Can you use spray foam Insulation under a slab?

When building a slab on grade, EPS rigid foam insulation is the most common choice of insulation, for performance and cost. I wouldn’t go as far as saying you 'couldn't' use spray foam below a slab, but I’m not sure why you’d want to. Spray foam for sure has uses where it is the best choice by a long shot, but under a slab is not the place. On this page you can see what we like and don’t like about spray foam and why – 

4th generation blowing agents for spray foam

If you need it as a vapor barrier, air barrier, or to insulate difficult areas like basement rim joists or to create a radon barrier in a crawl space it’s a great choice, but in most other applications, and certainly with a slab on grade there are better ways. When an installer sprays the polyurethane insulation they really go by experience to get the appropriate depth, and of course a skilled installer could get close, but it would still be an uneven surface to an extent. That would mean your mesh and mesh chairs don’t sit at an even level, nor does your rebar, and then the thickness of concrete may in turn vary as well, so you may need a little more, or it may be too thin in some areas. 

The only reason I could even think of to use spray foam would be if you needed to maximize the R value in order to meet height restrictions for your build, but if your allowance is down to a couple of inches that’s cutting it pretty close! 

All that said, there may be something I’m not thinking of and I’m curious, why were you thinking spray foam?