I'm interested in using SIP walls over an ICF foundation. For the garage the SIP would have to sit on the ICF stemwall. I understand I need to put a larger sill plate down first to carry the load from the SIP (the ICF foam can't bear any load).

My question is how much can that larger 2x sill plate "cantilever" out over the 2" of ICF foam? I would imagine that a 2x on it's side is not very resistant to shearing off. Won't 1/2 of the weight of the walls and roof be on the outer skin of the SIP which would bear on the sill plate cantilevering over the foam? The garage will be a single story with 4/12 roof trusses spanning 36' bearing only on the sip walls.

Or do I set the SIP wall back so there is very little "cantilever" (1/2 inch) and fur out the outside wall to match the ICF thickness?

I don't even know if they make such an animal, but a PT LVL would be perfect. Here is what an engineer specified on a recent project: The SIPS used were 6.5" wall panels. But this thing was a mess due to an unlevel ICF wall. And, you would have a better structural solution by having the anchor bolts penetrate through both the sill plate and the SIP bottom plate.

Thanks for the reply. That much overhang on the sill plate still has me a bit concerned - but I'm not an engineer. I don't really understand all the forces going on as the two wall skins transfer the wall and roof load through the two sill plates to the concrete.  

Since 80% of the ICf's that I sell are used for foundations, this question has come up numerious times. Some of the solutions are: 1. use 2x12Pt for plate, 2. use taper top Icf for last course, 3. use LVL with peel and stick waterproofing on underside along with sill sealer.

Your engineer will specify the sill plate if you have speced out the ICF's and the SIPs. The style of ICFs will make a difference. You can use the foam+concrete for superior performance.