by McDave » Sun Oct 29, 2017 11:10 pm
Radiant floor heat coils for bathrooms are typically 300 watts and 2-3 ft wide x 10 ft long. 300 watts is not a ton of heat but floor heat relies on mass to do a lot of work with little heat. They are slow to heat up but once warm they heat everything in the room, all the fixtures and furniture etc. above floor level are heated, and the heat isn't all at ceiling.
The coil would have to be inside with a moisture barrier between plywood and coil, then some sort flooring like a floating "wood" floor or tile. You would probably want to thinset the top of the coils to have a flat floor before flooring, also to add mass and spread the heat out. Also waterproofing the underside would be best, Rhinoliner etc. and insulating and skirting would be a huge help. But the heat is really nice, warm feet and the heat comes up past you as it rises, it is usually warmer at floor (where you are) than at ceiling (where you are not). If your pipes are all inside and you keep it around 50f then all should be ok. I can't tell you if it would work because there are a lot of variables I don't know, r-value of insulation, how much loss, location and expected lowest temps etc.
I have hot water radiant floor heat in my house and I love it, very efficient and silent but it does react slowly. You need to anticipate demand by OAT (outside air temp) and loss rate. But lets just say you have 1500 lbs of payload warmed up to 72f, it will take quite a while to cool off if OAT is 30f but not a lot to maintain 72f. When OAT starts to rise you need to anticipate and cut back heat so as not to overheat. Remember, you are heating mass not air. Mass reacts slowly.
McDave