To elaborate - some people have tried putting flexwatt underneath a "false floor" in existing cages and had good luck. If your have a wood cage, putting the flexwatt under the cage obviously won't pass enough heat through the cage floor because it's too thick and wood doesn't conduct heat enough.
I've heard of people putting thin flooring or shower stall material over flexwatt and siliconing it to seal it. There is a small risk of heat buildup getting too hot, but if you use a thermostat along with the flexwatt (which is a good idea no matter where you use flexwatt), it shouldn't be an issue.
I am just starting to use Flexwatt more than traditional heat pads and don't think Flexwatt would project heat downward from the ceiling enough to heat up 12 to 24 inches to the floor. It might heat 4 inches downward. Since heat rises, everything will collect at the top of the cage.
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Lauren Madar - OphidiaGems.com
1.0 Ball Python, 1.1 Hog Island Boas, 1.1 Hypo BCI, 1.1 Surinam BCC, 0.1 Green Tree Python