Condensation in a tent: where it comes from and how to reduce it

Seven in the morning, beside a lake, not a cloud in the sky. It didn’t rain last night, yet the top of your sleeping bag is damp. You brush the flysheet: it’s streaming, and three drops land on your neck. First instinct: curse the manufacturer and hunt for the hole. Wasted effort. This water didn’t get in. It was already in the air in the tent, and some of it came out of your own lungs.

On this point, the manufacturers agree. Hilleberg writes that there is no such thing as a condensation-proof tent [1], MSR that you will never fully eliminate it [2], Nordisk that you can limit it but never remove it completely [3]. That leaves three questions: where it comes from, what really reduces it and how to tell it from a leak. This guide fills out the shelter stage of our guide to your first wild camp.

The dew point, explained simply

Air always contains water vapour, an invisible gas. It can only carry so much, and that maximum depends on its temperature: the warmer the air, the more it can hold, as Météo-France, the French weather service, points out [4]. At that maximum it is saturated: that’s 100% relative humidity, the moment when fog, dew or frost are on the cards [4].

Now cool that air without taking anything away from it, and it edges closer to its limit. The temperature at which it gets there is the dew pointThe temperature to which air must be cooled, at constant pressure and water vapour content, for it to reach saturation., as defined by the US National Weather Service [5]. Any colder, and the surplus vapour turns back into liquid. Hilleberg gives the example of the car first thing in the morning and the glass of cold beer, misted on the outside [1]. I can think of no better physics experiment.

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It all depends on how humid the air is to begin with. The table below gives the dew point for a few common conditions, calculated with the formula published by the US National Weather Service [6].

Look at the last column: in air at 95% humidity, fabric about 1 °C colder is enough. At 60%, it takes a gap of 7 to 8 °C. That’s why the same tent stays dry one night and streams the next, without anything about it having changed. MSR puts it another way: the higher the humidity and the bigger the temperature difference, the faster condensation occurs [2].

Why the water settles on the fabric

That leaves the question of why the fabric is colder than the air. Under a clear sky, Decathlon explains, the flysheet radiates towards the sky and loses heat, until it is colder than both the outside air and the air in the tent [7]. It’s the mechanism behind dew on a lawn: ground that cools at night, and a clear sky that lets that radiation escape, where clouds would send some of it back [8].

Inside, meanwhile, your breath and body warm the air [2] and load it with vapour [9]. This air passes through the breathable fabric of the inner tentThe part you sleep in, with its groundsheet, often partly made of mesh., meets the cold flysheetThe waterproof outer layer pitched over the inner tent, which keeps off rain and wind. and condenses there; a cold floor plays the same part [2]. Hilleberg also points to the underside of the sleeping bag, and to the foot of the bag, which is less warm, when it touches the wall [1].

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Where the water comes from: seven causes

Nordisk names three big contributors: the ground, wet gear and you [3]. The rest comes down to the air and the spot you choose.

As for the calendar, our only reference point is French: Météo-France notes that in mainland France, air humidity is on average higher in December and lower in July and August [4]. As for the sleeper, Hilleberg points out that body type, sleeping bag and the heat you produce all affect condensation [1], which explains why your neighbour sometimes sleeps dry in an identical tent.

How to reduce condensation: air, site, ground

Getting air moving is still the most effective method, according to MSR [2]. The brand advises rolling up the porchThe area covered by the flysheet in front of the door, outside the inner tent; also called a vestibule. doors as far as possible, opening the top of the door zip a little and, with two doors, opening both a bit so that air flows through [2]. Decathlon has you keep the vents open even in the rain, with a gap between the ground and the bottom of the flysheet, to create a draught from bottom to top [7]. In dry weather, Nordisk goes as far as lifting the bottom of the fabric with a stone or a stick [3].

One caveat that quick tips forget: ventilation only works if the air coming in is a little drier than the air going out. When it’s more humid outside than in, Hilleberg writes, the vents actually increase condensation [1]. The brand adds that you can wake to a soaking ceiling even under a tarp, open as it is to every wind [1]. So don’t judge your vents on a single night of fog.

