Hey all, thanks for the insight. I’ve gotten completely stuck while approaching insulation during our attic remodel. The house is a 1949 story and a half with six inch rafters, it’s in climate zone 5a.
There’s no solid sheathing under the shingles of the house, just 2×6’s laid diagonally with about a 3/4″ gap between them. From in the kneewall you can see the tar paper between the boards.
Because of the spacing between boards, I think sprayfoam is out of the question. Dense packed cellulose was the original option I looked at, but the insulation company told me there isn’t enough space to both insulate and use a baffle to allow for ventilation.
Furring out the studs would make the attic ceiling too low and unusable as a bedroom.
My current thought is to do something like this: https://youtu.be/44dPWgoxhXw
Use two layers of rigid foam, r-13 each with a gap for air ventilation from the soffit to the ridge vent. My main concern is that r-26 isn’t going to be enough to keep the dew point inside of the insulation.
Thoughts, recommendations? Thanks!
Replies
Like the guy in the video, you can layer more foam board inside (below) the rafters, and then cover with drywall you screw into the rafter bottoms.
Another way is to insulate over the roof decking, and re-roof.
no such thing as a free lunch.
I did exactly what the guy in the video did, with great success. Except! After reading about how polyiso insulating board's R value is drastically reduced as it gets colder I used XPS for the exterior 4 inches, then polyiso (for the higher R value on the warm side of the insulation) for the 2" layer over the rafters. Here's a link from Corning (obviously they're biased, but they reference the original Building Science Corporation test to bolster their case for XPS). http://www.owenscorning.com/NetworkShare/EIS/10019949-FOAMULAR-XPS-vs-Polyiso-Tech-Bulletin.pdf