vapor barrier

I think the emergency blanket would provide more warmth and weighs less.

and only costs a few $$$ :-)

Reflective mylar emergency bags are fragile, so if you plan on wearing or storing your boots in your bag, don’t expect them to last more than 2 or 3 mights. Also, they are crinkly and the noise will plague you every time you move. The warmth of reflective layers is over rated. I think you are better off with a fabric you can tie into the bag and have a decent adjustavle closure on. They are cheap, but so are mattress bags that can be cut and heat sealed to shape.

about the mylar being fragile, my dad used a space blanket as a ground cloth at philmont this summer and it dident tear once. he brought an extra just in case, but dident need it.