There is a small ledge, maybe 2″ wide at the top of the foundation wall where I could try to fasten rigid.  My problem is there are obstructions all around the perimeter on this ledge and the sill plate, primarily electrical wires fastened to the front side of the sill plate (where the rigid should but up against) and some steel wall bracing.  There isn’t enough slack to relocate the wires to be in front of the assembly.

My question is can I box in this space?  What I mean is set a piece of foam on top of the plate and bring it out far enough to meet up / sit atop the 2″ piece of wall foam that will hang vertically?  That would create roughly a 2×2 air pocket in that void between the sill plate and top of my concrete foundation wall, so I don’t know if this is a potential water vapor or other issue.  I would think so long as all the foam is taped and sealed properly, this should work (similar to how one might box in a light or penetration in an attic).  The weak link is going to be where the actual floor joists are as I would either have to notch each piece of foam, put a piece of rigid in there, or just use canned foam in those areas.

**after reading another post on this topic, I should also note that there is no capillary break and the plate is not PT.  The house also sits very low and there is some evidence of water (before I owned it) – not any wood rot yet but might be close in parts.  Would leaving this “air chamber” reduce the risk of accelerating rot, were it to get wet again?  

Thanks,

Michael