FHB 190 Remedies for Energy Nosebleeds
In this Oct/Nov 2007 article author Bruce Harley points out that for cantilevers, the of the underside should be sheathed and caulked where many are covered with a vinyl soffit.
My whole house is elevated up in the air (for a view) and is essentially oe big cantilever. Local builders and building companies suggested I insulate the underside and cover with a breathable fabric… similar in purpose to the above mentioned vinyl soffit. Their arguement is our environment is so moist that doing anything else, over time the insulation won’t be able to breathe and become soggy and not effective.
After doing as locals suggested the house floor is still brutally cold and I am inclined to try sheathing the underside of the house. I currently have 2×10 floor joists with R30 bats covered with a breathable mesh.
What do you think? Sheathe or not to sheathe? I am leaning towards giving it a go. Should I sheathe with hard foam and caulk or with plywood/OSB and caulk?
Nathan In Juneau, AK
Replies
Huh, I hadn't read that article thoroughly--the picture they use for an example of an uncovered overhang is next door to a house I worked on. The new shingles under the electric service I installed when the service got changed.
Anyway, there is no reason the underside of your house shouldn't be just as airtight as your walls or roof. If allowed to "breathe," the batt insulation isn't doing squat. The best solution is to use foam, either closed-cell spray foam or rigid foam panels sealed in place with expanding foam. It's impervious to moisture and eliminates air flow.
If you decide to stick with fiberglass, seal up the underside with housewrap like Tyvek or Typar, and cover it with something to keep critters out like plywood or asphalt-impregnated homosote.
What about foam insulation board nailed under the joists, taped together, and then something like T1-11 under that? I'd have more faith in the foam than in Tyvek as an air barrier, and you could also reduce bridging.
No need for the t-111 really. Treat it like a soffit old style and use ac ply. Or does t-111 have something over ply that I'm missing?
Just thinking of something semi-finished looking. AC would be fine.
Maybe I'm not enough of an abstract thinker, but I'd love to see a photo of your house. Cuz I'm not really gettin' it.
Edited 4/25/2008 7:42 pm ET by Biff_Loman
We did an addittion on piers last year and did r30 with 2" pink foam and taped.
Sealed up all of the edges then strapped it and had solid vinyl sophitt installed over it.
Plywood would work to as well as certain types of gypsum board.
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