What are you guys using for insulation when you have a Radiant staple down and Gypcreet over. I can't put any insulation in the joist cavity below & I feel like I need some... Have you been using a Bubble wrap / reflective insulation and staple to it?
I am guessing you have finished ceiling below. Can you blow in insulation?
First off bubble wrap will not work in a radiant system. The "reflective" value only works in an air space. Even then it is suspect and not allowed by most codes. The other thing to look at is the compressive rating. If the product will compress under load, the gyp will crack. If you can afford the space rigid insulation would work.
Why are you looking for a high mass system? I would take a hard look at a product like warm board. You can put rigid insulation under it and eliminate the thermal "flywheel effect"