Double stud wall with plywood panel siding
Hi everyone!
I recently purchased a house in climate zone 7 in Alaska and was planning on upgrading in the insulation on the house (built in 1990). I have been reading quite a bit about double stud walls and really like the performance to cost ratios. One thing I am really worried about is moisture. I have lap siding on the front of the house (which I assume has some type of rain screen) and plywood panel siding on the other three walls. I believe the plywood panel siding was also used as the sheathing for the exterior walls.
Should I be concerned about moisture build up on the panel siding? all of the diagrams I have seen so far usually have sheathing with an air gap and then siding.
Currently my plan is:
1. 1/2 plywood panel siding.
2. 2×6 16 in. on center exterior wall.
3. R23 rockwool
4. 2×4 16 in. on center interior wall.
5. R15 rockwool.
6. Intello plus membrane
7. 1×2 furring strip.
8. 5/8 drywall.
any recommendations are appreciated. Thank you!
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Replies
Especially in your climate, if you manage moisture at the interior wall and let the walls dry to the exterior there shouldn't be any problems.
It looks like you're planning on a wall that is about 16" thick. At that thickness you might find blown cellulose to be more cost-effective.
I don't like plywood siding without sheathing, there's no good way to flash the window and door openings.
Thanks for the reply, total wall thickness should be around 11-12".
I think I misread the original post thinking there would be three layers of rockwool. Even at 11-12" you should look into blown insulation.
Zone 7 is very cold. Generally double stud walls in cold climate should have a rain screen under the siding. I would run your wall through this calculator and see what comes out:
https://www.ubakus.de/en/r-value-calculator/
The outdoor temperature you use should be the mean temperature of your 3 coldest months. The inputs are metric so you'll have to do a bit of conversion to get the right units.
You might have to move the vapor barrier to the middle of the wall for it to work.
I was with you until the last sentence. Why the middle of the wall?
Two reasons.
A mid wall VB is easier to detail air tight and less chance of trades putting holes through it.
The mid wall location allows it to work as a vapor throttle, this changes the vapor profile across the wall. I recommend setting it up in Ubakus to see what this looks like, this can significantly reduce the moisture load on the sheathing. This also allows allows the wall to dry in two directions which makes a bit more robust.
Of course this is all in theory, so I can't comment on how well it works in real life but at least on paper it seems to be better.
I would worry about the lack of a rainscreen. I've seen a lot of T1-11 that has rotted. It seems to work well for 20-30 years but then it starts to fall apart.
Double stud walls build up moisture in the sheathing toward late winter/early spring here in CZ6; then vapor drive slows to a crawl as the outdoor dewoints get close to indoor dewpoints. When the sheathing is getting saturated with spring rains it's not doing much drying to the exterior. A rainscreen gap allows the sheathing to dry while the cladding gets wet from stormwater.
Great project!
A couple points. First, a rain screen is a must. Your siding will last longer and driven rain is less likely to defeat your taped plywood sheathing.
Dense pack cellulose is the best choice but I have carefully layered in fiberglass batts keeping costs down.
If you line up your inner and outer studs you can fit ripped pieces of rigid foam between them and your batts just slide in, minimizing awkward gaps in your installation.
A different approach altogether is to avail yourself of the CCHRC wall system or another variant of the Canadian REMOTE building system. You still will need the rain screen but overall easier to detail and great for "deep energy retrofits".
I have done both double wall projects here in northern MN and the CCHRC wall in the Michigan UP. I have no hard data but all three buildings perform well and the two residences have ERV's , a must with good tight buildings.
Good luck up there!
Oops, the Canadian predecessor to the REMOTE system is RESIST. An alternative to double wall approaches.