of the order of 7%. The thermal conductivity of the samples, derived from the thermal resistance measurements on the basis of the measured thickness, was well correlated with the density, although the variation with density was larger than that obtained in previous studies. The conductivity of the wool–hemp samples was not significantly different from that of the wool samples at the same density. Moisture uptake produced an increase of less than 5% in the conductivity of the bonded wool insulation for an increase in absorbed moisture content of 20%. The thermal resistance was 1.6% lower on average for samples oriented in the horizontal plane rather than the vertical plane, but this difference is not significant. Copyright # 2005 John Wiley & Sons, Ltd. KEY WORDS:

thermal insulation; thermal conductivity; wool–hemp; guarded hot-box

1. INTRODUCTION Natural fibre such as wool has become well established in the buildings thermal insulation market despite its higher cost relative to man-made vitreous fibre (MMVF) materials. There is, however, relatively little published information on the thermal properties of wool-based insulation products. Symons et al. (1995) and Trethowen (1995) have presented data on the conductivity of bonded wool insulation available in Australia and New Zealand for densities in the range 10–70 kg m3. The purpose of this paper is to provide additional data for New Zealand commercial sheep-wool insulation, including the influence of orientation on the thermal resistance. Results are also presented for wool–hemp mixtures and for the influence of moisture content on the thermal resistance of wool. n

Correspondence to: C. G. Carrington, Department of Physics, University of Otago, P.O. Box 56, Dunedin, New Zealand. y E-mail: [email protected] z Now at Faculty of Technology, The Open University, Walton Hall, Milton Keynes MK7 6AA, U.K. Contract/grant sponsor: New Wool Products Ltd

Copyright # 2005 John Wiley & Sons, Ltd.

Received 8 July 2004 Revised 11 January 2005 Accepted 2 February 2005

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Environmental issues linked with the growing demand for natural fibre insulation have been reviewed by Schmeitz (2001) who noted user’s perceptions of reduced health risks compared with MMVF materials. Ballagh (1996) made similar observations on the use of wool instead of MMVF for sound absorption. These authors cited examples which suggest the use of MMVF materials involves increased public health risks. Engholm and von Schmalensee (1982), for example, found the incidence of bronchitis among non-smoking workers in the Swedish construction industry exposed to MMVF was 2.8 times higher than that for non-exposed people in the same industry. Petersen and Sabroe (1991) showed that MMVF was a significant cause of irritation of the eyes, skin and upper respiratory tract for Swedish construction workers. Twothirds of those working daily with mineral wool reported frequent symptoms of irritation, and Lambre´ et al. (1998) found that rats exposed to certain stone-wool fibres exhibited a significant dose-related occurrence of cancer. Schmeitz (2001) also suggested that the embodied energy of sheep-wool is smaller than that of equivalent MMVF materials by a factor of 5–8. This is consistent with the data of Alcorn (1998) who determined the embodied energy of recycled sheep-wool insulation to be 139 kJ m3 at a density of 9.5 kg m3, compared with 970 kJ m3 for fibreglass insulation at a density of 32 kg m3. On this basis Skates et al. (2001) has estimated that the difference in the embodied energy of a light-weight construction New Zealand house built to code would be approximately 20 GJ less for recycled sheep-wool insulation compared with fibreglass. Skates et al. (2001) also showed that the embodied energy in New Zealand houses could be a large fraction of the lifetime energy requirements of the house. For houses well-insulated with mineral wool, located in the warmer regions of the country, the embodied energy of the house could represent up to 85% of its total lifetime energy requirements. As an environmental indicator, the embodied energy of wool insulation makes no allowance for sheep ruminant methane emissions. The question therefore arises, is the embodied energy an appropriate indicator for the environmental cost of producing sheep-wool insulation? In New Zealand wool insulation is composed primarily of recycled wool (Newton, 2002) and the embodied energy in producing the material is, by convention, allocated to its first use (Alcorn, 1998). Recycled wool is used because of its availability and because it is cheaper than new wool. In this situation the embodied energy therefore provides an appropriate indicator of the environmental cost of sheep-wool insulation materials in New Zealand. This does not necessarily apply to other countries where new wool may be used for building insulation. A factor favouring sheep-wool insulation, also noted by Schmeitz (2001), is that wool has a capacity to absorb some 30% additional moisture content without a major loss of thermal resistance. This claim was not substantiated by Schmeitz (2001), nor was any comparison made with MMVF insulation. 2. EXPERIMENTAL SYSTEM 2.1. Guarded hot-box Samples for testing consisted of sheep-wool insulation and wool–hemp mixtures in the form of bonded insulation batts. The thermal resistance of the samples was measured using a guarded hot-box operated substantially in accordance with ASTM C236-89 (1990), as specified in NZ standard 4214:1997 under which the measurements were conducted. The system, shown in Figure 1, was designed for materials with thermal resistance in the range 1.0–3.0 m2 K W1. Copyright # 2005 John Wiley & Sons, Ltd.

