How to insulate my attic — getting conflicting advice and going crazy!
Climate zone 4a, 1920s house with little insulation, so bills are high, drafts are strong, and humidity is low in winter. I’d prefer a conditioned attic as there is HVAC up there and we might finish it someday. I’ve gotten quotes for open cell and closed cell on the attic roof deck; each guy swears the other guy’s product will ruin my roof. Neither are quoting enough insulation to reach the recommended R value in my area. Batt insulation lady says she can’t insulate the roof deck because there’s no ridge vent and recommends making the envelope at the attic floor and piling insulation over the the ducts up there. It’s about time for a new roof, so I was wondering about rigid foam on top of the roof deck, but a local contractor acted like that was the stupidest thing he’d ever heard. It’s been so hard to figure what’s right— and we’re talking substantial $$ any which way— I’ve just been paralyzed. Any help appreciated!
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Replies
Rainyseason,
The best option is to install R-49 of rigid foam above the roof sheathing. Often you can use reclaimed foam to make the project more affordable (and greener). See this article for more information: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing
R49 above the roof deck is overkill and can be awkward to install. When going with rigid foam above the roof deck you really only need ~R35-R36 to meet code performance on a U-factor basis. Continuous insulation not thermally bridged by the more thermally conductive rafters delivers the same heat transfer performance to the assembly at the lower center-cavity R.
In zone 4A as long as more than 30% of the total R is above the roof deck it's fine to install open cell foam or fiber insulation snugged up to the roof deck on the underside. Assuming full-dimension 2x6 (more common than milled 2x6 in the 1920s) there is enough room for about R22 of half-pound density open cell foam or mid-denstiy fiberglass batt, which means with as little as R9.5 of rigid foam above the roof deck the roof deck would be fairly well protected from interior moisture drives. With R15 above the roof deck (the IRC prescriptive) you could install up to ~R35 between the rafters if you have 2x10s.
If doing it under the roof deck 2" of closed cell foam is enough to protect the roof deck without eliminating it's ability to dry toward the interior completely, but you still need to pay attention to the R-ratios (30% minimum for the foam layer.)
Rainy Season,
I agree with Dana's advice. Here are links to two relevant articles:
How to Install Rigid Foam On Top of Roof Sheathing
How to Build an Insulated Cathedral Ceiling
Thank you all so much! The roof is constructed of true 2x6s in most areas and 2x4s in a small dormer area. Now to find a roofer willing to do this!
Have you had a Blower Door Test? All this discussuon about insulation will not help your drafts; air sealing will. For the record, I like Dana's recommendation, but the drafts and low humidity have other solutions - seal the huge leaks in your house. A good BDTestor will show you where those drafts are coming from and suggest how to fix them.
Yes, we’ve had a blower door test and were advised to seal the rim joist and insulate the attic, and many other small sealing projects. That’s why I first investigated spray foam as I hoped it’d both air seal and insulate. I did a cut&cobble of rigid foam and great stuff on the rim joist, which has helped with drafts.