Well it's a lot less work if your concrete isn't poured yet because you can integrate the radiant floor system. However, a radiant flooring system also requires some form of heater, like a pump/radiator that heats water and then circulates it through the pipe system... I don't think it's something just anyone can do and would require a specialist or an architect to draw plans and figure out how the system will be integrated. It can be costly, but pays for itself over time, especially if you use solar heating.

Radiant Heating is definitely one of the best ways to heat a space because of the fact that no energy is lost through the ducts. When a surface (thermal wall) is warm, it transfers heat into a space very efficiently, and because hot air rises, and when a space is warmed by hot air, usually the hottest air is near the ceiling (in other words: lost). A thermal wall/heated floor, will retain heat.

From what I read though, it seems that the garage was already in place and only insulation and drywall(?) was added...

 

Take a pic.

 

as a guide, my attached garage with NO insulation stays about 20-25 degress above the outside temp.

 

drop your pants and see what happens after an hour. that should be a good gauge.