Hi, I have a two storey house with a basement.  The house is heated with infloor radiant in the basement slab and under the first floor joists.  My question is how should i insulate the floor joists.  My radiant pipes are suspended 1 1/2" from floor.  My plan was to staple reflective foil 2" below pipes.  I have a problem doing this because i have open web floor joists.  So it would be difficult to seal.  My next plan was to put r-19 insulation with a reflective foil towards the pipes, then staple my 4' reflective barrier to bottom of joists.  Or i thought of stapling my reflective foil to bottom of floorjoists without the r-19 batts.  That would leave an 13" airgap as my joists are 13".  Would this be too large of an cavity to heat.. would lose some of my effienicy.  Any thoughts would be greatly appreciated. thanks in advance.

First, you need to be sure that plateless joist heating will do your job. Did anyone do a room by room load calc to verify that you can get the output you need from naked pipes to meet your load?

If not, that is what needs to be done.

once you get to insulation, putting reflective UNDER your fiberglass isn't all that helpful. Putting it at the bottom of the joists is useless. You want a real R13 or so starting about 3" down from the subfloor IF you stick with the suspended tube app, so it is in the middle of the airspace. I would use rigid foam if possible, but a batt product works too.

Reflective foil under anything but a roof or a turkey is not all that effective. It was not made for radiant floor applications and is not used by professionals. I wish people would give up on this silly gimmick and heed your advice Rob.

Heat load analysis, experienced specification of effective heat emitters (I no longer use suspended tube) and a specific design temperature. If your contractor doesn't speak the language, find a new one. If you don't have a contractor, at least find a certified designer before wasting good money on tankless water heaters, magic boxes and the ubiquitous "bubble foil".