Ok i get that bit just fine.... So for an extended outting for more than 10 to 15 days in -0' a vapor barrier would have some use, and would help. Sooner or later even that bag will be over come. When I was living out during those 3 years I used wool blakets as well as down bags. it got to be -50' at night and for a bit maybe as warm as -40' at hight noon. At times everything needed sunning, to wick out mositure.... I figure any thing would need it. Thxs for the reply... Mac