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?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part