Foil on, or better under, the tank depends on the room where the tank is. In the office, which is my QT room, it gets rather cold as there's a large window and the biggest wall is exterior. For that reason, one of the 10g tanks I have in here is set on a piece of the foam with foil wrapped around it. The foam and foil help keep the heat from the UTH up where it belongs, but the foil isn't directly over the UTH so the pad can still 'breathe'
Basically, put the insulation on the back and sides of the tank and foil treat the screen first. If you still have temp stability issues, then you can think about insulating underneath.
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