If you're referring to the expanded PVC sheets that is what most of the cage manufacturer's use. No issues in using it with flexwatt. It is fire retardant in that it won't burst into flames just from a high heat source but can burn if exposed to direct flames from a prolonged exposure to fire.

It's pretty easy to make a shallow channel in it for the flexwatt using either a router with a mortising bit or even better a power planer.
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BigT
There is a difference between ignorance and stupidity. The ignorant can be taught, stupidity is beyond our control.
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