i suspect the 2" gap would be a minimum recommended distance for IR reflection. More should not matter. For standard foam installation without heat in the floor, some recommend 3/4" minimum with 1 1/2" preferred. The dust will be a problem regardless. By the way, you are losing heat through the uninsulated floor joists
And the install is MUCH harder by installing it in between the floor joists. Regardless of how it is done, the fire fighters would like to have the foam covered by something like drywall or plywood so if there is a fire, they do not have poisonous fumes or molten foam dripping down on you or them. I used construction adhesive and glued painted osb to the foam and installed it as a unit in part of the basement.
As an aside, i did not use the metal plates the manufacturer recommended, again no problems. I had nails sticking slightly through the floor, so i bought some rolls of 12" wide corrugated cardboard and stapled this to the underside of the floor (will slow down the heat transfer slightly but less than everything above it any ways). I had checked first with a piece of cardboard that i pressed hard with my thumb and the nails did not poke through. Long nails that missed joists etc., i either cut or more often stuck a piece of 1" x 1" foam insulation end wise onto the nail. That worked well to prevent getting the nails poking into the tubing during installation. I was worried about being able to rearrange tubing because we were going to renovate at some point so i used scrap cutoffs from siding my barn to sandwich the tubing against the floor (two runs of tubing per cavity so the screw was in the middle of the board with a piece of tubing to each side) - was supposed to be temporary and then i was going to rent a stapler. Worked so well, i never changed it.
During a subsequent renovation, a contractor who was warned about heat in the floor did shoot a long nail through some blocking on the floor, missing the floor joist completely and going through a tube. Sealed pretty well! Was several years later before we saw it when we removed the insulation for access for some wiring.
A few years ago we renovated the rest of the house and ran it the same way minus the cardboard since it was not necessary. Since that was an exposed crawlspace (post foundation), we used 4" of foam (2 layers of 2" foam, staggered seams) and covered it with PT plywood. Probably overkill. That whole addition that we gutted and redid is so well insulated (fiberglass in the stud cavities, 2" of foam with an air gap on the inside) that the snow does not form icicles, etc. and takes nothing to heat. Now if only the other part of the house was that well insulated.
Ken
