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.
Thanks
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Replies
Spray foaming the entire attic floor just for air sealing is a waste. You don’t need to seal the middle part of sheets of drywall, the drywall itself is an air barrier. You really only need to air seal specific spots prone to leakage like electrical boxes, holes for wires and pipes, and gaps around top plates. There may be other spots too depending on the design, but the idea is the same — air seal key spots where it’s needed.
You could probably just use one of the two-part spray foam kits to coat all the top plates and other trouble spots (note that many recommend against spray foaming recessed can lights even if IC rated). This would probably be a lot cheaper.
Bill
Lawrence,
Here is a link to an article that discusses the usual approach (as Bill mentioned): "Air Sealing an Attic."
I am also a builder in WI and agree an entire spray foam layer over the ceiling plane is a complete waste. I would also favor cellulose loose fill attic over fiberglass any day of the week although I do install BIBS walls and it works great.
My houses test below 1 ACH 50 and I can tell you it's not difficult to do, I dont use a lot of tapes, OSB ceilings or an excessive amount of spray foam, but I do seal as we build and that is the most important thing to do.