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PNNL latest Commercial models don't align with the ASHRAE 90.1-2019 code

asked 2024-02-22 10:14:15 -0500

Urvi Shah's avatar

updated 2024-02-22 11:37:34 -0500

I observed that the latest commercial prototypical buildings don't align with code ASHRAE 90.1-2019. (specially exterior walls). I tried to compare it with the older models which I had downloaded in 2023. I see that the latest medium office building has been assigned with three different wall U values which wasn't the case in older models. Has anyone faced this issue? What is the reason behind this change?

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(just guessing here - link to idf?)

90.1 has almost always had 4 above-grade wall categories/requirements (e.g. "mass", "steel-framed"). Depending on the climate zone, the steel- and wood-framed categories sometimes have the same U-factor requirements. This would boil down the actual number of wall requirements to 3 in some instances. So when I read "3" different wall U-factors, I assume all 3 requirements are likely being evaluated (e.g. GJ vs $). PNNL may have simply picked 1 (out of 3) in previous models, e.g. "steel-framed" for medium office? Hoping PNNL staff can chime in.

Denis Bourgeois's avatar Denis Bourgeois  ( 2024-02-22 14:55:39 -0500 )edit

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answered 2024-02-23 18:18:11 -0500

updated 2025-06-02 13:29:17 -0500

The prototypes were recently updated to address Addendum av, which characterizes the new thermal bridging requirements. The addendum contains data sets representing cases where thermal bridging is mitigated and un-mitigated. Since the 2022 prototypes represent mitigated thermal bridging (in climate zones 4 - 8, where required), the 2019 prototypes were updated to represent un-mitigated thermal bridging. Overall average de-rated u-values were determined on a floor-by-floor basis for multi-story prototypes, so that's why there are three different u-values for the ground, middle, and top floor of the medium and large office prototype.

The calculation of overall u-values that incorporate the thermal bridging assumptions will be detailed in a forthcoming Final Determination report.

Edit: Here is the Final Determination report.

Edit2: This response details the calculations involved in derating the wall u-factor for the Small Office prototype in CZ5.

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Ah! Good to know!

Denis Bourgeois's avatar Denis Bourgeois  ( 2024-02-24 05:12:43 -0500 )edit

Hi Eric,

I checked 2019 and 2022 medium office prototypes in Buffalo. int_slab_floor is used for gournd, mid and top interior floors. Could you please let me know where I can find three different u-values for the ground, middle, and top floor in ASHRAE901_OfficeMedium_STD2019_Buffalo and ASHRAE901_OfficeMedium_STD2022_Buffalo idf files. Thanks

Eric's avatar Eric  ( 2025-05-20 17:02:58 -0500 )edit
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@Eric : unless an actual floor slab is exposed (e.g. a 2nd-story cantilevered floor above a recessed ground-floor, see example pics here), it wouldn't be derated using 90.1 2022 un/mitigated PSI factors. Depending on the adopted convention, exterior walls are (mostly) singled out for derating. I think @ericmartinpe is simply referring to "floors" here as a synonym for building stories - not floor slabs.

Denis Bourgeois's avatar Denis Bourgeois  ( 2025-05-21 07:40:35 -0500 )edit
1

Denis is correct, unexposed floors are not derated for thermal bridging. For the Medium Office prototype, only Window-Wall interface and Cladding support are accounted for in the thermal bridging calculations.

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).

ericmartinpe's avatar ericmartinpe  ( 2025-06-02 13:34:11 -0500 )edit

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Asked: 2024-02-22 10:14:15 -0500

Seen: 393 times

Last updated: Jun 02