Double exterior wall question
Since I already own the OSB sheathing for our new central Indiana home, I’ve been stressing over all the concerns with using OSB in a double exterior wall assembly. I’m trying to come up with a solution that keeps my sheathing on the exterior of the double-wall assembly – and prevents it from turning to oatmeal!!
Is there a concern with assembling the exterior wall as follows:
1) Frame 2×4 exterior structural walls. The sole plates would sit on top of foundation. The interior walls would rest on top of floor system.
2) Apply membrane (Tyvek, 6 mil poly or even 45 mil EPDM?) to outside of wall framing – from top of foundation and terminate above attic insulation. This would serve as the air barrier and vapor retarder – the purpose for this is to reduce the high levels of summer humidity from migrating toward air-conditioned space.
3) Install vertical 1×4 strapping to wall studs (on top of membrane). No horizontal strapping – only insect screens at bottom and top. The ¾” air space allows air flow between the exterior sheathing and the membrane/wall cavity insulation.
4) Install OSB sheathing
5) Install water barrier (Tyvek or 2 staggered layers felt paper)
6) Install vertical 1×4 strapping on top of water barrier (Again – no horizontal strapping – only insect screens at bottom and top). This allows air flow between cement lap siding and water barrier/OSB. (Also allows me to gun-nail OSB and furring rather than screw through add’l thicknesses of an exterior foam board assembly)
7) Install horizontal cement board siding.
8) Inside: Install 3” rock wool insulation against the vapor retarder and fill the remainder of the wall cavity w/dense packed cellulose. Cover interior walls/ceilings w/drywall.
I believe this technique will keep the OSB as dry as possible, but am concerned with potential for winter humidity coming from within the home and condensating on the vapor retarder. I could use insulation netting instead of the vapor retarder, but am more concerned with drawing the summer humidity into the walls.
Any thoughts on this?
Thanks
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Replies
Matt,
Your plan to install a 6-mil poly or EPDM vapor barrier on the exterior side of the insulation is unusual, to say the least -- especially since you live in Indiana. Your plan might even be described as "potentially disastrous."
In winter, you'll get condensation on the interior side of the vapor barrier, and the water will drip down and form puddles on your bottom plate. After just one season, your bottom plate (and the bottoms of your studs) will be covered with mold and beginning to rot.
Martin - I appreciate your (surprisingly fast) response.
There's no "Plan" yet – just thinking out loud and researching options.
The 12/09 FH article: 6 PROVEN Ways To Build Energy-Smart Walls (https://www.greenbuildingadvisor.com/fine-homebuilding/six-proven-ways-build-energy-smart-walls ) sold me on the benefits of a double-wall assembly.
Since I prefer to avoid using any type of foam (off-gassing, flammability, reduced R-value at reduced temps, insect tunneling, buying yet MORE petro, etc.), Wall Design # 1 made the most sense to me for new construction.
Wall Design # 1 specs the interior paint as the vapor retarder, taped/caulked OSB as the air barrier, and off-the-shelf house wrap and rain screen. This seems like an easy, fast and affordable approach for constructing a wall assembly that gets you in the neighborhood of R-40. R-40 buys you comfort and better re-sale 10 to 15 years from now.
Wall Design # 2 uses twice the OSB, 50% more vertical framing, and implied by the sketch, a few miles of continuous acoustical sealant to secure the mid-wall polyethylene to the OSB. Aside from the fact that you couldn’t shift the existing floor system inward to support the assembly “during retrofits” so interior “walls can remain intact”, this is actually a great remodeling idea for many existing homes – and the mid-wall poly idea does (did?) make sense to me.
I should’ve stopped reading at that point…
(The evolution of wall assemblies has made the wall selection process similar to researching and buying a new PC – you eventually force yourself to stop looking after you’ve made your decision to avoid second-guessing yourself into madness!)
After reading GBA articles describing the concern that (below dew-point) OSB/Plywood becomes saturated when installed on the exterior of a double-wall system, and other articles that recommend to NOT use polyethylene in walls of an air-conditioned home… I began to question my earlier decision on wall design.
