Mylar has a few characteristics that make it both useful and problematic as a blanket material. It holds static electricity so things cling to it like dust lint etc.,. It is waterproof, so it can be a shelter from rain or sleet. However, when used alone as a blanket, mylar does not breathe so any moisture produced by your body will be trapped. If it’s cold, that moisture can lead to hypothermia- when you’re wet, heat is dissipated much more quickly from your body. If you are clothed or have a cloth (wool or polartec or down is best) blanket, use the mylar as an additional outer cover. You can leave part of the mylar open so it breathes rather than building up moisture underneath. i recommend the double strength mylar with one side mirror finish and the other safety orange. There are also mylar “sleeping bags” that can provide a quick warm up, but again, moisture build-up can be a problem. I honestly don’t know how flammable mylar is, but keeping it away from open flame is a must. Despite these drawbacks, mylar is a cheap, lightweight essential for any bug out bag.