Tongue and groove interior walls smart membrane required?
Cottagebuilder
| Posted in General Questions on
We are planning to do pine tongue and groove walls on the interior of our house. Climate zone 6.
Our 2×6 wall assembly from exterior to interior is going to be:
Pine clapboard
1×4 strapping
3in of polyiso
7/16 zip sheathing
R-22 TimberHP batts
Tongue and groove
Would a smart membrane be required before we install the tongue and groove? I thought the exterior insulation would prevent condensation from forming on the sheathing and a smart membrane would not be required but I’ve been reading around the Q&As here and now I’m not so sure.
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Replies
Cottagebuilder,
You don't need a vapour-retarder, but the walls do need an interior air-barrier to stop warm inside air making its way though the batt insulation to the foam. That's a lot more important on roofs, but it's still good insurance on walls using hybrid insulation.
So I'd say including an interior vapour-retarder isn't strictly necessary, but is definitely useful.
If you're in the US, this is a rare case where I disagree with Malcolm. The IRC model for CZ6 requires a class 1 or class 2 vapor retarder, unless you have enough exterior insulation, in which case you can use a class 3 vapor retarder. (The IRC really means a high enough ratio of exterior R-value to overall R-value, but that's another matter).
The reason for the class 3 vapor retarder, I've come to understand, is that the more indoor air and moisture can circulate more or less freely through the batt insulation, the more likely you are to have moisture accumulating on the interior of the sheathing.
I learned about this after writing a Fine Homebuilding article about flash-and-batt insulation, after which I received many complaints from people using that system, which from a building science viewpoint is similar in most ways to using exterior foam insulation. The readers were experiencing large amounts of condensation on the face of the foam. I learned that the ratios are not particularly robust, meaning they only work if moist, indoor air can't get to the sheathing, at least not much of it. Ideally the interior membrane would be airtight, but the system has some resilient built in so just installing a variable permeance membrane behind the boards is enough.
On a few projects I had the entire ceiling drywalled and primed before putting up boards, for the same reason; fortunately variable permeance membranes came along shortly after those projects.
Edit: I actually agree with Malcolm that reducing air movement into the cavity is by far the more important reason to add something behind the wood. But beyond that, code requires a vapor retarder.
Michael,
I don’t think we are disagreeing, I just phrased my answer poorly. I learned from you that these assemblies need a good interior air-barrier. Installing that will provide a class 3 vapour-retarder. Assuming enough exterior insulation, that should be sufficient.
Malcolm, at the same time you were commenting, I was adding my edit. I agree that we agree ;-)
Thank you both.
I do not plan to do drywall under the tongue and groove. What other options are there for a class III vapor retarder? Or should I use a class II vapor retarder like Intello? If so, are there any options that are more affordable? Intello cost for a whole house adds up quickly.
Certainteed MemBrain performs reasonably similarly to Intello; the biggest difference is that it's more delicate and prone to tearing. It's about half the price of Intello. 1/2" drywall, painted, is almost always the least expensive way to finish the interior. If you're using wood, the extra cost for a membrane is something you need if you want to avoid moisture problems (and meet code).