Can closed-cell foam insulation be used to entirely fill double-stud exterior-wall cavities?
Can closed-cell foam insulation be used to entirely fill double-stud exterior-wall cavities (i.e., about 7″ deep) for an entire wall? Other than higher cost, does this plan suffer from technical drawbacks?
Background:
Master bedroom to be expanded over an un-heated garage. In the process, remove all sheet rock and fiberglass-batt insulation in the existing bedroom. Wall of addition will be continuous with the existing exterior wall (1/2” plywood sheathing); a second 2×4 stud wall will be constructed on the interior side of the existing addition, and house wrap (e.g., Tyvek) used as the exterior WRB (water-resistant barrier). Exterior foam-board insulation cannot be added due to existing siding on remainder of house. Exterior siding is asbestos (9”x48”); bottom of each course of siding is held off the course below by a ¼” wooden wedge strip (a horsefeather). Exterior of the addition will use cement board that matches the existing asbestos siding. This arrangement permits drying to the exterior. House is located in climate zone 5 (New Jersey). Interior side will be finished with ½” gypsum; no separate vapor barrier to be installed (closed-cell foam to provide this function).
The underside of the addition (the portion over unheated garage) and the attic will also be insulated with closed-cell foam.
Alternatives:
There appear to be a few alternatives to the above proposal.
1. Use closed-cell foam only on the perimeter of the external stud cavity to ensure a good vapor seal, leaving the main portion of the external sheathing visible. Completely fill both stud cavities with fiberglass batts, attach a vapor barrier to the interior-stud face, and finish with ½” gypsum.
2. Use closed-cell foam only on the perimeter of the external stud cavity to ensure a good vapor seal, leaving the main portion of the external sheathing visible. Completely fill both stud cavities with loose-fill cellulose, attach a vapor barrier to the interior-stud face, and finish with ½” gypsum.
3. The use of solid foam sheets on the exterior is not a viable option since the addition needs to align with the existing exterior wall.
4. Other alternatives?
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Replies
My preferred option would be to spray 2" closed cell foam in all cavities and finish insulating with dense-pack cellulose. No vapor barrier, just drywall.
Martin,
As long as the supplier of the closed-cell foam provides documentation showing that the foam has been approved for installation at 7 inches of thickness (some brands have, and some brands haven't, due to fire safety testing issues), there is no reason you can't do it.
Your concerns about "vapor seals" and "vapor barriers" are misplaced. There really aren't any vapor-diffusion concerns with these walls. Pay attention to air sealing and you'll be fine; there is no reason to worry about vapor diffusion.
Almost all closed cell foam has to be installed in 2" lifts to limit shrinkage issues and fire hazard during installation. ( The curing is an endothermic process, and if layed on too thick in any one pass the R value of the outer layers "cook" the middle, which creates both the shrinkage and fire hazard.)
The high-R of closed cell foam is wasted whenever it's between studs. The difference in whole-wall R value on a 2x6 studwall between an open-cell stud fill between an open-cell/cellulose vs closed cell foam is about R2. Doing the cavity fill with the cheap stuff and adding 3/8" more depth to closed cell in the space between the studwalls.
Closed cell foam also uses blowing agents with severe greenhouse gas potential- I try to design it out to the extent possible until the newer less damaging stuff is out there.
Dana,
I think you meant "exothermic," not "endothermic."
I need an editor! (Thanks!)