they absorb heat better through the belly, but i dont think it actually matters, any heat source will work either way, although it my be hard to put a lamp on the bottom of the cage lol
I have either back heat or side radiant heat in all of my racks and have not had any issues. I love back heat. It gives a very good gradient and also warms the air and not just the bottom of the tub. Granted it is good to have decent temperatures in your room (i.e. at least 75 degrees) but it has gotten down to 65 at times in mine and the thermostats were only running at around 60% power. Either way is fine. In my opinion back heat is safer though, being that the tape isn't sitting directly below the animal and has a little bit of air space between. But both work very well and there isn't truly a right or wrong way to do it.
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I use belly heat and back heat in my racks, my 41qt rack get's belly heat and 28qt and smaller have back heat. Im also in FL and my room is never lower than 76-80.