ADz
Thru Hiker
The amount of energy lost as radiation from the outside of your sleeping bag is proportional to its surface temperature. It going to be warmer under you where it is compressed than above you where it is lofted. Some of that radiated energy will be reflected back to your sleeping bag by a silvered surface under you. The main value in a silvered surface above you would be the degree to which it inhibited convection and conduction of body heat to the cold air above. This will be a bigger part of what laying a space blanket over will do for you than the reflection of radiated heat. But an impermeable silvered surface above you will form condensation on the underside, which if bad enough, will dampen your sleeping bag and reduce it's loft, losing you more than you'll gain from it.
I would never use a foil blanket above me. I've used them in past to just possibly insulate mat from cold floor?