Will attaching 1″ rigid foam to the inside of a stud wall create an effective thermal break?
Srockwl
| Posted in Energy Efficiency and Durability on
I am getting ready to complete a timber frame project in Northern Minnesota that I started several years back. I left it with 3.5″stud wall envelope sheathed in OSB thinking that I would insulate with spray foam. Since then, I’ve read about thermal bridging and have come up with a wall system that I believe will address that issue, give me an acceptable R factor and not create a second condensation surface. That is: board & batten siding (locally milled tamarack), OSB, R-15 fiberglass, 1″ rigid foam, 1/2″ sheetrock. I would appreciate comments.
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Replies
Wade,
If you want to install rigid foam, the best place to put it is on the outside of your wall, not the inside. That way the foam will keep your OSB warm.
Of course, that means that you'll need thicker foam than what you plan to use. In your climate zone (zone 7), you'll need to install rigid foam with a minimum R-value of R-10 if you install the foam on the exterior side of your 2x4 walls. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.
By the way, I hope that your 2x4 walls aren't located between the posts of your post & beam frame. That's a bad approach.
Martin, hundreds of homes near me have rigid foam under drywall here. And now I see exterior foam use that to me is going to be worse than all past builds. Builders are using less than the amount you all recommend.
I see future problems, time will tell. Inside foam I am thinking has no problems here in 6a if too thin. Martin, Peter Yost thoughts?
Thanks Martin, (The stud walls ARE outside the timberframe.). So it appears that 1.5" of polyiso on the outside of the OSB will keep it warm. I would then use horizontal 2x2 firring strips to hang the board and batten siding because it is a heavy product. Is the thermal bridging through the firring strip a concern? Does it matter if I use R-13 or R-15 in the stud walls? With this system, I've seen recommendations that the vapor barrier be left off from under the Sheetrock to allow drying from the inside. Is this something you would recommend?
Wade,
Q. "Is the thermal bridging through the furring strip a concern?"
A. The furring strips are entirely outside of the insulation layer, so there is no thermal bridging through the furring strips.
Q. "Does it matter if I use R-13 or R-15 in the stud walls?"
A. Only if you care about R-value. If you want the additional R-2, go for the better insulation. You could also consider installing dense-packed cellulose between the studs. Dense-packed cellulose does a better job of filling all the nooks and crannies, and usually performs better than batts.
Q. "With this system, I've seen recommendations that the vapor barrier be left off from under the Sheetrock to allow drying from the inside. Is this something you would recommend?"
A. Yes, I wrote about that recommendation in the article that I linked to in a previous answer (Calculating the Minimum Thickness of Rigid Foam Sheathing). For more information, you may also want to read How to Install Rigid Foam Sheathing.