Hi, Worthy,

I don't envy you guys in the frozen north. Your assemblies are much less forgiving. Assuming relocating to a more southern climate isn't an option for you, I wonder if you've considered using a liquid applied air barrier in lieu of the spray foam. You should get the same benefits of air sealing around penetrations, brick ties, etc. but without any insulating value.

Haven't done it yet but only a matter of time, even in a climate much warmer than yours. Even hot climates benefit from exterior insulation which considerably reduces thermal bridging.

With portable spray rigs becoming more affordable and a set of 55 gallon spray foam under 2K, it's an increasingly competitive market and attractive solution. The last 5 homes we have used spray foam under the slab, right on top of the gravel and love it compared to doing traditional way of piecemeal foam sheets. It's a superior way to insulate and takes the place of liquid applied weather barrier. Carpenters usually install foam sheets which can get expensive when you include time, tape and fasteners. This is tougher to account for than easy cost of spray foam per foot from spray foam contractor.

I know you follow Dr. Joe's writings on the subject. What is the plan for interior vapor barrier because without it, you run the risk of moisture problems from exterior foam being too thin.

What is the R value of 1" of this stuff?

Looking at ccspf of R7.5, plus fg batts between the studs to meet Ontario Building Code (OBC) min. of at least R24.

Here's the architect's drawing for XPS on the exterior. (Instead of the polyethylene pillow on the rim joists, I will use ccspf. As well, if I use ccspf for the walls, the Blueskin can't be used as the heat of the spray destroys the adhesion, says the spray contractor.)

Ratio of exterior to interior insulation.

I've built several homes in our Climate Zone 5 with 1" XPS on the exterior, unfaced fg batts on the interior. I lived in one for six years and kept up with that buyer for a few years. No visible problems.

Here's Dr. Joe: "I grew up with this wall in Ontario - 2x6 wall with R-5 foam sheathing (Photograph 2) coupled with unfaced fiberglass batt cavity insulation covered with an interior 6 mil poly air/vapor barrier (Photograph 3). This wall is currently being built all over Minnesota and Wisconsin. It is still being built in Ontario. And it works." *

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spray foam under the slab

I hadn't heard of that before the spf contractor made that point too. Thanks for the confirmation! I will decide when closer based on price.

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a liquid applied air barrier in lieu of the spray foam

As you say, no insulation value to that. What I typically see here on high-end infills is simply housewrap over sheathing.

relocating to a more southern climate

Actually, before the upper end housing market collapsed here, we were all set to move south--to New Hampshire. We're not fans of heat. Or alligators!

"With portable spray rigs becoming more affordable and a set of 55 gallon spray foam under 2K, it's an increasingly competitive market and attractive solution. "

We are most likely doing a flat roof and spray foam above the deck protects from hail as well as seals penetrations etc.

I actually considered buying a machine and doing the whole house with spray foam and then selling the machine once the house is built. if the prices come down some more I might still do it.