Regarding the U-value of the exterior wall in the proposed model, I see the LEED calculator provided a hint "Proposed construction assembly U-factor should be as-designed and consistent with Appendix A of ASHRAE 90.1". When I checked Appendix A, it seems like in the proposed model, we just need to obtain the R-value of the insulation and from there, we look up the corresponding U-value in table A. Is that the correct way to determine the U-value of the exterior wall for the proposed case? I thought we have to calculate U-value from each layer of wall assembly as shown in the design drawing, not looking up from Appendix A.
My second question is if a dry warehouse is unconditioned and not semi-heated, then will the U-value of its wall be modeled the same as the design? Table 5.5 only applies to the envelope of conditioned and semi-heated space, but yet mentions un-conditioned space. So I don't know how to set up the U-value for an unconditioned building in the baseline model. Thanks for your advice.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
August 24, 2022 - 11:50 am
You do not calculate the overall U-value of an exterior wall assembly by determining the U-value of each layer and adding it up. This would tend to overinflate the assemply U-value since that is a point calculation and not a calculation of the U-value of the whole wall assemply.
Appendix A tables include the effect of the framing members on the overall assembly U-value and is a far more accurate representation of the thermal properites of a given wall assembly. For example take a look at the difference between the assembly U-values for a steel framed wall vs a wood framed wall with the same amount of cavity insulation. For LEED you must use Appendx A and it is by far the most accurate way to determine the assembly U-value.
Insulation levels in uncondtioned spaces are not regulated. So for an unconditioned space I would model the baseline identical to the proposed design.