There's quite a price difference ($9 for OSB vs. $26 for CDX plywood. I'll have double stud walls (R-42) and am worried about condensation on inside of ext. sheathing. Holladay, I believe, says plywood is safer. What say you...??? THANKS -michael

For that price difference, consider alternatives that also help with moisture performance. For example, an inch of EPS foam insulation, a rain screen, MemBrain on the interior, cellulose.

Also consider plywood on the corners (for racking) and just EPS elsewhere (both covered with furring strips). Or steel straps and EPS (no plywood).

Plywood is safer. On the other hand, what has been found so far is that the OSB is getting damp but drying out, so the jury is still out on whether the OSB is safe to use long term. That said, it is ultimately dependent on how tight the house is overall, and the wall in particular. Do a blower door prior to insulating, use a smart membrane on the interior and achieve a ACH50 of under 1.0 and you should be good with the OSB.

Vented rainscreened siding and eave/rake overhangs of at least 1' per story go a long way toward keeping the OSB dry. On the exterior of a thick wall assembly the OSB is far more susceptible to exterior bulk water than it will be from interior moisture drives, especially if you use air-tight methods on the interior side of the assembly.

Using cellulose as the cavity fill also buffers much of the wintertime moisture accumulation safely in the insulation (without losing function.)

Plywood is less susceptible to moisture overall, and plank sheathing is less susceptible than plywood.

An inch of EPS won't dramatically raise the average temperature of the OSB on a R42 cavity fill, but it won't won't reduce the drying rate TOO radically, but some. The OSB would probably run drier if you skip it- just be sure to have at least 3/4" of rains-screen depth, and vent it properly both top & bottom, paying special attention to venting it below window openings (where occasional bulk wetting is more likely) , and be obsessive about the flashing details.

Thanks to all for your responses.

I plan on getting the walls (inside and outer wall) as tight as I can. I was leaning toward using Roxul insulation.

What about Zip System sheathing. It's about $19 per sheet, but nice for taping the seams. It's still OSB, though. Otherwise I was looking at plywood and Siga Wigluv tape for the seams.

As for rainscreen (under cedar lap) I was going to use Tyvek Drainwrap (dimpled housewrap). The cedar people recommend a dimpled wrap. They say 3/4" furring isn't necessary(?)

So if I used Zip, I'd have a double water barrier.....

Thanks again!
-michael

You can use ZIP sheathing with the ZIP tape or OSB or plywood with SIGA tape. I disagree with "the cedar people" in that while the 3/4" air space may not be "necessary" from their standpoint, it WILL help the wood to dry evenly and will prolong it's life as well as the life of the coating . Some folks use Cedar Breather, another petrochemical solution, but it's hard to get a flat surface on your siding. 1x wood boards go on fast, are inexpensive, local, renewable, and most important, work.

I'm with Bob- a 3/4" air gap is a heluva lot better capillary break than anybody's housewrap, and moves a lot more drying air through it to boot.

From the Western Red Cedar Lumber Association website:

For maximum performance, the Western Red Cedar Lumber Association recommends DuPont™ Tyvek® DrainWrap™ for installing Western Red Cedar siding.

So I think I'll skip the furring. Maybe if I lived on or near the coast.....

Thanks for your thoughts.
-michael

Posted By pbrane on 20 Aug 2014 08:53 PM
From the Western Red Cedar Lumber Association website:

For maximum performance, the Western Red Cedar Lumber Association recommends DuPont™ Tyvek® DrainWrap™ for installing Western Red Cedar siding.

So I think I'll skip the furring. Maybe if I lived on or near the coast.....

Thanks for your thoughts.
-michael

Skipping the furring is a bad idea unless you're in a true arid zone (Mojave desert, perhaps?).

The tannins in cedar used to break down the polymer in housewraps of the 1980s. I'm hoping they have mostly fixed that, but would still be reluctant to put cedar in direct contact with a polyolefin housewrap, given the history.

While DrainWrap is a better capillary break than smooth housewraps, it's nowhere near the capillary break of an air gap. It offers orders of magnitude lower convective drying for the OSB.  It's useful for "innie" window mounts on foam-over assemblies where the Drainwrap is between the sheet-foam and sheathing, but when the housewrap is next to the siding in the stackup even 1/8" of air gap is far superior than tight to the wrap.

The Western Red Cedar Lumber Association's concern is primarily for protecting the siding (and maybe the sheathing), whereas your concern would be the overall moisture resilence of the assembly.  Drainwrap offers nowhere near the drying capacity of a rainscreen gap. Rainscreens are pretty cheap insurance.

Put the OSB on the outer face of the inner wall with NO exterior sheathing, just house wrap over studs and mineral wool bats. For absolute minimum condensation risk to sheathing on the outside of inner wall use aligned framing. Sheathing in this position should be the air barrier eliminating the need for airtight drywall.

3/4" Furring strips are the easiest and best way to dry your cladding out after rain events. Besides it also extends the life of the finish applied to the cladding because it drys out so quickly.

It would be interesting to see numbers for Drainwrap/Stuccowrap such that one could predict where it is adequate.

Also interesting would be numbers for the thermal effect of a 3/4" air gap directly against Roxul Comfortboard (without an air barrier between them). Some air must flow through the insulation. Also, does it hold water?

ComfortBoard IS is fairly high density at ~8lbs per cubic foot, and is extremely air-retardent, if not a true air-barrier, and it's not as if you can tape the seams to make it a quasi-air barrier.  But it's probably not going to suffer performance loss without air barriers both sides of it- even their standard density batts are pretty good in that regard.

It tests out at about 30 US perms  from a water vapor permeance point of view.  That's more vapor permeable than Typar housewrap, but less permeable than standard Tyvek Homewrap:

Like other rock wool products it is fairly hygrophobic to liquid-water- moisture will bead up rather than wick.  

So "...does it hold water?"

Not much. It can't violate the laws of physics from a condensation/frost-formation point of view, but it doesn't have the adsorption characteristics of cellulose or fiberglass- it rejects bulk water fairly well, and has a very favorable drying rate even when liquid moisture forms in it (or on the sheathing.)

How about foam insulation? It can help keep the heat away.

Posted By DylanMitchell on 23 Sep 2014 12:20 AM
How about foam insulation? It can help keep the heat away.

Sure, but foam also impedes drying toward the exterior. You would have to use enough foam to keep the mid-winter sheathing temperature average at about R38- 40F or so, and with R42 of cavity fill it would take a LOT of foam to keep the sheathing that warm.  

Rigid rock wool is extremely vapor permeable, and will not impede drying.  So if you are building a stack-up where the exterior-R is too low for dew-point control at the sheathing layer, it's much better to use rock wool than foam.

EPS foam is permeable enough that it can be used on the exterior without needing lots of it. While I wouldn't do it, even 1" of XPS is common - generally without problems.

On the other hand, nothing breaths better than no sheathing. So why use it? There are other ways to address racking and thermal bridging.