IMHO, the most important thing is to install at least 2" of rigid foam board and vapor barrier UNDER your slab. Even if you do NOT install radiant in-floor heating your floor will be warmer and dryer.
Radiant, in-floor heat is the most COMFORTABLE form of heat, especially on a slab. You can use natural gas, propane, oil or even wood as a heat source. The downside is it is not cheap to install. Operation costs depends mostly on how well you insulated.
Stay away from electric resistance heat. Cheap to install, but you will go broke running it in winter.
Cheapest to install and operate is probably gas or propane forced air.
One of the things I'm finding buy accident is that my uninstalled slab appears to be working very well for the way I use the garage and I'm not sure if it will be a plus or a minus.
What I found in having an unconditioned shop over the past several years is that the temperature inside the garage is significantly warmer in the winter and significantly cooler in the summer than outside temps. I attribute that to the slab in contact with the ground acting as a giant heat sink maintaining a more neutral mother earth temperature.
For me this may be a plus. and I say maybe so don't run to the bank with this This is because of the way I use the garage. For example today when I got up it was 10° F outside but 42° F in the shop. Heat was shut off at around 8:00PM the night before. Now if I my intent was to keep the shop at a year round temperature of 68° F, I would say absolutely foundation insulation is required and radiant heat would be the best solution. But my intent is to keep the shop at around 40 in the winter and AC off in the summer unless I'm in it which is not most of the time. So right now it looks like the heat wont even be coming on at all when I'm not in it.
In this situation I would say radiant heat would not be good at all because it heats the floor first before the shop but my hot air will work fine although I may have somewhat cold feet as the floor heats last.
Now if I were to have insulated my floor and was gone for my typical 2 days the concrete would have been able to absorb ambient air temp and I would have come back to a cold or hot shop and the unit would run longer to catch up.
So your thoughts did I win by accident or am I doing to pay heavily for the mistake of not insulating the slab.
