Critique of cathedral ceiling and wall design – Swedish Style
Planning on building a 1-1/2 story, well insulated home using 14” I-joists as the rafters. Then running 2 x 4” horizontally. The I joist cavity will be air sealed. The 2 x 4” cavity will not. I will have an ERV. This idea is a mix of concepts seen on greenbuildingadvisor.com, buildingscience.com and the USA new wall info provided by Gregory La Vardera’s site on Swedish framing/insulation – http://www.lamidesign.com
After seeing all the horror stories regarding trapped moisture, I’m seeking advice to help me in building my own home. I’ve attached an image of the ceiling and wall designs I’d like to use.
I like the idea of the bulk of the insulation cavity being well sealed and free of penetrations that would allow moisture to enter from inside the home. The horizontal 2 x 4 cavity would not be air sealed or have an inside moisture barrier of any kind. Any moisture that enters there would dry to the inside.
Questions on roof construction and insulation:
– Can I use the less expensive 2.2” EPS (R10) for making my air channel vent under my roof sheathing? Or do I need to use the more dense, vapor barrier XPS 150?
– The 2 x 4 horizontal cavity will be filled with batt insulation. All electrical and water supply pex will go in this cavity. The only penetrations of the air sealed I-joist cavity would be for plumbing stack(s) and a chimney for a wood stove (yes, I’m providing fresh air for it).
-With this vented cathedral ceiling design, are there any concerns with dew point/condensation on the inside of the I joist cavity?
The walls will be standard 2 x 6 with blown in insulation and the same horizontal 2 x 4 cavity filled with electrical, pex water supply and batt insulation. Membrain would be used as the air/moisture barrier between the 2 x 6 wall joists and the 2 x 4 horizontal cavity.
– Any concerns?
Thanks much for any advice you can share.
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