Off-Grid Cabin Roof Retrofit
Building seasonal off-grid 1 room cabin. Primary reason to insulate the roof is to make the cabin a little cooler in summer and retain warmth from small wood stove in shoulder season. No winter usage. No central heat or air. The other important point is the cathedral ceiling design and aesthetic choice want to see the roof-cut roofing boards from interior, so all insulation and sheathing will be sandwiched between the roofing boards and top layer of shingles. I am assuming there is no ridge vent.
What is the suggested build up from roofing boards up?
Taking into consideration the avoidance of condensation buildup between layers, adequate R value, and adequate WRB properties.
Will this work build up work (from bottom up)?
1. 5/4″ roofing boards (for aesthetic look to interior)
2. Rigid foam
2a. Does it matter EPS, XPS, PolyIso? With or w/o foil backing?
3. Plywood 5/8″ sheathing
4. Roofing underlayment / felt paper
5. Shingles
?? Do I need additional WRB (air or vapor?) between #1 and 2?
Thanks
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Replies
Do I need additional WRB (air or vapor?) between #1 and 2?
It really depends on how quickly you plan on getting it built. It sounds like you'll be doing much of this yourself. In that sense, I think you'll have at least a small gap between nailing up the roof sheathing, and installing the insulation. It's pretty quick, easy, and cheap to staple down a few widths of #15 felt to make sure everything below is protected in the case of a popup shower. On the other hand, if you can knock the roof sheathing, insulation, and upper sheathing out in one day, it can probably be skipped.
Will this work build up work (from bottom up)?
1. 5/4″ roofing boards (for aesthetic look to interior)
This is fine
2. Rigid foam
2a. Does it matter EPS, XPS, PolyIso? With or w/o foil backing?
The most common use here is Polyiso. It's higher R value per inch means you need less thickness of it, which makes the installation of the other materials easier. The fact that it's foil face means little here, since there's no void space between the sheathing above it.
3. Plywood 5/8″ sheathing
This is fine.
4. Roofing underlayment / felt paper
Also fine.
5. Shingles
Also fine.
I assume you plan on insulating the walls and possibly floor as well? Without cooling, the place will still get to nearly ambient temperatures, eventually. It's just that it takes longer to do so.
Thanks.
I am considering #15 felt at layer between #1 and #2 for the reasons you suggest. Overall, I am concerned about making a condensation trap that would impact the 5/8" plywood sheathing sitting on top of the rigid foam. I am thinking foil faced is more vapor impermeable than other faces. Does my 5/8" sheathing need a direction to dry from condensation? If so, would that suggest no-foil on rigid foam or firred-channels for drying?
It's true that it would make it hard for the polyiso to dry, but since there is no conditioning of the space, there's unlikely to be a huge gradient in either humidity or temperature that would allow drying in either direction in the first place.
I was going to suggest a double roof above, as its not that much extra in cost compared to what you are doing, but it does take quite a bit longer to build. The layers in that case would be (from outside to inside)
1. Shingles
2. tar paper
3. 7/16" osb ***
4. 2x3 on the flat
5. polyiso
6. tar paper
7. boards
#3 depends on your rafter spacing. If it's 16" OC, 7/16 is plenty because of the decreased distance between supports. If it's 24" I'd stick to 5/8 plywood or 19/32 osb. A sheet of 7/16" is certainly easier to hoist up by yourself.
That'll let your layer between the underlayment dry, and it will decrease the net heat flux into the interior compared to the version with no vent space.