METHOD OF IN-SITU MEASUREMENT OF THERMAL INSULATION PERFORMANCE OF BUILDING ELEMENTS USING INFRARED CAMERA Shinsuke Kato 1, Katsuichi Kuroki 2, and Shinji Hagihara 2 1 Institute of Industrial Science, University of Tokyo, Meguro-ku, Japan 2 Japan Testing Center for Construction Materials, Souka-shi, Japan ABSTRACT Specific and effective measures for international issues, such as prevention of global warming, energy saving, reduction of environmental loads, are urgently required. The improvement of the insulation efficiency of the house and the building is demanded as the part. For that purpose, it is also necessary to determine whether installed insulating materials have been installed properly, and whether they can actually demonstrate the expected performance. On the basis of the results of various surveys, the infrared method was proposed as nondestructive in-site measuring method enabling simple measurements on site. Laboratory experiments and a verification experiment using an actual building at the site were performed for a method of measuring the thermal insulation performance of building elements in actual large structures by the infrared method (Hagihara et al. 2006 and 2007, Nakamura et al. 2006). The results confirmed that measurement accuracy can be sufficiently guaranteed. KEYWORDS Thermal Insulation, In-Situ Measurement, Detached House, Coefficient of Heat Transfer Sensor, Infra-Red Photograph INTRODUCTION Purpose In measurement of the thermal insulation performance of building elements such as walls, etc. in actual large buildings, simple, high accuracy measurement is quite difficult due to variable factors such as diurnal changes in air temperature, heat due to sunlight, and the like. Therefore, recently, a comparatively simple technique for obtaining the thermal insulation performance of building elements has been proposed by measuring the indoor-side surface temperature and the surface coefficient of heat transfer of the surface of building elements using an infrared camera, taking advantage of the high temperature measurement accuracy and improved functions of these devices. MEASUREMENT PRINCIPLE Heat transfer at the inside surface of a wall which is in contact with the outside air during a heating period can be expressed by the following equation, considering convection and radiation. q ( Ta Ts ) + r ( Tr Ts ) = c α α (1) where, q : Heat flux from interior (W/m 2 ) α c : Convective surface coefficient of heat transfer (W/(m 2 K)) α r : Radiant surface coefficient of heat transfer (W/(m 2 K)) Corresponding Author: Tel: + 81 3 5452 6431, Fax: + 81 3 5452 6432 E-mail address: kato@iis.u-tokyo.ac.jp
Eq. (1) can be rearranged as follows: T a : Indoor air temperature ( ) T s : Wall surface temperature ( ) T r : Average temperature of radiation from indoor surfaces other than walls ( ) q ( α c α r ) α c Ta + α r Tr = Ts + α c + α r (2) Assuming Tn ( α c Ta + α r Tr ) ( αc + α r ) = /, α = α c + α r, Eq. (3) can be obtained from Eq. (2): ( Tn Ts ) q = α (3) where, α : Total surface coefficient of heat transfer (W/(m 2 K)) T n : Environment temperature ( ) T s : Wall surface temperature ( ) Accordingly, if α, T n, T s can be measured, it is possible to obtain q. In this case, T s is measured with an infrared camera, and α is measured with a device that serves as a surface coefficient of heat transfer sensor in the vicinity of the wall. Alternatively, α is measured with a heat flow meter, which is attached to the wall surface. T n is a fictive temperature, but in indoor environments, it is permissible to assume that substantially Ta =T r. Therefore, there is normally no problem if the air temperature T a is measured. However, strictly speaking, the radiant heat from indoor surfaces must be considered; hence, the globe temperature is used here. Alternatively, the temperature of a black curtain suspended in midair can be measured using the infrared camera, and this can be regarded as the approximate environment temperature. Because actual wall surfaces do not have a uniform temperature, but rather, display temperature variations, the heat flux from walls can be calculated by the following equation by obtaining a surface area where the temperature distribution is uniform. ( Tn Tsi ) Q = qi Ai = α Ai (4) where, Q : Heat flux from entire wall (W) q i : Heat flux from i part (W/m 2 ) A i : Area of i part (m 2 ) T si : Surface temperature of i part ( ) If the air temperature of the outside air can be measured, it is possible to obtain the coefficient of overall heat transmission of the wall by assuming a quasi-steady state. [( Tn Te ) A] U = Q / (5) where, U : Overall thermal transmittance (W/m 2 K) Q : Heat flux from entire wall (W) T n : Indoor environment temperature ( ) T e : Outdoor environment temperature ( ) A : Wall area (m 2 )