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THERMAL CONDUCTIVITY OF WOOL

900 400 Guard box External radiation shield Fan and heater unit

Gib board

Fan and heater unit

800

Metering box

1350

Fan Test sample

Metering box baffle Fan

Fan and heater unit Thermopile junctions (a) 900 400 Guard box

Thermopile junctions

Fan and heater unit

External radiation shield Test sample Gib board Metering box baffle

1500

Metering box

950

Fan and heater unit

Fan and heater unit (b)

Figure 1. (a) Plan of the guarded hot-box. External dimensions are shown in mm; and (b) side-elevation of the guarded hot-box, omitting two metering-box fans. External dimensions are shown in mm.

The metered surface of the sample was 714.7 mm wide and 870.3 mm high and the overall sample area, which included the guard surface, was 1200 mm wide and 1350 mm high. Samples could be oriented in either the vertical or horizontal plane. This description applies to the vertical orientation. The heat-flow measurement was carried out using a composite wall consisting of the sample located on the cold-side of a plaster-board lining. For determining the sample conductivity the surface temperature on the cold-side of the plaster board was used. The cold-side of the test sample was defined by a non-woven polypropylene permeable sheet stretched across the sample Copyright # 2005 John Wiley & Sons, Ltd.

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to which the temperature sensors were attached with adhesive tape. The sheet supported the sample for measurements made in the vertical orientation and was left in place for horizontal measurements. The guarded hot-box assembly as a whole was located in a temperature-controlled chamber which provided both the cold heat-sink for the sample and a stable thermal environment for the guard-box. Time variations in the cold-side temperature of the test sample were typically less than 0.18C, the period of the fluctuations being in the order of 40 min. Both the metering-box and guard-box were heated using enclosed electric elements through which airflow was maintained by DC-powered fans, as shown in Figure 1. Two fan-heaters were used in the guard-box to minimize spatial temperature gradients. To reduce radiant heating by the metering-box heater, and to ensure unidirectional air flow down the sample, a baffle was used in the metering-box (Figure 1). The temperature difference between the top and bottom of the sample was normally less than 1.08C. On the cold-side, fans maintained an air-flow in the upward direction so that the temperature change matched that on the warm-side. Not all measurements reported here were carried out using fan-assisted air movement on the cold-side, but no difference was found between these measurements and those with fan-assisted airflow. 2.2. Instrumentation Type-T thermocouples with a wire diameter of 0.25 mm were used for temperature measurement. Either six and 12 thermocouple junctions were used for metered-surface temperature measurements and tests showed that the accuracy of the results was not affected significantly (Ye, 2001). Thermocouples were also used for air temperature measurements on the hot- and cold-side of the sample, for the temperature of the hot and cold radiation baffles and for representative temperatures in the guard space. To control the guard-box heating, a thermopile consisting of 16 pairs of junctions was installed across the walls between the metering-box and guard-box in accordance with ASTM C236-89. All thermocouples were calibrated in situ over the range 0–358C using a stirred water-bath, the temperature of which was measured with a traceable reference thermometer calibrated to 0.018C. Thermocouple calibration data was recorded directly by the data acquisition system and a cubic calibration curve established for each thermocouple. The overall accuracy of each thermocouple sensor, including the instrumentation and data acquisition chain, was normally better than 0.058C for periods of several months. The system for measuring the power input to the metering-box fans and heater was calibrated using a Hewlett Packard model 34401A digital multimeter for precision voltage and current measurements (Ye, 2001). The metering-box heating was controlled by a PID temperature controller based on a thermocouple sensor on the test sample surface. The guard-box heaters were controlled by a second PID controller for which the guard-box/metering-box thermopile provided the error signal. The controller achieved a mean temperature difference across the metering-box wall of the order of 0.0018C and the metering-box temperature variations were approximately 0.028C. Variations in the power input to the metering-box were typically less than 0.3%, with occasional excursions approaching 1%, and drifts of less than 0.5% over a period of 30 h. In accordance with ASTM C236-89 procedures, temperature and power data was recorded for a minimum of 16 h under steady-state conditions and data samples were taken at intervals of 15 min. Final results were produced using the 8 h of data recorded in the middle of the 16 h test period. The thermal resistance, R, of the sample under test was then calculated using the average Copyright # 2005 John Wiley & Sons, Ltd.

