I used ductwork insulation. Its 2" insulation with a foil backing. It acts as an insulator/sealer for my room, and it reflects. Might not be as good as mylar for the reflection, but in my case, I kill three birds with one stone. My room is sealed, keeping the stink in, and its easier to keep the temperature where I want it, and it acts as a reflector. Lined the walls and ceiling with it, and sealed all the seams with the aluminum tape they sell with the insulation.