Spray foam insulation in low-slope shed dormer roof
I would like to insulate a shed dormer roof for a second story bathroom. (Located in Oregon, Zone 4C). It is a low slope, unvented roof assembly, 2×6 construction. The roofing material is 3-ply roof system (self-adhering SBS modified bitumen) on 5/8″ OSB. The size of the roof is approximately 10’x7′.
Due to the limited cavity space created by the 2×6 rafters and blocking, my plan is to use spray foam insulation directly on to the underside of the roof sheathing. The rest of the roof on the house is vented. I plan to make this section of the roof over the bathroom airtight and self contained with foam.
My question is: I believe to achieve R21 it would be approximately 3 1/2″ of foam. This leaves about 2 inches of space above the green board ceiling. Should I leave this gap? I have read it is inadvisable to put any type of a vapor barrier on the green board ceiling. Is this correct? Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Michco,
Most building codes require insulated ceilings in Climate Zone 4 to have a minimum R-value of R-49. So you want more than 3.5 inches of insulation.
If you are using closed-cell spray foam, you should be able to install 5 inches of spray foam in your 2x6 rafters -- meaning that you'll end up with an R-value of something between R-30 and R-32.5 . That is still shy of what you want, but better than R-21.
If you end up with a small air space between your greenboard ceiling and the spray foam insulation, the air space will do no harm. You don't need to install a polyethylene vapor barrier -- closed-cell spray foam is already a vapor barrier.
Thanks for your response. You have answered my concerns succinctly.