Dudes,
I have been into at least 20 houses where people have bought into the bubblewrap scam for use under slabs IN PLACE OF PROPER XPS and the under performance has been very noticeable, and in many cases, the slab has had to be either ripped out or major work around the perimeter had to be done to fix it.
For 20 years I used radiant barriers (foil faced building paper, not bubblewrap) behind walls. It is with an appropriate air space, usually 1.5" min because it allows for plumbing, electrical to be placed without breaking the barrier which is also the vapour barrier so it was properly caulked.
Foil faced insulation :: NRCan :: Advisory paper
Attic placement is iffy and especially below radiant tubing under the subfloor, I think, is a waste of materials and effort. I don't like suspended radiant tubing anyway, so i would never do it anyway.
I'm still looking for the report done many years ago about using the radiant barriers directly under the slabs. It doesn't paint a good picture.
IMO, the wall is the best place for the radiant barrier to be used.