So, I've read that 90-80F is a very good temp gradient but I'm wondering if it should be above or below the substrate? I'm using aspen at the moment and when it's 90F below the substrate is barely 81F above it, cool side still remains at 80F. I'm not using much aspen either, like 1/4 inch or so. So below the substrate I'm having 90-80F temp gradient and above only 81-80F (room temp is 82F). I should say that he never burrows under the aspen either, he only stays in its hide so this is why i'm worried about the temperatures above the substrate.