1) The info from Table N1102.5.1 https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing) “spells out the MINIMUM R-values for exterior foam TO BE SURE that moisture won’t accumulate in a wall”.
Per the Tabel, my Zone 5 requires R-5 on a “2x4 wall” and 50% more (R-7.5) for a “2x6 wall”.
a) Am I correct in presuming I’d need a minimum of R-15 to R-17 (3.25” of XPS foam or min 4” inches rock wool) outside of a 12” thick wall assembly (cavities filled w/12” dense-packed cellulose @ R-3.5 per inch = R42)?
b) The Table-spec’d R-5 foam represents 38% of the R-13 value of the fiberglass inside a 2x4 wall; R-7.5 foam is pushing 40% of the R-19 glass in a 2x6 wall.
c) Pointing this out because the table doesn’t identify the R value of the wall – only the framing member sizes within the wall.
2) I’ve also read in the GBA blogs that a good rule of thumb is to keep 2/3 of my R-value on the exterior of my sheathing.
a) Neither of the two examples above represent 66% of the inner wall’s R value (?)
(Trying to avoid a discovery 5 years from now that I built something that “almost worked”)
As an FYI, there are links within your pages that bring you to exterior wall designs that appear contradictory to designs that you currently promote (?)
I’ve been a subscriber to FH since the early 80’s. Aside from the fact that most of the “Fine” homes are well beyond the reach of most of the taxpayers who subsidize the manufacturers that develop ideas, solutions, etc., the magazine has never been anything less than enlightening and inspiring.
As far as the Pro GBA membership is concerned, I consider 15 bucks a month a bargain for the novice contractor to minimize bumping his head for the next 10+ years, but rather pricey for the seasoned pros that chime in and provide a caliber of FREE ADVICE that results in increased membership (and brings commercial guys like myself back to a more interesting conversation!).
Thanks again - you have a great thing going on!
Search here for Sunrise House, and see what Lucas Durand has going on as well: two examples with a good track record. You'll see other examples here, too.
Matt,
The thicker the insulation on the interior side of your OSB sheathing, the colder your sheathing will be.
In most cases, exterior rigid foam is only used for 2x4 or 2x6 walls -- not double stub walls. The reason is simple: if you have 12 inches of cellulose on the interior side of the OSB, it would take tremendous amounts of rigid foam to get the OSB above the dew point. And in any case, you don't need any more R-value -- you're already at R-40.
The whole point of choosing the double-stud wall approach is (usually) to avoid rigid foam.
If you really want to calculate how thick your foam would have to be to keep the OSB on the exterior side of a 12-inch-thick double-stud wall above the dew point in January, I have written a blog that provides instructions for making the necessary dew point calculation: Are Dew-Point Calculations Really Necessary?
Martin,
I understand that the thicker the cavity insulation is, the colder the outboard sheathing will be.
“In most cases, exterior rigid foam is only used for 2x4 or 2x6 walls -- not double stud walls. The reason is simple: if you have 12 inches of cellulose on the interior side of the OSB, it would take tremendous amounts of rigid foam to get the OSB above the dew point. And in any case, you don't need any more R-value -- you're already at R-40.”
I don’t want to use foam. The only reason I asked about the requirement for exterior foam is because I’m confused by what appears to be a contradiction in how you address moisture concerns w/exterior sheathing among varying wall assemblies.
"The whole point of choosing the double-stud wall approach is (usually) to avoid rigid foam."
Your Double Wall Design # 1 assembly does not insulate the cold/outboard side of the OSB. ( https://www.greenbuildingadvisor.com/fine-homebuilding/six-proven-ways-bu... )
• The warm-side vapor retarder/air barrier consists of nothing more than sealed and painted drywall. What moisture/vapor that does escape the living areas during winter should still migrate through the cellulose and condensate on the OSB – shouldn’t it?
• Here’s the contradiction: a 2x4 or 2x6 wall requires exterior foam to keep the OSB above the dew point. Your R-40 Double-Wall Design # 1 will result in OSB temp that is much colder. Please explain why there’s no concern w/the R-40 DW design or the inevitable moisture that would condensate on the OSB.
Thanks
Matt,
I never said there was no concern. Read about all of the concerns here: How Risky Is Cold OSB Wall Sheathing?
