Roof for small cold-climate A-frame "writing room"
Hi Martin --
I'm helping a friend build a small, simple 10x12 A-frame in northwestern MA (Lanesboro). It will be in the woods a hundred yard from his house, and mostly be used as a "writing room" to escape the house, but occasionally as a bunk house for guests who have overstayed their welcome (like me). Single room for the bottom, half loft on top.
No permit required, so no great concerns about code. Low-voltage lighting from a solar panel and no running water, so generally low humidity. Reclaimed single-pane windows, so our insulation goals aren't that high. But we'd like something that can be kept warm for a few days even in the middle of winter with a tiny wood stove, is comfortable the rest of the year with minimal heating, and won't fall down or rot.
We're building it now in the snow, and would like to get it enclosed in the next week . We have walls and rafters up, and it's time to put on a roof. I think we have a good handle on the "ideal" from your site and Lstiburek, but we're trying to understand what corners can be cut given budget and the limited use of this building. The current roof plan (from the top down):
4" Blue foam XPS (2 x 2", overlapped, taped seams)
2"x6" rafters @ 18" filled with fiberglass bat
Some air barrier
1) Should we add battens and an air gap between the plywood and the foam, or are we OK presuming everything should dry out seasonally? It's plywood not OSB, and 4" of XPS is still slightly permeable.
2) Screwing through 4" of foam and hitting a rafter is hard. If we put 2"x4" sleepers on top of and perpendicular to the rafters, life becomes easier, but then we only have 2" of foam in places. Think we could get away with it?
3) It's a tiny roof, but steep (slightly greater than 12:12, 51%). This also makes the 2"x4" cross pieces appealing. If we don't use the cross pieces, is there a good and safe way of building this? We have scaffolding up on both sides up to the eaves.
4) We could add another layer of plywood on top of the rafters. This would make it slightly easier to work on, but reduces the inward drying we're hoping will cover up for our cut corners. And it's more plywood we'd have to buy. Would this layer make much of a difference?
5) In your great cathedral ceiling article (http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulat...) you say "the biggest air-barrier blunder is to install tongue-and-groove boards as your finish ceiling without first installing taped gypsum drywall". How about Tyvek, or some other vapor-permeable housewrap?
Thanks, and Happy New Years!
Posted Wed, 01/01/2014 - 15:04
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