double stud wall refit
We are renovating a small rectangular home in climate zone 7
We have removed the entire inside of the building to the four exterior 2×4 walls
There is buffalo board type sheathing under the exterior siding.
I have been reading a lot of articles including on GBA about double stud walls.
My thought is to put 2×4 ROCKWOOL batt type insulation in the existing exterior wall and then sheathe it on the inside with 1/2″ plywood, leaving the existing buffalo board and siding on the outside. I will seal all the plywood by using construction glue in addition to fasteners.
I will then build a second 2×4 wall further inside of this, leaving a 5″ gap which I will fill with spray foam. Then, I will wire the inside wall and insulate it with ROCKWOOL 2×4 batts as well and then finish it with 1/2″ drywall.
I am seeing this as a wall with high R value that has little thermal bridging and can dry to both sides of the foam
I’d appreciate any advice available. Thanks.
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This is undoubtedly about the highest R-rating wall which can be built
with today's technology. It will end up about 12 inches thick (stud face
to stud face) and with an actual R-rating between 55 and 60 - very robust,
little or no possibility of moisture problems, but somewhat expensive.
Congratulations on a superior design - GOOD LUCK !!
Spray foam is something you try to design out of a build. It is very expensive and not the greenest option.
If you want foam between the two stud walls, I would look at sheets of EPS or polyiso. Roofing polyiso can be had reclaimed which is much cheaper than new stock. For any foam inside the house, make sure to do a sniff test, some of the reclaimed stuff that is left out in weather can smell pretty bad.
Instead of the spray foam or rigid, you can use regular batts. Any high density ones can be put sideway into the gap and work just as well for a fraction of the cost. For sealing your plywood air barrier you want to use a quality tape (ie ZIP, 3m8067, Tescon Vana). This is much less work than dealing with caulk and gets you a more robust air seal. Also figure out how to tie this air barrier into your foundation and the ceiling. In a retrofit, this might mean a bit of spray foam in these areas after all.
The plywood does work as a mid wall vapor throttle layer but it is a bit too far on the outside for Zone7. This assembly would still need a warm side vapor retarder, one of the many smart membranes is good for this plus can also serve as you backup air barrier.
A less expensive but more space efficient alternative would be to insulate your 2x4 wall, but add 2” to 4”’of EPS inside of that. Add a layer of reflective vapour barrier. 2x3 strap vertically. Drywall over strapping. Wiring and plumbing in the warm cavity. We used this method in a zone 7A commercial retrofit and it worked extremely well. Heating costs (NG) for 9000 sq/ft in a -20C average month were only $180.
This creates an easy gap for services with a radiant reflective layer and is easy to build for code approval here in Ontario: It’s hard to quantify the R value of the radiant reflective gap, but the building results over about 7 years were very impressive. A subsequent renovation revealed zero moisture issues in the walls at year 6 . Receptacles were installed sideways on the 2x3 horizontal strapping.
The attached pics show a few images as I added a few windows to a film studio area (zero windows) that was being repurposed to a dance studio. The entire 9000 square foot building exterior walls were treated this way.