Egads! That's the article that sparked this circular conversation :--)
Matt since you own the OSB, here is how you can use it relatively safely and avoid foam.
Use the OSB to sheath the INNER wall's outer surface! With a 2x4 walls and a 3 1/2" or maybe 3 3/4" space between walls. If some form of exterior sheathing is desired use fiberboard, it is vapor open so drying to the outside is maintained. if you take this option it's best to align the inner and outer framing which will assure that the first condensation will be mid cavity where there is over twice the r value on the outside of the OSB. Stagered studs would only add about r1 and make the moisture issue happen at a far higher outdoor temperature. Such a wall will have no condensation with indoor temperature of 70 f and 40 f dew point (about 35% RH pretty high for winter) until outdoor temperature is below -20 f
I plan on building this way with Roxul. The construction order is problematic with 2 sheathing layers and insulation between. Either the outer wall is erected unsheathed with it's sheathing hung after insulation or insulation between walls must be installed before the inner wall is erected..
I'm leaning toward the second and using 7 1/4", r30 bats in the outer wall with strips, 1 1/2"x 3 3/4" of XPS behind each stud. Even with 16" OC framing I'll be over r40. To totally avoid foam the strips could be Roxul for a small cost saving and r 1.3 hit in thermal performance Another option is erect the outer wall unsheathed then add netting and blow in cellulose add an exterior WRB, furring and siding. With the OSB on the outside of the inner wall it is the primary air barrier so it needs to be caulked and sealed appropriately. OSB is itself a "smart vapor retarder" with a perm of about 1 when dry and increases permeability as it gets moist. It gives a "service cavity" on the inside and removes any need to attempt to seal the drywall.
Jerry, that's interesting about aligning the studs, I hadn't thought of that.
A couple of thoughts on deciding the construction sequence. I would imagine the primary consideration is which wall is load bearing. i also think that if the inner wall is to be air sealed, building it second might make this process a lot harder.
Malcolm,
The load bearing wall needs real structural headers while the other essentially needs only cosmetic headers. I don't see how which wall is load bearing affects construction sequence. Making the inner structural allows inspection at a more convenient time. I have concerns about wood shrinkage being greater in one than the other. The wall with the most cross grain wood will shrink more. Since, in my case, my outer wall sits on the concrete of a basement and the inner sits on a floor supported by a wood wall on the inside of the basement it's clear that the inner will shrink more so I've designated it as load bearing. I plan on putting an EPS strip 1" thick under the outer wall to serve as a " crush" strip as shrinkage occurs.
As to air sealing being more difficult, I don't see how it would be. I plan to use construction glue under the plywood or OSB and under the plate on the sub floor and in each corner as they are assembled. With additional acoustic calk and spray foam during testing, which will start before the service cavity is insulated or closed..
Which wall is load-bearing may influence the sequence of insulation. You suggest that the outer wall could be sheathed and insulated before the inner ones were built. But given that you probably don't want to do any insulating before having the roof on and the project dried in, this option isn't available if the inner wall is the load-bearing one.
Construction and air-sealing the inner wall will be more difficult if the outer perimeter is already in place. Presumably you are building and sealing the sheathed wall on your floor deck as you will have no access to the sheathed side once the wall is raised. That means you can not build the wall in sections but must raise the entire length at once. If you intend to extend the sheathing below the bottom of the wall as an air-eal to cover the floor system and sill plate, the outer wall will make raising the inner one very difficult.
Malcolm,
I agree it's best to sheath the outer wall after the roof to avoid water loaded insulation which may take a long time to dry. I'm now thinking that I should build BOTH walls as load bearing and use the EPS to assure reasonable load sharing. This too argues for delaying sheathing the outer wall to satisfy the inspectors. Another benefit of delaying the outer wall's sheathing is it allows access, from the outside, to the inner wall's air barrier for additional caulking. I've changed my mind!
I doubt that the moisture condensation benefit of aligned studs in double walls with embedded moisture control layers is appreciated by many. I've never seen it discussed but I have seen several discussions of the small hit to thermal performance, some even showing rather elaborate 3 d simulations.