Self-Adhered WRB for Spray-Foamed ICF Foundation
I’m building a timberframe/SIP house on an ICF daylight basement in coastal AK- I have a question about the deck ledger. I know that one usually puts a self-adhering membrane behind the ledger, but I am wondering if that is appropriate in my case.
I’ve attached a schematic of the wall, there are I-joists on the ICF basement, and the timberframe/SIPs are sitting on top of that. Most will likely notice a nonstandard arrangement- the triple sill plates. That is because we wound up pouring the slab after the footers, which took away 4′ of height from the original plan- so we added the extra 2x6s to raise the second floor up and get that back.
That’s a bit of an air sealing nightmare, so the plan is to spray foam from the inside to both insulate and air/vapor seal- that idea is directly from Lstiburek’s SIP book.
My concern is this: With the peel and stick membrane on one side, and the spray foam on the other, there is two vapor barriers, sealing up the cavity with the rim joist and I joist and leaving nowhere for water to go. Water obviously shouldn’t be getting in there, but everything got wet when we built it- it’s been drying out since then, but it will surely take a long time for everything to dry (as much as that happens in coastal AK, perhaps a better term is “equilibrate”).
Will I be better off to forego the peel and stick membrane on the exterior so that the joists can “dry to the outside”, like the SIPs do? That is what makes sense to me.
Thanks for your thoughts,
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