Wall assembly opinions Zone 5/6
can_home
| Posted in General Questions on
Hi all. First, I wanted to say what an invaluable resource this forum has been for research. Big thanks to everyone who contributes with their expertise. We’re considering a new home in Zone 5/6 and the builder we would work with uses this assembly in most of their builds
7/8″ horizontal corrugated metal siding 1″x3″ vertical wood strapping @ 16″ o/c Spunbonded polyolefin paper air barrier
1 1/2″ wood fiberboard
1 1/2″ wood fiberboard
9 1/2″ engineered vertical wood joists
9 1/2″ cellulose fibre insulation
1/2″ OSB sheathing, taped as air/vapour barrier
2″x 4″ wood stud @ 16″ o/c
3 1/2″ mineral fibre insulation
1/2″ gypsum board
I’m unsure if the air barrier is a proprietary product but I can probably request some more details. I’ve read through countless threads on wall assembly pros and cons here but it would be great to hear other’s opinions about potential downsides with this.
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Replies
can-home,
I'm not sure much depends on what type of WRB you use on the exterior. I haven't heard of the product you are suggesting. As long as it is approved, and has a reasonable perm rating it should be fine.
What is the advantage of using the wood fibreboard over either OSB or plywood as sheathing?
One of the main benefits of double walls is being able to use very little framing, while taking advantage of the gap between them to greatly increase the levels of insulation you use. I don't see any advantage in using the 9 1/2" studs over either 2"x4"s or 2"x6"s.
OSB without a coating does not meet the code requirements for an interior vapour-barrier.
Hi Malcolm:
The description in the NBC is a bit complex (as you discussed so thoroughly in https://www.greenbuildingadvisor.com/question/poly-and-canadian-building-codes). I think some of the rationale around the drying capacity of this assembly is nicely described here https://www.ecohome.net/guides/2289/interior-sheathing-as-an-air-and-vapour-barrier. I'll need to confirm with the builder the specific OSB product they use but I'm assuming that this conforms to code in Quebec where they build. I believe the rationale for fiberboard as sheathing is its higher vapor permeability for external drying.
can-home,
I should have started my comment by saying what you are proposing should perform very well.
OSB has a perm rating of 2, or double what Canadian building codes require as an interior vapour-barrier. Using it is probably as simple as having to apply a layer of paint, but it is something to confirm at this stage - and I agree, practically it isn't important.
I was thinking you needed the exterior sheathing for structural rigidity, but you have that layer of OSB in the inside. Again I agree: fiberboard is a good choice.
That leaves you with the 9 1/2" studs. I'd be interested to hear your builder's rational for using them.
Your assembly is similar to a typical Larson truss detail. Are the "engineered vertical wood joists" I-joists? Are they supported below or are they screwed to the 2x4 studs?
Spunbonded polyolefin means an open-mesh rain screen such as B. Obdyke's Slicker Classic. Without punctuation, I'm not sure what they mean by "paper air barrier." It could be an airtight paper product pre-applied to the rain screen mesh, it could be #30 felt paper over an airtight membrane, but my guess is it's Pro Clima DB+, a paper-based WRB which can be taped for airtightness.
Assuming it's any of those, it's an excellent assembly in terms of building science principles. It is relatively resource-intensive which is not ideal if you're trying to minimize your embodied carbon emissions. But it should work well.
Looks like a great wall to me. Definitely doesn't sound like a budget option though, if you can get that wall for a reasonable price it will work great.
A standard dense packed 11 7/8 I-joist wall would get you close enough to similar R value for much less money.
Is there any rating for an I-joist in a vertical position? Or do you need to have 2x4's to carry the load as well?
One of the I-joist manufacturers used to publish ratings for them for walls, can't seem to find it now.
As with anything I-joist, you need an engineer to sign off on it anyways, I can't see one having an issue especially with the ones with 2x4 flanges.
Intuitively the 2x4 flange should be able to support as much as a vertical 2x4. But agreed that an engineer would have to sign off, and I wonder if an engineer would be a little hesitant. A lot of the time the flanges are build up from pieces of smaller wood, and the engineer gets to thinking about the intended direction of force and starts to doubt...