I have an 8" concrete slab with XPS (rigid foam board) insulation - 6" wide, 16" tall - around the perimeter. The operator who back-filled around the site compacted things a little too aggressively and cantilevered the top of the XPS out from the top of the slab. There is a further 8" of EPS beneath the slab which may be what allowed the bottom of the XPS to move inward. There is now a gap of up to 3" between the outer edge of the slab and the inner edge of the XPS.
This is going to make finishing over the XPS challenging and the gap will also need to be filled, probably with spray foam, to keep cold air away from the slab.
Is there some way I can bring the XPS tight against the concrete (short of digging everything up and trying again)?
One idea that's been suggested is to drive some concrete screws through the XPS with large washers and use these to cinch the XPS tight up to the concrete. This seems like a great solution except that I'm unable to find nylon concrete screws long enough to do the job. Longer metal screws are available but create a thermal bridge from outside (cold) to the slab which I'd prefer to avoid (having just put a ton of XPS around the slab to insulate it ...).