hybrid wall insulation consisting of 2″ rigid foam + 2×4 fiberglass batts?
I was flipping through some of my back issues of Fine Homebuilding, and readng different articles about convection currents in wall cavities (worse with fiberglass insulation; non-existant with spray foam), spray foam insulation, and one about using 2″ rigid foam instead of OSB sheathing on frame construction 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 is messy and next to impossible to remove easily once it’s in the wall cavity.
– Using 2″ rigid foam instead of OSB 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 interio/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 2×6 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)
Is there any reason this couldn’t work? I never hear of anyone promoting this approach, and wonder why it’s not commonly done. Thanks for any insight!
Replies
Start this off w/ some food for thought. Item #2 seems strange to me. Your foam is only partially filling the cavity. You still install fiberglass batts ... so you still have the potential of convective heat loss. Also installing foam like this would be a royal pain. Cutting, fitting, etc. Wow a lot to do there. While it can be done, I wouldn't tackle it unless I really felt it was really necessary.
Consider high density fiberglass batts. It will work fine ... IF you take the time to do a good job installing it. Cut it to fit ... +1/4" -0". Fill all cavities.
Another good option: cellulose. Many here would really promote it as probably being one of your best options all things considered. It is recycled material, has a reasonable R-value, is economical/inexpensive, and will help minimize convective heat loss, and it 'naturally' fills all cavities (this may not be absolutely true, but it comes very close).
A colleague of mine started doing 1/2" polyison sheathing on his houses. He let in the sheer bracing (e.g. metal strap or wood diagnonals). He kind of liked it ... he could easily find the studs to nail siding to (personally I don't think that is a big problem anyway). This approach cut the thermal bridging affect of the framing and added the additional ~ R-3.5 and left his framing/structure w/ a conventional thickness for doors, etc.
As far as cellulose and future changes ... should be few issues. But I think it can be tough to second guess what the future has in store for e.g. foam insulations or any other for that matter. It'd be a royal pain no matter what. With all the focus on safe products, I'd venture to say that current products that have been in use for a long time, now (e.g. XPS, EPS, polyison, urethanes, etc) are probably reasonable bets. I know nothing about the icynene stuff or the aerogels, though.
Others will undoubtedly add more food for thought.