Thermal Bridging Calculation for Wall U factor of ASHRAE 90.1 OfficeSmall Prototype

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Your calculation is missing the thermal bridging due to the parapet. The parapet length is the length of the roof-wall intersection, which is equivalent to the perimeter of the building (302.82 ft). The psi-factor for wood-framed wall parapet is 0.32 (unmitigated and mitigated are the same factor).

Therefore, the total unmitigated heat flow impact (∑ΨiLi + ∑Xinj) is 440 * 0.15 + 302.82 * 0.032 = 75.69. Divide this by the total opaque wall area (2341 ft2) yields the Thermal Bridge Adjustment for AGW U-factor of 0.03233 (unmitigated) or 0.0227 (mitigated).

The derated u-factors (assuming 0.051 u-factor requirement in CZ5):

2019 (unmitigated): 0.051 + 0.032 = 0.083

2022 (mitigated): 0.051 + 0.023 = 0.074

The thermal bridging assumptions and calculations used in the prototypes will be detailed in a paper presented at the 2026 Winter ASHRAE conference (pending acceptance).