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Open-cell spray foam: 0.5 lb or 1.2 lb?

Nola_Sweats | Posted in General Questions on

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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  1. Expert Member
    Michael Maines | | #1

    Nola, the 0.5-lb foam will give you the same R-value with less material than 1.2-lb foam, so the embodied carbon will be lower. I wouldn't consider 1.2-lb to be structural--at best it would help keep the roof glued together in a storm, but 0.5-lb foam will do that as well.

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