| Résumé | Typical analyses of heat transfer across building envelopes consist of determining the thermal resistance of the assembly. The thermal resistance, or R value, is determined through test methods, calculation methods, or numerical simulations. In complex or novel wall assembly configurations, thermal resistance is required to be determined with experiments that use a guarded hot box (GHB) test apparatus according to ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus. One scenario in which complex heat transfer occurs is in a furred airspace in contact with a low emissivity, as the combined effects of natural convection and radiation dominate the heat transfer across the space. Convection and radiation heat‐transfer effects are much more sensitive to the variation in surface temperatures, orientation, and aspect ratio of the airspace. This paper presents the results of ASTM C1363 GHB tests of a full‐scale (8 ft by 8 ft [2.44 m by 2.44 m]) wall assembly containing a furred airspace, with one surface having low emissivity in two configurations. The first configuration is with ¾‐in. (19‐mm)‐depth strapping oriented vertically and spaced at 24 in. (0.61 m.) on center, creating four identical furred airspace cavities approximately 8 ft high by 24 in. wide. The second configuration consists of rotating the same wall assembly by 90°, creating four identical horizontally oriented furred airspace cavities. Each configuration was tested for three exterior temperature conditions: −20°C, −25°C, and −30°C—all with an indoor temperature of 21°C. Additionally, the experimental results were compared with results of the ISO 6946 Annex B.2 calculation method. The results from the tests did not show a significant difference in thermal resistance results between either the exterior temperature differences or when comparing the effects of airspace orientation. This highlights some of the challenges when trying to differentiate small differences between wall assemblies with GHB testing, especially when the experimental uncertainty is considered. |
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