What is the best way to build a 2×8 exterior wall and achieve R-40?
I am building a house near Toronto, Ontario. Winters are cold, and we do see our share of -20 or colder weather. Summers have been somewhat hot in last couple of years.
My wall section looks like this right now from exterior to interior.
Stucco>1″rigid foam board (taped seems)>2×8 studs @ 16″OC>2″ closed cell spray foam> R24 Batt>6mm poly>drywall.
I am worried about any moisture getting in the wall so that is why I am considering doing 1″ rigid foam board from the outside and poly from the inside.
Can anyone tell me if they have done anything similar.
This will be my last house and I am not too worried about spending little bit extra to achieve R40 value.
Thank you.
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Replies
Slobo,
I'm glad that you are aiming for a high-R wall. A few comments:
1. It makes little sense from an economic or building science perspective to frame with 2x8 studs, for two reasons: (a) 2x8s are more expensive than two parallel 2x4 walls, and (b) 2x8 walls have lots of thermal bridging through the studs. So you might consider double-stud walls (two parallel 2x4 walls with a gap of 3 to 6 inches between the 2x4s) insulated with dense-packed cellulose insulation.
There are lots of articles on these topics to read on GBA. Here's a link to one that discusses 2x8 walls: Choosing the Right Wall Assembly.
2. Your plan to install 1 inch of rigid foam on the exterior of a 2x8 wall won't work, because the rigid foam layer isn't thick enough to prevent moisture accumulation (condensation) in the sheathing and wall assembly during the winter. You are in climate zone 6 (see map here), so a good option (if you really want to use exterior rigid foam) would be 2x6 walls with 3 inches of exterior rigid foam. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.
A wood sheathed 2x4 16" o.c. /R15 rock-wool wall with 2" of foil-faced polyiso outside the wood sheathing then 2" of EPS between the polyiso & stucco with no interior side poly would deliver a bit over R30 whole wall performance at about the same thickness as your 2x8 wall, and with much higher thermal performance and VASTLY higher moisture resilience. The coldest wood is the center-cavity sheathing, only about 40% of the way into the insulation from the warm interior, and isolated from the high moisture drives of the stucco.
A 2x6/R24 16" o.c. wall with 3" of exterior foam (instead of the 2x4 wall with 4" foam) comes in just shy of R30 whole-wall at a comparable wall thickness. It would still cut it without an interior vapor retarder, but with less margin. With this stackup the center sheathing is colder, about 40% of the R from the exterior rather than the interior. A smart vapor retarder such as Intello Plus or MemBrain would make it as resilient as the 2x4 wall (or more), and would be cheap insurance.
The 2x8 wall with 1" foam and interior polyethylene is effectively a moisture trap, since the wall can only dry into the vent cavity between the stucco and foam. The foam rigid foam itself is a vapor retarder (permeance depends on foam type) and the closed cell spray polyurethane is a Class-II vapor retarder at about 0.5-0.7 perms. If you build that, the 2" ccSPF is the first condensing surface, and about 40% of the way in from the exterior- it won't need the an interior vapor barrier for dew point control any more than the 2x6 wall outlined above. (Here too smart vapor retarders are a good idea, but not absolutely necessary.) But the thermal performance would be be in the low to mid 20s for whole-wall R due to the extreme thermal bridging.