Never tried the reflective stuff. Probably would be more efficient but I'm wondering how it would hold up. The area is quite windy at times and even the tarpaper would blow and tear sometimes. I'm also not interested in squeezing the last % of efficiency from the thing either. I've got a lot of space back there so if I ever decide I need more heat I'll just add more pipe and roofing. Can you imagine a couple acres covered with 1/2 black pipe?
Al