Insulating attic with foam and blown insulation
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.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
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
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.
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
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.