To build or to buy, that is the question

[Scubaf250]

Just curious, while I was looking on YouTube for rack ideas I have seen people sandwich the flex watt in between the foil tape. (Layer of foil tape on the shelf, flexwatt, layer of foil tape covering the flex watt completely) Would this be the way to go? The foil tape can withstand temps up to 250 degrees and the PVC board if flame retardant. Maybe someone can point out how this wouldn't work, I know YouTube is not the best place for rock solid information.

I think it's so that the tubs won't rub on the flexwatt and eventually wear through the plastic coating and cause a short/fire/shock/some-kind-of-exposed-electrical problem. Not sure why under it though. Maybe for the same reason or it's just a practice people carried over from wood racks?