My shop is a detached double car garage. I have an Easy Radiant HeatWave. it has 8' of combustion chamber that does a wonderful job of adding a bit of warmth to the shop. As for the floor, I laid down some dimple waterproof membrane and then two layers of 5/8" OSB. The floor is cool, but never cold on the feet. The dimple gives an air break between the cold cement and the warmer wood ontop. As a bonus, it is also waterproof so any moisture that might wick up from the concrete can easily evaporate or re-absorb into the concrete pad.

It all comes down to fore thought in design.

Anthem: AVM60, Fosi DAC-Q5
Axiom: ADA1500, LFR1100 Actiive, QS8, EP500, M3, M3comp, M5