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Community and Q&A

Insulating a Flat Unvented Garage Roof

alex_coe | Posted in General Questions on

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:
5/8 or 3/4 Ply
Rigid foam
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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  1. Jon_R | | #1

    Unless it's low GWP XPS, I'd call around more - you should be able to find EPS or polyiso.

    If not, this may be an odd case where less insulation is better.

    1. alex_coe | | #2

      There are 2 building suppliers in the area. 10k population. It is what it is. Polyiso takes a performance hit in cold climates and costs more, so I think that's why there's no supply or demand for it here. No Zip sys, Advantech, or many other things here unfortunately.

  2. brian_wiley | | #3

    I definitely feel you on the scarcity of polyiso. I'm in Boise (pop. 275,000) and I have trouble getting polyiso, to the point where I've had to talk to managers or other sales people just to get them to understand specifically what I'm asking for.

    That said, EPS seems to be far more common. While it may not be something they stock, I have always been able to order EPS through my local masonry yard. That might be an option, and it was a nice one in that I could order varying thicknesses in specific dimensions. The cost was only $0.32 sq ft at 1-inch thick, which may make any delivery fees or trouble worth it.

  3. Expert Member
    Akos | | #4

    If you are not heating the place, there is not much point for going crazy on the insulation. Even something as little as R15 exterior insulation can really take the edge off in the summer and help keep the place warmer in the winter. Usually the bigger issue in most garages is lack of insulation and air leaks through the garage doors, which are pretty expensive to fix.

    For insulation, I would look at any commercial roofing supplier. You should be able to get roofing polyiso for less than half the cost above.

    After you have the rigid above the roof, you can always adds batts and finish the ceiling if you want to use the space down the road. In zone 5, for condensation control of the roof, you need to have 40% of the total R value above the deck, so for R15 rigid, you can put up to R22 of fluffy insulation between the rafters.

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