Sleeping bag wet? How to prevent it

A wet sleeping bag is rarely the result of rain. On multi day trips it is almost always condensation and sweat, building up over several nights, without you noticing until you get cold. This article explains where that moisture comes from and how to keep it out.

Why a wet sleeping bag is dangerous

A sleeping bag does not heat you. It traps air, and you heat that air. That still air sits in the loft, the volume that down or synthetic fibres build up once you shake the bag out.

Moisture destroys that loft. Wet down clumps together and loses most of its insulating value. Synthetic fill keeps more of its insulation when wet, but its performance drops clearly too. Water also conducts heat away many times faster than air: lying in a clammy bag is physically a different thing from lying in a dry one.

On a single summer night that is annoying. On day five of a winter trip it is a safety problem, because you stop recovering between days and your reserves run down.

Where the moisture comes from

How to keep it dry

Never breathe into your sleeping bag

However tempting it is at minus twenty: keep your mouth and nose outside the bag. Use a hat, a buff or the bag's collar to keep your head warm, but breathe into open air. On a week long trip this is the single habit that really makes the difference.

Ventilate, even when it is cold

Always leave two openings in your tent so air can move through. One degree colder and dry beats one degree warmer and wet.

Get in warm and dry

Lying down chilled does not work: your bag can only hold on to what you produce. Move briefly before you get in, put on dry sleep clothing you use for nothing else, and eat something.

Protect it during the day

A waterproof compression sack in your pack, and an extra dry bag for river crossings and packrafting. A groundsheet under your tent keeps rising damp out.

Air it every morning

Twenty minutes inside out in sun or wind takes the night back out of it. Do this every day, even when the bag feels dry. Moisture builds up unnoticed.

The vapour barrier liner

A vapour barrier liner is a waterproof inner lining you place inside your sleeping bag. The idea is simple: you keep your own sweat on your side of the barrier, so it never reaches the insulation.

On long winter trips with a down bag that is a serious advantage. Without a liner such a bag gets slightly wetter every night, and after a week you notice.

Be honest about the downside: you are lying in a non breathable layer and it feels clammy. Wear it over thin base layers rather than against bare skin, and only use it when the cold justifies it. For a summer trip in the Ardennes it is overkill.

Wet anyway, now what?

• Never wring out a down bag. Press the water out gently and shake the fill loose as soon as it starts to dry.
• Sun and wind do most of the work. Hang it inside out with the footbox towards the sun: that is usually where most of the moisture sits.
• If it is only slightly damp, just get in. Your body will dry it partly, as long as the temperature allows.
• If it is soaked in freezing conditions, use a bivvy bag or emergency blanket and sleep in all your dry layers. Always tell your guide.

Further reading

• How do I buy a sleeping bag? for the choice between down and synthetic.
• The impact of sleeping outside for what a night outdoors does to you.
• Escape The Arctic, the expedition where these rules matter most.