Slab edge insulation and expansion joint all in one?

A homeowner in Maryland planning a garage and workshop build ran into a disagreement with his architect over foundation details. His plan was to use 2 inches of rigid foam insulation (either Comfortboard 110 or Foamular 250) around the slab, including a strip between the slab edge and the stem wall (the short foundation wall the slab sits against). That foam would do double duty: insulate the edge and act as a gap that lets the slab and wall move slightly without cracking. The architect objected, arguing the foam would get crushed over time and leave a gap, and wanted a separate, purpose-made joint strip added alongside the insulation.

Other builders weighed in with a different read. One pointed out that what's needed here is technically an "isolation joint," not an "expansion joint," and that an ordinary garage slab isn't going to move enough to crush rigid foam, aside from some minor shrinkage as the concrete cures. A second reply ran the numbers: concrete's rate of thermal expansion means a 24-by-24-foot slab would shift less than a sixteenth of an inch across the seasons, with drying shrinkage in the same small range. Their conclusion was that the foam alone should hold up fine.

For anyone insulating a slab edge in a similar build, the exchange is a reminder that a single layer of rigid foam commonly serves as both insulation and the joint material where slab meets stem wall, and that real slab movement is usually far smaller than people assume.