Using closed-cell foam in a double stud wall
Joe St Denis
We live in north Central Alberta, zone 7-8. We want to build a 1 1/2 story house with a walk-in basement. Hillside home on a lake lot with west, north, and part of south basement wall open.
I have spent hours reading on how to best insulate walls and ceilings (which I find can be and has been challenging). After reading many articles I was planning to build a double stud wall using 1/2 inch plywood on the inside stud walls which we could cover later with wood or thin gyprock. Use scissor trusts for a cathedral ceiling, also lined on the inside with plywood.
Then to solve many problems insulate both from the outside with closed-cell foam to within one inch of the surface to allow for venting cover with a rainscreen and install siding and metal roofing.
Your thoughts would be appreciated.
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Also it would solve any air sealing problems,not require an air barrier. The only additional sealing concerns would be doors, windows and utilities.
Joe,
Your suggested approach would work. It has two disadvantages:
1. It is expensive. The main reason that builders choose to build double-stud walls is so that they can use an inexpensive and environmentally friendly insulation (cellulose).
2. Closed-cell spray foam uses a blowing agent with a high global warming potential; that's why green builders try to minimize its use.
A house with double-stud walls and scissors trusses can have wall and ceiling cavities that are as thick as you want, easily accommodating almost any thickness of cellulose insulation.
Putting half-inch OSB, detailed as an air barrier on the exterior side of the inner studs, sheathing the exterior with half-inch CDX and doing it all of the insulation with cellulose or blown fiberglass makes for a much cheaper, greener and more resilient assembly. The OSB would always be above the dew point of the conditioned space air, and when dry has a vapor permeance nearly as low as code demands for interior vapor barriers in Canada. When it's moisture content rises, so does it's permeance, but in a fairly linear fashion. CDX has a slightly higher vapor permeance than OSB, but becomes quite a bit more vapor open than OSB when wet, so as long as the siding is rainscreened it will reliably pass through any moisture that gets by the OSB.
This is a fairly common stackup for PassiveHouse & PassivHaus designs that makes smarter use of the lower vapor permenace of OSB relative to plywood. Both are variable-permeance "smart" vapor retarders, but OSB is more vapor tight than plywood, making it a better choice as the interior vapor retarder for a plywood-sheathed assembly.
Putting the OSB on the interior side of the interior studs just under the gypsum would work too, but it would have many more electrical & plumbing penetrations to air seal. Whether it's worth the extra trouble of putting it on the exterior side of the interior studs to gain the long-term and more reliable air-tightness is up to you.
Cheaper than OSB would be to install a membrane type smart vapor retarder (Intello, MemBrain, etc.) directly under the gypsum, or on the exterior side of the interior studs (if you can figure out a good way to install it there in an air-tight fashion.)
Thanks for the advise. I will re consider and look at other options.