Do you use n undertank heater with this setup? This is al lot like what I wanted to do, using the Great Stuff to make the base, but I worried that the UTH would either not heat all the way through the foam and keep it hot enough, or the Great Foam would get too hot.
What I did is place slate tile down directly on the bottom of the tank over the uth. I then used grout to fill in the cracks. There are parts of the uth that do heat the foam, but so far I haven't noticed any sort of melting or extreme temperature fluctuations. I have a compact grow light over part of the tank too for the plants. Right over the tile where the uth is the temp was getting about 101-102 degrees, so I covered it with paper towels and repti-carpet over that to bring the temp to a nice 97-98.
The cave over the heater has a ridge inside where the temp stays about 94-95, and about 90 on top of it. The cool end with the growlight on stays about 78, at night the cool end drops to 70 w/the light off (With the warm end the same as during the day). So far it's been this way ever since I made it. I could lift the tank up off the counter it's on a little more and that would probably also lower the warm end temp a few degrees if needed. Next year when I cool her for breeding I will turn the uth off and use a normal fluorescent for the plants on the cool side.
So no, no melting. If you put great stuff right over the uth I would run the heater to find if it gets warm enough, if it doesn't, you can always cut the foam to make it shallower and run another heat test.
Oh and the reason the foam is so deep is because there's a couple hides it's hiding.
