Permeance of sheathing in double stud walls – how important is it?
There’s a lot of articles about double stud walls out there – enough to convince me that it’s a good assembly in many cases.
One of the things that I have read about on here and elsewhere is the need for a double stud wall in zone 5 or higher to be as vapor open as possible. To that end, it would seem like CDX + WRB is going to be preferable to OSB or Zip in terms of overall permeability on the outside.
However, I’ve seen a fair number of examples of folks using Zip or OSB on the exterior sheathing. Most recently the “sustainable build” series, where Zip is being used on the exterior of a double stud assembly in upstate NY. Given that Ben Bogie is an advocate for double stud assemblies and has done a lot with them, I trust that he is confident in the choice of materials, but it does seem to run counter to choosing the ‘easiest drying’ materials.
In the long run, is it especially significant to choose CDX over OSB or an OSB product, when considering a double stud assembly? Or would the permeance of the sheathing be secondary to the quality of the air sealing + ventilated rainscreen details?
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I'd say it's one of the least important things to consider; very little moisture will make its way to the sheathing, especially if an interior variable permeance membrane is used. It's easier to make ZIP sheathing airtight than it is to make CDX airtight, due to the voids at CDX perimeters, and airtightness is important. I like ZIP and specify it often. But with double-stud walls I still spec CDX, partly because it's slightly more vapor-open, but more importantly because it will likely hold up better than OSB to repeated wetting and drying cycles over the course of decades. But if a knowledgeable builder such as Ben Bogie wants to switch to ZIP sheathing, as he did on the one double-stud project we did together, I have no problem with the substitution.
Michael,
Do you think the permanence of the sheathing, and it's ability to dry to the exterior through including a rain-screen gap, becomes a lot more important if batts rather than cellulose are used?
Or to put it another way: Do you think you can compensate for the loss of the moisture buffering cellulose brings by using an assembly that has greater drying potential to the outside?
Malcolm, good question. I am really only comfortable with cellulose in a double-stud wall that has sheathing, regardless of the other details, because the wall WILL accumulate some moisture and the borates in cellulose protect the framing and sheathing while the cellulose redistributes the moisture. But with a fully vapor-open exterior, such as a Sarking membrane and no sheathing, and a well-detailed rain screen, I'd be ok with fiberglass or mineral wool.
I appreciate you taking the time to respond, Michael. I apologize if it only leads to me asking more questions (spoiler: it will).
If working with a builder who may be capable but not particularly knowledgeable/focused on high performance techniques/building science, would you favor one approach over another to mitigate any shortcomings?
Also, I saw that you mentioned an interior variable permeance membrane. I assume you're thinking of a product like Intello, which also functions like an air barrier? In that case, would your choice of a WRB (if not using Zip) change to balance out the costs of detailing the interior WRB?
In a scenario with an interior membrane, it seems to me like decoupling the exterior wall assembly from services is probably a good idea. Do you typically detail a service cavity of some kind if an interior membrane is being used?
mrfpgh,
The interior variable-perm membrane is primarily functioning as a vapour-barrier. It's there to regulate the moisture movement through the wall.
Depending on what you are using as your primary air-barrier (typically the sheathing), it may make sense to detail the interior membrane or the drywall as a secondary one, but if you decide to do that it's pretty easy to seal around plumbing and electrical boxes without resorting to a separate service cavity.
Another option is to move the variable-perm membrane part way into the wall - on the outside of the interior framing, or the inside of the exterior wall. That way it is undisturbed and protected. If you do that you could call the interior wall an insulated service cavity.
Thanks, Malcolm.
What I'm expecting is that we will have either CDX or OSB for sheathing, and that at least part of that will be wrapped with a black mechanically attached WRB meant open joint rainscreen assemblies. The rest of the house, could be Zip or could be CDX, or could be OSB. It seems like CDX might be optimal. From what I understand it is an additional cost, but not a huge one.
Part of the reason for a service cavity is that I don't know how well I will be able to rely on my builder to ensure good air sealing for things like electrical boxes. The other part is that I will be planning on doing a fair bit of casework after occupancy, and having the air barrier relatively 'safe' from harm during that work seems like a good idea. Am I overestimating the difficulty in effective air sealing here?
mrfpgh,
If the sheathing is the air-barrier, and the variable-perm membrane is being used primarily as a vapour-barrier, sealing penetrations in it aren't that important. Unlike air-barriers, vapour-barriers work on a percentage basis, so even quite large holes don't appreciably affect their performance.
The problem I see with dedicated service cavities is that they often don't end up containing much. My suggestion would be to look at what will actually be in each wall and only build them in areas that warrant it. The expense and loss of space doesn't make much sense if all that would penetrate the membrane are a couple of electrical boxes - especially if it isn't the air-barrier.
Response to #3: Haha, that's why I don't do consulting these days, because every answer leads to two additional questions! That question is tough to answer because it depends on what the builder is used to doing and where their blind spots are. I recommend asking them to read and watch videos--Ben Bogie has some good ones here and at ProTradeCraft.com.
Yes Intello is one variable permeance membrane. It's one of my favorites but there are a few alternatives. They can be detailed as an air control layer but I have repeatedly found it much easier to make either the sheathing or a self-adhered WRB airtight.
Do you have someone experienced as part of your build? There are a lot of variables that would benefit from having someone on site who knows what they're looking at.
I agree with Malcolm regarding service cavities. They are nice to have and I have used them, but rarely do these days. Using the sheathing or WRB as the air control layer is a simpler approach, in my opinion. But if you have the space and money, having a dedicated service cavity is a nice luxury.
Malcolm,
Your point about loss of space and expense is well taken. In addition to the boxes, though, don't the cables themselves have to be run through the packed-full-of-cellulose framing (or between them, in a double stud assembly)? And does that create an obstacle to future wiring modifications? I realize the initial wiring would happen before insulation.
mrfpgh,
What kind of modifications to the wiring are you envisage doing, and does it have to occur in exterior walls? Adding wiring to service cavities still requires an open access route from the panel, access to the stud bay, and some way to move it between bays. If you have some idea of where things might go in the future, you could always run conduit to those spots.
We used dense pack cellulose in the space exterior to the interior stud wall. Inside the interior stud wall we used fiberglass batts. Almost all of the wiring and plumbing is inside the interior stud wall, the outside of which has an air/vapor retarding membrane. So.if we ever need to run wires in the wall, we can.