I understood that the efficiency of TC Ceramic is best when exposed to the sun, not underground, inside a roof, or under another coating.

Yes, the best application is on an external roof at 15 mils/.4mm. – for reducing solar heat. With thermal conductive heat, then you have to use more product to block the heat. This is the same with all insulations. Moisture will not impact TC Ceramic’s™ insulation effectiveness as with other materials.