Y'all need to remember that things like heat tape and fluorescent lights, that produce heat (actually it is the *ballast* of the fluorescent fixture that produces most of the heat), are designed to work in a particular sort of environment (i.e. their intended use) where heat is being shed at a certain rate. If you insulate them by embedding or surrounding them in stuff they weren't designed to be embedded or surrounded in, they get much hotter, sometimes too hot for the materials and cause a fire.
Heat tapes are a pretty common cause of fires; MISUSED heat tapes are by far the commonest source of heat-tape fires; and putting insulation over heat tapes not designed to be used under insulation, or wrapping it over itself or closer than the mfr says, is probably about the most common WAY to start a fire by misusing heat tape.
Just don't do it, eh?
Pat