Vapor Barrier, MemBrain or nothing?
Building my own home this summer and struggling on which route to go.
Current plan is 2×6 framing. Tyvek sealed and taped on outside. 1″ closed cell on the inside followed up with R-19 stuffed in the remaining cavity.
I live in Southern Minnesota.
Based on research it seems as though I could forgo the vapor barrier, I could use one, or I could use a product like MemBrain?
If I don’t use one are the measures or other precautions I should take? I’m trying to get a tight seal on the house with the foam. Plan on doing 1″ in the attic as well, but also be economical.
Another route I thought of going which is a whole lot less economical but would be more efficient is to go to 2×4 framing. 3″ close cell on inside. 1-1/2″ rigid or maybe rockwool on outside with 1″ strapping on outside for siding. If I go this route there is just the Tyvek on the outside and no vapor barrier on the inside correct?
Replies
Hi Parker,
Flash-and-batt or flash-and-fill walls are not ideal in a new home. They require financially and environmentally expensive closed-cell spray foam and yet still do nothing to mitigate thermal bridging. A better approach is a wall with exterior continuous insulation. This type of wall reduces heat loss through framing, allows you to use whatever insulation you'd like in the cavities, including much more environmentally sensitive cellulose, and keeps all of the framing withing the conditioned building which is more durable.
That said, if you do go with a flash-and-batt or flash-and-fill assembly and have the appropriate R-value of closed-cell foam for your climate, then you don't need an interior vapor retarder beyond the painted drywall.
If you install closed-cell foam inside the sheathing, you should keep the exterior vapor-open and have a vented rainscreen siding detail so the siding and the sheathing both have a certain drying path. In other words, exterior mineral wool would work; most rigid foam would be a bad idea in this assembly.
I'd recommend this article for more info and help designing your walls: Walls that Work
What Brian said. You'll get a lot more bang for your buck with exterior insulation. And you should look into products other than XPS foam because the global warming potential of its blowing agents make it unlikely to ever repay its carbon debt through energy savings. Polyisocyanurate would be a better way to go. Or Rockwool. Or Gutex, if you have the money.
I also live in Southern Minnesota. You want some sort of vapor barrier on the inside if you are not completely filling the cavity with sprayed in foam. Whether you use a plastic vapor barrier or MemBrain, or simply give the drywall a coat of oil-based paint after installation is up to you. MemBrain claims to be somehow "smart", allowing moisture through when the AC is running, which is a good idea if it works, but in southern Minnesota the AC season is not (yet) so long that this should be a big concern.