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measured power input to the metering-box, P, the mean temperature difference across the sample, DT, and A, the metered area of the guarded hot-box, using Equation (1). ADT ð1Þ R¼ P The power, P, was corrected for the residual heat-leak between the guard-box and meteringbox, as described below. Results were accepted only when the thermal resistance obtained in two successive 4-h periods did not differ by more than 0.4%. 2.3. System calibration When the guarded hot-box was at equilibrium, small residual heat-flows remained between the guard-box and metering-box due to imperfect averaging of the temperature differences between the surfaces and to non-uniformities in the airflows and the walls. An acceptable method for guarded hot-box calibration is outlined in ASTM C236-89 (1990), based on the method of Orlandi et al. (1983). This procedure requires the guard-box temperature to be lower than the temperature of the cold-side of the test sample for part of the test. The method is therefore inapplicable when the external environment of the guarded-box is at the same temperature as the cold-side of the test sample. The procedure used here employed linear extrapolation to determine the power input to the metering-box under steady-state conditions when the mean temperature difference across the metered area, DT, was zero. To apply this method the power input of the metering-box heater and fans was determined at equilibrium for different values of DT. The guard-box temperature was set to track the metering-box temperature, as described above. To ensure that the thermal conductivity of the test sample was not affected when DT was changed, the hot- and cold-side temperatures were varied together so that the mean temperature of the sample remained constant at 15.08C. The residual heat-leak between the metering-box and guard-box was then determined by extrapolating the measured power input to the metering-box to DT=0. Figure 2 shows the results of applying this procedure to a sample of expanded polystyrene, 50 mm thick. Because this calibration method has not been recognized previously, additional measurements were taken to establish its validity. In particular, the data shown in Figure 2 includes measurements with fan-assisted airflow on the cold-side, and with passive air movement due to room air circulation. The intercept at DT ¼ 0 indicates there was a residual heat-flow of 0.084 W into the metering-box from the guard-box. The mean difference between the linear fit to the data and the data points is 0.02 W, the largest difference being 0.06 W, and the expected error in the intercept at DT ¼ 0 is 0.02 W. Within this error the same intercept is obtained using both linear and quadratic fitting to the data. The thermal resistance of the sample, which is obtained from the gradient of the straight line fit to the data divided by the metered surface area, A, is 1.34 m2 K W1. The corresponding thermal conductivity of the sample, l; was determined using Equation (2): l ¼ d=R

ð2Þ

where d is the mean thickness of the test material. The value obtained for the thermal conductivity of the polystyrene test sample was 0.0372 W m1 K1 and the expected error due to scatter in the data was 0.5%. A limitation of this method for measuring the guard-box heat-leak is the error due to differences in the temperature of the sample across the guard-box–metering-box interface. Copyright # 2005 John Wiley & Sons, Ltd.

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9 Metering-box power (W)

8 7 6 5 4 3 2 1 0 0

2

4

6

8

10

12

14

16

18

Mean temperature difference (K)

Figure 2. Total measured input power to the metering-box as a function of the mean temperature difference across the metered area of a 50 mm polystyrene sample. For each measurement the mean temperature of the sample was 15.08C. Closed symbols represent data obtained with fan-assisted airflow on the cold-side of the sample. For open symbols the coldside airflow was induced by the room air-circulation fans.

