Question for Martin Holladay
Back on Oct 13, 2015 this guy asked a question about venting his attic/roof with rigid foam on the outside of the sheathing.
He decided to go with a vented roof with the insulation on the interior (between the floor joists) instead of attaching it to the exterior.
In comment #10 you suggested:
“If you are planning a vented roof assembly with cellulose insulation between the rafters, you can improve the R-value of the assembly (and reduce thermal bridging through the rafters) by installing a continuous layer of rigid foam on the interior side of the rafters“.
I was wondering what exactly you meant by that. Did you mean to say the joists rather than the rafters? In a traditionally vented attic space with insulation piled between the joists wouldn’t applying rigid insulation to the interior (underside) of the rafters be the wrong location?
I ask because I am building a small house with a traditionally vented attic (eave and ridge vents) and an airtight drywall ceiling between the attic and heated interior. Would it be a good idea to include the rigid foam on top of the joists or between the drywall ceiling and blown-in cellulose? Which is better?
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Replies
I'm not Martin but I think I know what he meant: By "vented roof assembly" he meant an assembly with insulation between the rafters, and a 2" or so vent channel between the insulation and the roof deck. So the roof assembly, up with the rafters, not a attic is what is vented.
In your vented attic, it's much easier. Once you are piling the insulation higher than the joists, you no longer have the joists creating thermal bridging. There's no need buy any foam, which is good news for your wallet and for the planet. Just put in plenty of blown in cellulose.