The bottom floor of our 33 year old house in the mountains of Utah is half basement, half 2 car garage.  We are going to convert the garage to living space (one big room) and are considering radiant heat.  Reasoning is that since we have to pour cement to get the floor level (garage floor slopes out), we it would be nice to add radiant heat.  Size is approx. 650 sq. ft (28'x22').  Ceiling is high (9'+), so we have plenty of room to work with there.  We would likely just do a decorative finish on the concrete to save $$ and no extra insulative layers (other than area rugs).  Would like to do a lot of the installation ourselves to save $$.  Looking for feedback on our plans.  I'm guessing we'd need a vapor barrier, foil, insulation, then wire mesh, tubing and concrete.   Current water heating is done with electric water heater and house heating is propane furnace.  We are considering geothermal in the future.  Is this a good fit for our plans?  Cost effective?  Ballpark figures would be appreciated.

Forget the foil- it provides ZERO thermal benefit in a full-contact sandwich like that (and only measurable-but-marginal benefits even with a substantial air-gap at radiant-slab temps.)

A poly vapor barrier can be useful, but may not be necessary if it's already well drained and has had no moisture problems. (But it's cheap, if you're a "for good measure" type of person.)

For slab insulation, EPS will be substantially less espensive per unit R than XPS, but needs to be ~ 25% thicker to achieve that R. But since you have the headroom, that would be the way to go. Two layers of 1.5" EPS (~R12) for a total of 3" would be enough for your climate & subsoil temps, but if the walls are very well insulated with minimal glazing 2" (R8) might be enough.

Do a real manual-J type heat-load calc before committing to a tubing layout, but with minimal window area the BTUs per square foot that the radiant needs to deliver on design-day will likely be pretty small. You might have to take some educated guesses as to the outside design temperature if you're up in the mountains, unless you have a weather station within 30miles at pretty-much the same altitude to work from. Nearest-city design temp from an ASHRAE tables probably won't cut it.

Thanks for the reply. Your insights are valuable. I will be looking into it further.