Wall A Outside Ts αc αr Ta Indoor side Tr Ts i Ai Fig. 1 Measurement principle STRUCTURE AND CALIBRATION OF SURFACE COEFFICIENT OF HEAT TRANSFER SENSOR Fig. 2 shows an outline of the surface coefficient of heat transfer sensor. In order to obtain a uniform temperature distribution, two copper sheets with a thickness of 0.5mm are used in the sensor. A heat flow meter with a thickness of 1mm is inserted between the two copper sheets and glued to the sheet on the surface side of the sensor. The copper sheet which forms the surface of the sensor has emissivity of 0.95 or higher in the long wavelength region, and is given a smooth finish free of roughness. A plane heater is mounted on the back side of the two copper sheets, and its back surface is insulated with extruded polystyrene foam having a thickness of approximately 25mm. As a standard, the size of the heat flow meter used is approximately 50mm 50mm. The sensitivity of the heat flow meter is 0.005mV/(W/m 2 ) or higher. Calibration of the surface coefficient of heat transfer sensor is shown in Fig. 3. Calibration was performed under the same conditions as with the heat flow meter attached to a heat sink. In measuring the heat transfer rate, measurements were performed assuming the between surface and air temperature difference to be approximately 4K. It was found that measurements are substantially independent of the direction of heat flow, regardless of whether the surface is heated or cooled.
insulation copper plate insulation copper plate plane heater heat flow meter (50 50mm) thermo couple 25 200 25 25 3 25 150 25 Fig. 2 Outline of surface coefficient of heat transfer sensor 10 9 8 7 6 5 4 heat condition (plane heater) 3 heat condition(heat sink) 2 cooling condition(heat sink) calculated value by reference document 1 analysis result 0 0 1 2 3 4 5 6 7 8 Difference of temperature (K) Fig. 3 Results of calibration of surface coefficient of heat transfer (natural convection) Surface coefficient of heat transfer[w/(m 2 K)] EXPERIMENTAL STUDY Laboratory Verification Experiment Using Model Wall (Experimental Comparison with Hot Box Method) A model wall (wooden wall) was constructed, the overall thermal transmittance was measured in accordance with ISO 9890, and a comparative study with the infrared method was carried out. An outline of the test wall (dimensions: 1980 x 1980mm) is shown in Fig. 4. Construction of the glass wool thermal insulation was performed so as to intentionally create insulation defects (see Photo 1). A infrared thermograph of the wall with insulation defects is shown in Photo 2. As shown in Table 1, the overall thermal transmittance obtained by this method was 0.75W/(m 2 K). The surface coefficient of heat transfer obtained with the coefficient of heat transfer sensor at this time was 9.76W/(m 2 K) near the center of the test wall. The overall thermal transmittance when the same wall was measured in accordance with ISO 8990 was 0.78W/(m 2 K). Comparing the two, this shows good agreement with the overall thermal transmittance measured by the infrared method. The error with the standardized overall
thermal transmittance, adjusted to that in infrared thermograph measurement of the surface coefficient of heat transfer, is 3.8%. From this result, it can be concluded that the infrared method is an adequate measurement method from the viewpoint of accuracy. Outside 20 100 9 Indoor side gypsum boards(9mm) siding glass wool (50mm) house wrapping sheets 80 444 444 444 444 80 1980 framing elevation cross-section Fig. 4 Wall model (wood-structure post-and-beam wall) Photo 1 Insulation work with intentional insulation defects Photo 2 Infrared thermograph of wall in Photo 1
Table 1 Results of measurement of overall thermal transmittance (laboratory experiment) Infrared Measurement method ISO 8990 method Heat transfer area A (m 2 ) 3.9204 Generated heat Q + = Q H QF (W) 58.98 - Calibrated heat Heat flow rate through test wall Environment air temperature in isothermal chamber Environment air temperature in hot box Environment air temperature in low temperature chamber Environment air temperature differential Thermal resistance Q Q l (W) 2.04 - N = Q Ql (W) 56.94 57.47* θ Ga ( ) 19.4 20.5 θ Ha ( ) 19.6 - θ Ca ( ) 0.6 0.9 θ = θ Ha -θ Ca (K) 19.0 19.6 R θ A = (m 2 K)/W 1.31 1.34 Q N Overall thermal transmittance Standardized overall thermal transmittance** Difference