Exterior Wall Insulation
Hi, I am in the process of designing a home that will be one floor, with a mono slope roof and full basement in Zone 7 in Ontario, Canada. This will be a 4 season lake home and retirement home for myself and spouse and as such, we are looking at a design that will be very thermally efficient and to stand the test of time.
We plan on heating it with a hydronic radiant floor system with a concrete pour over and using a propane boiler (no NG in area and electricity is very expensive). We are planning on installing a solar hot water heat to offset some of the thermal load and to use the concrete floor as a heat sink for during the night.
We have a family friend who owns a spray foam company and they have offered to spray our house for only the cost of the materials and no labor. I like the idea of spray foam (still working to find one that has the least amount of environmental impact) as it provides structural rigidity, a vapor barrier and a high R value per square inch. Our walls will be 2×6 walls and we will look to do 5” of spray foam. We have a target of R40 for the walls. The windows haven’t been decided upon but we are looking at a triple glaze window from several manufacturers at the moment.
My question is, what is the best way then to insulate the exterior of the house and to minimize the heat loss through the wall studs? I was thinking of a rigid board insulation but am uncertain how best to design the exterior portion of it. We are using a cedar siding and some stone work for exterior cladding.
Any suggestions are appreciated.
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Replies
Focus on whole wall R value and the cost to achieve it. It includes heat loss through the wall studs, so no need to worry about this separately.
Also consider wall moisture performance.
Thanks Jon, appreciate the feedback. Given our family member that has a spray foam business and his offer, we are going to use them for insulating the interior of house.
What I’m uncertain of is what is the best way to get to R40 and, like you said, in a way that properly addresses moisture performance.
Thanks.
Since you’re getting an unusually good deal with spray foam, maybe try something unconventional: use pieces of 2x2 to support the exterior sheathing 1.5” off of the exterior sides of the studs. Use maybe 4-5 pieces along each stud, with each piece cut to maybe 2-3” long, so that there is mostly air gap between the exterior side of the stud and the sheathing. Use long screws to secure the sheathing through the 2x2 spacers and into the studs.
When spray foam is applied, that 1.5” gap will get filled along with the wall. If we assume R6/inch of spray foam, that gets you up to about R49 for the wall (5.5” in the 2x6 wall cavity plus 1.5” in the gap created by the 2x2s), if the wall is completely filled. If they install closed cell in the usual way with about 1-1.5 inches of empty space on the interior to avoid the need for trimming the foam, that gives you just over the R40 you’re looking for.
The gap created by the 2x2 blocking gives you about R9 worth of foam over most of the stud’s area to limit thermal bridging, similar to what exterior rigid foam would do. The limited area of the 2x2s would be all that is left penetrating the foam. This method would allow one pass of spray foam to complete all the insulation in the walls at once. The downside would be the labor to put up the blocking and reduced shear strength on the walls which might be something you’ll need to have an engineer check depending on the design of the house.
Bill
Or similar on the non-structural interior side. Or with foam strips along the studs on the interior side. Or maybe even staggered 2x4s on 2x8 plates.
I wouldn't be too fixed on exactly R40 walls - what counts is the carbon impact and cost of the entire house. I'd be very surprised if a heat pump didn't beat propane. BEopt should be useful.
The two options that have reasonable environmental impact are:
HFO blown closed cell foam. (second best)
Water blown open-cell foam. (Best for low environmental impact, but it's "moisture open" so the design with it takes some care.)
What you want to avoid is HFC-blown closed cell foam, which has severe climate impact.
HFO foam materials more more expensive than HFC foam, but not by much and the installation cost will be less, which your friend should appreciate even if you don't see the benefit directly.
+1 for preference given to water or HFO blown spray foams.
To reach R40, you either need lots of continuous exterior insulation or a double stud wall with a combination of closed cell and open cell foams. For walls in zone 7, at least 45% of your R value should be closed cell in the cavities or continuous exterior insulation (R18). The remainder can be open cell or dense packed cellulose or fiberglass, etc. More closed cell or exterior continuous insulation is safer from a moisture perspective, but costs more and will have higher environmental impact.
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies