Some folks here use vapor barriers. I did, and found a different system. Basically, it does keep body moisture from venting through ( and reducing the loft) of the insulation and bag. This also keeps in the warmth that would be wicked out . Does it work? Yes, very well. My complaint was leaving the bag in a sauna like sweat and hitting the cold morning air. A good website that compares systems and materials is www.wiggys.com If you have a present system a add on vapor barrier is a good investment. I did this with the premium down bag from my wasted youth. My present system from the above website uses a different mousetrap.