"Thanks for taking my question, David. I'm building my own home here in northern NY and have spec’d out closed-cell foam insulation for my non-vented roof deck.
So far, so good.
My insulation contractor has suggested a cost saving option for me. He suggests an initial pass of closed-cell foam product (about 1"+) to ensure a tight moisture barrier against the roof deck. Then finish the space with the cheaper open-cell product in amounts sufficient to obtain proper R-values.
This option could save me $2,500 versus having the full blast of closed-cell throughout.
Have you heard of this option? Is it legitimate? I can run it by my building codes officer, but he's only going to want more data and an engineer/architect sign-off.
I thought you might have an opinion before I started that process?
As you have found out, sprayed foam may eliminate the code requirement for ventilation, but it's also much more expensive. I've seen several situations such as yours where the insulation contractor recommends sprayed foam only where it's absolutely necessary.
Your question really is a code question. The Connecticut codes I use allows elimination of ventilation if rafter bays are filled with "sprayed-in" foam insulation; there is no requirement for open or closed cell foam.
This may not be true in your area, and you should talk to your local building official.
Good luck with your project,
David Moore, AIA
Original Home Plans
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