I have FPX36 insert with a ~18" hearth which is made of marble. That's on top of cement board, grout and frame structure stuff. When I put my hand on it, I can barely keep it there 1-2 seconds, then I zap it with IR gun and it sez 140, yikes...is that an issue? It's usually dependent upon how much wood is burning, and how close it may be to the window for a lot of radiant heat, but dang that sure seems awfully hot!