Exterior insulation on climate zone 6/7
Hello,
I’m planning to reside a house built circa 1960s in northern Michigan climate zone 6/7 (boarder line).
I’d like to add continuous exterior insulation.
Has 2×4 walls with:
Outside 1/2-inch plywood-tar paper-wood siding.
Inside foil face fiberglass insulation-drywall-paneling, and wallpaper over all of that.
There is also an addition with clear polyethylene behind the drywall.
Based on the that climate zone I’m in. I would need a minimum R10 exterior insulation to prevent any condensation problems in the walls right?
Would 2 inch XPS foam over a tyvek drain wrap or tyvek and home slicker 1/4 thick work for some place for the moisture to go, or would I need to go with something like 2.5 inches of Rockwool that is vapor open.
There is no air conditioning in the house although someday there may be.
Thanks, Dan
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Replies
My biggest concern would be getting a double vapor barrier if you use XPS on the exterior. You have poly inside some areas, foil-faced fiberglass in some areas (the foil facer, if not perforated, is probably a vapor barrier), and what I'd assume is vinyl wallpaper -- yet another vapor barrier. That's a lot of interior-side vapor barrier materials. Adding an exterior vapor barrier would eliminate any drying path for the sheathing.
All that said, our friends up North in Canada commonly build with interior polyethylene and don't have problems, and you're in a similar climate zone to much of lower Canada. Adding exterior rigid foam is going to increase the temperature of that sheathing, which should mean LESS issue with moisture. I think you'll probably be OK, but rock wool would be safer, since it's very vapor open. The downside to rockwool is that it's the most expensive rigid insulating material commonly used.
Bill
Agree with Bill (vapor open would be best) - until TimberHP (US based) comes out with their rigid board product, you could also look at this product (made in Germany):
https://475.supply/products/gutex-multitherm
Thanks Bill and begreener,
I'm still a bit perplexed. After reading the articles from Martin Holladay "Calculating the Minimum Thickness of Rigid Foam Sheathing" and "Rethinking the Rules on Minimum Foam Thickness"
and "Mind the Gap, Eh!" from Joseph Lstiburek https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh.
In the "Mind the Gap article he mentions a scenario where a wall cavity could have high density closed cell foam in it and foam board on the outside. His solution to this is to have a gap between the plywood sheathing and the foam, using crinkle warp or a bumpy WRB.
I don't plan non redonning the wall cavities for another 10-15 years or so but,
In my wall with the foil face insulation, clear polyethylene and wallpaper would that be equal to the same problem as closed cell foam?
I intend to us either LP smart siding with furring strip or Vinyl
If I was to use Vinyl, I could tape the seams of the 2-inch foam and us long enough nail. No furring strips (I know it's not permitted by the manufacture, but it has been done by others and have not had issues)
Thank you Dan
The rules for foam assume non-permeable rigid foam, which means no outward drying. If you use rigid mineral wool on the exterior, you have lots of outward drying, since mineral wool is very much more vapor open than any of the rigid foam materials are. This means you don't have the same restrictions as you have for the exterior rigid foam mentioned in those articles. The same would go for a double studwall or any other assembly that differs significantly from the assemblies described in those articles.
I don't see crinkle wrap being a problem if you want some additional drying ability here. Crinkle wrap allows for a very small amount of ventillation, so a little bit of drying. It wouldn't hurt you here, and might help.
Your interior wall assembly should be pretty vapor closed, with the layers of foil and wallpaper (which is likely moisture impermeable vinyl) restricting drying towards the interior. While your exterior rigid foam is going to restrict outward drying potential for the assembly as a whole, it will also keep the outdoor sheathing warmer, which will help limit moisture migration into the wall. Moisture drive in heating dominated climates is from the interior towards the outdoors, and I think in your case, the additional rigid foam is likely to help you (by keeping the assembly warmer) than it is to hurt you (by reducing exterior drying). If you go with rigid mineral wool though, you get the "keep it warmer" advantage without the "reducing exterior drying" disadvantage. The tradeoff is the much higher cost for the rigid mineral wool compared to rigid foam.
Bill
Thank you Bill