Dear Tim,
I think it is more a function of light that you are using the foil for, the heat will be reflected, and therefor the back wall will not absorb the heat to radiate at night, so you may get cooler night temperatures. I once saw a plan for a lean-too greenhouse that had plastic tanks filled with water stacked all along the back to provide heat at night via a heat pump, looked like a good idea to me!
Brian.