How much warmer is a tent than outside at night?

The air inside a vented tent at night is usually only about 1 to 3 degrees C (2 to 6 F) warmer than the air outside. On a clear night it can even be colder. The warmth you feel comes from shelter, not trapped heat: the tent blocks the wind and the cold sky. The 10 to 20 degree F figures that get repeated online come from measurements taken while someone is awake and moving, or from a thermometer in the wrong place.

What "warmer than outside" actually means#

The question is about air temperature: a thermometer hanging in the middle of the tent, out of the sun, compared with one at the same height outside. The gap between those two readings is the number this page is about.

Three terms decide that number, so here they are in plain words:

• Vented tent. A tent with at least one vent or door gap open, as most people pitch it to keep the inside dry, so warm inside air is always being swapped for cold outside air.
• Air change rate. How many times an hour the whole volume of air in the tent is replaced. A 2 person tent holds about 2 cubic metres of air, so twenty air changes an hour means 40 cubic metres an hour flow through it.
• Radiative cooling. Every surface gives off heat as infrared light. On a clear night the sky sends very little back, so a tent fly facing it ends up colder than the air around it. Clouds and tree cover send more back and mostly stop this.

Feeling warm is a different thing. It depends on how fast your sleeping bag loses heat, and that depends on wind and on what the bag "sees" overhead. A tent changes both of those a lot and the air temperature very little, and mixing the two up is where the big numbers come from.

Worked through: one sleeper in a 2 person tent#

Take one person in a single wall 2 person tent. Suppose the night is cloudy, so the sky is not pulling the fly colder, with a light breeze of 2 m/s (7 km/h, 4.5 mph). The vents are open, as tent condensation recommends.

Heat in. A sleeping adult produces about 73 W, the figure used in why you sleep cold. Not all of it heats the air:

• Breath and skin release 40 to 70 g of water an hour. Take the low end, 40 g, and turning that water to vapour uses about 27 W. That heat leaves with the vapour and comes back only where the water condenses, which is mostly on the fly.
• An R-4 pad lets about 13 W into the ground, on the figures in sleeping pad R-value.
• That leaves about 73 minus 27 minus 13, or 33 W, to warm the tent.

Heat out through the air. Air at 0 degrees C weighs 1.292 kg per cubic metre, and it takes 1.006 kJ to warm one kilogram by one degree. So each cubic metre carries 1.3 kJ out for each degree it is warmer than outside. At 20 air changes an hour, 40 cubic metres leave each hour, which works out to about 14 W for each degree of difference.

Heat out through the fabric. The fly is thin, so its own insulation hardly counts. What slows heat is the thin layer of air clinging to each face. For still air, de Dear and colleagues measured a heat transfer coefficient of about 3.3 W per square metre per kelvin. For moving air they give 10.3 times the wind speed to the power 0.6, which is about 15.6 at 2 m/s. Add the two faces in series and the fly passes about 2.7 W per square metre for each degree. A 2 person dome about 1 m high has roughly 8 square metres of fly, so that comes to about 22 W per degree.

The answer. Together the tent loses about 36 W for each degree of difference, and it gains 33 W. So the air settles about 0.9 degrees C (1.6 F) above outside. That is the whole of the "tent is warmer" effect from one sleeper on a cloudy night.

The general rule: heat in divided by heat out#

The worked case comes down to one line: the temperature rise equals the heat reaching the tent divided by what the tent loses for each degree. You can move either side.

Four things fall out of the table.

Closing the vents barely helps. With one sleeper, shutting everything raises the air from 0.9 to 1.3 degrees above outside, because the fabric loses more heat than the vents do. Even with zero air exchange, the fly alone would hold the rise to about 1.5 degrees. What closing up does change is humidity, which is why a sealed tent is wet by morning.

A second sleeper doubles the heat in. It roughly doubles the rise, to about 2 degrees C with the vents open. In calm air with the vents half open, two sleepers reach about 3 degrees C.

A solid inner tent helps a little. It adds two more air layers between you and the outside, which in the model lifts one sleeper from 0.9 to about 1.5 degrees. Seeber's measured average was 2 F (about 1 degree C) more than a single wall. A mesh inner adds almost nothing, because air passes straight through it.

The big numbers need everything at once. Two people, a solid inner, still air and closed vents can push the model to about 6 degrees C. Roger Caffin, writing in the same Backpacking Light thread, puts his double skin tents on snow at "about 5 C warmer". Seeber points out in reply that Caffin's tents had two people in them. Large gaps are real, then, but only in the setup most likely to soak the tent with condensation.

