Tent condensation: where the water comes from, and how to stop it

A wet tent inside is usually physics, not a defect. A sleeping adult releases roughly 40 to 70 g of water an hour through breathing and skin, so one person puts 0.3 to 0.6 litres of water into a shelter overnight and two people approach a litre. That water condenses wherever a surface falls below the dew point of the air touching it, and on a clear still night the flysheet is exactly that surface.

The overnight water budget#

At rest, an adult loses water continuously through respiration and through the skin, at roughly 40 to 70 g per hour depending on temperature, humidity and body size. Over an eight hour night:

• One person: 8 times 40 to 70 g, so 320 to 560 g, or about 0.3 to 0.6 litres.
• Two people: 640 to 1,120 g, or about 0.6 to 1.1 litres.

That water has to go somewhere. It leaves as vapour in air that moves out, it condenses on a surface, or it is absorbed by clothing and bedding, which mostly means it condenses later. There is no fourth option.

Other sources add to the budget. Wet boots and rain shells evaporate their surface water. Damp ground is a continuous supply through an uncoated floor or under a tarp with no floor at all. Cooking is the largest single addition: boiling half a litre puts up to 500 g of vapour into the air, one person's entire night in about ten minutes.

Why the fly gets wet: dew point and a sky that is very cold#

Condensation is not caused by humidity alone. It happens when a surface is colder than the dew point of the air touching it. Dew point is the temperature at which that air saturates: cool a surface below it and the excess water leaves the vapour phase.

Two things make a flysheet the coldest surface available. It is thin, with almost no thermal mass, so it tracks its energy balance closely. And on a clear night it radiates long wave energy to a sky whose effective radiating temperature is far below air temperature. That net loss pulls exposed fabric a few degrees below the surrounding air, the same mechanism that puts frost on a car roof while a thermometer at the same height reads above freezing.

Inside, the air is warmer because you are heating it and far more humid because you are humidifying it. Warm humid air meeting fabric colder than ambient is the entire mechanism. Nothing has failed.

This explains a pattern people notice but rarely name: cold nights show more condensation for the same moisture, because cold air cannot carry it. At 0 degrees C a cubic metre of saturated air holds under 5 g, so a two person tent of roughly 2 cubic metres can hold about 10 g in its air. Two sleepers produce eighty times that overnight.

Why no fabric can breathe the water away#

Take two people producing 800 g overnight, inside air at 10 degrees C and saturated at 9.4 g per cubic metre, outside air at 5 degrees C and saturated at 6.8. Every cubic metre of air exchanged carries out at most the difference, about 2.6 g.

800 divided by 2.6 is about 310 cubic metres of exchange over eight hours, or roughly 39 cubic metres per hour. For a tent of about 2 cubic metres that is nearly 20 complete air changes an hour, all night. That is a breeze through the shelter, not a vent panel, and it is why a zipped up tent on a still night is wet by 3 am whatever the fabric is.

This is also why a waterproof breathable single wall shelter still runs wet. The vapour transmission rate of any membrane is small against 800 g in eight hours, and the driving force collapses when the outside air is nearly saturated too. Breathability is the wrong size of solution for the problem.

Cross flow, not holes#

Ventilation only works if air moves, and air moves through a shelter for two reasons: wind pressure and buoyancy, since warm moist air is less dense than cold air outside. Both need an inlet and an outlet in different places.

• A low inlet. A door open a hand's width at the bottom, a fly hem lifted off the ground, a vestibule cracked at the base.
• A high outlet. A peak vent, a hooded ridge vent, or a door unzipped from the top down.
• Separation. Inlet and outlet as far apart and as different in height as the shelter allows. Two vents at the same height in still air exchange almost nothing.

Opening one vent in an otherwise sealed shelter does almost nothing, which is why people conclude venting does not work. Buoyancy driven flow needs somewhere for replacement air to enter, exactly like a chimney. Pitch height matters too: a fly hem set 5 cm off the ground on the upwind side is a continuous inlet the length of the tent, and moves more air than any vent panel. Site choice compounds this, and pitching and site selection covers where to put the tent in the first place.

What to open, in which weather#

The correct venting action is different in each of the four conditions you actually meet. This table is the decision rule we use.

The first line is the counterintuitive one. People expect calm clear nights to be easy and vent least on exactly the night the fly is coldest relative to the air. The wet morning then gets blamed on the tent.

Single wall, double wall and where the water ends up#

A double wall tent does not stop condensation, it relocates it. Vapour passes through the mesh or breathable inner, condenses under the cold fly and runs to the ground on a surface your sleeping bag never touches. You made the same litre of water, but you never meet it.

A single wall shelter puts that condensing surface directly over your face. Good ones manage it with generous venting, steep walls that shed drips to the perimeter rather than onto the sleeper, and enough width that a bag never touches fabric. Wall angle is a real specification, and part of the geometry argument in how to choose a tent.

