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 ...).

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    If you would close a 3" gap by pulling the XPS to the slab with screws, wouldn't this break the rigid foam? If the XPS has been "cantilevered out" by filling, doesn't this mean that the slab is not vertical as far down as the bottom edge of the XPS, that is, the exterior face of the slab is undercut at some distance down? If so, the XPS would seem to go down too far. You refer to this as 6" XPS. Do you mean it is 6" thick or 6" from top to bottom? How thick is this XPS? Aug 16, 2017 at 22:18
  • It seems like it has enough give to be bent into place. I can push it in with my hands and it seems alright - it just pops back out again when I let go. I think that, indeed, it is probably not vertical all the way down. I can only guess at what's going on down there, though. The XPS is about 16" tall (top to bottom) and 6" wide (air to slab). Below the slab is an 8" tall layer of EPS. The bottom of the XPS is meant to be even with the bottom of the EPS. Aug 17, 2017 at 1:16
  • Rather than screws into the concrete slab could you use adhesive and use some sort of bracing to hold the XPS to the face of the slab while the adhesive sets? Of course, the XPS would then be held by its surface and maybe the XPS doesn't have the internal strength to resist the tension. If he XPS goes all the way up to the top of the slab, will it be exposed and visible? The wooden sill for the structure will be only on the concrete slab, so there would be a lot of XPS projecting from the side of the building. Will the insulation on the outside of the wall be 6" thick to be in the same plane? Aug 17, 2017 at 21:22
  • Screws through large plastic or stainless steel discs or strips may be necessary to hold this 6" thick layer of XPS to the outside of the slab. Just be careful to not hit any water supply or drains that are in the slab. It sounds like you are constructing an ultra insulated house? Where is this in Vermont? Aug 17, 2017 at 21:35
  • Northeast Kingdom. Building zone 6 or so. It's meant to be a tight, well-insulated structure, yea. The XPS does come up to the top of the slab and it is going to end up extending beyond the plane of the walls, unfortunately. It will be covered with aluminum or stainless steel. The adhesive is an interesting idea. I have no idea about the XPS's tension strength either though. As far as the screws go, if I could just find some nylon screws long enough we'd be all set. The longest I've seen in my searching is 4.5" though. Aug 17, 2017 at 23:45

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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.

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