Rigid board insulation for an unvented cathedral ceiling: fill the void?
Hi there, first post.
I’m building a tiny cabin, Pacific NW, right on the boarder between zone 4/5, 40miles
east of Portland.
I have vaulted ceiling/gable roof. No interior walls in yet.
I have enough rigid board insulation to fill the void (iso). The sheathing was sealed to the rafters with adhesive sealant when laid down. Blocking at the soffets was sealed with silicone. There is rolled mineral roofing over that, all edges sealed with lap cement. All this was done with the anticipation of filling the void with pink fiberglass rolled insulation, with drying to the interior.
I was given enough 2″ and 3″ iso board to fill the void between the rafters. Great.
My question: Do I fill the void completely with this foam,
can I stuff the 3″ board flush to the rafters, (leaving an unvented airgap of 2 1/2″ above the iso), then overlay everything with the 2″ iso for a higher r-value? (rafters insulated). I don’t have enough iso to do both.
There will be no penetrations in the ceiling sheetrock, no can-lites, no skylites, nada. There will be no air path from the walls. I would prefer not to vent this space, as this will just increase the risk for imperfections that the mice will eventually exploit (I hate mice.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part