Dense-Packed Double-Stud Wall – Sheething or Membrane?
Starting a build shortly with a 12″ double stud wall dense packed with cellulose. The interior wall is 2×6 and load-bearing, and has Engineered shear panels located inside. The exterior wall is 2×4 and has no Engineered shear value, it’s just there to hold the insulation and siding, similar to a Larsen truss design.
I was originally planning to sheet the walls with Zip or ordinary OSB+adhered membrane, but ludicrous material prices has led me to ponder the value of that sheething and to consider another approach I’ve seen used; a sheet-less wall with a dense-packable WRB membrane on the exterior.
My original plan was to build a windowless sheethed box and do my first round of air-sealing before any penetrations were involved or any plumbing, electrical or insulation had started.
Using just a membrane on the exterior, what pitfalls am I potentially setting myself up for? Concern: similar to a vapor retarder installation, is whacking several thousand staples through this exterior membrane going to compromise its airtightness? I’ll be some screwed if I fire up the fog machine and there’s an air leak or two at every staple – I’ve got my sights set on an extremely airtight enclosure.
Are there any other considerations I’m missing? Looking to leverage the experience of builders who are more experienced than I (first time builder/homeowner here) as well as those who may have used this approach before. Any case studies or build blogs you can refer me to?
Thanks!
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Replies
Lance,
It's something that's been done successfully for a while. The first blog on GBA I remember using a sheathing-less approach was Lucas Durand's: https://ourhouseuponmoosehill.blogspot.com/p/details.html
I've never done a building with just studs, but I did do several last summer which only had strapping. The drawbacks I encountered almost all had to do with the process, not the finished product:
- It took a lot more thought to provide adequate backing for penetrations, trim, flashing, tapes, etc than if you had sheathing.
- The WRB is susceptible to wind damage in a way it isn't usually, and was very hard to install well.
- There is nothing to rest a ladder on, especially once the WRB is up.
- As well as shear, sheathing has other structural benefits - the walls become diaphragms, and are more resistant to high winds. It also adds a layer of redundancy that is one of the beneficial features of stick-framed, sheathed buildings.
- On the plus side, without sheathing the wall is a lot more resistant to moisture concerns.
I tend to agree with you though - I'm not sure the drawbacks are serious enough to warrant the extra labour and expense right now.
Thanks for your thoughts, Malcolm.
Backing for penetrations etc.: I was planning to use OSB in select areas behind the WRB for exactly that reason. Granted, there will be many penetrations I cannot plan ahead for entirely (mainly electrical I'm thinking), but the big ones like ventilation and plumbing vents should be able to be planned for. I would have blocking between the studs to hold the OSB flush with the outside of the stud wall, then apply strapping to the outside to keep the WRB from pulling away from the OSB during packing. Does this sound reasonable?
Wind damage: Absolutely, but I'm hoping a product like Mento Plus (or similar) with a mesh weave will be reasonably durable when stapled on. I'm also planning to use strapping to keep the WRB solidly in place.
Ladders: Never thought of that one! Maybe I'll have to screw a piece of strapping across the top of my ladders so they bridge the studs when leaned on the wall. This is a big tip - thanks!
Structural benefits of sheething: I really like the idea of a sheethed wall, especially for pest resistance. I would plan to have some permanent cross-bracing in the walls in addition to the Engineered shear panels, which should hopefully add plenty of shear resistance to the whole assembly. It will also make construction much easier/safer until all the shear details are in place (my shear panels will go in after insulation and vapor retarder, on the interior side of the load bearing 2x6 wall).
Between the house and garage I'm looking at roughly 6,000 sqft of wall area. A quick price check of 7/16 Zip panels shows $72/sheet in the US. In Canada I wouldn't be surprised if that was closer to $90/sheet, so I'm probably looking at $18,000 for a Zip wall system including tape and screws. Compare that to about $4,000 for Mento Plus, and the Zip System represents about a $14,000 premium in materials, not including labor.
Is there a $14,000 benefit to having a sheethed wall vs. a membrane wall. This is the question...
You can also look at gypsum sheathing.
The 1/2" stuff is usually under $1/sqft. Easy to nail up with a roofing nailer, as simple as drywall to cut. About the only thing annoying about it is that it is itchy. 3M 8067 sticks great to the surface, so easy to tape the seams to seal it up. Standard house wrap over the whole thing for WRB.
This would give a solid vapor open exterior sheathing plus a proper critter barrier for not much more over a fancy membrane. With the right nailing pattern it also works as structural bracing so you'll have a much stronger structure.