Rigid foam as sheathing: OSB bracing on inside only?
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I’m interested in potentially using 1″ polyiso foam as the exterior sheathing layer on my upcoming small home, replacing the house wrap and 1/2″ OSB that is currently planned.
Based on the reading I’ve done and videos I’ve watched, there are a number of ways to properly brace the walls without a full OSB or plywood skin. Those include “let-in” braces and partial panels and quite a few other ways. As far as I can tell, though, those all reference stiffening the walls from the outside of the exterior walls. But what about doing that on the inside of the wall?
I ask because I’m going to create a service cavity on the inside of the exterior wall and it will be secluded from the insulated 2×6 part of the wall with OSB. So essentially:
Layer 1: 5/8″ drywall
Layer 2: 2×3 service cavity with horizontal boards
Layer 3: 1/2″ OSB sealed fully air-tight
Layer 4: 2×6 with cavity insulation
Since the entire exterior wall will be covered with 1/2″ OSB, just on the inside instead of the outside, will that suffice to honor any building code to brace the walls?
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Replies
You can certainly use interior OSB for bracing as well as exterior OSB. Functionally, they would be the same. If you are in a high wind and/or seismic area, the current codes have very detailed bracing requirements and you would be best served by an architect or engineer familiar with them. There are some continuity requirements that are more difficult with interior bracing, but not too bad if you take account of them during design. For the most part, your continuity of air barrier will push you into similar design solutions.
Thankfully I'm not in either a high wind or seismic zone (Phoenix, AZ), so the extra bracing requirements don't apply. That would have made this entire idea a non-starter. I am reluctant to involve any solution that requires an engineer since that will wipe out any potential cost savings -- at best, not having the extra layer of OSB would save me maybe $400.
What continuity requirements are you referring to? I'm not familiar with that phrasing in this context, so I'm not sure what you are talking about.
In cool/cold climates, it would be better to avoid low-perms (like most polyiso) on the outside.
Agreed. I'm in Phoenix, AZ, where the high temperatures make polyiso an ideal choice.
The sheeting on the outside generally overlaps the joint from the floor framing and the wall framing tying them together. How will you replace that structure?
How airtight will your OSB to subfloor and ceiling joints be?
How many 2x3 will you need to buy for each outlet or switch in your service cavity? My guess is 8.
Walta
1. I'm not 100% certain what you mean by the overlap. This house will be on a slab. The outside sheathing was never going to overlap onto the slab, if that's what you mean. The only overlap was going to be some metal flashing (to help with termites) and some XPS foam.
2. My plan is to make the interior of the house insanely airtight, using the a lot of the concepts learned while building a incredibly soundproofed home theater.
3. Not sure what the 2x3s in the service cavity have to do with this?
Kurt,
What is the reasoning behind wanting to move the OSB from the exterior to the inside of the wall?
Typically the sheathing is moved inwards for two purposes: To protect it from being too cold and not drying adequately, or to protect the sheathing (air-barrier) from penetrations by moving it to the middle of the wall assembly. If you are adding exterior foam, the OSB on the exterior would be protected from the cold, and if the sheathing is your primary air-barrier, moving it to the interior, where it won't form a continuous plane, makes it harder to seal.
Picking up on Walters point - I see the service wall as perhaps something to re-visit. Targeting areas where there are a lot of plumbing and electrical services might make sense, but most exterior walls don't have enough services to warrant providing a whole cavity for them when they only represent one or two penetrations. Horizontal service cavities also preclude running most plumbing through them, and from a practical standpoint, framing up a 2"x3" wall with horizontal members is a lot more time consuming and expensive than using studs run vertically.
1. It's no so much moving the OSB from the outside to the inside as much as just not installing the outside layer in the first place. The design with the service cavity already has the layer of OSB (albeit a 1/4" layer). Since it's already going to be there, my thinking is that it would save me a few hundred dollars by just increasing the 1/4" to 1/2" and not installing the outside layer at all.
2. Why would having the OSB on the interior cause it not form a continuous plane? That's the second reference to continuity and now I'm worried that I'm missing something fundamental.
3. This is a very small house (700 sq ft; one bedroom; one bath) and ALL of the plumbing services are on the exterior walls, along with quite a bit of the electrical. There simply isn't very much interior wall space to even put services on, with such a small overall space.