Such transverse gradients could create heat-flows across the perimeter of the metering area which could scale with DT. However this error is not considered to be significant here, since the surface temperatures of the sample exposed to the metering-box and guard-box were typically within 0.058C. A temperature difference of 0.058C would have produced a heat flow of the order of 0.08 W, equivalent to an error of 0.25% in a typical power measurement. As an independent check of the guarded hot-box calibration, the conductivity of the sample which provided the data shown in Figure 2 was measured by an independent laboratory using a LaserComp Fox 600 heat-flow meter that had recently been calibrated (Stanford and Cox-Smith, 2000). The accuracy claimed for that measurement was 2%. The result, 0.0365 W m1 K1, was 1.9% less than the value obtained using the guarded hot-box, 0.0372 W m1 K1. The difference between these two measurements on the expanded polystyrene sample indicated that the systematic error in the thermal resistance obtained by the calibrated guarded hot-box described here is unlikely to exceed the expected error in the LaserComp instrument plus the difference in the measurements, say, 4%. As a further check on the calibration of the system, measurements for a second sample of expanded polystyrene were compared with measurements made independently in a third laboratory with a guarded hot-plate (Willix et al., 1998). The average conductivity of the sample for six hot-box measurements was 0.0379 W m1 K1. The corresponding result obtained using the hot-plate apparatus was 0.0374 W m1 K1, a difference of 1.3%. These two comparisons with independent measurements on expanded polystyrene samples suggest that the expected error in the measurement of the thermal resistance by the guarded hotbox is of the order of 4%, an acceptable value under ASTM C236-89 guidelines. This estimate is based on the reproducibility of the measured data, as illustrated in Figure 2, and on comparisons with measurements made on two different test samples by two independent laboratories using different equipment. Copyright # 2005 John Wiley & Sons, Ltd.

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3. EXPERIMENTAL RESULTS 3.1. Dry material measurements Measurements are reported here for 15 commercially manufactured bonded fibre insulation batts. Of these samples, seven were composed of New Zealand sheep-wool, seven consisted of New Zealand sheep-wool blended with hemp–fibre and one sample consisted of sheep-wool from a U.K. source. None of the samples were selected to be representative of insulation materials from the sources cited. The New Zealand wool consisted of recycled fibre composed predominantly of coarser wool grades with the fibre diameter estimated by the supplier to be in the range 35–40 mm. The fibre matrix was bonded with an acrylic resin containing boron salts to provide resistance to biological attack and to act as a fire retardant. The resin and salt binder comprised typically 12% of the mass of the matrix (Newton, 2002). The composition of the wool–hemp blend varied from 40% hemp for low density insulation to 85% hemp for high density. The hemp fibre used in the blended samples was obtained from Wales and People’s Republic of China. The thermal resistance was determined with the plane of each sample oriented both horizontally and vertically, the direction of heat flow being upwards for the horizontal measurements. Prior to measurement, the samples were stored in the cold-side environment for a period of typically 3 weeks. This condition was selected because the exposed surface of the samples was in the cold chamber, and this was considered to be representative of the actual application of the material in New Zealand buildings. The cold-side temperature was normally in the range 4–68C and the relative humidity was 80–90%. Table I presents measured values of the thickness, density and thermal resistance of the samples tested. The sample thickness was measured by placing the sample between two sheets of expanded polystyrene lying horizontally and determining the mean spacing of the polystyrene

Table I. Summary of measurements for the thermal resistance of wool and wool–hemp insulation samples.

Material

Wool source

Thickness (mm)

Density (kg m3)

Resistance horizontal (m2 K W1)

Resistance vertical (m2 K W1)

Wool Wool Wool Wool Wool Wool Wool Wool/hemp Wool/hemp Wool/hemp Wool/hemp Wool/hemp Wool/hemp Wool/hemp Wool

NZ NZ NZ NZ NZ NZ NZ NZ NZ NZ NZ NZ NZ NZ U.K.

