Roof Insulation for Cold-Dry Climate
I’ve build a 12×13 tiny house in northern New Mexico, at 8000 ft. It gets pretty chilly in the winter here, and as you probably imagined, it’s a very dry climate. I have an unvented ceiling with no attic, so if I understand right, spray foam is my only option. Unless I want rot… The thing I’m having trouble wrapping my head around, is what happens to the heat that wants to escape, when it can’t through the ceiling? Wouldn’t it just end up in the walls, unless they’re spray foamed as well? My original plan was spray foam ceiling, batts in the walls, and 6mil plastic over the batts on the interior. Would that cause me any problems? If it’s helpful, on the exterior, it’s just t1-11 on studs. No moisture barrier. Anyway, thanks!
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Heat itself doesn't really pool up like water. Air can (sort of), which is why air barriers are often more imporant than insulation. An air leak can loose a lot more heat than an area of less than optimal insulation, for example.
You could put exterior rigid foam on the roof, so foam above the rafters, and the actual roof (shingles, etc.) above that. You'll need some layers of sheathing in there for strength. That gets you around the need for spray foam. If you don't want to take your roof apart, then spray foam is probably the best option.
Walls with batts are usually fine. I would use a vapor retarder and not "6 mil plastic" (which I assume to be polyethylene). Even in a dry climate, it's best to not put vapor barriers in unless they are really needed. You do need an air barrier though, but that doesn't mean you have to use poly. What are your interior walls? Drywall is a good air barrier, but T and G boards are not.
Heat doesn't "want to escape". Heat just goes through stuff based on thermal differentials, always seeking equilibrium. Insulation doesn't "trap" heat, insulation just slows the transfer of heat from one side to the other. Walls don't have the same moisture issues as unvented roofs do, which is why insulating walls isn't as critical as insulating roofs in terms of exactly how you put things together.
Bill
Thanks for the response! I actually put a layer of 1.5” foam board on top of my roof sheathing, but I put a 2x4 furring strip across the middle like a dummy. Not very air tight… I’m pretty opposed to re-doing it. I was thinking… what if I put in batt insulation in my ceiling, then 6mil plastic, drywall, vapor barrier paint, then t&g on top? The purpose of the drywall would just be to further create a barrier. I’m basically just trying to brainstorm a cheaper solution.
For my walls I was planning on drywall. Just curious, why don’t you recommend the poly sheeting? I was just figuring that such a small space, with a shower, sink, dog, and human would need it considering all the moisture.
I’m still a little confused as to how the walls don’t typically have condensation issues. I understand that most heat exits the roof, but what about the areas of wall right next to the ceiling?