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It is critical to know or measure the installed insulation profile in metal building roof and wall systems to determine the effective thermal resistance with acceptable accuracy. Guarded hot box tests and finite element models can be precise, but are accurate only if the modeled cross-sections correctly represent the insulation system as practically constructed. In cooperation with others on the Metal Building Task Group reporting to the ASHRAE SSPC 90.1 Envelope Subcommittee, we developed recommendations for modified thermal transmittance values for metal building insulation systems to reflect current common construction practices as explained in this paper. The current version of ASHRAE 90.1 over-estimates the thermal performance of “over-the-purlin” insulation for roofs and walls by approximately 20%.

Units: Dual