70 119 95 95 112 90 82 100 90 90 93 118 114 100 45

13.6 9.6 13.0 10.9 13.5 25.9 13.2 14.6 14.2 9.9 12.4 10.7 11.9 18.1 12.3

1.53 1.77 1.88 1.61 2.16 2.60 1.74 2.04 1.95 1.57 1.83 2.07 2.12 2.33 1.01

1.67 1.79 1.92 1.65 2.18 2.68 1.74 2.06 2.00 1.60 1.85 2.01 2.16 2.30 1.05


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sheets. The mean sample temperature was 13.3 18C and the temperature difference across the samples was in the range 14–178C. The table shows that the thermal resistance was normally smaller when the plane of the sample was oriented horizontally (heat-flow up), but the difference between measurements made in the horizontal and vertical orientations was small, being just 1.6% averaged over all measurements. This is rather less than the root-mean-square-difference between the values obtained for the two orientations, 2.8% and it is not significant. Figure 3 shows the thermal conductivity derived from the data in Table I as a function of the sample density. The values for the horizontal and vertical measurements have been averaged. The data for samples containing NZ-sourced wool have been used to establish a correlation (Equation (3)) for the conductivity, l; in W m1 K1, in terms of the sample density, r; in kg m3 using the same form as that used by Symons et al. (1995). The equation, which represents the least-squared percentage difference between the measured data and the fitted curve, is shown by the full line in Figure 3. l ¼ 1:097 103 r þ 0:7667r1 0:0239

ð3Þ

A factor which may have influenced the measurements reported here is the method used to hold the edges of the samples. This is most likely to have had an effect on the low-density samples. An edge restraint was required to support samples during measurements with the samples oriented vertically. This support, which was used also for the horizontal measurements, was achieved by applying tension to the edges of the cover sheet on the cold-side. The influence of this procedure on the measured thermal resistance was tested for one low-density sample by relaxing the edge constraint when the sample was oriented horizontally. The effect of this change was to increase the measured thermal resistance by 3.5% (Ye, 2001). This suggests that the uncertainty in the measurement of the thermal resistance should be increased from the value, 4%, estimated above, and that 7% would be a more realistic. This value has therefore been adopted for the uncertainty in R and l for the wool and wool–hemp samples.

Conductivity (W m -1K-1).

0.07

0.06

0.05

0.04

0.03 5

10

15 Density (kg

20 m-3)

25

30

.

Figure 3. Thermal conductivity of insulating batt samples as a function of density at a mean temperature of 13.38C. The line represents Equation (3) which is fitted to the data for NZ sourced wool. Symbol key: closed circle, NZ wool; open square, mixed NZ wool and hemp; open circle, U.K. wool. Copyright # 2005 John Wiley & Sons, Ltd.

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3.2. Wet material measurements To assess the effect of added moisture on the performance of medium-density wool insulation, the thermal resistance of one NZ wool sample was measured under four conditions with different moisture content. Measurements were made in both the horizontal and vertical orientations and the test procedure was based as far as possible on ASTM C236-89. However this standard does not address the problems of measurements with enhanced moisture content where dynamical effects due to moisture migration are significant. Before the wet tests started, the thermal resistance of the sample was first measured dry. The dimensions of the sample were 1400 1200 82 mm and the initial weight was 1.846 kg. The thermal resistance of the sample was then re-measured with moisture added in steps of 10% by mass up to 30%. This was achieved by spraying water onto one surface of the sample, which was then sealed inside a plastic film envelope. The wrapped sample was then mounted onto the guarded hot-box metering area, with the wetted side adjacent to the hot-box. The condition for the sample was monitored using the measured temperature and heating power of the metering-box. The time required to establish a steady-state condition was typically 2–3 days, depending on the moisture content of the sample. In these tests the hot and cold surface temperatures were maintained at 20 and 58C, respectively. Further details of the sample preparation and measurement procedures are described by Ye (2001). When the sample was placed onto the metering area, moisture began to diffuse from the hotside to the cold-side. With the sample in the horizontal position, water in excess of the capacity of the material to absorb formed an evaporation and condensation cycle. For the vertical orientation the excess moisture condensed out on a cold wall of the plastic wrapper and became lodged at the bottom of the wrapper envelope. For 20% added moisture there was 3.4% condensate; for 30% added moisture there was 10% condensate. The absorbed moisture in the three measurements with added moisture was therefore taken to be 10, 17, and 20%. Table II summarizes the moist-sample measurements and Figure 4 shows how the change in average thermal conductivity varied with the added moisture absorbed.

