Using closed-cell foam in a double stud wall
Joe St Denis
我们住在7-8区艾伯塔省中部。我们想建造一个带有步入地下室的1 1/2故事的房子。西部，北部，北部和南部地下室墙壁开放的湖边房屋。
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.
您的想法将不胜感激。
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此外，它将解决任何空气密封问题，不需要空气屏障。唯一的额外密封问题是门，窗户和公用事业。
Joe,
Your suggested approach would work. It has two disadvantages:
1.这很昂贵。建筑商选择建造双块墙的主要原因是，他们可以使用廉价且环保的绝缘材料（纤维素）。
2.闭孔喷雾泡沫使用具有较高全球变暖潜力的吹风剂；这就是为什么绿色建筑商试图最大程度地减少其使用的原因。
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.