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NC-2009 EAp2: Minimum Energy Performance
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U-value for Green Roof using Appendix A
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Lawrence Lile
Chief EngineerLile Engineering, LLC
76 thumbs up
December 17, 2015 - 2:45 pm
Given that the green roof can and will be wet, I'd argue that it provides little or no R-Value. We are left with the other layers.
My ASHRAE fundamentals book is dated, however these numbers should still be good
Outside surface, assume 15 mph wind, horizontal R0.17
Green roof Assume wet, R factor 0
R value for roofing membrane material about 0.33
R Value for steel metal deck * 0.69
R value for interior horizontal air film 0.61
My ASHRAE fundamentals has a short paragraph on panels containing metal - it mostly recommends testing them, but says some have measured out at R0.69.
Overall this is a terrible roof at R 1.8. You can refine these numbers with better sources, but mostly the insulation here is air films and the rest won't add up to much. You'd do well to convince the builders to add about ten inches of foam under the rood membrane, or whatever your local code requires, because this won't do well compared to a baseline building.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5916 thumbs up
December 17, 2015 - 4:09 pm
Agreed the soil and plants have no U-value. When wet and at certain outdoor conditions I might argue that they actually have a negative U-value and accelerate heat loss/gain.
Santiago Rodriguez
Revitaliza Consultores65 thumbs up
December 17, 2015 - 4:48 pm
Thanks for your reply. The building is already finished and there will be no adjustments in the building envelope. Since the project is located in Mexico City there is no much need for insulation. So if I assume the value of R 1.8 that Lawrence explained, is this the value I should use for the model? Or do I still have to use Table A2.3 of Appendix A?
Thanks in advance
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5916 thumbs up
December 17, 2015 - 4:53 pm
Does not sound like the structure will have an impact on the overall U-value so no need to use Appendix A.