Air sealing between second floor and attic?
We’re in the process of designing our home and talking to various builders. One in particular takes building science seriously and takes a pretty robust method to air seal between the second floor ceiling and the attic. He recommends 2″ of closed cell spray foam over the entire attic floor (top of the drywall) which would seal the perimeter, drywall seams, and any penetrations for light fixtures. Then 15″ of blown fiberglass over the top of the foam. Sounds great, but it’s also a bit spendy. So the question becomes, is it really necessary to take it to that extreme, or can it be accomplished another way? He has done a project where he put 6mil poly under the ceiling drywall, then address any penetrations individually (can’t remember what product he used to seal them though). Does this sound like a good alternative? What’s best practice to seal off the attic? The rest of the house would have exterior foam insulation and no interior vapor barrier.
For the record, I’m in southeastern Wisconsin, climate zone 6.
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