Double 2×4 bearing wall for 2-story construction?
I am designing a 2 story house on a insulated shallow foundation. (15″ wide footing to the exterior grade beam). Doing an R40 cellulose filled cavity utilizing advanced framing methods.
As apposed to an outer 2×6 24″oc wall to carry the upper story and roof, why not make the inner and outer walls 2×4 with the inner studs supporting the second floor structure and the outer studs bearing the exterior walls and roof?
Given that the foundation already provides bearing to a full 12″ wall it would seem to me that the advantages would include less framing, more insulation, shorter joist spans, elimination of thermal bridging through joist ends, and easier fire blocking between inner and outer walls.
I am thinking that this could be considered to meet the code proscribed wall framing description.
Another strategy I am considering adding would be to have the first floor double studs made up as “stud trusses” that could cantilever out over the foundation foam to to keep the rain screen drainage plane outboard of foundation foam. This I presume would require a design professionals sign off.
What are your thoughts?
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Replies
Jonathan
There are countless iterations of double stud walls using every conceivable combination of the inner and outer walls to carry the loads.
There are advantages to using the inner wall on two storey buildings as you can keep the floor framing back from the exterior limiting thermal bridging at the rim joist.
Using both walls to support loads brings up what might be unnecessary complications. Once both walls are load bearing then both walls will need headers over openings. They will also both need to be framed at 16"oc, and the outer wall will have exceeded the height for a 2"x4' unsupported wall.
You will also have to deal with having the sheathing that provides shear support only being attached to one component of the load bearing structure.
Thanks Malcolm. Makes sense but let me, at the risk of seeming a total idiot, argue the case a bit further.
Regarding headers, my North and South walls are the bearing walls carrying floor joists and roof trusses. There are only 2 small openings in the North wall so the doubling of headers is only an issue on the south wall.
In thinking that the walls can be 2x4 24 oc I am refering to Table R602.3(5) (IRC). Which I am attempting to interpret as allowing the the second floor to be supported by the inner wall and the roof structure to be carried on the outer wall.
Regarding wall height I am not sure I understand what "laterally unsupported" means but I was thinking a plate at the second floor level provided lateral support. Can someone clarify?
Finally in regards to shear bracing if the inner wall was considered an interior bearing wall will it need bracing?
I don't want to compromise quality of construction/safety but in the interest of cost control and being green I am interested in minimizing material usage wherever reasonable.
Johnathan,
I'm not as familiar with your code as my own so it's worth looking at my comments with that in mind.
Our code prescribes that the structural elements in load-bearing walls be continuous. So it would not allow the outer wall to have a plate at mid-span. This makes sense, as it then has a hinge point. However the wall is too tall to use continuous 2"x4". So either way it would need to be tied back to the inner wall if it is load bearing.
Sheathing the outer walls provides shear resistance to them and the roof, but what keeps the inner walls and floor system (i.e. the whole rest of the house) from moving laterally?
There are engineered solutions to both problems. Probably they include tying the two walls together. It's worth finding out what will be required as it will affect both the potential for thermal bridging and your air-sealing details.
Many double wall assemblies decide to designate one wall as being load-bearing. If it is the outer wall then things become simple, although you lose some benefits at the rim joist. When it's the inner one a lot of people prefer to sheath this wall, both for shear and to act as a protected air-barrier and vapour resistant layer. The downside is you are faced with deciding what you use as a second layer of sheathing on the outer wall. The choices aren't that palatable. Another layer of plywood is expensive, fibre board is hard to work with and provides a poor substrate for fasteners, and forgoing the sheathing altogether in favour of a robust WRB is difficult to detail.
Lot's of choices, all with knock on consequences.
Here is a drawing, from one wall of my house. The two walls are tied together by the subfloor of the upper level. The rim board is insulated as part of the lower level wall cavity. The outer wall frame is 2x6, to accommodate the outer rim board to which the deck brackets attach and because it overhangs the foundation by 2" to accommodate exterior foundation foam.
Thanks Dick. It is about what I would like to do except without needing to hang a deck off of my walls I wonder if the outer wall could be 2x4 construction. Your drawing points out that as the inner wall continues above the floor it is no longer a bearing wall and therefore double headers are no longer needed. I think the subfloor tie-in detail should satisfy anyones need for lateral support.
I do like your overhanging the foundation insulation but I have an idea of flairing out the siding at the bottom with a few courses of roofing slate I have available.
Jonathan, I am not a structural engineer or builder, although my wall design was approved by a structural engineer. It is my understanding that if the outer wall studs bear directly over the foundation without overhang, then 2x4 @16"oc is adequate for one- and two-story platform frame construction. The only problem normally would be that in single-wall construction a 2x4 by itself won't hold enough insulation other than closed cell foam, so that a layer of external insulation would be required. If I had to design and build my house over, I suspect I would opt for putting all foundation insulation on the inside. That would have let me use 2x4 studs on the outer wall, except in places where I have exterior deck attached.
As to headers on the interior upper level wall frames, that would depend on details at the top of those walls. If the top plates of inside and outside frames are at the same level, there could be some distribution of roof load onto both walls, despite any intent to have the roof bear only on the outer wall.
Jonathan,
Dicks' details are a good solution - and we know they work as he has been in his place for a few years now and has provided periodic updates. One concern if you use dimensional lumber for the floor joists, and not I joists like he did, is the possible differential settlement between the two walls.
As Dick said our code precludes using 2"x4"s at 24" o.c for two storey construction. Even if your's does, advanced framing relies on the stacking of structural elements to eliminate the loads on the top plate. That probably means spacing the studs at 16" to support the floor loads, and it's probably a good idea to run the 24" spacing to support the trusses by their manufacturer before firming up that spacing too.
Good luck with your build!