Replies

1. 
   FrankD | Feb 10, 2025 01:02pm | #1

   I don't know what version of the IRC you are under, but the 2024 IRC allows you to have as little as R-5 continuous insulation over a 2x6 wall in climate zone 6 as long as you have a variable vapor retarder (such as CertainTeed MemBrain) on the inside.

   With the 2021 IRC, you could have the same minimum R-5 continuous, except that you could use any class II vapor retarder. A variable product would give a greater margin of safety though.

2. Expert Member
   BILL WICHERS | Feb 10, 2025 01:23pm | #2

   I usually go by Martin's classic article for this kind of thing:
   https://www.efficiencyvermont.com/Media/Default/bbd/2019/docs/presentations/efficiency-vermont-bbd-rethinking-the-rules-on-minimum-foam-thickness.pdf

   R11.25 for exterior rigid foam on a 2x6 wall is what you want for zone 6 according to Martin's article. I would use 2" of polyiso here, which would give you R13. If you're trying to keep within a budget, see if you can get reclaimed or "factory second" polyiso for this, which will save you a significant amount of money over regular "new" material. Note that you can use either foil faced or fiber faced polyiso here.

   I would NOT use flash and batt here. I would not use flash and batt anywhere, actually, since it's generally a waste of money. Flash and batt does nothing to address thermal bridging of the studs, which means it can never perform as well as continuous insulation can. That's on top of the fact that it's the least green way to go, and adds an additional step AND CONTRACTOR to the project. If I was going to use spray foam in the walls, I'd do full fill open cell, but I'd rather use dense pack cellulose over spray foam here.

   My preference would be exterior rigid foam and mineral wool in the walls. It sounds like you're trying to cut down on costs though, in which case I'd look into the reclaimed/factor second polyiso. You can also use fiberglass batts instead of mineral wool, which will be a significant cost savings. You can usually get R21 high density fiberglass batts without the need to special order material, and that gets you pretty close to the R23 of mineral wool at a lot less cost.

   Bill

3. 
   AmyMoses | Feb 10, 2025 03:30pm | #3

   Thank you Bill. That's a great article you linked to. Disconcerting at times, though very educational.

   I too would prefer not to go the flash and batt route. We are doing it for the cathedral roof though, so the contractor is suggesting it as the sub will already be there.

   So to reiterate, R5 exterior insulation on a 2x6 walls meets code only with a class 1 vapor retarder (poly sheeting or variable perm membrane) on the “warm” (interior) side of the stud wall. See below. Is #2 an option? I've attached a detail.

   If we’re not installing a class 1 vapor retarder, there are two options:
   1. At least R11.25 exterior insulation on a 2x6 wall. OR
   2. R5 on a 2x6 wall but you have to have ventilated cladding (ex. strapping behind siding) and you have to use fiberboard sheathing (ex. Homasote) that will properly release any moisture. No plywood or OSB for sheathing.

   Thanks,
   Amy

   
1. Expert Member
   BILL WICHERS | Feb 10, 2025 03:47pm | #4

   #1 is fine.
   #2 I'm not so sure about. I never use fiberboard for anything aside from sound deadening in floors sometimes. Plywood and OSB are both fairly vapor open too, and allow for outward drying. I would expect a wall assembly with an interior smart vapor retarder to perform OK if it can also allow for outward drying, which could be no exterior insulation, or could be a vapor open continuous insulating material like rigid mineral wool (Comfortboard or similar). The downside is that rigid mineral wool is very expensive as rigid insulating materials go.

   With a cathedral ceiling, your options are more limited so sometimes flash and batt is the only way to go. I would check if it's possible to put vent baffles in, since if you can, you can avoid the need for spray foam and also have a safer overall assembly. If you are stuck with spray foam as your only option (which is usually the case when you can't vent the roof assembly), then I would consider putting in a layer of closed cell spray foam sufficiently thick for your climate zone, then filling the remainder of the cavities with open cell spray foam which will be trimmed flush as part of the installation process. It's also possible to use batts in place of the open cell spray foam, but the open cell spray foam often saves time.

   Bill

4. 
   AmyMoses | Feb 11, 2025 08:54am | #5

   Bill,
   If we go with #2, and install plywood sheathing, would you recommend that mineral wool insulation continuous insulation would also need to be R11.25 or greater? Since the comfort board, as you noted, could allow drying to the exterior, can we use 1"? Thank you. Amy

1. 
   Malcolm_Taylor | Feb 15, 2025 07:07pm | #7

   Amy,

   Codes aside, if you go with permeable exterior insulation (mineral wool), but less than the suggested safe level of 11.25, the sheathing can fall below the dew point, but that isn't a problem because of its ability to dry to the outside. That's just like the more common walls we have always built with no exterior insulation.

   You don't need to use fiberboard. Plywood acts as a variable-perm vapour-retarder, and will open up to allow more drying when damp.

5. 
   edels | Feb 15, 2025 04:17pm | #6

   Great article!