How to Reduce Condensation in a Bivy Sack With a Vapor Barrier Liner

A bivy sack is a bivouac sack, which is an ultra-lightweight sleeping bag designed to protect the user in a variety of conditions, including extreme cold temperatures, without the need for a tent. A vapor barrier liner fits inside the bivy sack and is made from a material that does not allow moisture or air to pass throughout it. The vapor barrier liner protects the bivy sack from your perspiration and creates an environment that actually causes you to perspire less while sleeping with your body wrapped in a nonbreathable membrane.

Instructions

    • 1

      Shake the bivy sack to fluff the insulation fibers. The fluff creates air pockets that create the insulation that protects the user from external conditions. If on a multiday trip, ensure that the bivy sack has had a chance to dry out every couple of days or as needed. When condensation moistens the fibers, they become weighted down and the bivy sack loses its ability to maintain maximum warmth.

    • 2

      Place pads between the bivy sack and the ground. Protecting the sleeping system from ground moisture is a simple way to eliminate condensation inside the bivy sack. A plastic trash bag, pine needles or a polyfoam mat are each suitable options depending upon actual climate conditions and your desired comfort.

    • 3

      Insert the vapor barrier liner into the bivy sack. Most vapor barrier liners are designed for use inside a bivy sack sleeping system as many bivy sacks are constructed with an outermost laminated or treated layer that resists moisture. In extreme conditions of persistent rain or sleeping in wet snow, an additional external vapor barrier may enclose a bivy sack that was not designed for contact with extreme moisture.

    • 4

      Wear wicking underclothes and a balaclava. Slip into the vapor barrier liner wearing long-john style undergarments, balaclava, socks and gloves that have the ability to pull perspiration away from the body. A balaclava is a hood that has an open face and fits over the head and shoulders. Do not wear additional clothing while sleeping inside the vapor barrier liner as this can lead to excessive perspiration that adds to condensation inside the sleeping system and dampens your clothing.

    • 5

      Draw the strings of the opening at the head of the bivy sack close around the face but do not cover the nose and mouth. Breathing inside the vapor barrier or the bivy sack increases the condensation inside, which limits your ability to remain warm. Never close the vapor barrier around your nose and mouth, to avoid the risk of suffocation while sleeping. Instead, tent your face with a bandanna or square of thick fabric like denim or polar fleece. Hold the center of the fabric square so that it tents your face as the drawstring of the bivy sack is closed to hold the fabric in place and away from your face while sleeping. Zip the bivy closed from the inside when finished.