Best Insulation for Unvented Attic in Hot Climate
I live in Zone 5, Utah. I just had my 1925 8/12 pitched hipped-gabled bungalow re-roofed, having added an attic cathedral-ceiling loft which will be sheetrocked and painted. Four layers of old shingles came off, R20 rigid polyiso went on top of the original structural 1” hardwood skip sheathing, then OSB, synthetic underlayment, dark thick fiberglass TL shingles. Rafter bays are 2x4s, does it matter if these are filled with open cell spray foam vs net and blown fiberglass? I am not sure if it matters in this situation that one is air permeable and the other is not, and I read conflicting information on their ecological attributes.
thank you for your help!
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Replies
Hi Ken. I have no doubt you’ll get quite a few opinions here. Of course you don’t really have to worry about condensation with your exterior foam. My very amateur opinion is always to have at least one side that is more air and water permeable in case of leaks or water in the future.
Best of luck.
Cory
With 2x4 rafters and R20 above the roof, you can pretty much fill the rafters with anything.
SPF is not really worth it in this case since the condensation control is handled by the rigid above and not increasing your assembly R value that much.
Regular batts or dense pack is the way to go.
You need at least one very good, tested air barrier layer - with two being of some value. If you don't have that already, then it's down to the drywall layer or spray foam.
If the polyiso has taped seams and is detailed at the edges as an air barrier, then it doesn't matter what you do inside. Netting with blown fiberglass or cellulose would be fine, as would open cell foam, but the FG or cellulose would be cheaper and more environmentally friendly.
Note that your stackup only gives you about R35. Current code for Zone 5 is R-49. It is always best to aim for code minimums, even on retrofit work, if possible. You could get there by adding a couple of inches of EPS or GPS to the interior. GPS uses less space but costs more.
Hi Ken,
An unvented insulated roofline (or cathedral ceiling) always needs a layer of air impermeable insulation, which you have in the form of exterior rigid foam on the sheathing. Therefore, your rafter bays could be filled with blown-in cellulose, which, environmentally speaking, is better than spray foam or fiberglass. In any case, excellent air sealing is key. You might want to read this article by Martin Holladay: Blown Insulation: Fiberglass vs. Cellulose.