PSI of foam for sub-slab insulation – GreenBuildingAdvisor

Hi all! We’ve finally started our build, and the footings, foundation walls, and waterproofing are all in place. Exterior foam on the walls goes on next week, and we were planning to do the sub-slab foam as well, but my builder’s come to me with a concern. 

His concrete sub had a bad experience doing a pour at a school that used foam beneath the slab, and it’s made him very skittish about projects that use anything other than super-dense foam. 

From what I have found here (Dana had a good post a few years back) and elsewhere (a foam salesman telling me the concrete guy had no idea what he was talking about), 25 PSI XPS or EPS should be more than strong enough for a residential pour. The concrete sub wants something like 100 PSI foam, which is insanely expensive. 

Does anyone have resources I can show to my builder to help reassure him we don’t need bulletproof foam to keep the slab safe? Thank you!