I have seen YouTube Videos that show interesting tests of this type of bag. They seem to work to surprisingly well. However they provide no insulation whatsoever, so you must have an insulating pad over cold ground. Because they do not allow moisture in or out of the envelope they do become damp inside when used. The sack is a better design than the simple "space blanket" for several reasons. Overall, the light weight, the windproof, waterproofness of the bivvy sack alone make it worth carrying when you don't have a tent along. I don't know anything about the durability.