What the measurements show#

Few people have logged tent air against outside air all night. The best public record is Stephen Seeber's August 2024 study for Backpacking Light: 23 overnight tests in five tents, under mostly clear to partly cloudy skies, with winds from calm to 20 mph and air from the mid 30s F to single digits. The full results are for members only, so this page uses the free summary and Seeber's own replies in the public comments.

His summary findings are blunt:

• Outside air temperature "is the most significant determinant of interior tent temperature."
• The heat from a person in a sleeping bag "has minimal impact" on the inside air. The warmer the bag, the smaller that effect.
• On clear nights, heat radiating to the sky cools the walls below outside air, "causing the interior air temperature in the tent to drop below outside temperatures."
• Double wall tents "are not much warmer than single-wall tents."

In the comments, Seeber describes two nights in detail. In a double wall tent, the inside jumped to 16 degrees F above outside while he got settled, then fell back to about 4 F warmer once he was in his bag, and later to about 2 F. In an X-Mid 2 with the inner removed, the inside started 4 F colder than outside before he got in. Once he was settled it sat about 1 F colder, then about 1 F warmer near dawn as the outside air kept falling. His later summary: a tent "will tend to stay within a few degrees of outside ambient temperature, regardless of number of tent walls or fabric type."

A reader, Jerry Adams, posted probe thermometer readings from three nights in a tent. The air inside was 6 F warmer on a calm cloudy night with the door wide open. Under a clear sky with partial tree cover, it was 1.5 F warmer on a windy night and 7 F warmer on a calm one. Those are crude spot readings, as he says himself, but they sit in the same range.

Makeshift shelters show the same pattern. Tan and Tan summarise a winter study by Thapa and others of six types of shelter built after the 2015 Nepal earthquake. Mean night air was 10.3 degrees C inside and 7.6 degrees C outside, a gap of 2.7 degrees C.

Why people say 10 to 20 degrees warmer#

The big figures are not invented. They are real readings taken at the wrong moment or in the wrong place.

• Awake and moving. A person moving about, not yet in a bag, heats the air fast. Seeber's readings jumped 14 to 16 F when he entered the tent or came back in, then fell once he was still in his bag. Anyone checking a thermometer at bedtime sees that peak.
• Thermometer placement. Seeber's floor sensor always read warmer than his sensor near the ridge. A thermometer close to your body, or clipped to a pack in the vestibule, reads your heat, not the tent's air.
• The wrong outside reading. A forecast low is for a weather station, not your pitch. A hollow can run several degrees colder than open ground nearby, as why you sleep cold explains.
• Stoves and heaters. A stove or gas heater puts out more than a thousand watts, against 33 from a sleeper. That is a heated tent, and a different question.

Where a tent's warmth really comes from#

A tent keeps you warmer mainly by protecting the outer surface of your sleeping bag. Heat leaves that surface by convection to moving air and by radiation to whatever is overhead, and a tent cuts both. Call this wind shelter and sky shelter.

Wind. In still air, de Dear's coefficients give a bag's outer surface a resistance of about 0.128 square metre kelvin per watt. That counts natural convection (3.3) plus radiation to surroundings at air temperature (4.5). In a 2 m/s (4.5 mph) breeze the convection coefficient rises to about 15.6 and the surface resistance falls to 0.050, and at 4 m/s (9 mph) it falls to 0.036. So the breeze strips away 0.08 to 0.09 square metre kelvin per watt.

ISO 23537 rates bags from measured thermal resistance. In the three season range, each added square metre kelvin per watt is worth about 31.5 degrees C of comfort rating. On that scale, a 2 m/s breeze costs about 2.5 degrees C of rating and a 4 m/s breeze about 2.9. The real cost is likely higher, since wind also pushes through the bag's shell and fill and that is not counted here.

Sky. Sleeping in the open under a clear sky, your bag radiates to the sky and gets little back. Adams measured his bag's surface 4 F below the air temperature that way. In a tent, the bag faces fabric at about air temperature instead. Seeber found that Dyneema fabric lets much of that infrared through, "akin to standing in front of an open window". He also found it "simply does not matter much" for someone in a sleeping bag rated for the night.

The rating already assumes shelter. ISO 23537 tests bags indoors, in a climatic room with a set air speed. A tent does not add warmth on top of the rating; going without one, in wind or under a clear sky, takes warmth away from it. That is why the sleeping bag temperature rating should be read as the number for a sheltered pitch.

