Replies

1. GBA Editor
   Martin Holladay | Apr 17, 2017 02:13pm | #1

   Terry,
   Unless you live in Alaska or the colder regions of Canada, the use of interior polyethylene isn't recommended (especially if your house will be air conditioned).

   I've never heard of anyone using 3/8-inch OSB sheathing. Why did you choose it instead of the more common 7/16-inch OSB?

   The use of 2x8 studs is unusual. Because of thermal bridging through studs, it's rare to get enough of a thermal performance improvement from 2x8s to justify the high cost of the studs.

   For more information on the airtightness of OSB, see this article: Is OSB Airtight?

   -- Martin Holladay

2. 
   brendanalbano | Apr 17, 2017 02:14pm | #2

   What's your climate zone?

   What led you to that particular wall detail? It's somewhat non-standard, but that might be for very good project-specific reasons! Sharing your thought process will help you get better answers. You might be able to get the same level of efficiency in a cheaper wall.

   - Why 16" O.C. for your studs? With such deep studs I'd imagine you could do 24" O.C.
   - Why the interior poly? Depending on climate zone you might either not need it, or be better off with a smart vapor retarder. Or the poly might be right. Depends on climate zone.
   - Can you do 4" or 5" of exterior rigid mineral wool and a 2x6 stud interior? Shifting more of your insulation to the exterior is always good.
   - Have you considered Zip sheathing instead of Tyvek on OSB? That solves your worries about airtightness on the OSB if you're concerned about some OSB not being as airtight as other OSB.

   Not knowing all the details of your project, what do you think about this:

   Wall: metal siding, strapping, 4" (or 5"? 6"?) Roxul Comfortboard, Zip sheathing with taped seams, 2x6 studs 24" OC, Roxul (or cellulose or whatever) in cavity, 1/2" drywall. (No service cavity because no interior poly to protect).

   Attic: R80 blown cellulose, taped OSB, 2x4 service cavity, 1/2" drywall.

3. 
   user-6156962 | Apr 17, 2017 02:29pm | #3

   Hi Martin and Brendan
   Yes I completely forgot to mention I live in climate zone 3 in Northern Canada(the coldest zone we have). The house will be air conditioned, and we do have to use interior poly here.
   The 3/8 OSB is the standard practice here on walls regardless of performance desire of the wall system.
   We compromised on the envelope, by trying to balance exterior insulation, interior insulation and labour with total performance of the wall. Not saying it's perfect from a thermal bridging perspective, but the theory is the 3" exterior Roxul will counteract that?
   I read the OSB article prior to posting, but still couldn't figure out if the exterior Tyvek would solve or diminish this problem.
   Thanks again.

4. 
   brendanalbano | Apr 17, 2017 03:00pm | #4

   Could you do 2x6 @ 24" OC with 4" exterior mineral wool? Would this save you money? It's a more common stud size, and 3/4" thinner.

   Unless I did my math wrong, whole wall R-value will be about the same as your 2x8 @ 16" OC with 3" exterior mineral wool.

5. 
   user-6156962 | Apr 17, 2017 03:40pm | #5

   Well it would save on lumber, but the 24" OC isn't done in this part of the country, building inspectors aside, I don't know if I want to worry about the framer figuring that out. The Roxul doesn't come in 4" widths to my knowledge, so we'd have to do 2 layers, along with fasteners, etc.

6. 
   DanCK | Apr 17, 2017 04:01pm | #6

   Hi Terry, I'm in a zone 2 in Canada and have planned for a scaled down version of your wall design seeing as it's not quite as cold as zone 3.
   Have you thought about using a staggered 2x4 stud wall instead of using 2x8 lumber. Even if you filled your service cavity with insulation you would gain R-value and be able to use 2x6 lumber, although this wouldn't be a good idea with the A/C and 6 mill poly combination. (You should think about a different method of air barrier)
   I've never seen 2x8 precut lumber for a wall either but that doesn't mean its not available.
   The 24" framing should be no issue for the framer either.

7. GBA Editor
   Martin Holladay | Apr 17, 2017 04:27pm | #7

   Terry,
   For an extended discussion of the drawbacks of 2x8 studs, see Choosing the Right Wall Assembly.

   For a discussion on wall design, see How to Design a Wall.

   -- Martin Holladay