Per 2015 IECC table R402.1.4 if the R-value for the wood frame wall is R-13 cavity insulation in climate zone 2, the U-factor is 0.084. However, per table A3.4.3.1, the cavity insulation R-13's U-factor is 0.089. I am confused why the cavity insulation U-factor from 2015 IECC is different from what is calculated in ASHRAE 90.1?

Thank you all

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Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 04/17/2020 - 15:55

I don't know why it is different but the difference is pretty marginal and should not significantly affect the project's energy use. Does the IECC value include other aspects of the building assembly? The Appendix A tables assume some of the components of a finished wall assembly. The U-factors include R-0.17 for exterior air film, R-0.08 for stucco, R-0.56 for 0.625 in. gypsum board on the exterior, R-0.56 for 0.625 in. gypsum board on the interior, and R-0.68 for interior air film, vertical surfaces.

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FAQs about EAp2 :

2015 IECC Table R402.1.4 VS. ASHRAE 90.1-2013 Table A3.4.3.1

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