Winter Sleep & Clothing Systems

During the deep winter I use vapor barrier liners (VBL's) in order to reduce evaporative heat loss and prevent my insulation from "collapsing." VBL's contradict modern emphasis on breathability -- they are made of non-breathable fabrics that do not permit the transfer or moisture.

To understand how VBL's prevent insulation collapse, let me explain two scenarios rating to sleeping bags assuming you were not using VBL's. In warm conditions your perspiration moves to the outside of the bag and evaporates, keeping the insulation mostly dry. But in cold conditions (starting at around 15-20 degrees) your perspiration moves towards the outside of the bag but hits the dew point before getting there (it's so cold that the dew point is actually inside your insulation). The perspiration turns into moisture and soaks the insulation; with every passing night your sleeping bag becomes wetter, heavier, and less able to insulate.

If you wear a VBL, you can prevent your perspiration from entering your sleeping bag insulation at all. And the perspiration does not just build up inside the VBL -- your skin senses that the "micro climate" between it and the VBL is fully saturated with moisture, and so it stops perspiring in that area.