Trapping moisture in the wall
Hello all, I stay here on the east coast of Canada in a moist, cool climate and am looking into rigid insulation to boost the overall r-value of the wall and to deal with losses over thermal bridging. This is for a new-build.
The standard wall construction for the last while for here is gyprock, 6 mil poly, 2×6 frame with fiberglass insulation, osb or plywood, homewrap and then vinyl siding. This combination works very well but there’s lots of room for improvement, espically on the insulation front.
I’m leaning toward using closed-cell spay-in insulation in the wall cavity, then osb, rigid insulation, housewrap and vinyl siding.
My big question is whether I would be creating a vapor barrier on the inside and outside of the wall by installing the sprayed in closed cell insulation on the inside of the sheathing and then installing rigid on the outside? I’m afraid this will trap moisture as the frame of the house dries out. Anybody have any insight into this topic or experience any issue before?
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If you limited the closed cell foam to 1" it's permeance (typically ~1.2 US perms) would fall just outside the Canadian code definition of "vapour barrier" but would be more than sufficient for protecting the sheathing against interior moisture drives in winter (no 6 mil poly needed or wanted) and will still have some drying capacity toward the interior.
In a Canadian Maritimes climate if at least 1/3 of the total R is exterior to the sheathing, the sheathing is protected from interior moisture drives by having a sufficiently higher temperature (averaging above the dew point of the interior conditioned space air), even without the closed cell foam. Cheap insurance with that stackup would be to use a smart vapor retarder such as Intello Plus or 2-mil nylon (Certainteed MemBrain), which would meet Canadian code for a vapor barrier when the air in the wall cavity is dry (under 40% relative humidity at the interior side), but becomes vapor open if the humidity in the cavity reaches mold potential levels. With a smart vapor retarder you can even cheat the 1/3 -R rule by a bit.
If you used R23 Roxul in the cavities it would take 3" of EPS on the exterior to be protective on it's own. If that's too fat a wall for you, going with 2" rigid rock wool would be lower performance, but would preserve drying capacity toward the exterior, and you can still skip the interior side vapor retarders- it would have maximal drying capacity in both directions. But rigid rock wool is more expensive than rigid foam (at least in my neighborhood.)
Daniel,
Here is a link to a climate zone map for Canada. If the map is to be believed, you are in Climate Zone 6.
Here is a link to an article that explains the rules for installing exterior rigid foam insulation: Calculating the Minimum Thickness of Rigid Foam Sheathing.
If you are building a 2x6 wall in Climate Zone 6, the minimum R-value of your exterior rigid foam layer is R-11.25. As Dana explained, this can be achieved with 3 inches of EPS or polyiso. (Green builders try to avoid using XPS, because XPS is manufactured with a blowing agent that has a high global warming potential.)
I think that installing rigid foam on the exterior side of your wall sheathing is a great approach to improving the thermal performance of your wall. I usually advise any builders who take this approach that they shouldn't use any spray foam insulation between the studs. Instead, use cellulose, mineral wool, or carefully installed fiberglass, all of which are vapor-permeable products that allow wall sheathing to dry inward.
If you have your heart set on using spray foam between the studs, use open-cell spray foam (which is vapor-permeable), not closed-cell spray foam (which is fairly impermeable unless you limit its thickness to 1 inch or less).
Hello Martin and Dana. Thanks for all the advice. I should be able to find a balance between high r-value walls and keeping the moisture out.
Roxul's comfort board looks like it will do the trick nicely. It is expensive but will pay for itself eventually. Their brochure claims that's it has a high permeable rating as well. That should work OK with the closed cell spray in and let the sheathing breath.
Thanks
Daniel