Posted By Birdman on 04/14/2009 6:10 PM
Once again, I agree with Dana. I don't think a VR on the exterior of the interior suds is remotely practical - or necessary. As Dana said, airtight drywall construction will do a ton of good to keep airborne moisture out of the wall. A VR, even just under the drywall, will work in both directions ie it will block most moisture from going into the wall from the warm side but it will also inhibit drying of the cellulosic "sponge". I'm not ambivalent about this - I think a VR on the warmside of this wall is a BAD idea. I still think that you'd be well advised to do "flash and blow" meaning spraying the exterior wall with a layer of open cell foam (NOT closed cell). This will definitively seal the exterior against wind driven infiltration and, if the thickness is adequate, will keep the dew point somewhere in the thickness of the foam thus precluding condensation within the cellulose.
I think you're focusing too much on the issue of keeping moisture from getting INTO the wall and not quite enough on letting it get out, once it inevitably gets in (and it WILL get in). The open cell foam on one side of the cellulose and the airtight drywall (only) on the other side will allow drying in both directions. I would go so far as to recommend skipping the vapor barrier paint and using regular (low VOC of course) paint or even one of the clay finishes that "breathe". Also, plywood is more permeable than OSB so despite the additional cost I'd go with plywood sheathing.
As far as framing the wall I would approach it not as a true double stud wall but I would use 2x4's for the exterior side an rip 2x4's in half for the interior "wall". I'd construct an on-site jig to allow you to place a 2x4 and a 2x2 in the jig then connect them with 4 plywood gussets with a nail gun and glue. I would then use these assemblies just like studs to frame the exterior walls flat, stand them up and build the next deck on top of them. Once completely framed I'd get the roof tight, windows/doors in, housewrap the exeterior walls then foam the inside of the sheathing covering the 2x4's. Then maybe compartmentalize the cavity to allow packing the cellulose, drywall and fill the cavity from the top. I'd consult first with the cellulose installer to see how he wanted to specifically approach it so he's comfortable that he can get his density right.
Hope that helps. The really good news is you're asking all the right questions now!!!
Got any data/studies that demonstrates a need or benefit to high permeability inward-drying in cold climate zones?
There's a
strong body of evidence that keeping permeated (not just air-transported) moisture out of the wall is a good thing. In very cold or cold climates (zone 6 & higher in this map:
http://www.buildingscienceconsulting.com/designsthatwork/images/climate_zones.gif ) inward-drying would occur for only a few months of the year at best, yet outward permeation through the interior wall will occur for more than half the year. The colder the climate, the more signficant this becomes, which is why Canada's building codes require a separate-easy-to-verify vapor-retarder layer. Yes, it IS important that the wall be able to dry to the outside, but it's at
least as important to apply measures against the biggest humidity migration paths at their sources, which remain:
1: Interior side air barrier (achievable with good drywall and utility sealing technique)
2: Capillary break at the foundation top (I like the sheet copper solution to wide walls, but standard sill gasket also works)
3: Internal vapor retarder (paints are usually good enough.)
Exterior AIR-barriers can also be important, but you correctly assert that outward drying capacity has to be preserved- an exterior vapor retarder is a BAD idea in cold climates, whether there's an interior vapor retarder or not. Standard 1/2" OSB has a permeability of ~2.5 under high humidity- it's a very weak vapor retarder when exposed to high humitidy or active wetting (wind driven rain, etc, but it drops to ~0.5 when dry- it's a moving target, and acts somewhat as a humidity-diode from external vapor-drives- it allows in-ward diffusion, then blocks outward vapor escape as it dries. (But it's not much worse than plywood- both OSB & CDX permeability varies with actual humidity
conditions- between 0.5-0.7perms when dry, between 2-3perms when it's
internal humidity is high.)
Housewrap or fiberglass-reinforced fiber-faced iso sheathing with taped seams (eg. Dow Sturdy-R) will provide as-good an air barrier as a skim-coat of half-pound foam on the exterior. If we want to maximize the out-ward drying capacity of the layup (probably a good idea in very cold or cold climates) you're correct-OSB is less than ideal, but plywood isn't significantly better. R5 (1") rigid fiberglass reinforced iso has a ~3perm rating under all condtions- almost an order of magnitude more permeable than dry CDX, and at the highest-end of high-humidity 1/2" CDX/OSB.
Tyvek-type houswraps probably aren't necessary if one uses reinforced iso sheathing, but WOULD be necessary for plywood or OSB to protect the woody material from wetting from the exterior and allow the sheathing itself to dry. They run ~10-60perm ratings (depending on which product) and don't present much impedance to outward-drying.
Even if you go with the flow-through concept, relying on the wicking & hygric buffering of the cellulose for mold-protection, in cold climates ALWAYS make the interior wall less permeable than the exterior sheathing. Were they equal permeance, the annual outward drying capacity will be far greater than the shorter season of inward drying. (The converse would true in the hot-humid parts of the southern US- the eastern parts of zones 1-3 in that climate-zone map.) Standard latex- primed sheet rock has about the same permeability ~2-3 about the same as high-humidity OSB, but far higher than dry OSB. Painting with a low-permeance paint to bring it down to 1 or less will restore the annual balance to outward-drying, which is correct for most of the northern US (& all of Canada.) But using reinforced fiber-faced iso sheathing instead of OSB or CDX will pretty much guarantee excellent outward drying capacity no matter what you use for interior finishes/vapor retarders.
In zones 4&5 you can probably get away with just about anything as long as there some hygric buffering (which you get in spades with cellulose insulatio) and drying capacity of 1 perm or better in SOME direction, either interior or exterior.
I'm not clear how ripping down 2x4s saves you anything (it's a bit labor-intensive). Some Larsen Truss designs use one studwall as the structural wall using heavier framing, the other primarily as a partition to contain the insulation, others rely on both interior & exterior studwalls as structural support for roof/upper floors. But key to their thermal performance is the very minimal thermal bridging between the inner & outer walls- just the tie elements.
I'm a bit surprised that the used TJIs instead of open web trusses in the
Smith House:
http://www.greenbuildingadvisor.com/sites/default/files/images/PassiveHouseInstitute1.gif The webbing of a TJI is many-fold higher thermal bridging than in a Larsen truss:
http://www.greenfret.com/images/house_truss_lg.jpg There may have been valid cost &/or structural reasons for doing so, but the performance of the wall will be measureably less than a Larsen truss of equal thickness. They also went with blown fiberglass instead of dense-packed cellulose, offering NO hygric buffering, and no exterior-wall thermal mass benefit to speak of (although by the concrete slab provides a reasonably amout of interior thermal mass.) Blown fiberglass also has settling issues similar to those of low-density cellulose- I'd be curious to know the rationale behind those design decisions. (I'd assume that from a whole-lifecycle, embodied energy & sustainable-greenliness of fiberglass would be somewhat less desirable than cellulose.)