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

asked 2025-05-27 09:44:16 -0500

Eric's avatar

updated 2025-05-27 14:49:55 -0500

Hi,

I calculated the unmitigated and mitigated exterior wall U-factors for the ASHRAE 2019 and 2022 Small Office prototypes due to thermal bridging. As shown below, my calculated U-factors differ from those in the PNNL Small Office prototype. Does anyonmeknow if there are any issues with my calculations? BTW, I used 0.051 as clear-field thermal transmittance of the wall assembly.

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Thanks,

Eric Wang

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  • default 0.099 PSI (2022) + unmitigated 0.150 PSI (2019) = wood-framed construction. Is the PNNL Small Office prototype also using wood-framed construction?
Denis Bourgeois's avatar Denis Bourgeois  ( 2025-05-27 10:45:04 -0500 )edit
Denis Bourgeois's avatar Denis Bourgeois  ( 2025-05-27 10:45:41 -0500 )edit
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ericmartinpe's avatar ericmartinpe  ( 2025-06-02 12:08:35 -0500 )edit

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answered 2025-06-02 13:15:56 -0500

updated 2025-06-02 13:19:21 -0500

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).

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Hi Eric, Thank you so much for your explanation. Shaojie Wang

Eric's avatar Eric  ( 2025-06-02 21:58:29 -0500 )edit

Shaojie, if you believe this answers your question, it is customary to accept it by clicking on the checkmark (below the up/down vote). This also helps others in the future.

ericmartinpe's avatar ericmartinpe  ( 2025-06-03 16:55:51 -0500 )edit

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Asked: 2025-05-27 09:44:16 -0500

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Last updated: Jun 02