4. DISCUSSION The average absolute difference between the measured conductivity of the NZ wool samples and the fitted correlation (Equation (3)) at the same density is 4.9% of the measured values. For these samples therefore the conductivity is well correlated with density. Symons et al. (1995) and Trethowen (1995) also obtained a good correlation with the density of wool-insulating batts sourced from Australian and New Zealand material. Symons et al. (1995) provided the following correlation for the conductivity of sheep-wool, which is applicable to material densities in the range 9.9–70 kg m3: l ¼ 2:05 105 r þ 0:286r1 þ 0:0269

ð4Þ

The data presented by Symons et al. (1995) was obtained at a mean sample temperature of 258C, some 128C warmer than the data presented here. Based on ASHRAE (2001) data for the temperature dependence of the conductivity of fibrous insulation, the thermal conductivity obtained by Symons et al. (1995) is expected to be larger than it would be at 138C by approximately 6%. To compare this expression with the data obtained in this study, Figure 5 Copyright # 2005 John Wiley & Sons, Ltd.

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Table II. Influence of added moisture content on the steady-state thermal conductivity for wool insulation. Added absorbed moisture (%)

Resistance horizontal (m2 K W1)

Resistance vertical (m2 K W1)

Average conductivity (W m1 K1)

1.74 1.69 1.68 1.65

1.74 1.71 1.70 1.67

0.0471 0.0482 0.0485 0.0494

0 10 17 20 5

% increase in conductivity

4

3

2

1

0 0

5

10

15

20

25

% absorbed moisture

Figure 4. Effect of additional absorbed moisture on the thermal conductivity of a NZ sheep-wool sample.

shows Equation (3), represented by the full line, and Equation (4), less 6% to correct for the temperature effect, is shown by the dashed curve. The data points for NZ wool are shown with error bars of 7%, the expected uncertainty in the measured data. Figure 5 shows that there are systematic differences between the results presented here and those obtained by Symons et al. (1995), when the density is less than 13 kg m3. For the material tested here the conductivity is higher at lower density and lower at higher density, the cross-over between the two correlations occurring at 15 kg m3. At the upper and lower density extremes the difference is 19 and 9%, respectively. Both Symons et al. (1995) and Trethowen (1995) have emphasized the highly variable performance of wool insulation, and that density alone is not generally sufficient to characterize the conductivity of the material with confidence. In addition, there are differences in both the method of testing and the material. The samples tested by Symons included some 20% polyester melt fibre binder whereas a cured acrylic binder with added salts was used for the material tested here. The variability in the conductivity of the samples tested is highlighted in Figure 5 by the result obtained for insulation using U.K.-sourced wool. The conductivity for this sample is significantly less Copyright # 2005 John Wiley & Sons, Ltd.

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Conductivity (W m -1K -1).

0.07

0.06

0.05

0.04

0.03 5

10

15

20

25

30

Density (kg m-3).

Figure 5. Thermal conductivity of NZ wool insulating batts as a function of density at a mean temperature of 13.38C with 7% error bars. The heavy full line shows Equation (3). The dashed line represents Equation (4) less 6% as a correction for the difference in the temperature at which the measurements were made. Symbol key: closed circle, NZ wool; open square, mixed NZ wool and hemp; open circle, U.K. wool.

