The Battle Against Condensation
Condensation in tents has bedeviled designers ever since Julius Caesar set out to conquer Gaul. Condensation occurs when warm, moist air encounters a cooler surface and is cooled below the dew point, the temperature at which the air is saturated with moisture and dew begins to form. In a tent, your breath is the most common (and unavoidable) source of warm, moist air. A person can exhale a pint of moisture per night. Evaporation of sweat and moisture from rain-soaked clothing further humidifies the air and increases the threat of condensation. Cooking inside a tent adds still more water vapor to the air, but that's by far the least of your worries. Running a stove in a poorly ventilated tent poses an extreme danger of poisoning by carbon monoxide. Cooking in any tent, even with the door and windows wide open, creates a serious fire hazard and should be avoided.

Nearly all tents sold today fight condensation with a double-walled design that uses a waterproof rain fly over a non-waterproof tent body, called the canopy. In theory, warm, moist air will rise through the uncoated canopy and condense on the fly. In temperatures above freezing, it will then roll down the inside of the fly and drip onto the ground instead of the occupants. In temperatures below freezing, the moisture will congeal into frost. If the wind shakes off the frost, it will land on the canopy and slide harmlessly down to the ground. The theory works, in most conditions, although you then have to deal with a wet or frosty rain fly in the morning. Be sure that any tent you're considering provides a good separation between the fly and canopy. If they touch, either when the tent is buffeted by the wind, or when you brush up against the tent wall, condensation on the fly is sure to penetrate the canopy.

In severe cold, as well as in certain other conditions of cool temperatures and high humidity, the canopy's temperature can be below the dew point, so that moisture condenses on the canopy instead of passing through to the fly. I still remember waking up at 17,200 feet on Mt. McKinley to find a veritable snowstorm raging inside the tent. Moisture from three deep-breathing bodies had condensed to a thick frost on the canopy. As the tent trembled under assault by the ever-present wind, the frost sifted down and covered everything. Opening our tightly closed sleeping-bag hoods even a crack permitted a cascade of ice crystals to pour onto our faces. The only solution was to use the whisk broom we'd brought for just this purpose to dust off our sleeping bags and gear, then sweep the floor and shovel the piles of frost crystals out the door.

Tent-makers also try to prevent condensation by providing ventilation in various ways. The problem is providing a way to let warm air escape without permitting rain to enter. Vents that penetrate the canopy but not the fly do little good. Opening the door can help if the fly extends as an awning over the door to prevent the entry of rain. Opening doors or windows on opposite sides of the tent (if the tent is so equipped) permits cross-ventilation, which works well if the night is graced by a breeze. The best venting system combines roof-level and floor-level vents, so that warm, rising air can naturally flow out the top vent while cooler air flows in the bottom. Very few tents have a high/low venting system, however, because such vents are hard to seal against rain.

One group of tents that do have such a vent system is made by Stephenson Warmlite Equipment in Gilford, New Hampshire. Stephenson tents are unusual in another way: both walls of his double-walled tents are made of waterproof material. The idea is to prevent condensation by keeping the inner wall warmer than the dew point. Stephenson tents try to achieve that by capitalizing on the "storm window effect," in which two panes of glass (or tent fabric) trap still air between them. That still air acts as an insulator, keeping the inner layer of fabric warm. To further increase the inner layer's temperature, Stephenson uses an aluminized fabric that acts as a radiant-heat barrier. I've used Stephenson tents on three expeditions to the Alaska Range and found that they keep condensation to a minimum. One final virtue: Stephenson tents use pre-bent, large-diameter poles that are some four times as strong as the typical flexible aluminum poles found in most other tents, yet exceedingly light. The two-person Stephenson tent weighs just over three pounds. On the negative side, the tents are expensive, and the lightweight coating on the ultra light fabric will wear through more quickly than heavier coatings, requiring more frequent touch-up with seam sealant.

Another maverick in the tent business is Todd Bibler, a mountaineer turned paraglider-pilot who designs single-walled tents out of Todd-Tex – essentially, Gore-Tex by another name. Until 1986, W.L. Gore made a Gore-Tex tent fabric that several manufacturers used in a variety of tents. Unfortunately, those tents could not pass the rather arbitrary fire-retardancy tests that conventional tents could, and so could not be sold in a handful of states. Eventually, Gore took the fabric off the market. Most tent manufacturers abandoned any attempt to make tents of waterproof/breathable material, but not Bibler, who found a way to produce his own version of Gore's tent fabric. Bibler tents are well-made and very lightweight but quite expensive. I’ve used Bibler tents extensively in both winter and summer and found that the fabric breathed well; condensation was minimal.

Next Page: Choosing a Tent

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