in overall thermal transmittance due to measurement method K = 1 W/(m 2 K) 0.76 0.75 R K ' W/(m 2 K) 0.78 - (%) - 3.8 *: Heat flow rate calculated from the relationship between the surface temperature in infrared thermograph and the surface coefficient of heat transfer measured by the surface coefficient of heat transfer sensor. **:Overall thermal transmittance when the thermal resistance of the indoor-side surface(0.12(m 2 K)/W) in measurement in accordance with ISO 8990 is adjusted to thermal resistance of the indoor-side surface(0.10(m 2 K)/W) in measurement by the infrared method. Verification Experiment with Wall at Site (In-situ Measurement Experiment) The thermal insulation performance of a wall in an actual large residential building (north side wall not exposed to direct sunlight) was actually measured, and the effectiveness of this measurement method was verified. The cross section of the actual large wall is shown in Fig. 5. This is a conventional (Japanese-type) wood-structure post-and-beam building. As thermal insulation, the wall is filled with glass wool 75mm in thickness. Measurements were conducted over a period of 5 days. The measured values of surface temperature, air temperature, surface coefficient of heat transfer, etc. are averages of values measured at intervals of 10 minutes from midnight until 6:00 in the morning during the measurement period. An infrared thermograph of the surface temperature on the indoor side of the wall is shown in Fig. 6. The average surface coefficient of heat transfer during the measurement period was 12.66 W/(m 2 K). The measured value of the overall thermal transmittance obtained from these results was 0.413 W/(m 2 K). The total thermal transmittance (design value), considering the column parts and thermal insulation parts based on the composition of wall materials, was calculated as 0.437 W/(m 2 K). Thus,
the two showed good agreement. Outside Indoor side measuring 測定対象 object Fig. 5 Cross section of wall (wood-structure post-and-beam construction method) AR01 30.0 30 29 28 27.5 Fig. 6 Infrared thermograph of wall surface temperature (square shows measurement range) Table 2 Results of measurement of overall thermal transmittance (in-situ measurement) Season Autumn Measurement period Nov. 2 7, 2006 Insulating material Thickness mm 75 Air temperature on outside 11.6 Air temperature on indoor side (globe temperature) 30.0 Average surface temperature of wall 29.4 Surface coefficient of heat transfer on indoor side 1) W/(m 2 K) 12.66 Average overall thermal transmittance Measured value 2) Design value 3) K A' W/(m 2 K) 0.413 K A W/(m 2 K) 0.437 Difference ( K A' K A) / K A 100 (%) -5.5 1) Result of measurements using surface coefficient of heat transfer sensor (average value of measured results from 0:00a.m. to 6:00a.m. during the measurement period). 2) Calculated using average values of measured results of temperature and surface coefficient of heat transfer from 0:00a.m. to 6:00a.m. during the measurement period. 3) Calculated value including general parts and heat bridge parts in object area of infrared thermograph.
CONCLUSIONS Laboratory experiments and a verification experiment using an actual building at the site were performed for a method of measuring the thermal insulation performance of building elements in actual large structures by the infrared method. The results confirmed that measurement accuracy can be sufficiently guaranteed. Using infrared camera, measurement is easy and, with this method, the thermal insulation performance of building elements can be quantified simultaneously with visual understanding of irregularities in the insulation construction. Therefore, infrared method is considered an effective measurement method. ACKNOWLEDGEMENTS This research is done as commission investigation of New Energy and Industrial Technology Development Organization (NEDO). REFERENCES 1. S. Hagihara, et al. (2006) In-Situ Measurements of Wall Thermal Performance Part 2 Method of Infra-Red Photograph [Constitution and Calibration of Surface Coefficient of Heat Transfer Sensor], AIJ Summaries of Technical Papers of Annual Meeting, C-2,.121-122 2. Y. Nakamura, et al. (2006) In-Situ Measurements of Wall Thermal Performance Part 3 Method of Infra-Red Photograph [Experiment of Wall Model on Laboratory], AIJ Summaries of Technical Papers of Annual Meeting, C-2,.123-124 3. S. Hagihara, et al. (2007) In-Situ Measurements of Wall Thermal Performance Part 5 Method of Infra-Red Photograph [In-Sutu Experimental Validation], AIJ Summaries of Technical Papers of Annual Meeting, C-2,.87-88