I was flipping through some of my back issues of Fine Homebuilding, and reading different articles about spray foam, convection currents in wall cavities (worse with fiberglass insulation; non-existant with spray foam), and one about using 2" rigid foam instead of OSB sheathing to improve insulation performance. Which got me to thinking...
I want to build a guest house on my property next spring, and am debating what type of insulation to use. Spray foam performs the best, but is expensive, and becomes a permanent part of the structure. The former is not a big deal, but the latter concerns me because of the long history of finding out years later that certain building materials are defective and a health hazard (e.g., asbestos, urea formaldeyhde foam, mold in open-cell foam, etc). With a spray foamed house, the structure is basically a loss if you have to remove the foam later for health reasons.
All the insulation types seem to have downsides:
- Spray foam performs well, but is messy, needs to be professionally installed, and is next to impossible to remove easily once it's in the wall cavity.
- Using 2" rigid foam instead of OSB on the outside of the framing is suboptimal because of noise transmission and the fact that it doesn't strengthen the framing. OSB or plywood sheathing not only provide a surface for exterior cladding, but add to the rigidity of the shell and cut noise transmission between the interior/exterior.
- Just using fiberglass batts gives you flexibility for abatement later, but is a poor insulant and the worst solution for convection currents that siphon away heat
My question: Could a hybrid solution be used that balances out the pros and cons of each? Here's what I have in mind:
I want to build a guest house on my property next spring, and am debating what type of insulation to use. Spray foam performs the best, but is expensive, and becomes a permanent part of the structure. The former is not a big deal, but the latter concerns me because of the long history of finding out years later that certain building materials are defective and a health hazard (e.g., asbestos, urea formaldeyhde foam, mold in open-cell foam, etc). With a spray foamed house, the structure is basically a loss if you have to remove the foam later for health reasons.
All the insulation types seem to have downsides:
- Spray foam performs well, but is messy, needs to be professionally installed, and is next to impossible to remove easily once it's in the wall cavity.
- Using 2" rigid foam instead of OSB on the outside of the framing is suboptimal because of noise transmission and the fact that it doesn't strengthen the framing. OSB or plywood sheathing not only provide a surface for exterior cladding, but add to the rigidity of the shell and cut noise transmission between the interior/exterior.
- Just using fiberglass batts gives you flexibility for abatement later, but is a poor insulant and the worst solution for convection currents that siphon away heat
My question: Could a hybrid solution be used that balances out the pros and cons of each? Here's what I have in mind:
- Conventional 2x6 framing and OSB sheathing on the outside (adds rigidity, cuts noise transmission, provides a solid cladding surface)
- Use 2" rigid foam (vapor permeable) on the inside wall cavities, right against the OSB sheathing (performs better than fiberglass batts, and cuts down on convection currents and heat loss)
- Fill the rest of the cavity with 2 x 4 fiberglass batts (adds to the R-value and provides flexibility if I ever need to get into the wall cavities again)