Thanks for your comment - a friend here who is a local builder said the same thing; he suggested that I use a foil with about a 2-3" airspace stapled up below the subfloor in the joist space (which is 2x12), and then put fiberglas in the space below that; or, that I simply use a thicker batt.  I am somewhat confused, however; the reflecting foil was one of the things that several people on this forum seem to be suggesting is important.  Is it?  Does the reflecting of the radiant heat really matter, or is it simply about R-value?

Thanks

I don't recommend foil in upward facing applications, typically. but you can argue about it until you are blue in the face, in the end we are all guessing.

I tend to trust R value over reflectivity, in any case. foil won't hurt but in no case would I substitute foil for real insulation in that application.

Is this different from a foil-based material? I've seen some arguments that a radiant system will benefit from a foil-based insulation with an air space, to reflect heat back. A friend here who is a contractor does not think that's likely to make much difference, but suggested the 1" poly isocynante, because it has the foil backing (and is thin). He suggested that I put it up in the joist space w. a couple of inches of airspace between it and the underside of the subfloor, and then put fiberglass underneath it to fill the (2x12) joist spaces. Does that sound like overkill?

Nevermind - just saw your reply to prev. post - thanks!