A resilient double-stud wall for the West coast
After reading Martin’s excellent blog and the interesting discussions that followed on the potential vulnerability of sheathing in thick walls, I wonder wether this assembly might result in a good resilient wall for the PNW climate?
– Drywall
– 2″x4″ load-bearing framing insulated with batts or cellulose.
– plywood sheathing as air seal and to provide shear for seismic.
– 2″ gap (insulated as above)
– 2″x4″ framing (insulated as above).
– 1″x4″ skip sheathing at 12″ o.c.
– WRB
– Rain screen strapping.
– Siding.
The sheathing and air sealing are buried within the wall avoiding potential mechanical and moisture damage. The exterior is extremely permeable. The assembly has good drying potential to both the interior and exterior.
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Replies
Malcolm,
So, a 9-inch wide double-stud wall with "skip sheathing"? Sounds resilient to me. Choose a top-quality WRB.
Malcom,
I have a feeling that Martin is providing some good-natured sarcasm in regards to the "skip sheathing".
Aaron,
Not really. I'm more familiar with skip sheathing on roofs (usually under cedar shingles or slate) than walls, but there is no reason that Malcolm's plan won't work.
Martin and Aaron,
Here on Vancouver Island our code mandates both structural sheathing and a rain screen gap.
Given that, it seems like a good idea to have the sheathing double as an air barrier and to place it where it won't be vulnerable to wetting or damage.
Several projects featured here have eliminated exterior sheathing entirely and substituted a WRB. My experience applying vertical rain screen strapping to the houses I have built is that you often want it in locations that don't coincide with the wall framing, especially around openings, penetrations and where larger trim is located. I'm also not comfortable relying on tape to seal flashing to the WRB.The skip sheathing would provide a good base for the strapping and a permeable substrate for both the WRB and allows it to lap any flashings.
The next two house I've got are probably too big to try this on. I need something small where I don't lose my shirt if my numbers turn out to be off. Have to see that comes through the door this winter.
Martin and Malcolm;
Interesting idea. This and Martin's excellent article on double stud walls are very timely.
We live in Southern Ontario, Zone 6, near Sauble Beach, in a clearing in the woods. Humid in summer, cold and dry in winter.
We have built a rammed earth addition (main house) to our garage that we have been living in for 2 years. It is being topped off with a double stud wall.
Our first phase of the house was all insulated outside the 2x6 walls. After having some sizable ant colonies move into our house, we have lost enthusiasm for external foam. They love it. Hence the rammed earth for the lower two stories and a new stick wall method for the upper. We are using 2x3 walls for the drywall and 2x6 walls for the structure. Roxul between the two walls and in the exterior wall. Plywood for sheathing. Rain screen strapping and shiplap wood siding.
Questions about Malcolm's idea:
What about interior vapour barrier?
Martin, what's a top-quality WRB? We all use Tyvek here. And do you recommend Malcolm's system as much as the more traditional exterior sheathing with rain screen strapping?
Thanks,
Paul and Julia MacDonald.
Malcolm,
With 475 we provide several first quality European WRBs, can also ship to Canada. The SOLITEX MENTO line is super airtight and waterproof - see this blogpost about its monolithic vapor open layer, it's 33' watercolumn and ASTM test.
I wouldn't use skip sheathing. It creates air-pockets behind the WRB that could lead to convection since they would be impossible to insulate. If you need to deal with odd spaced elements, a 'flying' batten approach on the exterior of the WRB could be an option and use less wood (see photo). Regarding your flashing issue, you could also use plywood window boxes to support flashing (see photo) or most likely a combination of the two.
Paul and Julia,
To use Malcolm's idea, which is similar to what I proposed in (a href="https://www.greenbuildingadvisor.com/blogs/dept/musings/monitoring-moisture-levels-double-stud-walls">Martin's blogpost, you would use plywood/OSB on the exterior of the interior wall. This most likely means that this wall should become load bearing. This sheathing now can be taped airtightly, after which you can build a 2x exterior wall, add insulation, and use a good quality WRB (like the SOLITEX MENTO we supply at 475). Feel free to contact us with question or see our blog for projects, details and examples
Malcolm,
Floris advised, "I wouldn't use skip sheathing. It creates air-pockets behind the WRB that could lead to convection since they would be impossible to insulate."
You are considering using either batts or cellulose as insulation. If you choose cellulose, there won't be any air pockets, because the cellulose will fill the gaps. Cellulose will perform better than batts.
Paul and Julia,
Here in the rain forest carpenter ants are ubiquitous and I've battled infestations both in my own house and in projects I've built. Maybe treated foam is the answer but I'm sure wary after my experiences too.
In the wall I've proposed the sealed plywood sheathing on the exterior of the inner wall acts as an air barrier and vapour retarder, so no interior vapour barrier is needed. Our code may be a bit more flexible in its approach than yours. Many specify that the vapour retarder be located closer to the interior than it is in my wall but practically between the drywall and the plywood there won't be much of a vapour diffusion problem.
Florus, I take your point about the possible convection using skip sheathing. My problem is that the vast majority of siding types require vertical not horizontal strapping for support, and our code mandates that a continuous drainage plane must be maintained behind the siding, so the first layer of strapping must be vertical too - precluding your flying strapping.
Any ideas to get around this?
How important do you think the convection problem would be with skip sheathing given the the voids are horizontal not vertical allowing convective loops?
Malcolm,
For what its worth, I think you're on a pretty good track with your proposed assembly.
I might consider substituting 2x3s for the 2x4s on the non-load bearing side of the assembly.
I don't think there is any perfect solution so compromising, say, unlimited nailing opportunities for cladding and trim is not such a big deal if you gain a more robust assembly in other regards.
I think your concern is prudent, but I think these things are very manageable with the right amount of forethought in the details and as long as whoever is doing the building is willing to "play ball".
Here's how I provided support (and nailing opportunities) for some of the penetrations in my house:
http://ourhouseuponmoosehill.blogspot.ca/2013/01/ventilation-duct-supports.html
I've got about half the siding and trim up on my house now (over the WRB with no exterior sheathing at all) and so far so good - I have yet to run into a situation where I didn't provide something to nail into.
Incidentally, (I'm not sure if you remember my describing to you the cladding system for my house in another thread) I decided to improvise a rainscreen after all.
Because the the rainscreen wasn't pre-planned, I ended up ripping the battens from 1/2" plywood (would have preffered to use 1x4s) and manufacturing window trim to suit.
The 1/2" rainscreen gap results in about a 1/4" gap behind the siding since the insulation does "belly out" slightly against the WRB.
Sorry, I don't have any better photos at the moment, but here's a wide view of what the siding looks like now:
Lucas, thanks that is very helpful. I'll have to keep mulling this over.
Malcolm,
What function does the skip sheathing serve? You've mentioned serving as a nail base, is that all? If so, other solutions as Lucas suggested should be far less costly. If it serves as "wind barrier" fiber board would be almost as vapor open, far less labor intensive and should be considerably less costly. Also a 2" gap between walls limits the insulation choices, excluding my favorite, Roxul comfort bats. A gap of 2 & 1/2" or 3 & 1/2" would allow the use of comfort bats intended for steel studs to totally fill the gap.