If this is timber-framed construction you can spend a fortune on foam cavity fills with out much performance gain. Expanding polyurethane foams (open or closed-cell) are good at air-sealing, but it's not the only way. Injection foams (including Airkrete) not so much, but are still light-years ahead of batt type insulation. Sprayed/blown fiber insulation can be as-tight as injections foams if "dense-packed", usually at a fraction of the cost. In general it's easier to air seal with closed-cell foam than open-cell, but nothing should be taken for granted. Air sealing is more cheaply done using caulking & tapes (and small amounts of foam), but it requires a lot of detailing.
The thermal bridging of ~R0.85/inch wood robs the assembly of it's center-cavity performance. R20 (fiber or foam) in a 2x6 framed wall only gets ~R14 performance after the framing is factored in. R15 in a 2x4 wall only delivers R10-R11. Save the foam budget for the outside of the sheathing, where it does the most good, since it puts real R-value over the thermal bridging timbers. A 2x4 wall with sprayed cellulose is ~ R10, but add 1" of rigid polyiso or closed cell spray foam and you're at R16- better than a 2x6 full of open cell foam, and about par with 2x6 24" on-center with closed cell foam, and at a fraction of the price. (With 1" pink or blue XPS you'd be at R15.) And in a KY climate that 1" of foam would also be sufficient exterior R to prevent wintertime moisture accumulation in the sheathing, while powerfully mitigating summertime moisture drives from the exterior. With as little as an inch of exterior foam interior vapor barriers/retarders would not be necessary, and in fact would be best avoided.
More exterior foam==even better performance, but even with thin foam don't just nail through it to hang the siding. Furring strips through-screwed to the studs can support the siding more securely and will do less thermal bridging (nails & screws are thermally conductive- use the maximum spacing that gets 'er done.)
In KY there's a long-term financial rationale for whole-wall Rs as high as R25 if more-economic methods are used. (See table 2, p10: http://www.buildingscience.com/documents/reports/rr-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones KY is in zone 4.) A 2x6 wall with spray-cellulose + 2" of rigid polyiso (or 3" of EPS) on the exterior gets you there. Both the structural sheathing and the rigid foam can & should be detailed as air-barriers. With 2-3" of foam you can use 24" spacing on the fasteners for the furring unless you're going with stone veneer or something. See:
http://www.greenbuildingadvisor.com/blogs/dept/musings/fastening-furring-strips-foam-sheathed-wall
Exterior foam on timber frame may not be very common in KY yet, but 2x4 construction with R13+ 1" of XPS or iso is code-minimum. As a general rule (and one to take seriously), batt insulation installations are rife with performance-robbing errors (even with well-intentioned installers), and are to be avoided. It's a lot harder to screw up blown or sprayed fiber insulation. Low density wet-sprayed cellulose works well and is more air-retardent than low-density blown/sprayed fiberglass, but when dense-packed to 1.8lbs density or higher the new-school micro-fiber f.g. are at least as good as low-density cellulose, and comparable to dense-packed cellulose.
The difference between open & closed cell polyurethane is primarily density- about 0.5lbs per cubic foot for open cell, 2lbs for closed cell. But closed cell is also fairly vapor retardent (resists moisture permeation), where as open cell is fairly vapor-open, but not as vapor-open as fiber insulation. Both are extremely air-retardent- convection loops within the material simply can't happen the way it can with lower density fiberglass (everybodies favorit straw-man punching bag.) Where and how you use these materials in an assembly matter in terms of how rapidly the rot-suscepetible wood can dry, or accumulate moisture. Closed cell is 4x the material, but only ~1.7x the R value per inch compared to open cell foam. But in a stud bay it hardly matters from a whole-wall R point of view due to the thermal bridging.
From a moisture point of view, if you're not putting up an inch of exterior foam you'd need to use vapor-retardent latex primer or some other not-too-strong interior vapor retarder (do NOT use poly sheeting or foil!) to limit wintertime moisture accumulation in the sheathing, but depending on how you do the siding that could potentially lead to summertime moisture issues since it keeps the AC from drying out the stud cavites. One solution to that is to build a 3/8" or greater air space between the exterior of the sheathing and the siding (aka "rainscreen") and skip the interior vapor retarder. With the rainscreen in place the ability of the sheathing to dry toward the exterior in winter is good enough to keep up with interior moisture drives through semi-permeable standard interior latex paints. But simply going higher R with exterior foam oiutside the structural foam is still a superior solution.
|