Sorry, but J Jay's got it wrong. Reflective insulation properly installed is an excellent choice for radiant heating. The "cool air theory" doesn't incorporate the basic concept of heating with radiant units -- that heat transfers across air spaces by radiation, and does not heat the air space. The aluminum foil surfaces reflect 97% of that radiated heat back to its source. If, over time, the air space gains some heat and the insulation is properly installed, it will not "leak" out of the cavity.