If the insulation dries out quickly OK, but if its so dense that it
dosent, I wouldnt.

I thought that celotex was closed cell, so couldn't get soaked? Am I
mistaken?

Also, would it break down over long periods? I have seen no sign of moisture
ingress through the roof during these last weeks of rain, so it wouldn't be
a deluge of water, just spots, at the most. I could always use foil backed
stuff facing upwards. This could even reflect excess summer heat radiated
from the tiles back up. (The insulation would be sufficient that no useful
solar gain in winter would ever occur).

Christian.