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Community and Q&A

Insulating attic with foam and blown insulation

Colby Forester | Posted in General Questions on

I am currently building a house in North Alabama, Zone 3, and conflicted on how to insulate the attic space. I originally planned to encapsulate the attic in foam but all my duct work is in the basement so I saw no need close in that extra space. I’m aiming for great air sealing so thought about the idea of putting 1-2″ of closed cell foam across the entire attic floor for air sealing/insulation and then blowing in insulation on top of the closed cell foam. I thought that would also help with insulation dust from making its way into the house. Is that a good or bad idea? Is there a better recommended option?

Thanks in advance for your comments.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Colby.

    The air sealing strategy you describe would work just fine, however it is unnecessary and requires economically- and environmentally-costly closed-cell spray foam. It's fairly straightforward to detail your drywall celing as an air barrier and to seal additional thermal bypasses before blowing much more wallet- and climate-friendly insulation like cellulose into the attic. Check out these articles:

    How to Insulate an attic floor
    How to Hang Airtight Drywall

  2. Expert Member
    Dana Dorsett | | #2

    The 2" closed cell foam is an expensive environmental disaster, relatively speaking. Assuming there isn't a cheaper/better way to air seal, as little as 3" of half pound open cell foam is sufficient for air sealing, using only 38% of the polymer of 2" of closed cell foam (75% the polymer of 1" of closed cell foam), with no climate damaging blowing agents (it's blown with water), putting it on par with low density fiberglass insulation from a CO2e per R point of view.

    Half pound open cell polyurethane is a bit less than half the damage per R of HFO blown closed cell foam shown in that chart.

    Cellulose is CO2e negative, since it is sequestered carbon, using recycled/reclaimed feedstocks and very little processing energy.

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