Insulating a Flat Unvented Garage Roof
Hey GBA, I’m trying to decide how best to insulate this roof deck for the attached garage in my house build.
CZ5 16°F / -9°C Design temp
12 x 22′ attached garage
R22 wall insulation
Unvented flat roof to serve as second floor deck
The garage will be unheated most of the time. Occasionally I’ll turn the heat on in winter to use as a work shop.
The only thick rigid foam insulation my building supplier carries is XPS. Could I do a split insulation roof with some above then a layer of R14 rockwool in the joist cavity? R14 in the joist cavity plus R15 above would get me to about R26 effective for $3.04/sf… which would be 43% of the R value in the joist bay. Would that be fine? Would I need to add an interior vapor retarder or just use airtight drywall?
What would be a good cost effective target for a garage / occasional shop?
The roof deck is 264sf. These are my cost/options:
R15 3″ XPS: $2.28/sf = $602
R17.5 3.5″ XPS: $3.07/sf = $811
R22.5 4.5″ XPS: $3.50/sf = $924
R14 3.5″ Rockwool: $0.76/sf (doable in joist cavity?)
R22 5.5″ Rockwool: $1.20/sf (problematic ratio for joist cavity use?)
This is the roof assy I have in mind (top down):
Flexstone layered urethane deck coating: https://flexstones.ca/flexstone-uses/sundeck-coatings/
5/8 or 3/4 Ply
1/2″ ply or OSB w/ taped seams & polyurethane adhesive around perimeter.
Joists (empty or insulated?) (rim joists sealed to top plates)
Membrain SVR (only if insulating joists)
Drywall (only if insulating joists)
Would a Smart Vapor Retarder (Membrain) be the right choice for the walls?
Any insight to a solid cost effective solution here would be great. Thanks guys!
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