Open-cell spray foam: 0.5 lb or 1.2 lb?
Is there an advantage to 1.2-lb open-cell spray foam over 0.5lb open-cell spray foam, assuming they are blown to identical R-values? For example, does the denser 1.2 lb open-cell foam have any of the structural characteristics of the 2-lb closed-cell foam? Or is there some other advantage of 1.2? Or disadvantage?
Situation: I’m going to use open-cell spray foam on my roofline to create a conditioned attic, to bring my ducts and attic-stored items into the building envelope. This is a retrofit of an older house in Zone 2A, Gulf Coast.
I know about the potential moisture issues of open-cell thanks to this site, so I will ventilate the attic either with an a/c duct or a low-cfm vent and return, per Joe Lstiburek’s recommendations. The 0.5-lb foam will be sprayed in multiple layers to bury the rafters, and the 1.2lb will at least include a flash coat of any exposed framing to limit thermal bridging.
I ruled out closed-cell because no one around here uses the friendlier blowing agent. Also, no contractor will use rock wool or denim batts or other greener options, and I don’t want to do it myself.
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