The one survival experience with the older foil type blankets left me worse off than without-I was young, and didnt understand the mechanics of heat generation, and put the foil blanket around me, then a fleece blanket over that. I dozed off, only to wake up later to condensation that had frozen to the blanket. Not a really pleasant experience...
I have an AMK bivy bag, and, should it be used, I think I would stuff leaves, pine branches, or something else in there, to give some insulation. The heat sheets I could wrap up temporarily till I get a fire going, then build a shelter from there...and use it to reflect heat.