Load-Bearing Stud Over Exterior Rigid Foam
Richard_Schl
| Posted in General Questions on
I am considering constructing a double stud wall with ICF Foundation as outlined here:
The one thing I am worried about is the outside load bearing stud being placed over outside foam. Is this an issue or does it transfer the weight over the joist?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
It depends. Where the joists (or floor trusses) cantilever out they can take the uniform loads, but p0int loads may be a problem. Where the real difficulty comes is the other sides where the joists run parallel to the foundation.
Richard and Malcolm,
I am trying to decide on a similar wall and foundation for a double wall build instead of another exo-insulation job like I live in. The idea of hanging the load point past the concrete as happens when building to the ICF face gives me only slightly more pause than making the inner wall the load bearing one. Each way seems to be working against traditional load path practices.
The most current issue of Fine Homebuilding shows one double wall with the inner wall being non-load bearing, which seems the most common. One of the wall contest entries on GBA used floor trusses applied to the load bearing wall ala Larsen wall. This would make it possible to set the load path of the bearing wall dead center of the concrete to minimize the overhang past the ICF face. Still a bit of nightmare to detail the bottom and filling cellulose from the outside seems fraught with issues.
Would the idea of using brick ledge ICF blocks turned to the inside be a viable way to provide a wider concrete contact point for oversized sillplates? Or would the extra surface be mostly for show since the joist contact with a normal 2x6 sill plate/rimboard is enough. For the sides parallel to joist ends at least, it might provide a solid way to set a wide sill plate that could then support two rimboards blocked together to better take the load of the two wall plates.
Maybe upgrading the floor decking to 1" be a less drastic way to handle the possible uneven loading in general? Also, would blocking between the rimboard and first joist on the parallel sides be sufficient if roof loads are not on those sides?
I have even thought about using LVL flat as super sill plates, but they aren't pressure treated. My main problem for loads is the house will be two story and we have to deal with 60lb snow load as well. The per foot loading can get pretty high.
Roger,
Wouldn't it be easier to forgo the ICFs that are causing all the dilemmas, and insulate the inside of a conventional concrete foundation wall of any thickness you wanted?
With ICF, what would be simpler is to hang the floor joist on the inside from a ledger (there are specific hangers for this).
You can then go with a an interior load bearing wall that sits on the joists with the exterior non-load-bearing wall bearing partially on the ICF.
You would have to have an engineer spec both details though.
A much simpler approach would be to skip the double stud and go with exterior rigid insulation.
If you must have cellulose, you can also have an engineer spec TJIs for your walls instead of double stud.
If the exterior stud is load bearing, is the joist is just essentially cantilevered over the concrete wall and the load from the second floor/roof above will transfer through the end of the joists into the concrete below? Maybe just have to locally reinforce the joists to support the loads?
Richard,
See my post (#1) above. How do you deal with point loads, and what happens on the walls where the joists run parallel?
Akos and Malcolm,
I agree that there are easier ways than I postulated. I used exterior insulation on my own home, which was a bit of a pain. The thermal performance is great, but I was limited by weight factors to a two coat stucco exterior finish. One plus, I didn't need to putz about inside with rim board insulation. The next house is for one of my children and they want siding which drives me to look at double wall for exterior fastener simplicity. I am pondering a work around on the foam/siding problem, but more on that another time.
Also, I would insulate on the inside of the foundation for ease, but again I like the results I got by putting foam on the outside of my current foundation. More detailing and cost, but my man space in the basement is warm and dry. The ICF idea was driven mostly by getting to have exterior foam built in. It is cold for enough of the year here to make not having frozen concrete under the first 10" of your floor decking preferable.
The engineer I am working with doesn't roll his eyes too much while I cook things up. I am not sure if he has encountered the idea of using TJIs as wall elements. I will ask.
I’m currently in a similar situation trying to conceptualize the union of an ICF basement wall, to a floor truss to a double stud wall.
My double stud wall might be sheathingless (as detailed on BSC website) or have structural sheathing on the exterior face of the interior wall.
I’d prefer to have the floor trusses hung off of the inside of the ICF to help ensure a continuous thermal break.
The next issue is figuring out the load path from interior load bearing wall of the double stud wall to the ICF. One option might be an ICF block with a double taper top. Nudura’s 8” core block would have a bearing surface of 12”. There would be a thermal penalty as the foam tapers down to only 5/8”
Akos’s suggestion reminded me of something Ben Bogie did on a recent project. Here is a video of what I think is at least similar to what Akos mentioned: LVL Ledger-Hung Floor.
This is currently my concept for the details between an ICF basement wall, floor trusses and a wide double stud wall. (Details are missing including bottom plates, capillary breaks and other things)
I’ll be bringing this to my architect to get his opinion on the load path.