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Old 11-21-2011, 07:14 PM   #1
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Insulating options for flat roof

Hi all, I'm converting my detached 12' x 22' garage into a shop space and need to insulate it (from the inside). It has a flat roof made up of 2x10 joists. There's about a 1' overhang on one side, but the other is right on the property line so there isn't any overhang on that side, nor do i have access to it.

Insulating the walls with batting and then vapor barrier over top is pretty straight forward, but what are my options for the roof? I'm concerned about it needing to vent properly. The climate where I am ranges pretty wildly.

Can I just get the cavities between the joists spray foamed and then tie it in to the vapor barrier at the walls with tuck tape?

What about using fibreglass batts? Can I get away with just installing soffits on the one side and maybe drilling holes through the joists at the other end for cross ventilation? Or would I need to add vents on the roof somehow between each joist?

Looking forward to some good advice


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Old 11-22-2011, 08:22 AM   #2
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Spray foam is long accepted as a suitable insulation for hot/insulated roof decks.

Fiberglass just won't get it done at that depth or application.

Put up your vapor barrier and have your spray foam contractor tie right into it with foam.


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Old 12-05-2011, 03:57 PM   #3
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Thanks Windows on Wash. Had an installer take a look. Ended up going with 1/2 lb in between the joists because I guess the code folks in my area like seeing that on ceilings because it lets water through in case of leaks, rather than rotting out the wood. Then, I'll have to put vapor barrier over top...
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