Believe it or not, one of the most important features of the standard sleeping bag/quilt is that it's permeable. This is important, because your body gives off quite a bit of moisture (the technical name is "insensible perspiration") during the night. If either the outer or inner shell of the bag/quilt is waterproof, the bag will trap moisture inside.
Of course a number of us use a vapor barrier inside the sleeping bag in really cold weather, which adds to the warmth. However, you don't want anything inside that vapor barrier except a suoer-moisture-conducting base layer. Any insulation you may want to add on an extremely cold night needs to be outside that vapor barrier, or it will be soggy (and not very insulating) by morning.
I strongly recommend, from long experience, that whatever vapor barrier you use (and that foil layer you're contemplating is a vapor barrier) be separate from the sleeping bag. On warmer nights, you won't need it, and it will feel like a Turkish bath inside. The borderline here is different for different folks--for me it's about 30* while for others it's more like 20* or 15*. On extra cold nights, when your quilt/bag is marginal, you'll need to be able to add insulating clothing over the top of your vapor barrier, not something you can do if the barrier is part of the sleeping bag shell.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey