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Zone 6 R40+ Wall Assembly With Inside Sheathing

dgsmills | Posted in Energy Efficiency and Durability on

I’ve been trying to figure out an approach for my walls, but I’d like some more informed opinions.

Background: I’m doing a DIY build of a home in the interior of British Columbia, Canada next year, and trying to nail down the wall assembly to hit at least R40. It’s a 2-storey home in Zone 6 with heavy snow load in the winter and increasingly warm summers. Heating and cooling will be handled by mini-splits. Spring and Fall deliver a fair bit of rain [we’re adjacent to a temperate rainforest], but it’s not particularly windy. Windows are triple-glazed throughout. Walls are 9′ – 11′ tall on the main floor, and 10′ – 13′ tall on the second floor.

I’ve modeled different wall assemblies in [kind of like an online WUFI] and have narrowed it down to a couple of options. In both cases, the inner wall surface is 1/2″ plywood, outer cladding is steel on vertical furring strips, and assembly is described from inside to outside.

Approach #1 – 2×6 with 2×4 inner wall, 24″ OC – R45
 – 2×4 non-structural wall filled with rockwool
 – 3/4″ T&G plywood sheathing, taped seams
 – 2×6 structural wall filled with rockwool
 – Pro Clima Mento 1000 WRB
 – Rockwool Comfortboard 80 – 3″

Approach #2 – 2×10 with 2×2 inner wall, 24″ OC – R42
 – furring strips to provide runs for electrical and plumbing
 – 3/4″ T&G plywood sheathing, taped seams
 – 2×10 structural wall filled with rockwool
 – Pro Clima Mento 1000 WRB
 – Rockwool Comfortboard 80 – 1.5″

Approach #2  can easily be pushed to R50+ by changing to an insulated 2×4 wall on the inside.

I’m leaning toward the 2×10 option because the Comfortboard is pricey enough to make the 2×6 version materials more expensive @ R45. I looked at double 2×4 walls, but local pricing for 2x4s longer than 8′ has gone haywire, and bizarrely, it would be the most expensive option.


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  1. Expert Member
    Akos | | #1

    One more option to throw out there for you.

    2x10 walls as you start, cross strap this on the outside with horizontal 2x3s on edge 17.5" OC and fill with 2.5" Rockwool AFB. AFB is pretty much the same as comfort batt but for commercial steel construction. Much cheaper and higher R value than 1.5" rigid MW. Because the 2x3 are at 90deg to the studs, it gives you almost all the assembly R value increase of continuous insulation without having to deal with the semi squishy rigid MW boards.

    Your metal siding can also be screwed directly to the 2x3 saving strapping out the wall.

    P.S. Walls without exterior sheathing can make great homes for critters. Make sure to figure out your critter barrier, this needs to be continuous across any cross section same as your air barrier.

    1. dgsmills | | #2

      AFB is fully 1/3 the price of the Comfortboard 80 at the same R-value! That's a fantastic alternative.

      Critters are definitely a thing in the area. Mostly bears and ravens, but I'm sure there are smaller bugs that would love a nice place to nest. The cladding guy is local and has likely seen it all, but I'll ask for a specific solution.

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