Perfect Wall for Park Model, Followup
This was the proposed assembly :
1/4″ Wood T/G decorative planking over 2×4 studs
Smart Membrane Vapor Barrier
R-15 Rockwool Batt
3/8″ exterior ply sheathing w/ WRB (eg Benjamin Obdyke Hydrogap).
1″ Polysico or XPS foam board over WRB exterior.” *
3/8″ air, water gap as rain screen drainage plane
3/8″ Siding
After doing further research I noted that some folks use GPS (Nucor) Type 2 Rigid form board because of its better permeability (3.1) for drying than XPS (nearly impermeable). As it will also serve as the rain screen behind the cladding I am unsure of which is better and if the GPS should be faced or left plain.
Am I overthinking this?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Deleted
I think that is a great wall in almost any climate. The permeable foam is a better option. Unfaced XPS, the micro perforated GPS and permeable polyiso allow for a bit of drying to the exterior which makes it a more robust wall.
Make sure to tape the seams of the plywood as your air barrier.
If this is sitting on a metal frame, make sure to build a fully insulated structure above it and set your walls on that. Any metal connecting the inside to the outside is a big thermal bridge and can create issues with condensation and mold especially behind cabinets. You want insulation or wood between you and the outdoors everywhere.
Thanks. I will taking floor and roof design as a followup. But it answers the questions I had.
It will be easier to build with a 3/4" rainscreen gap, which would allow the use of 1x4s or ripped strips of 3/4" plywood as furring. You wouldn't have to get all the siding fasteners aligned through the foam this way either, which would make it easier to hang the siding.
My preference would be to have an interior side air barrier other than a smart membrane, but it's possible to detail that smart membrain as an air barrier if you're careful. Using one of the more durable smart membrains here would make it easier to keep the air barrier intact during the construction process.
Bill
I can see the advantage of 1x4" furing strips as attach points for siding but the challenge here is to keep the walls 6" thick so as to provide a 12' wide external width of the "house", giving at least 11' wide interior width. Absent an expensive and thin 1/2" airgel product to replace the exterior 1" foam board I think I am stuck with a 3/8" rain screen "gap" behind the siding..
As to the smart membrane, perhaps a sheet product (other than heavy drywall) would be better? I beginning to think simple 1/4" ply rather than decorative planking on the interior wall frame is easier and a better air barrier?
Thx...
Mark
Just to fill in some blanks here, I think you're getting a bit confused by thinking about the foam as the inside surface of your drainage plane because it is also the inside surface of your vented rainscreen cavity. That's not entirely the case. With your design, the foam sits *within* the rainscreen cavity. The cavity is the entire space between the back of the siding and the front of the WRB. It's the WRB that stops water from getting in the walls and the foam just provides a bit more wall insulation. This is one reason why it's best to have some permeability in the foam. If (when) water gets behind the foam, bulk water will eventually drain out the bottom but you would like the rest of the water to dry to the outside rather than just sitting there or (worse) drying into the walls.
So unless I am reading you incorrectly, it would seem that EPS or GPS would be preferable foam board products than the much less permiable foam board options (XPS and Polysio). I am also still uncertain if a side of the foam should be faced on none, one, or both sides in my design.
I have noted that some writers say the exterior foam board face can be used as the drainage plane, without specifing if the surface has a facing or added WRB. So I am a bit confused on that aspect.
Thx
Mark