How much closed cell spray foam do I need?
I’m building a home in northern Wisconsin on the southern edge of zone 7. I’m planning on R40-45 in the walls (double stud) and R60-65 in the ceiling (parallel chord trusses). I had planned on using dense pack cellulose but there are lots of valleys and no good way to ventilate them. So I’d like to have “the appropriate amount” of closed cell spray foam applied to the inside surface of the exterior sheathing (1/2″ walls/5/8″ roof OSB) and fill the rest of the wall/ceiling cavities with dense packed cellulose. I assume the R value of the spray foam needs to be enough to prevent condensation in the cellulose – no interior vapor retarder. Also using this approach would it be wise to not use synthetic roofing underlayment, i. e. DiamondDeck?
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