I haven't put this bag to the TRUE test yet (sub-zero), but Im certain it will do just fine. Im amazed by the build quality, materials, and construction. The loft is amazing, no joke 9 inches easy! Its expensive, yes, but worth every penny. For me, it was important to have a bag that would allow me to sleep well during the bitter cold winter nights. It's the equivalent of a one night stay at a 5 star hotel, except this bag will bring me more joy than any 5 star hotel ever could, for many years to come.