Themal Board
I know this is likely moe than you wanted to know, but I have been looking into this stuff or a job. You lay out thermal board on top of the slab. Its a pocessed wood poduct, like OSB, with channels for PEX tubing, then lay som kind of floating floor or even real hardwood over the entire thing.. You use a heat exchanger to collect heat fom you domestic water heater, and BANG.. Hydronically heated floors. If you have a 400 square foot garage, it would be about 2,500-3,000 to do the whole thing, which will pay for itself over the years while you are not buying 100 dollar heaters and paying high gas or electric bills.