Are you talking about that shiny mylar sheet "space blanket" you can buy at Walmart? Or the 'emergency rescue blanket" things that are mylar with a little insulation between the layers maybe?
I use the cheap mylar sheet 'blankets' to block wind and create minimal dead air space under my hammock, which works fine. I've used it as ground cover too, but it doesn't last very long, is really noisy, and shreds after a few nights. I consider the "thermal reflective" attributes of the stuff worthless except as a big light reflector...I've rolled the blanket ends onto my trekking poles and made a HUGE signal mirror, which actually worked well! I carry one or two of the things in my pack for their utilitarian value....but......
Making a bag out of it would make a noisy, unbreathable, nightmare unless you figured out a way to vent or perforate the stuff. Nope, I wouldn't waste my time with it, but you might come to other conclusions.
paul, texas KD5IVP