Survival bags/blankets don't do away with the need for insulation but they do give you a very fast wind / waterproof layer which can be a lifesaver. I always consider it an aid to building a shelter rather than an absolute shelter - although I think anyone who expects to build a shelter from natural materials better assume he has a lot of time / energy / daylight available - my SHTF scenario being injured with night falling relies on getting my butt off the ground and wrapping myself in a survival blanket