I am considering underfloor (subfloor and hardwood flooring) screw up type extruded heat transfer plates. I then plan to follow up with closed cell insulation underneath the plates and subfloor. My concern is that since the plates do not fit perfectly to the subfloor I could then get the foam insulation between the plates and subfloor. Remember that the foam insulation starts as a liquid and fills any void. To that end I think I will cover the area where the extruded aluminum heat transfer plates are with reflective foil and tape that to the subfloor. I only care that the tape holds long enough to get the foam insulation done since closed cell insulation will not sag. My installation is only for bath floors and a central area in my family room. I already will have geothermal heat, but just want this for the special comfort it gives. I will supply the pex tubing with solar heated hot water. I do realize that putting this system 1.5" below the interior surface will give a lag. I am using the extruded plates to avoid the tic tic you get with staple up aluminum sheets. Any suggestions or any experience doing this?