Seeber's own conclusion is the practical one: the tent is "a secondary system for keeping you warm". The bag and pad do the work, and a tent that is 1 degree warmer will not let you carry a lighter bag.

Make your tent warmer, in order of what works#

1. Pitch out of the wind. It is the largest effect on the page. Use a windbreak or the lee of a slope, and pitch the fly low on the windward side. Pitching and site selection covers how.
2. Pitch under trees, not in a hollow. Tree cover cuts radiative cooling of the fly, and Seeber calls a cloudy sky or dense canopy "the cheapest and lightest way" to raise the inside temperature. Low ground on still nights collects cold air and undoes it.
3. Keep air moving through the tent slowly, not fast. Seeber found ventilation has "minimal impact" on inside temperature as long as it does not become a breeze through the tent. In wind, keep openings small, as the venting table in tent condensation says.
4. Share the tent. Two sleepers roughly double the heat in, the only free way to move the air figure by a further degree.

Measure your own tent#

You need two cheap digital thermometers, ideally ones that log readings.

1. Match them first. Seeber's advice: leave both in the same place for at least an hour, then note or set an offset so they read the same.
2. Hang the inside one mid-air, at about chest height when lying down, away from your body and the walls. Not on the floor and not in a pocket.
3. Hang the outside one at the same height. Place it a few metres from the tent, in shade, away from the car and any fire.
4. Read at 3 am, not at bedtime. Wait until you have been still in your bag for an hour, because bedtime readings catch the peak from moving around.
5. Note the sky and wind, clear or cloudy, calm or breezy. The same tent will give different gaps on different nights.

Expect 1 to 3 degrees C. If you see much more, check for a heat source near the sensor.

Common mistakes#

Buying a lighter bag because "the tent adds 10 degrees". It does not. Size the bag for the forecast low at your pitch, using the comfort rating.

Choosing a double wall tent for warmth. Choose it for condensation management and a dry inner surface. On Seeber's averages it adds about 2 F, not enough to pay for its weight.

Bringing a heater in to close the gap. A flame inside a tent is a carbon monoxide risk. Read the Mr. Heater Buddy in a tent first.

Where this answer stops applying#

This page covers an unheated tent at night, with people asleep in bags. It does not cover tents with a stove or heater, where the heat in is many times larger and the safety rules come first. It does not cover snow caves or quinzees, which are thick walls of snow, not fabric. Daytime is different too: in the Taiwan tests, between 09:30 and 14:30, a tent ran 3.1 to 6.0 degrees C above an ambient 36.3 degrees C. For a larger tent, run the same rule with its own fabric area, air volume and number of sleepers. The model's coefficients come from a human manikin, not from tent fabric, so treat its results as an estimate that agrees with the measurements, not as a measurement itself.

Frequently asked questions#

Is it warmer inside a tent than outside?#

Slightly. With the vents open, the air in a tent with sleepers in it usually sits 1 to 3 degrees C (2 to 6 F) above outside. Under a clear sky it can drop below outside, because the fly radiates heat to the sky. A tent keeps you warmer mostly by blocking wind and sky, not by warming the air.

How many degrees does a tent add to a sleeping bag?#

None to the rating itself. ISO 23537 already tests bags in a sheltered climatic room. Sleeping without a tent takes warmth away. On our arithmetic a 7 km/h (4.5 mph) breeze costs about 2.5 degrees C of rating, and a clear sky adds to that. So a tent protects the rating; it does not extend it.

Is a double wall tent warmer than a single wall tent?#

A little. Stephen Seeber's measurements found double wall tents about 2 F (1 degree C) warmer on average. A solid inner adds more than a mesh one, because mesh lets air straight through. The gain is too small to carry a lighter bag, so choose a double wall for condensation, not for warmth.

Why does my tent feel colder than outside?#

On a clear, still night it can be colder. The fly loses heat to the sky as infrared and drops below the air temperature, which cools the inside air. Seeber measured a tent 4 F colder than outside before he got in. Pitching under trees or cloud stops most of it.

Does closing the tent vents keep you warmer?#

Hardly. In our model, closing every vent raises the air from about 0.9 to 1.3 degrees C above outside for one sleeper. The fabric itself loses more heat than the vents do. What closing up does do is trap the water from your breath, so the tent gets wet.

How much warmer is a tent with two people in it?#

About twice the rise of one person, roughly 2 degrees C (3 to 4 F) with the vents open. Two people in a tent with a solid inner, in still air with vents shut, can reach 5 degrees C or more. That setup also produces the worst condensation.