Can exterior foam be combined with interior spray foam?
CSolar
| Posted in Green Building Techniques on
If I have walls and/or an unvented cathedral ceiling that are insulated with closed-cell spray foam from the inside, can I also have rigid foam on the outside of the sheathing to limit bridging?
Just wondering if it’s somehow a bad idea to have the sheathing sandwiched between two layers of foam.
Thanks, Chris.
(This is for Ottawa, Canada, equivalent to zone 6A.)
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Replies
Chris,
It's best if the plywood or OSB can dry out in at least one direction. So if you are going to install rigid foam on the exterior side of the sheathing, use open-cell spray foam (which is vapor-permeable) on the interior, not closed-cell spray foam.
You can get away with up to 2" of closed cell foam on the interior and still retain some drying capacity. If you went with Icynene's water blown version (MD-R-200, NOT the MD-C-200) you could even go to 6"/R30 on the interior and still be over 0.6 perms. At 3"/R15 it has about the same vapor permeance as 1"/R6 of most other closed cell foams. If you go the MD-R-200 route, 3" would be the minimum necessary to reach a permeance low enough to limit wintertime moisture accumulation in the roof deck.
Open cell foam is FAR more vapor open, and would require more exterior foam to protect the roof deck from wintertime moisture accumulation- the ratio of exterior-R to cavity R would really matter. With closed cell foam on the underside the R-ratio doesn't matter nearly as much as the total vapor permeance of the spray foam. Too much foam (==lower permeance) and it becomes a moisture trap, but with too little (==higher permeance) the ratio of exterior to interior R comes into play.
OK, thanks. This is a bit of a minefield at times :)
Having just read today's feature on ComfortBoard IS, I'm wondering if a layer of that very permeable product could be used on the exterior of this unvented roof assembly; that is, applied over the sheathing with another layer of OSB/plywood on top as a base for the shingles. Would that be able to dry to the exterior, or would it be hampered by the shingles/underlayment/etc.?
Chris,
If you are installing asphalt shingles, and you want your roof assembly to dry to the exterior, you would have to include a vented air space between the insulation and the the underside of the top layer of roof sheathing (the plywood or OSB). It's been done -- it's not impossible. You just have to build it.