Nothing leaked. The fly is intact, the seams are sound, and it did not rain.

Everything you own is damp anyway. The inner is beaded, the foot of the sleeping bag is wet, and when you brush the fabric a small shower comes down.

You did this. Specifically, you breathed.

The arithmetic of a night in a tent

An adult exhales roughly 40 grams of water an hour while asleep, in air that is nearly saturated at body temperature. Over eight hours that is around 320 ml per person — over half a litre for two.

Now put that into a two-person tent, which holds perhaps 2 to 3 cubic metres of air. Add whatever is evaporating from wet boots, damp clothing and the ground underneath.

The dew point inside climbs steadily all night. Meanwhile the fly is doing what every surface with a clear view of the sky does — radiating heat away — and ends up colder than the air around it.

Warm humid air, cold surface. There is only one outcome.

Why the fly is the coldest thing there

Tent fabric is thin, it has no thermal mass, and it faces straight up at the sky. It is essentially the ideal radiator, and on a clear, still night it will sit several degrees below the outdoor air temperature.

That is the same reason a car roof gets frost when the air stayed above freezing, and why skylights fog before other windows.

What actually helps

Ventilate, more than feels sensible. This is by far the biggest lever. Every vent open, both ends if the design allows, even when it is cold — a colder tent with air movement is drier and ultimately more comfortable than a warm sealed one that rains on you. Sealing everything up is the single most common mistake.

Pitch well. Avoid hollows, valley bottoms and ground beside water, where cold damp air pools and the local dew point is highest. A slight rise with some air movement is worth a lot.

Keep wet things in the porch. Boots, waterproofs and anything damp are moisture sources you can simply put outside the sleeping area.

Use the double skin as designed. The inner is meant to be breathable and the fly is meant to be where condensation happens. Do not let the two touch, and do not let your bag touch either — that is how the water gets from the fabric into your insulation.

Do not cook inside. Boiling water in a tent adds far more moisture than you do, quite apart from the other reasons not to.

Single-skin and rooftop tents

Single-skin shelters have one surface doing both jobs, so the condensation forms on the wall you are touching. Ventilation is the only real lever, and you should expect some dampness on a clear night.

Rooftop tents are worse than they look for exactly the radiative reason: raised on a vehicle with a completely unobstructed view of the sky, the fabric cools even more efficiently than a ground tent's.

Knowing which nights will be bad

The dew point spread tells you before you pitch:

  • Wide spread, breezy — minimal condensation. Ventilate normally.
  • Narrow spread, clear, still — heavy condensation. Maximise ventilation, keep gear off the fabric, and expect to pack a wet fly.
  • Spread near zero — the whole campsite is inside a cloud, and nothing will be dry.

The forecast shows both lines, which is worth a look while you still have a choice about where to pitch.