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NC-2009 EAp2:Minimum Energy Performance

Continuous insulation on the inside of the wall?

Our state energy code is based on ASHRAE 90.1-2004, which I believe does not require ci for exterior walls. ASHRAE 90.1-2007 requires R-3.8 ci for nonresidential walls. The project architect designed to the current code, but the project is registered in LEED 2009, so we need to meet the 2007 mandatory provisions for the envelope, including the ci layer. Unfortunately, the structure has been built, but the interior drywall has not been placed. Do you think we could be considered to meet the 90.1-2007 mandatory provision if we put an R-3.8 layer on the inside, under the drywall? We would still have thermal bridging through the ceiling / floor joists, but would break the thermal bridge from all of the studs on both first and second floors.

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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
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Tue, 04/24/2012 - 15:32

Hey Dave, The insulation levels in 90.1 are not mandatory so you are not required to meet them for LEED. Not meeting them will potentially affect your savings but perhaps you can make that up in other areas. 90.1 prescriptively requires continuous insulation but it is up to the designers to determine where is goes.

Tue, 04/24/2012 - 16:18

EAp2 reads "Comply with the mandatory provisions (Sections 5.4, 6.4..." And 90.1-2007 Section 5.4 seems to say to me that our insulation must comply with the "requirements found in Sections 5.8.1.1 through 5.8.1.9." And that seems to prescribe a layer of R-3.8 in the wall section. Or does the insulation column offer one way to meet the Assembly Maximum U value? But I could meet it another way (higher R insulation between the studs, for instance)?
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Marcus Sheffer
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7group / Energy Opportunities
LEEDuser Expert
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Tue, 04/24/2012 - 17:37

Sections 5.8.1.1 to 5.8.1.9 deal with the insulation material and in some cases the installation of that insulation. I don't see any section that requires continuous insulation. Which section in particular has you thinking it is prescribed? Unfortunately this does not get simpler, the reality is actually more complex. Note that the insulation values in Tables 5.5-1 to 5.5-8 are the prescriptive requirements, they are not mandatory. You could demonstrate prescriptive compliance by exceeding the overall U-value indicated in the tables. So the answer to your last question is yes but you are required to determine the overall U-value, not a point calculation. The U-values in these tables and the U-value of your proposed envelope assemblies is calculated according to 90.1 Appendix A. So for example a steel stud wall overall U-value must be calculated (and modeled) using table A3.3 to account for the thermal bridging in the wall assembly. So a 6 inch steel stud wall at 16" OC with no continuous insulation with R19 batts has an overall U-value of 0.109 (R9.2). All envelope assembly U-values must be calculated and modeled according to Appendix A.

Tue, 04/24/2012 - 17:42

My bad. I was looking at Table 5.5-5 and interpreting the R requirements to be mandatory. Thanks for your patience. And thank you for clarifying the task of calculating U values.
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Tue, 04/24/2012 - 18:07

No problem, you aren't the first to have issues understanding 90.1! Fun stuff eh . . .

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