Perfect Wall for Variable Location (Park Model Living)?
I am designing a custom RV Park model home for my retirement. It is basically a 12ft wide by 33ft long unit (400sq feet). In order to maximize interior space but also provide sufficient insulation the “perfect wall” should be no more than six inches thick. The challenge is that I am not sure which climate zone I will settle in (3, 4, or 5). Zone 4, in particular will be very rainy.
From the interior to exterior my proposed assembly consists of…
1/4″ Wood T/G decorative planking over 2×4 studs
R-15 Rockwool Batt
3/8″ exterior ply sheathing w/ WRB (eg Dorkin Delta Vent SA, Benjamin Obdyke Hydrogap, or Blueskin).
1″ Polysico or XPS foam board over WRB exterior.
3/8″ air, water gap as rain screen drainage plane
3/8″ Siding
While his provides a 5 and 7/8″ wall will it work for zones 3-5? The R value is a little low (approx 20-21) but acceptable. However I am concerned as to the impermeability of outward foil facing polyscio as opposed to XPS and if there should be a tiny drain “gap” between the ply sheathing and foam panel?
Finally, I might end up in a mixed cold/hot humid climate area. So I’m a little baffled as to what other vapor barriers might be appropriate, and if the foam board can serve as a rain screen plane (with or without foil faced outward) under cladding.
Thanks for your advice…
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Replies
Because wood plank wall finish is usually very leaky, I suggest installing a variable vapor retarder such a CertainTeed MemBrain on the inside. That will limit the moisture that can enter from the interior, while still allowing drying as necessary. If you seal it at the edges and around electrical boxes, it will also serve as a secondary air barrier. A wall with R5+ continuous insulation and a variable vapor retarder meets the IRC requirements for vapor control in climate zones 1 through 7.
If you tape the foam, it can certainly be your WRB drainage plane. You don't need a drainage gap behind the foam, but it wouldn't hurt to use one of the WRB's that allow slight drainage.
Thanks. I assume if the foam is foil faced outward to exterior it will still serve as a drainage plain.
+1 for Frank's recommendation for MemBrain on the interior. I would use the exterior sheathing as the primary air barrier, with seams taped and polyurethane sealant around the perimeter to seal it to the framing. While I do try to detail MemBrain air tight on my own projects, I don't trust it to be air tight enough to really be a good air barrier -- it's too easy to get a hole in it somewhere while finishing the wall, or having leaks around electricals. If you use the exterior sheathing as your primary air barrier instead, with the MemBrain acting only as a secondary air barrier, you're less likely to have leaks.
Bill
Thank you to both FrankD. and Bill Wichers. I hadn't given enough thought to air sealing, these tips address that.