bag water

Condensation trap

The building of Condensation traps or moisture traps (as they are more commonly known amongst Survivalists) is a survival skill to collect drinkable water.


Condensation traps have been in use since the pre-Incan peoples inhabited the Andes.

A method for gathering water in condensation traps is still taught within the Argentinian Army, for use by specialist units that are expected to conduct extended patrols in the arid border areas of the Andes.


Several methods of trapping condensation exist:

First method

This method was first used by the peoples of the Andes. A pit is dug into the earth, at the bottom of which is placed the receptacle that will be used to catch the condensed water. Small branches are placed with one of their ends end inside the receptacle and their other ends up over the edge of the pit, forming a funnel to direct the condensed water into the receptacle. A lid is then built over this funnel, using more small branches, leaves, grasses, etc. The completed trap is left overnight, and moisture can be collected from the receptacle in the morning.

This method relies on the formation of dew or frost on the receptacle, funnel, and lid. Forming dew collects on and runs down the outside of the funnel and into the receptacle. This water would typically evaporate with the morning sun and thus vanish, but the lid traps the evaporating water and raises the humidity within the trap, reducing the amount of water that is lost. The shade produced by the lid also reduces the temperature within the trap, which further reduces the rate of water loss to evaporation.

Modern method

Today, with the advent of plastic sheeting, the moisture trap has become more efficient.

The method is very similar to that described above, but a single sheet of plastic is used instead of branches and leaves. The greater efficiency of this type of trap arises from the waterproof nature of the plastic, which doesn't let any water vapour pass through it (some water vapour escapes through the leaves and branches of the first method). This efficiency requires a certain amount of diligence of the part of the user, in that the plastic sheet must be firmly attached to the ground on all sides; this is often accomplished by using stones to weight the sheet down and/or covering the edges of the plastic sheet with earth (such as that dug out to make the hole in which the trap sits). Weighting the centre of the plastic sheet down with a stone forms the funnel via which the condensed water will run into the receptacle.

Transpiration Method

Water can be obtained by placing clear plastic bags over the leafy branch of a non-poisonous tree and tightly closing the bag's open end around the branch. Any holes in the bag must be sealed to prevent the loss of water vapour. The sun heats the air in the bag, lowering its humidity and drawing water from the leaves; gravity then causes the water to run to the lowest part of the bag. Water is collected by tapping (see under "cask beer tap" ) the bag and then resealing it. The leaves will continue to produce water as the roots draw it from the ground.

In using this method, the water should be drained off every two hours and stored. Tests indicate that if this is not done the leaves stop producing water. Probably the heavy concentration of moisture-laden air reduces the effectiveness of the sun.

If there are no large trees in the area, clumps of grass or small bushes can be placed inside the bag. If this is done the foliage will have to be replaced at regular intervals when water production is reduced, particularly if the foliage must be uprooted to place it in the bag.

Efficiency is greatest when the bag receives maximum sunshine at all times. Exposed roots are tested for water content. Soft, pulpy roots will yield the greatest amount of liquid for the least amount of effort.

Condensation Trap Efficiency

It should be noted that condensation traps are not in themselves a sustainable source of water; they are sources for extending or supplementing existing water sources or supplies, and should not be relied on to provide a person's daily requirement for water, since a trap measuring 16" in diameter by 12" deep will only yield around 100 to 150ml per day. One method to increase the water output is to urinate into the pit before placing the receptacle in. This increases the moisture content of the earth, reducing the amount of water vapour that the earth can subsequently absorb.

Condensation Traps in Popular Culture

Condensation traps are common in survivalist fiction. The 1972 Movie Family Flight features a family forced to make an emergency landing in the Mexican desert. The movie shows, in great detail, the method for construction and use of the modern (plastic sheet) condensation trap. In the novel Life of Pi, a number of traps are towed behind a life raft.

Condensation Traps and Human Urine

It is well known among survivalists that using a condensation trap to distill urine will remove the urea and salt, providing one with drinkable water as a result.


Source for the transpiration method is from the Aids to Survival, Western Australia Police Academy, 1998, available for download at

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