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 …).
Best Answer
In searching for a suitable nylon screw, I came across threaded nylon rods which are readily available in lengths up to at least 6'. By cutting such rods I was able to obtain a stem of suitable length. These were inserted into holes drilled in the slab and fastened using epoxy. Then, a nylon nut (also readily available) was used to tighten a galvanized washer against the foam and pull it closer to the building.
This was moderately messy (drilling holes in the XPS for the rod and to countersink the nut made a billion little pieces of foam to clean up) and still involved the use of a bit of foam to seal over each nut/washer/rod.
Worse, the XPS was not willing to move all the way to the building in some places. Where there was less than a 2" gap the approach worked. For the largest gaps, the XPS began crushing beneath the washer before coming flush to the building.
The remaining gaps led me to choose to spray more to fill these gaps and then, rather than flashing it all with aluminum sheet metal, we used an elastomeric paint over a fabric to extend the house's water barrier over the foam and down to the earth and to provide some mild protection for the foam. The result doesn't currently look very good but with another coat or possibly two it may improve.
Overall it's not a very satisfying solution but we couldn't come up with anything better.