Rigid Foam
Steven Miller| Posted inGeneral Questionson
I’m need to insulate my second floor cathedral garage ceiling and live in upstate NY. It has 2×10 joists and I want to get the most bang for my buck. I’ve looked at closed cell spray foam and its just not in the budget. So my plan is to use 1 inch rigid foam with a 1 inch gap to create my baffles, then use Roxul R30 that should fill the 9.25 cavity perfect to give me a estimated R rating of about 35 and then cover it with OSB. My question is do I need a vapor barrier in between the studs and the interior OSB? Could I put another layer of 1 inch rigid foam on before the OSB to give myself about a R40 plus I think it would help with thermal bridging?
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Replies
Steven,
Q. "My question is: do I need a vapor barrier in between the studs and the interior OSB?"
A. I'm guessing that you meant to write "rafters," not "studs." The answer is: No, you don't need a vapor barrier there. Building codes do not require, and building scientists do not recommend, an interior vapor barrier in this location. Instead, the code requires a vapor retarder -- a less stringent layer than a vapor barrier. In your case, the OSB is a perfectly adequate vapor retarder.
也就是说,要有一个室内的空气barrier, and the OSB won't work for that purpose unless the seams are carefully taped or you do an extremely careful caulk job when you install the OSB. You need to verify the airtightness of this OSB layer with a blower door to avoid problems. Drywall probably makes more sense, because drywall usually gets taped (creating a better air barrier). You could also finish the interior with two layers (for example, taped drywall followed by OSB).
For more information on these issues, see these articles:
Do I Need a Vapor Retarder?
Vapor Retarders and Vapor Barriers
Q. "Could I put another layer of 1 inch rigid foam on before the OSB to give myself about a R-40? (Plus I think it would help with thermal bridging?)
A. Yes. That technique is described in this article:How to Build an Insulated Cathedral Ceiling.
-- Martin Holladay
OSB is already a "smart" vapor retarder, that becomes substantially more vapor-open when the humidity is high, but is fairly vapor tight when it's dry. With soffit to ridge venting of the 1" gap you'll be just fine as long as you make it reasonably air-tight to the interior.
"upstate NY" covers quite a range of climate zones (4A-6A), but only at the coldest of locations at altitude in the Adirondacks would a true vapor barrier be considered prudent. Everywhere else the vapor retardency of OSB on the interior would be fine.