Zone 6 exterior wall insulation retrofit
Hello,
I recently purchased a small (1400 SF), 1.5 story home in Nova Scotia (I believe this is Zone 6). It was built late-40s and the previous owners had purchased it shortly after it was built. They were the type to keep things well-maintained, but not to splurge on any sort of modernization. It’s functional for the time being, but I will be spending the next two years getting the house up to modern standards (I’d kill for more than two grounded outlets in the whole place). Time and money constraints mean I will be doing this one room at a time.
As I do each room one priority will be putting insulation in the exterior walls. I have not yet opened a wall, but based on how most homes around here are constructed, the current wall assembly is:
Interior finish (varies) -> plaster and lathe -> 2×6 framing with no insulation -> 3/4″ tongue-and-groove -> tar paper -> cedar shingles (painted)
The exterior shingles are in good shape so I’m in no hurry to tear them off to do an exsulation. I am therefore considering the following wall assembly done from the inside:
Interior finish (varies) -> air-sealed drywall (concrete board in bathrooms) -> foil-faced polyiso -> 2×6 framing with 5.5″ of rock wool -> air barrier sealed to framing in each cavity -> t&g -> tp -> shingles
I’m not yet sure if the polyiso will be 1″ or if I can afford to lose 2″ everywhere.
Does this make sense? Is there a better approach? Does that work to have an individual air barrier applied to the back of the sheathing in every cavity?
The first room will be taking some of the kitchen to create a second bathroom (house currently only has a bath), so I won’t have to worry about window detailing yet.
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Replies
Hi Ian,
Your proposed wall will work. The rigid foam may be easier to detail as the air barrier than the drywall.
Keep in mind that you will be creating a wall that needs to be able to dry to the exterior. It would be ideal if your wall had a ventilated rainscreen behind the siding, but maybe down the road, you'll reside the house. Keep in mind that when you do, you won't be able to add exterior rigid foam, unless you used something vapor open like mineral wool. Here's an article that you may find helpful: Walls with Interior Rigid Foam.
Another option would be to deepen the stud bays with rigid foam furring. If you went this route, you could add exterior rigid foam later. Here an article on how to do this: Breaking the Thermal Bridge.
Thanks for the feedback, and interesting idea about the rigid foam furring. You're correct that I currently don't have any sort of vented rainscreen behind the shingles.
If I went the route of the furring, I would not be using any sort of vapour barrier, but I'd want to still consider an interior air barrier (vapour permeable), yes? Do you have any products you like for that?
You could also strap the interior walls horizontally. This will mostly break the thermal bridge and give you some added depth for insulation. The fact your current wall is 2x6 is a stroke of luck, never see that in the US in older homes. Airtight drywall or one of the variable perm membranes will let your wall dry to the interior as well. Foam on the interior of the studs is hard to work with, I would avoid it.
So I was thinking more about the idea of future exterior foam. If I leave that option open one of the problems might be getting enough r-value on the exterior. If I do the strapping or furring methods I can get 7.25" of rockwool in the walls, which gets me R28. If I wanted to do exterior foam in the future, my understanding is that 2/3rds of the insulation value needs to be in that foam. This would mean a need for R56 in the foam?
Thanks for the suggestion on the variable perm membranes. I was just looking and we apparently now have MemBrain available in Canada, though it looks like it can be a little hard to find. In terms of air-tight construction, my concern is in regards to penetrations from user error down the line. The first room is the bathroom and there is no need to mount anything to that exterior wall, but later down the line I'll be doing other rooms and I could see things like wall anchors accidentally penetrating the air barrier. I guess one option is to install furring to give an air gap on the interior of the air barrier, but then I'm losing more floor area in an already small house.