Vapor barrier on double wall system
So in my ever evolving build I’m begining to question my plan to use an intelligent membrane in my double wall system. My original plan was to use an intelligent membrane on the outside of the interior wall of a 16″ thick double wall which would then be filled with dense pack celulose. The house is in zone 7A, central Quebec. I’m now questioning whether I’d just be better off using 3/8″ osb instead as it would be more robust and be able to take the pressure of the dense pack. Is there any glaring problems with this approach? Cost wise its a little bit cheaper and on the labour front all the labour is mine, so highly discounted. Windows are outies. The exterior sheathing is 1/2″ ply with a 3/4″ gap between the sheathing and the metal siding. The exterior sheathing has been treated as the air barrior.
Thanks in advance for your opinions,
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Replies
Geoff,
Are you planning to tape the inner layer of OSB, or just use it as a vapour barrier? I ask because I'm not sure 3/8" won't flex, especially if the framing is spaced at 24" oc. Other than that concern, which may well be unfounded, I think switching from the membrane will make the walls a lot less finicky.
The idea was to tape the joints, is that overkill? The interior stud spacing is 16" so I can't imagine that flexing would be a problem.
Not overkill, taping the joints is a good idea. You need to end up with a less than 1 perm vapour barrier somewhere to meet code. That probably means a vapour barrier paint on either the OSB or drywall - and will make it easier to maintain the ratios Jon has suggested.
Sounds like you intend to, but follow the moisture recommendations here. I'd go even further and have at least a 5:1 perm ratio (external:internal).
A second, interior side air barrier will be beneficial.
Jon R -
I am not sure how you apply the rules from this BSC resource on vapor retarders to Geoff's double stud wall assembly. The OSB layer becomes the first condensing surface, is warmed by the insulation to its exterior, and is a bit of its own smart vapor retarder (see attached data from relatively recent research on sorption isotherms for Huber OSB).
I am not sure what the recommendation should be for Geoff's assembly for the INTERIOR vapor retarder (just to the exterior of the interior sheathing (GWB). I bet using Lstiburek's representative interior temp and RH Geoff can get away with a Class III but using the Table in the BSC article, the conservative approach would be a Class II interior VR.
Peter
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Canadian building codes require a warm-side vapour barrier (up here that's <1 perm), so a class 111 VR wouldn't fly. The required vapour barrier can be moved as far in from the interior wall surface as design conditions allow.
Peter: The OSB may not be inside of all insulation, but I'd keep the principles in mind. The material at risk is the exterior plywood. It's cold, so make sure that it has much more drying (to the exterior) than wetting (from the interior). Where exactly the interior moisture is blocked makes little difference (as long as that point is always warm and there is no airflow creating a VB bypass). So my take: either OSB or GWB < 1 perm can be OK. Apparently this is also compatible with Canadian code (it allows some flexibility in vapor barrier placement).
From Geoff's description he plans to locate the vapour-barrier on the outside of the interior-studs in a 16" wide wall. Your description of what's at play sounds right to me.
Jon: The osb would be on the inside of the insulation. The wall assemby as proposed is gyps-2x4 wall(16" oc)-OSB (taped)- 16"dense pack cellulose-2x4 (16"oc)-1/2" ply-tyvek house wrap-3/4" spacer-metal siding. I'm aware that the double wall system comes with condensation potential, which is why I'm looking for drying potential on both sides of the wall. Malcom, you're better versed with canadain code, would the osb pass muster as a warmside vapour barrier? Also how "intelligent" is the really? Whenever I've tried to track down the permiability of OSB I've found such wide range of figures, that they seem meaningless. BTW Peter I didnt see a link in your response about the OSB, although I'd love to see the data.
Geoff,
OSB is "smart" in that it can go from 2 perms when dry to 12 when damp. I think it would be fine as is, but the code wants something < 1 perm as the vapour-barrier. That probably is easiest to achieve with a vb paint on the OSB. If you have to have a vapour-barrier, where you have it seems to me to be the best place. The wall can still dry both ways.
With that clarification, I would only do it as Malcolm suggests. Here is info on how variable OSB, plywood and a smart retarder are. VB paint would also be somewhat variable.
If you wanted to further decrease risk (I don't think you need to), you could consider fiberboard exterior sheathing.