We have put radiant heat into a new-construction SIP house, using tubing, aluminum plates, and plywood strips inbetween tubes, above the existing subfloor. (Not staple up, but like a DYI warmboard system.) Hardwood flooring will go on top. We are wondering if we really need to insulate below the floors, between the joists. The basement is a heated living space, with tubing inside the slab, so any heat that leaks downward from the main floor, instead of radiating upward, will go into a heated space, so there is no net loss in the building if we do not insulate under the floors. We are hesitant to use fiberglass, for cost and health reasons, and blue jean batting is very expensive, and it sounds like a hassle to hang it up beneath the subfloor. Can anybody tell us if there is a good reason to insulate between floors, other than maybe keeping the zones more separated?

The main factor in insulating radiant floors is back-loss. If you have too much, you may not overcome the load in the room you are trying to heat and you may overheat the room below (particularly easy in an insulated basement). I have consulted many homeowners on problem radiant floors that weren't design or installed properly (both professional and DIY) and I can tell you, tearing down sheetrock is no fun.

The RPA guidelines recommend R-5 minimum (sorry, bubble wrap does not qualify), but the real test is the heat load for the room being heated. If the load is low and outdoor reset used (as apposed to on/off or setback) you may get away with it. Some manufacturers of sandwich radiant floor systems allow installation without back insulation again dependent on a known heat load.

DIY radiant floor heating is much easier when you have a professional plan up front.

Since you are using a DIY floor with unknown output, insulation would be wise.

greengirl - further to Badger's comments - it sounds as though you will have at least as much if not more resistance to heat above your tubing than below. This is why Badger indicates the risk of overheating the basement. If you are not finishing the basement right away, you have the luxury of wait and see! If not, its a lot cheaper to insulate now.

Rock wool batting can be used in lieu of fiberglass, and has fewer "issues" (beyond being itchier to install) than fiberglass products. The fibers are denser than fiberglass, and are less prone to showing up as suspended particulates in the air, etc.

Alternatively, an inch of foil-faced rigid iso or EPS cut'n'cobbled foil-side-down to fit the joists wouldn't have to be a perfect fit to achieve reasonable isolation. Material cost for 1" of fire-rated iso would be ~80-90cents/square foot. An inch of EPS would be about half that, but code would require putting half-inch gypsum up on the ceiling as an ignition barrier.