Creating a major vapor issue with this wall system?
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I have a friend that is having an old farmhouse renovated. Half the building is old 2×4, half the building is new 2×6. They’ve already installed high efficiency windows and 1″ of XPS wrapped around the building. I was out the other day checking it out and he stated they’re going to be using closed-cell spray foam. I’ve been wondering to myself if this is potentially dangerous per this article that I’ve read:
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
Here is what he is doing:
Old walls – 3.5″ x R6 (closed cell SF) = 21 + 1″ XPS R5 = 26
New walls – 5.5″ x R6 (closed cell SF) = 33 + 1″ XPS R5 = 38
We’re in a Zone 7 climate.
He has the exterior wrapped in only 1″ of XPS… and will be using closed-cell spray foam not allowing the wall to dry to the interior. He is going to plaster walls on the interior.
Is this creating a monster or am I worried for him for no reason?
Thanks,
Mike
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Replies
Michael,
Your friend has chosen an oddball wall system -- I wouldn't have chosen it -- but I don't think there are any grounds for moisture concerns. Of course, it's important to verify that the wall sheathing is dry on the day that the spray foam insulation is installed. If the sheathing starts out dry, and the exterior WRB and flashing details aren't flawed, I imagine that everything will stay dry.
It's worth pointing out that 1 inch XPS has a permeance between 0.5 and 1.2 perm. That's not much, but it will allow a little bit of (very slow) outward drying.
If your friend wants, he could switch from closed-cell spray foam to open-cell spray foam. The open-cell spray foam would allow for inward drying.
Hi Martin, thank you very much! He would be relieved to hear that.
I thought it would be a better wall system with open cell foam, the only issue is that with only 1" of XPS and 2x4 or 2x6 walls, the final results with R3.5 per inch in open cell would be a not-terribly well-insulated wall. Any thoughts there?
Thanks,
Mike
The thermal bridging of the framing severely undercuts the performance of the closed cell vs. open cell cavity fill. The ~25% framing fraction is still only about R4 for the 2x4 wall, or R6.5 for the 2x6 wall, and with closed cell in the cavity the majority of the heat transfer is through the framing.
We're talking about an R2 difference between open vs. closed cell with the 2x6 wall, and about a R1 difference with the 2x4 wall, not more, despite a HUGE difference in the center-cavity R.
In a zone 7 climate it will need an interior side vapor retarder when using open cell foam. The cheapest way to go would be to use "vapor barrier latex" as the primer on the wallboard, reducing the vapor permeance to about 0.5 perms. But a more resilient way to do it would be with 2-mil nylon sheeting (eg. Certainteed MemBrain), which has variable permeance. With the 2-mil nylon the sheathing can dry about 5-10x as fast as through half-perm paint if it needs to, but if the sheathing is cold and absorbing moisture the air next to the nylon is dry, and the vapor permeance is about half-perm. When spring temperatures arrive and drive moisture out of the wood, the vapor permeance of the nylon rises to higher than standard latex paint. Latex paint runs about 3-5 perms, so the moisture gets out a LOT faster than it goes in. Material cost is 12-15 cents per square foot, which is cheap stuff compared to an inch of closed cell foam.
With most siding types, and half-inch gypsum a 2x6 16" o.c. wall (~25% framing fraction) with open cell comes in at about over R15.25 "whole-wall". Add the R5 of exterior foam and it's R20.25.
The same wall with 5.25" of R6/inch closed cell + R5 continuous comes in at about R22.5.
That's a ridiculously expensive R2.25, and in 19 houses out of 20 the cost difference is better spent elsewhere.
If there's a moral to this story it is this:
Spend the high R/inch foam budget on the exterior foam, where it's performance isn't being robbed by thermal bridging.
Hi Dana, thank you for the detailed explanation. To make sure I understand correct, I just want to confirm that you're suggesting that he forego the closed-cell and use open cell foam with MemBrain. Then, take the cost savings and put up additional rigid foam on the outside? If that is correct, how much additional foam would you suggest?
thanks
Mike
Michael,
I agree with Dana that it always makes more sense to install thick rigid foam on the exterior side of wall sheathing than it does to install any type of spray foam between studs.
If your friend could start from scratch, a good wall would include thick rigid foam on the exterior, and dense-packed cellulose between the studs.
Your friend's error -- and the ramifications of your friend's error -- are discussed in my article, How to Design a Wall.