1. Pitch on flat ground, away from long grass [3].
2. Move away from the lake or river [7, 9].
3. Where camping in woodland is allowed, trees help: temperature and humidity are more regular there, according to Decathlon [7].
4. Slip a footprint under the tent: Vaude, Nordisk and Hilleberg say it holds back moisture from the ground [3, 13, 14].
5. Tension the fabric without overdoing it: the flysheet mustn’t touch the inner tent, and creases concentrate the drops [7]. On a well-pitched tent, water runs down the flysheet to the ground [2].
6. Keep wet kit out of the tent, under a tarp for example [3].
7. Keep your sleeping bag and sleeping mat away from the walls [7].

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One last remedy, and one more roof: Will Copestake, a brand ambassador quoted in Hilleberg’s handbook, pitches a tarp over his tent, among other reasons because it reduces condensation [15]. The setups are in our tarp guide. Gram-counters can do their own sums.

Double wall, single wall: what each one changes

A double wall doesn’t get rid of condensation: it moves it away from you. The water settles under the flysheet and doesn’t get into the inner tent, Vaude explains [9]. Hilleberg describes the same principle: a waterproof flysheet, neither breathable nor air permeable, and a highly breathable inner that lets humid air escape and stops the drops [1]. Even so, the brand warns that the walls of the inner sometimes become the coldest spot, and get wet in their turn [1]. In a small tent, it adds, condensation is more noticeable, because there is less air and the walls are closer to you [1].

A single-wall tent has just one layer of fabric. At MSR it is waterproof and breathable, which helps solve the weight problem but complicates ventilation [16]; these tents are mostly found in mountaineering and among tarp-style shelters [16]. The head of the lab that examined ten tents for the Swiss consumer programme Kassensturz in 2022 summed up the constraint: a thin waterproof fabric doesn’t let air through either, so ventilation relies on openings [17].

The fabric counts too. Vaude asks you to pay particular attention to ventilation with silicone-coated flysheets, otherwise condensation increases [13]. Hydrostatic head, on the other hand, tells you nothing about condensation: it measures what the fabric keeps out, not the vapour it lets out. Decathlon notes, in fact, that a fabric made perfectly waterproof keeps the sleeper’s vapour inside [18].

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Condensation or leak: how to tell the difference

It’s an easy mistake to make. According to Nordisk, heavy condensation gives the impression that rain is getting through, because wind or a mere touch knocks the drops loose [3]. Hilleberg tells of the same misunderstanding: in the early 1980s, users of its large porches saw rain strike the fabric, dislodge the condensation and send drops falling, as if the shower were coming straight through [1]. Three clues help you decide.

In dry weather, the doubt doesn’t last: no rain, no leak, and both Decathlon and Vaude point out that condensation says nothing about how waterproof the tent is [7, 9]. If the water really is coming in along a seam, Which? sees a likely stitching fault, which should be covered by the warranty, and suggests testing the tent with a garden hose before you go [10]. Our guide to hydrostatic head explains why seams and zips often let water through before the fabric does.

When you wake: wipe, dry, pack dry

Once the water is there, MSR offers two solutions: wipe the inside of the flysheet with a towel, or take it off and spread it out in the sun [2]. Decathlon recommends a microfibre cloth and a gentle touch [7]. Hilleberg has a trick for the following night: wrap a waterproof jacket around the bottom of your sleeping bag [1]. Erik Gauden met that same damp flysheet again during three days of rain in the Monts d’Arrée, and he tells how he kept the wet from the dry.

Back home, get the tent out of its bag: MSR insists that it must be completely dry before you store it, as damp encourages a mould that can do irreparable damage to the fabric [2].

A good habit to get into comes down to three moves, before the tent is even out of its bag: get away from the water, spread out the footprint, keep one opening high and one low. And in the morning, have a towel within reach: the rest is down to physics, which has never read a spec sheet.

Worth watching

Oscar Hikes gives ten tips built on one idea: cut the humidity in the tent and the temperature gap between the air inside and out. A useful companion to the reasons set out here.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Guillaume Hoarau

Camping, wild camping, bushcraft and hiking

Guillaume Hoarau writes for Outfarer about camping, wild camping, bushcraft and hiking. A survival-skills enthusiast and a collector of old field manuals, he likes clear explanations, lists that earn their place and other people’s stories, which he tells well.

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