Two consequences follow. In a double wall tent, keep the inner off the fly: a slack pitch lets the layers meet and water wicks straight through the contact patch, which then gets diagnosed as a leak. In any shelter, keep the foot of your sleeping bag off the wall, because a damp bag foot costs real warmth, as described in sleeping bag temperature ratings. Tarps and hammocks change this again, and are compared in tent, tarp or hammock.

Site, ground and the things that make it worse#

Where you pitch changes the humidity inside more than any vent does.

• Low ground. Cold air pools in hollows and valley bottoms on still nights, so the coldest, dampest air collects exactly where the flat pitches are.
• Long grass and damp soil. Both evaporate upward all night into the space under your fly. Gravel, sand or short cropped ground is drier.
• Water. Within a few tens of metres of a lake, river or bog the air is more humid, and the night temperature drop is enough to condense it.
• Under trees. A canopy blocks the fly's view of the open sky and reduces radiative cooling, so the fabric runs warmer. The trade is dripping foliage.
• Wet gear inside. Shells, boots and a dog evaporate their surface water into the shelter. In sustained wet weather this becomes the dominant term, which is why camping in the rain is largely moisture management.

Condensation or a leak? Run this before you complain#

The two are routinely confused. This takes ten minutes at home.

1. Look at the pattern. Condensation is uniform: a fine even film over the whole inner face of the fly, heaviest at the coldest, most sky facing panels. A leak is local, following a seam, a stitch line or a pinhole, with a track running downhill from a single point.
2. Check which face is wet. Water on the underside of the fly with a dry outer face, under a dry sky, is condensation by definition.
3. Check contact points. Anywhere the inner touched the fly, or a bag touched a wall, will be wet through. That is wicking, not a leak.
4. Hose test the fly. Pitch it at home, hold newspaper under the suspect area from inside and run a hose over the outside for two minutes. Water at stitching is a seam problem; water through open fabric is a coating problem. The full diagnosis is in waterproof ratings explained.
5. Check the floor separately. Kneel on the floor with newspaper underneath while it stands in shallow water. Floor leaks are pressure driven and show only under load.

If steps 1 to 3 explain it, you have condensation and no repair will help.

Common mistakes#

Sealing the tent up because it is cold. That raises humidity, not temperature. Air inside a vented tent is only a degree or two warmer than outside anyway, and it is the wind protection that keeps you warm.

Cooking inside with the doors shut. Half a litre boiled is up to 500 g of vapour, plus combustion products that for a hydrocarbon fuel are mostly carbon dioxide and yet more water. Cook in the vestibule with the door open, and see camping stove types.

Packing a wet tent and forgetting it. Coatings hydrolyse faster in warm damp storage and mildew stains within days. Dry it at home, as covered in gear storage.

Wiping with the wrong cloth. A microfibre cloth or packtowel takes most of the water off in a minute and wrings out. A cotton bandana moves water into your pack.

Frequently asked questions#

Why is the inside of my tent wet when it did not rain?#

Because you put the water there. A sleeping adult releases 40 to 70 g of water an hour, so one person adds 0.3 to 0.6 litres overnight. That vapour condenses on the flysheet, which on a clear night sits a few degrees below air temperature because it radiates to the sky. An even film over the whole fly is condensation, not a leak.

Does a more expensive tent stop condensation?#

No. Condensation is set by your moisture output, the air temperature and the fabric temperature, none of which price changes. Better designs manage it: more and better placed vents, steeper walls that shed drips to the perimeter, and a double wall that keeps the wet surface away from you. Expect management, not prevention.

Should I open the vents when it is freezing?#

Yes, but by less. Cold air holds very little vapour, about 4.8 g per cubic metre at 0 degrees C, so moisture that cannot escape becomes frost on the inner face of the fly and falls on you when the tent is knocked. Keep small openings clear all night and check before sleeping that they have not frozen shut.

Is condensation worse in a single wall tent?#

The amount is the same, but you meet it. In a double wall tent the water condenses on the fly and runs down a surface you never touch. In a single wall shelter it forms directly above you, so venting, wall angle and keeping your bag off the fabric matter far more. Wide single wall shelters handle it better than narrow ones.

Can I cook inside my tent to stay dry?#

Cooking inside a closed shelter is the fastest way to soak it, because boiling half a litre releases roughly 500 g of vapour, about one person's entire overnight output. There are also carbon monoxide and fire risks that have nothing to do with condensation. Cook in the vestibule with the door open and the stove outside the sleeping area.

Does a groundsheet or footprint reduce condensation?#

Only slightly, and only if damp ground is a significant source for you. A footprint blocks evaporation from soil into the tent and keeps the floor cleaner and drier, which helps on grass or wet ground. It does nothing about the water you breathe out, which is the larger term on almost every night.

How do I dry a tent that is wet inside in the morning?#

Wipe the fly with a small absorbent cloth, inside and out. That takes a minute or two and saves carrying up to a kilogram of water. Then drape or pitch the fly in sun or wind at the first stop, separately from the inner. If it must be packed wet, dry it fully within a day or two to avoid mildew and coating damage.