Vapor Barrier Fabrics

Avoid using any waterproof or near-waterproof fabric on the outside of your sleeping bag since it will trap body moisture inside the insulation. This includes avoiding use of plastic sheets, tarps, radiant reflectors such as aluminized films, and those waterproof-breathable fabrics that are currently popular in bivy sacks or over bags that fit around a sleeping bag. The breathable, DWR-treated, tight-weave fabrics usually found on good-quality sleeping bags should be sufficient to block cold wind (thus blocking conductive and convective heat loss) while not trapping excessive moisture inside.

Remember to vent your moisture-rich exhaled breath to the outside, not inside your sleeping bag. In below-freezing temperatures, exhaled breath trapped inside a bug-net-covered hammock may condense and freeze safely out of the way on the inside of the bug net. In near-freezing temperatures however, this condensation may not freeze, or may re-melt and drip back onto the sleeping bag. Prevent this by not using the bug net or occasionally venting it during the night. Getting up once or twice during the night to answer the call of nature may not create sufficient ventilation.

Vapor barrier bags or sheets should never be used over the face since they can block breathing and cause suffocation. Remember the infant deaths caused by thin plastic bags when they first became widely available? VB fabrics should cover the body only from the neck down.

Once again, there are many choices in the battle for warmth. If interested in VB fabrics, you should master their use on short, mild-weather outings before committing to long or cold-weather trips.

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