Posted By Nashvegas on 15 Oct 2014 11:55 PM
Gut reno on a house in East Nashville. Stripped to studs. Walls are full dimension 2x4 with diagonally placed 1x6 yellow pine with brick veneer( 1" air gap between sheathing and brick) Large overhangs. Brick has been painted with 4 coats of paint. Thinking of flash and batt with OC foam to seal the 1x6 joints, and Roxul for sound attenuation.( fire house is a half block away). I was planning on a vapor barrier on the inside under the gyp. Is this a reasonable stack up?
Second question - attic was finished off, is stripped and will be redone. Problem is that rafters are full dimension 2x4. Collar ties have been reset at 10' height, and there will be knee walls, so I'm actually beefing up the overall strength by cutting down the open span. But trying to figure how to get at least some marginal insulation in the roof. Thinking of creating an air gap under the sheathing by running 3/4" inside the sides of the bays, and adding 3/4" Foilbacked Iso to gain some reflectivity. Then rip down 2x4 and add to the edge of the rafter to make up what I took with the previous assembly. Again use Roxul to finish off. Obviously, not ideal from an R value, but better than the total lack of insulation originally there. Would use Membrane to retard moisture and allow drying to the inside.
Lastly on the knee walls, was again planning on running Foilbacked ISO, this time on the exterior of the studs, picture frame with Dow foam, and finish with Roxul. Again use Membrane.
Thoughts
That's a TERRIBLE wall stack up for a brick veneer in TN! With an interior side vapor barrier you would be guaranteed to have chronic condensation inside the studwall cavities during the cooling season.
A flash'n'batt with 1" of CLOSED CELL foam would put a ~1 perm vapor retarder between the sheathing & interior, which would be sufficiently vapor retardent to allow the interior side to remain fairly vapor open, allowing the studs to dry toward the interior. With ~1" of closed cell foam and 3" of fiber in the full-dimension 2x4 cavity you would have more than enough foam-R for wintertime dew point control at the foam/fiber boundary. The sheathing would still have some capacity to dry toward the interior, but it's drying direction would be primarily into the cavity.
Batts are not the best choice for old full-dimension framing. Even if the stud spacing is a standard 16" o.c. the batts are designed for a nominal cavity width of 14.5", not the 14.0" you would actually have. Compressing it into a 3" depth instead of 4" isn't a problem (that increases the R/inch), but the narrower cavity width causes buckling & void issues. It's FAR better to use blown insulation in those cavities (either cellulose at 2.8lbs minimum density, or new-school fiberglass at 1.8lbs min. density.) In a full-gut situation this is best done blowing in mesh, and rolling it flat to the studs before hanging the drywall.
Even an all open-cell foam solution (4") would be a problem during the cooling season, since it's too vapor open at 4" to be fully safe, and you may end up with failing interior paint or mold in the drywall facers during the cooling season. But if you put an vapor barrier in there you would end up with soggy open cell foam, and potential rot on the studs.
On the vented insulated roof deck, code demands a minimum of 1" of air space between the roof deck and insulation, but 2" is even better. It also has to have full soffit to ridge ventilation to really work. Here again you can do OK going unvented with a flash'n'fill approach using closed cell polyurethane. The amount of foam-R needed depends on both climate zone and the total R. Nashville is in US climate zone 4A:
Per
IRC 2012 chapter 8, if you were building to the full IRC 2012 R49 code min for zone 4A you would need R15 for dew-point control at the foam/fiber interface, and you would skip the interior vapor retarders. But you're not going to be insulating to anywhere near R49. What is important is the RATIO of foam-R to total-R. R15/R49 is about 30% foam with about 70% fiber. If you're only installing R15 of fiber, an inch of closed cell foam is enough. If you're bumping that to R25 of fiber, 1.5" of closed cell would be in order. Even at 2" the vapor retardency of the foam would still be high enough for the roof deck to dry at a very reasonable seasonal rate, and even 1" would be sufficient to limit the wintertime moisture loading issues to something quite tame.
If you rip down some 1" x 2" rigid polyiso strips for the rafter edges to adjust rather than ripping down wood, you would cut the thermal bridging by more than half, which is a huge performance improvement, but you DON'T want to put foil faced iso over the whole interior side of an unvented roof assembly since it would create a moisture trap- the vapor permeance of the foil facers are WAY too low. You could do that with 1" unfaced EPS though, since at that thickness it has a vapor retardency no tighter than latex paint, and won't create a moisture trap.
On the kneewalls, it's far easier to air-seal the house if you just continued down the rafter line with whatever you do for the cathedralized ceiling section. If you insulated the kneewalls and mini-attic floor instead, you would have to meticulously build air dams for every joist bay running under the wall, and air seal each with can-foam or something, as well as any electrical/plumbing penetrations coming through the mini-attic floor. Doing it at the roof deck ends up being less work, fewer square feet of insulation and higher performance. On a retrofit, air-sealing each of the joist bays usually ends up being a fools errand (speaking as one who has played that fool multiple times.
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I'm not a big fan of closed cell foam due to the fact that it's blown with HFC245fa, which has very high global warming potential (about 1000x CO2) But when used in thinner layers like this for dew point and vapor retardency control it's still "worth it". Similarly XPS is blown with an even worse stuff (HFC134a, at ~1400x CO2), but both polyiso and EPS are blown with comparatively benign pentane (~7x CO2).