Best Insulation Practices in Quebec, Zone 7
I am helping a friend to rebuild exterior walls in Quebec that currently have little to no insulation. We’ll be replacing rotten studs and replacing a bathroom window and want to know the best way to insulate the walls. We are currently thinking about closed cell spray foam, on the interior, and plywood sheathing, tyvek, rigid foam, a 1-inch air-space, then using board and batten fastened to horizontal furring strips (which ride on top of vertical furring strips to allow for air and moisture flow).
Is this overkill? Or would a simple interior spray foam install be sufficient, without adding a lot of complications such as window installation (on the now much thicker walls)? Are there issues with using closed cell spray foam in conjunction with exterior foam board? (been reading some controversies about this).
What would you do?
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Replies
Rob,
Q. "Are there issues with using closed cell spray foam in conjunction with exterior foam board?"
A. Yes. For a long-lasting wall assembly, it's best if the wall sheathing can dry in at least one direction. That means that if you decide to install rigid foam on the exterior of your wall, you should choose a vapor-permeable insulation (fiberglass, mineral wool, cellulose, or open-cell spray foam) to install between the studs.
For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
It really depends on the thickness & types of spray foam & foam board. A flash-inch of closed cell polyurethane foam isn't sufficiently tight to qualify as a vapor barrier under Canadian code, but it almost is. (In practical terms it's "as good as..." ). Unfaced Type -II EPS doesn't get there until it's more than 3" thick.
Closed cell foam thermally bridged by studs is a waste of good foam though. While the center-cavity R can be quite high, the thermal performance of the wall is limited by the heat conducted through the framing fraction. A full cavity fill of R6/inch closed cell foam in a 2x4 cavity adds only about R2 to the whole-wall performance over what R13 cellulose cavity fill or R15 rock wool would deliver.
Save the foam budget for fatter exterior foam- it'll buy a lot more performance per dollar, and it buys more moisture resilience too. If you put 4" of EPS on the exterior it typically costs less than $2 per square foot, and with a cellulose or batt cavity fill and the described rainscreen gap you'd be at about R26-27 whole-wall. With 3.5" of closed cell foam the foam would cost you over $3 per square foot, and you'd have a whole-wall R of about R12 plus the R value of the exterior foam. With the closed cell foam cavity fill it would take 3" of exterior EPS to hit comparable performance to the 4" EPS + cellulose cavity fill wall.
With R15 rock wool and 3" of exterior EPS (R12-12.5) you would have sufficient dew-point control at the sheathing to build it without interior vapor retarders, and it would still have some drying capacity toward the exterior. With R13 cavity fill you'd make it with only 2.5" (R10-ish), but more exterior-R is always better.
If you're going to cheat those numbers to limit the thickness of the wall, using a more vapor-open smart vapor retarder under the interior gypsum such as Certainteed MemBrain would not compromise drying toward the interior during the spring & summer, but would limit the moisture diffusion in winter to safe levels for the plywood sheathing.
Thank you Martin! One more question: Is taking the time and energy to create an airspace between sheathing/tyvek and the finished siding worth it?
Rob,
Q. "Is taking the time and energy to create an airspace between sheathing/rigid foam/Tyvek and the finished siding worth it?"
A. Usually, yes. For more information on this issue, see All About Rainscreens.
The air space is even more important with board & batten siding, which often gets in more rain penetration than clapboards or ship lap siding.
Thanks Dana and Martin.... for every answer two more questions...
1. Would horizontal furring strips, with adequate kerfs for airflow, be okay... or should I go to the trouble of a base-layer of vertical furring and overlay rows of horizontal over that? Basically will horizontal furring strips only defeat the purpose of the rainscreen?
2. If we went with the 4" of exterior foam, w/rainscreen, I am having trouble visualizing trim details at the top of the wall where it meets the soffit. Would I trim that with an extra thick trim board and run the foam up to that.... or run the foam all the way to the top of the sheathing and put trim board over top of it as well as the siding? Would this area require flashing? How would I create ventilation for the top of the rainscreen..... Any suggestions or visuals would be great.
Rob,
Unless the wall you are talking about is particularly exposed to the weather (due to stingy roof overhangs or unusual height), or unless the house is in a very exposed location (a hill overlooking the ocean), I don't think you need two layers of furring strips (vertical + horizontal). The horizontal furring strips will provide you most of the benefits of a rainscreen gap, and the siding should dry readily after a rain.
The trim at the top of your wall (the frieze board) can be in the same plane as your battens. Or, if you prefer, you can use 5/4 stock for your frieze (installed on top of the siding boards) and 3/4" (nominal 1") battens, so that the frieze is proud of the battens.
For information on venting the top of a rainscreen, see the article I linked to previously: All About Rainscreens. Remember, though, that if you're just installing horizontal furring strips (rather than vertical + horizontal), there is no real need to vent the top of the rainscreen gap.
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