Exterior insulation/double stud wall/canadian building code
Hi everyone,
Building a home in Canadian Maritimes (climate zone 6). It seems most homes are now being built with R5 exterior insulation. We are attempting to build a “pretty good house” and trying to follow the design principles outlined in that book. I’ve read about concerns around condensation on the inside of the exterior insulation but given that an R20 wall with R5 of exterior insulation seems to work fine in our region is there any reason we can’t do a double stud wall with R22 +R12 batts with an inch of Vapor permeable exterior insulation like iko ener-air? (>1 perm). Appreciate the feedback.
-siding
-rigid insul
-air barrier
-plywood sheathing
-2×6 wall
-insulation
-6mm Vb
-2×4 service cavity
-1/2 inch gypsum
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Replies
Using R22 + R12 batts with an inch of vapor-permeable exterior insulation like IKO Ener-Air should work well in your climate zone. The key is ensuring you have a good air barrier and that the assembly can dry out if any moisture gets in.
Move the "rigid insul" from the exterior to the interior side of the
2x6 wall (make it foil faced polyiso"), skip the 6MM VB (you
don't need it because you now have the foil faced polyiso
in between the two walls and it's is a truly excellent VB all
by itself). In addition polyiso has a somewhat higher R rating
than most other rigid insulation products and the reflective foil
face will increase the R value of the 2x4 cavity wall by about 1.5 to 2.
Because the Rigid Insul lies between the two walls it's protected
from physical damage and you have eliminated the problems and
fiddley details associated with attaching siding over Rigid Insul. Running
a few calcs on the temps that will occur on those foil faces (at just about
any imaginable temperature and humidity conditions) will convince
you (or your engineer) that you will not have condensation problems
with this wall config.
Have Fun!
WinterViper,
Exterior insulation/double stud walls are really an either or choice for a few of reasons.
- Using exterior insulation adds a layer of complexity to walls that only makes sense if it is necessary to boost the R-value of the cavity insulation, or protect the sheathing from moisture concerns. Neither of these apply to double-stud walls.
- The main attribute of double-stud walls is that you can adjust the amount of R-value to whatever you want by adjusting the width of the assembly without using more lumber. If you want additional R-value it makes more sense to do this than use exterior insulation.
- Double-stud walls already have a thermal break, so exterior insulation doesn't help much in that regard.
Unless you have some compelling reason to use the continuous exterior insulation I haven't thought of, I would omit it - or as gtan suggested relocate it to the interior face of the inner-wall.