Does exterior foam sheathing create a second vapor barrier?
bryanw511
| Posted in Green Building Techniques on
What I never quite understood is that in cold climates (I’m in Wisconsin, climate zone 6) vapor barrier is required on the warm side of the wall (under the gyp). By adding exterior foam, aren’t you essentially creating a second vapor barrier which would not allow the wall to breathe or dry to one side? What’s the perm rating of 1″ XPS?
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I think you will find that a vapor barrier (vs vapor retarder) isn't required on the warm side. 1" of XPS is about 1 perm - EPS would be better.
See here for good wall designs that can dry to the interior.
https://codes.iccsafe.org/public/document/IBC2015/chapter-14-exterior-walls
Bryan, yes, and that's why 1. Vapor barriers are a bad idea on the interior (vapor retarders or variable-permeance membranes are good) and 2. Exterior foam should be thick enough to keep the condensing layer (usually the sheathing) above the dewpoint temperature.
In addition to Steve's link, I recommend reading this: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing.
You're correct, I made the same mistake many do, I confused barriers and retarders. Wisconsin has not adopted the IRC. We have our own code, the Wisconsin Uniform Dwelling Code. It requires a vapor retarder of 1 perm or less installed on the warm side of the wall. So, I guess my question is, how vapor permeable does any material on the cold side of the wall need to be in order for the wall to breathe or dry? I'm considering foam sheathing or possibly the zip system, but I'm unsure how vapor permeable they are.
Bryan, variable permeance membranes like Pro Clima Intello meet the ≤1.0 perm requirement, when the wall is dry anyway, which is when low permeance matters. Then they open up if the wall accumulates moisture, to allow drying to the interior. Basically the best of both worlds. Siga Majrex also meets the requirement, but in a slightly different way, and is not as open to inward drying as Intello. Certainteed Membrain also meets the requirement. I believe there is a contradictory element to the WI code but I don't recall exactly what it is. It would be nice if they joined the IRC with rest of the country.
I don't know of a specific exterior permeance requirement but have heard of a 5X rule--in a cold climate, the exterior sheathing should be at least 5X more vapor-open than the interior. That is roughly what you would get with Intello, at 0.13 perms (dry), and ZIP, at a bit under 1.0 perms (dry), but Zip (and other osb) opens up as moisture increases, to a maximum of 10 perms if I recall correctly. In a code-minimum wall, there is enough heat flow that Zip is safe. With thicker walls there is less heat energy available to push moisture through the wall, so a more vapor-open material like CDX is a bit safer, though with cellulose insulation it doesn't seem to matter, and with a low-perm interior membrane installed well you shouldn't have to worry about too much moisture getting into the walls.
With foam sheathing, the safest route is to make it thick enough, and detail it well enough, that no outward drying is necessary because the condensing surface is always above the dewpoint temperature.
I'd rather not go that extreme and add 4" of foam. I really just want to break the thermal bridging of the studs, not get an extreme R-value. I've seen the 5x rule as well. If we assume that, would this assembly be acceptable: gyp, polyethylene, 2x6 with batts, zip osb, 1" foam, fiber cement lap siding? My thinking is that the exterior is more permeable than the inside, therefore the wall has at least one direction to dry.
Don't use polyethylene. For less than recommended external foam, consider:
gypsum, air tight smart retarder, strips of polyiso along the studs, 2x6 with cellulose, taped plywood, Typar, furring, siding.
or
gypsum, air tight smart retarder, 2x6 with cellulose, taped plywood, Drainwrap, 1.5" EPS, furring, siding.
Which one is better is a good question (my guess is the second (due to warmer sheathing)).
You could also consider the second one with no plywood - just diagonal steel straps.
I wouldn't rely on the 5X ratio suggestion at low perms - there are other sources of wetting that you want to dry.
I agree with Jon on all counts. If you want to go with his first suggestion, search for the Bonfiglioli approach. I would suggest strips of Zip-R for this.
I like the Bonfiglioli approach. I also found these aerogel strips which really pique my interest as well (links below). They solve the problem of thermal bridging without the thickness of inches and inches of foam sheathing.
http://www.thermablok.com/thermal-insulation/residential-construction.htm
http://www.advancedinsolutions.com/products/proloft-®