The manufacture Calorique's, instructions in a few places say to always leave 1/8 inch air space when using flexwatt or it may overheat. Covering it with foil, over the heating surface seems to violate the specs. I would guess that there is a reason.

Aluminum is the most common heat shielding material I know of, it does reflect and dissipate heat.

I have never heard of covering the top of flexwatt the under side for sure to reflect heat up but not the top. I have seen runners to protect it once of twice.