As far as installing the strapping horizontal vs vertical. There does appear to be quite a bit of debate about that and to be honest, I may well switch that up later, depending on how the plumbing and electrical end up being run. I will choose whichever direction minimizes how many holes I need to bore through the studs.
On a normal structure, the exterior sheathing is part of the structure and serves to tie everything together. That includes the framing of the wall. The sheet of plywood/OSB connects to studs AND top/bottom plates, so the entire wall is locked together by the sheathing. By “overlapping the floor connection” Walter likely means the sheathing is connecting the wall and the rim joist together. This will help to prevent lifting forces from seperating the wall from the floor assembly.
Nails aren’t particularly strong axially, which is the force that tries to pull them out of a piece of wood, like you’d do with the claw of a hammer to remove a nail. Nails are much stronger in shear, which is like the two pieces they’re connecting sliding against one another. Studs and top/bottom plates are usually nailed from the top/bottom of the plates, or are toe-nailed diagonally at the bottoms and tops of the studs. Lifting forces can seperate these connections (that’s why hurricane prone areas require steel strap connections in these areas). The sheathing is attached from the side, so all those nails are in shear, which is much more resistant to the lifting forces. The sheathing also provides strength for “racking resistance”, which is where the studs would fall over like dominoes.
Hopefully my explanations/analogies make sense.
I’ll add to the other responses that the service cavity probably isn’t worth the effort. I probably have a unique perspective here too since I’m an electrical engineer, and my work usually involves designing the types of things you’d put in those service cavities. The horizontal members will require wiring to be run differently, and 2x3s will probably make the use of “deep boxes” impractical which may complicate some electrical details.
I think the potential negatives outweigh any benefits from the way you’re planning to build your walls. I think you’ll find it ultimately costs you more money too due to the labor costs. You’d probably be better off putting that money into thicker exterior foam.
Bill
Right, horizontal vs vertical members will all come down to what makes the most sense when the detailed electrical and plumbing plan is created. Yeah, the 2x3s will require shallow outlet boxes, but that's doable -- all of the exterior wall outlets in my current home need to have a 1-1/2" depth, so an extra inch is practically a luxury.
It definitely won't cost more from a labor perspective since I'm going to do 100% of the building myself. And BEopt has conclusively shown that extra insulation beyond what I'm currently doing is utterly wasted -- heck, even the one sheet of polyiso is only worth it to me for reasons beyond only insulation, since even that is scarcely worth it according to BEopt models.
In fact, BEopt modeling suggests that doing a better job of air sealing makes just as much of a difference as insulation, in my climate. That's the primary reason I want the service cavity. It allows me to have an exterior wall that is 100% airtight, save those specific holes that go all the way through (like windows, doors, etc). It's possible to make a 100% airtight wall with frequent plumbing and electrical inclusions, but it's much much harder.
Kurt,
I see you are in Arizona, so neither the cold-sheathing or the shear-loads are really in play. Peter L. who posts here, has shown us photos of the new construction common to Arizona, and it all seemed to feature walls that only had exterior sheathing at the corners, and foam elsewhere, so what you are proposing is a step up from the standard.
The references to the continuity of the OSB come from two directions. One is the concern over the sheathing forming a continuous plenum over the framing to tie everything together (as I said above, knowing where you are building, it isn't the issue it would be other places). The other occurs if the sheathing is going to be your main air-barrier. If it is all in one plane on the exterior, it's a lot easier to seal. Putting it on the inside makes the detailing around plates and rim-joists much more complicated and important.
If there are any doubts or code problems, I expect that some diagonal steel straps wouldn't add much to the cost.
The continuity I was referring to was the typical use of the sheathing to span across the connections from sill plate, across the band and up onto the studs. Similarly, across the floor bands, and connecting the top plate to the studs. With a single story house in a non-wind and non-seismic zone, none of that matters. If you install sheathing that ties the top plate to the bottom plate, you're done. You still need to bolt down the sill to the slab per the prescriptive requirements of the code, and fasten the rafters to the top plate. It's been so long since I did a house without high shear loads, I'm not as familiar with them, but I think there is simple prescriptive nailing requirements for nailing the rafters to the top plates.
In fact, you don't even need to sheath the entire wall. Simply bracing the corners with sheathing works if you're in normal zones. In some areas, it is common to sheath the corners and then apply exterior foam, using 1/2" thinner foam at the corners.