R-value is an effective, well established, and widely accepted metric for describing the thermal performance of building insulation materials, but it is an incomplete metric for describing the thermal performance of building enclosure assemblies. The thermal performance of any building enclosure assembly is highly dependent on the amount of insulation installed; however, it can also be heavily influenced by a number of other factors such as air leakage, thermal bridging, operating conditions, moisture content, and installation defects. These factors are not properly or completely captured when insulation R-value is used as the sole metric for assembly thermal performance. A consortium of insulation manufacturers, representing all types of insulating materials and all regions of North America, undertook a series of research projects to examine these issues and their impact on the prediction of the thermal performance of building enclosures.
This paper documents the development of the primary apparatus used in the research: a new hot box designed to accurately measure the true thermal performance of modern wall assemblies. The approach is based on ASTM C1363. However, a number of improvements and performance extensions have been made to facilitate the research. These features allow quantification of wall assembly performance that is more representative of actual in-service conditions.
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