than that predicted by the correlation obtained here for NZ wool (Equation (3)) by 16%, and is 9% smaller than that predicted by the temperature corrected Symons correlation, Equation (4). Figures 3 and 5 show that the conductivity of insulation containing sheep-wool mixed with hemp is close to that for sheep-wool insulation at the same density. Based on the correlation for sheep-wool, Equation (3), the average conductivity of the mixtures was not significantly different from that for wool insulation at the same density. The correlation obtained for the thermal conductivity (Equation (3)) allows one to determine the optimum quantity of insulation required to produce a given thermal resistance, subject to the constraints applicable in a specific application. For example, the data indicates that, for a given thickness of insulation, the thermal resistance has substantially reached a maximum at a density of 26 kg m3. Based to the equation given by Symons et al. (1995) (Equation (4)) the thermal resistance would reach a maximum at a density of 57 kg m3, outside the range for which Equation (3) is applicable. Alternatively, the question arises, is there an optimum thickness for insulation, given a fixed mass of wool per unit of surface area? Equation (3) indicates that the thermal resistance is maximized when the thickness is as large as possible for a given mass of insulation, provided the density is not less than 11 kg m3. For Equation (4) there is no such limit on the density within the range for which the equation is applicable. Measurements made on the effect of added moisture on the thermal resistance for a single sample (Table II, Figure 4) indicate that the maximum added moisture for this sample was not more than 20% under the test conditions. Compared with the dry sample, the conductivity increased by 4.8% when the additional absorbed moisture was 20% of the sample dry mass. There was a 1.2% difference between the thermal resistance measured in the horizontal and vertical orientations at this moisture level, which is consistent with the difference obtained under dry conditions (1.6%). Copyright # 2005 John Wiley & Sons, Ltd.

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5. CONCLUSIONS The thermal conductivity of sheep-wool insulation and wool–hemp mixtures in the form of bonded insulation batts has been measured using a calibrated guarded hot-box. The density was 9.6–25.9 kg m3 for the wool and 9.9–18.1 kg m3 for the wool–hemp mixtures. The measurements were made at a mean sample temperature of 13.38C and the expected uncertainty of the thermal resistance and conductivity measurements was 7%. The results for dry NZ sheep-wool show that the thermal conductivity is well correlated with the sample density, in agreement with Symons et al. (1995) and Trethowen (1995). The correlation shows that, for a given mass of wool per unit area of insulated surface, the thermal resistance is maximized when the thickness is as large as possible, provided the density is not less than 11 kg m3. Thus, although the thermal conductivity of the wool insulation increases as the density decreases, the benefit of increased insulation thickness outweighs the effect of increasing conductivity when the mass of insulation material per unit area is fixed, for densities above 11 kg m3. The wool exhibited maximum and minimum conductivities of 0.067 and 0.034 W m1 K1, respectively, at densities of 9.9 and 25.9 kg m3. The variation of the conductivity with density is significantly larger than that obtained by Symons et al. (1995) and Trethowen (1995) at low density. The conductivity of the wool–hemp samples was not significantly different from the wool samples at the same density. Thus the use of hemp as an extender for wool in thermal insulation has no significant influence on the insulation properties under the test conditions. The measurements indicated the thermal resistance was 1.6% smaller on average when the plane of the sample was oriented horizontally, with heat flow in the upwards direction, rather than vertically. This difference, which could possibly be attributed to buoyancy effects, is not considered to be significant. Moisture uptake produced a decrease of less than 5% in the thermal resistance of sheep-wool insulation for an increase in the absorbed moisture content of 20%. This result is consistent with the claim by Schmeitz (2001) that the thermal conductivity of wool insulation is not greatly affected by moisture absorption. ACKNOWLEDGEMENTS

The authors wish to thank Mr L. Newton, New Wool Products Ltd, NZ, for supporting this project, providing a scholarship for Zhihui Ye, supplying the material for testing and granting permission to publish the results. C. G. Carrington wishes to acknowledge the hospitality of the Centre for Sustainable Technology, University of Ulster at Jordanstown, during the preparation of this paper and is particularly grateful to Dr Philip Griffiths for valuable discussions. This paper is dedicated to the memory of Dr Colin Mark Wells who drowned as a result of an accident while swimming, Dunedin, 31 December 2001.

REFERENCES Alcorn A. 1998. Embodied Energy of Building Materials (3rd edn). Centre for Building Performance Report, School of Architecture, Victoria University of Wellington: NZ (ISSN 1172-563X, ISBN 0-475-50012-1). ASHRAE. 2001. ASHRAE Fundamentals Handbook. American Society of Heating Refrigeration and Air-conditioning Engineers: Atlanta, GA. ASTM C236-89. 1990. Standard test method for steady-state thermal performance of building assemblies by means of a guarded hot-box. 1990 Annual Book of ASTM Standards. American Society for Testing and Materials: Philadelphia, PA. Copyright # 2005 John Wiley & Sons, Ltd.

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