Once again I change my plans. I’m free to do so as no-one else is inconvenienced by it. 🙂
I was going to skip hanging the building felt paper altogether. I forgot, though, what my favorite website Building Science has to say:
“The drainage plane in this assembly is the building paper or building wrap. The air barrier can be any of the following: the interior gypsum board, the exterior stucco rendering, the exterior sheathing or the exterior building wrap.”
Since Oliver’s Nest will live in a very cold climate, the air barrier should be to the inside and latex-painted barn board attached to thin plywood will serve that function.
(Quote and pictures copied from from http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers)
In Oliver’s Nest, substitute ceder siding and metal roof panels for the painted stucco with paper bond break. They act similarly. Then the thin rigid foam panels, which were going to serve as the drain field. But the author recommends building paper (ah-ha!) as the drain field. Then, plywood sheathing and the wool inside the wood stud cavity. On the interior of the wall will be a very thin layer of plywood and latex-painted wood boards (fastened to the thin plywood), which act like the “Kraft facing on a fiberglass batt or a “smart vapor barrier membrane” combined with the latex-painted gypsum board. I researched and it’s a very fine substitute. The big change is that my drain field *was* going to be the water-impermeable rigid foam panels, instead of the more water-permeable building felt. This change allows a much easier exit for water vapor inside the wall assembly. It’s important. I’m glad I caught it.
As the rigid foam panels I have are very thin at a quarter inch, my walls are more like the above than the below illustration:
If I were able to use much thicker foam insulation, I would go with the above design, but I’m limited by house width and (mostly) financial constraints. If I had the funds, I’d go wide. After all, the roof is almost 10 feet across. In fact, now I’m wondering whether to use the foam panels at all, as they won’t provide a thermal break as much as the drain field does, rendering them pretty much unhelpful. They are a process I think I can safely skip. Very nice. I have a future project on my property that they were originally intended for anyway.
I went ahead and finished with the plywood sheathing across the South side. It ended up looking like this:
Ugly but functional. Now just the upper part of the North side and I can go forward with the building felt. I had a couple rolls from the Restore that got wet in a sideways rain storm, so went back and got a couple more for $5 each. Not happy that the originals got ruined, but so it goes when you don’t have an indoor space to build and store supplies. I’d recommend one.
Based on my past performance, it’ll take a couple to three days to finish getting up the plywood. I have no idea how long hanging the building felt will take. Hopefully not more than a couple days. We’ll see!
Oh! And the wool? It’s drying so fast that I’m fluffing a couple times a day instead of every couple days, so the whole process is going super quick. It’s looking like I won’t need the dehumidifier. Unless it starts raining again before I get this baby weather-tight. My poor knee.