Bivvy Bags And The Physics Of A Damp Morning

A bivvy bag is the simplest shelter available and the one most likely to leave its occupant damp. The moisture is usually not rain, and knowing its source determines whether the format works.

A sleeping body is a humidifier

A person loses a considerable quantity of water overnight through breathing and through the skin, even doing nothing. That water leaves as vapour and has to go somewhere.

In a tent it disperses into a large air volume and out through vents. In a bivvy bag it meets a surface a few centimetres from the sleeper.

Where that surface is below the dew point, the vapour condenses on it. The result is a wet inner face and a sleeping bag damp on the outside by morning.

Breathable fabrics move less than expected

Membranes pass vapour by diffusion, driven by the difference in humidity across the fabric. That works well when the outside air is dry and much less well when it is not.

On a still, humid night the gradient is small, so the transfer rate falls exactly when the need is greatest. The fabric is behaving as designed and the sleeper is still wet.

Cold makes it worse. A cold outer surface condenses vapour inside the fabric before it can escape, which is why frost outside frequently means water inside.

Ventilation beats fabric specification

An open hood moving air over the sleeper removes more moisture than any membrane, because bulk airflow carries far more vapour than diffusion does.

The trade is warmth and the risk of rain entry, which is why most experienced users pair a bivvy with a tarp. The tarp keeps rain off and the bivvy stays open.

That combination is lighter than a tent, drier than a bivvy alone, and it explains why the two items are so often carried together rather than as alternatives.

Site choice does most of the work

Cold air pools in hollows and valley floors, so those are the places where surfaces drop below the dew point first and condensation is heaviest.

A slight rise, a spot with air movement, or ground under a partial canopy will be several degrees warmer overnight and noticeably drier by morning.

Long grass and damp ground add their own evaporation to the air immediately around the bag. Stone, gravel and short turf are consistently better surfaces for the same reason.

The format suits short trips and clear nights

A bivvy accumulates dampness night after night, because a bag that never fully dries starts each evening wetter than the last and loses loft steadily.

For one or two nights, or where mornings reliably bring sun to dry kit, that accumulation never becomes limiting. Over a week in unsettled weather it usually does.