Why does my air mattress or sleeping pad go soft overnight?

Usually it is the cold, not a leak. Air cooling inside a sealed bed loses about 0.35 kilopascals of pressure per degree C, and the pressure actually holding a sleeper up is only about 1.6 kilopascals, so roughly 5 degrees of cooling cancels the lot. Stretching of new vinyl is the second cause. A real leak loses air at a steady temperature with nobody on the bed.

The two numbers people mix up#

The ideal gas law relates absolute pressure to absolute temperature. At constant volume, cooling from 20 to 9 degrees C (293.15 to 282.15 K) multiplies the absolute pressure by 282.15 / 293.15 = 0.962, a fall of 3.8 percent. A 15 degree F drop from the same start (8.3 K) gives 2.8 percent.

Those percentages look small, which is why two wrong answers circulate. One says the cold makes the air inside "condense", which dry air does not do at any temperature a campsite reaches. The other overstates the loss: a cool room cannot take 10 or 15 percent off an airbed's volume. The ideal gas law allows 3 to 5.

Firmness comes from neither number. It comes from gauge pressure, the excess of the pressure inside over the atmosphere outside. The atmosphere at sea level is 101.3 kPa, and the gauge pressure in a bed is a small number on top of it. Cooling acts on the whole absolute pressure, so a small percentage of a big number is taken out of a small one.

How small is the gauge pressure? Physics gives a floor. An inflated envelope supports you over the area you press into it, at a pressure equal to the gauge pressure inside. We model contact at 0.45 square metres, as in our R-value guide, so a 75 kg sleeper needs:

75 kg x 9.81 m/s2 / 0.45 m2 = 1,635 Pa, or about 1.6 kPa

That is 1.6 percent of the atmosphere.

The arithmetic of a cool night#

A bed sealed at 20 degrees C and 101.3 + 1.6 = 102.9 kPa absolute, cooled to 9 degrees C at constant volume, would fall to 102.9 x 0.962 = 99.1 kPa: 2.2 kPa below the atmosphere. A fabric envelope cannot hold a vacuum, so the volume gives way instead, and you sink until there is enough contact area for whatever pressure remains.

The rule that falls out: each degree C of cooling removes about 0.35 kPa (102.9 / 293.15), so about 4.7 degrees removes as much pressure as holds a 75 kg body up.

Volume is where the 3.8 percent bites. A raised queen airbed about 152 by 203 by 46 cm holds 1.42 cubic metres, and 3.8 percent of that is 54 litres. The unloaded sides stay tight, so most of that volume comes out where you lie; spread over 0.45 square metres it would be 12 cm of extra sink. The real figure is smaller, but that is how hips find the floor at 4am with no air lost.

Why a pad blown up by mouth goes softer than an airbed#

Cascade Designs, which makes Therm-a-Rest pads, says that when a pad is inflated by mouth "the air going in is warm and, as it cools to the ambient temperature, you can lose quite a bit of pressure". Two things make it worse than the airbed case.

The first is the starting temperature. Mansour and colleagues measured exhaled breath at 31.4 to 35.4 degrees C across 340 measurements. Air at 34 degrees C cooled to 0 degrees C loses 11.1 percent of its volume on temperature alone, three times the airbed's 3.8.

The second is water. The same study measured breath relative humidity between 41.9 and 91.0 percent. At 34 degrees C the saturation vapour pressure of water is 5.3 kPa, so breath at 80 percent humidity carries about 4.3 kPa of vapour. Air at 0 degrees C can hold only 0.61 kPa. The rest condenses inside the pad, and liquid water takes up almost no room.

So a 10 to 15 percent loss is real, but it belongs to a mouth-inflated pad on a cold night, not an airbed in a cooling room. A 183 by 51 by 7.6 cm pad holds about 71 litres, and 13 percent of that is 9 litres, on a pad only 7.6 cm deep.

Treat these as upper bounds, since some breath cools during inflation. A pad topped up with a pump sack starts at tent temperature: filled at 15 degrees C and cooled to 5, it gives up 3.5 percent, the same as an airbed.

New vinyl stretches, and that looks exactly like a leak#

Intex's support article is direct: the material "stretches to its correct shape", the bed "will not lose air but there will be more room for the air", and you "will sense a loss of pressure". The owner's manual for its built-in pump airbeds calls this "normal for the first few days of use". The fix is to fill the airbed "at least 2 or 3 times and let it rest for a few hours each time", after which "the difference will be less". Do it at home, not on a first night in a tent where stretch and cooling arrive together.

Stretch is worst on the first inflation and does not reverse. Cooling recurs every cold night, and a bed that softened at 4am is firm again by noon in a warm tent. The manual makes the reverse point too: in heat the air "will expand significantly and the airbed will become hard and may burst", and Cascade Designs tells owners to partially deflate a pad before leaving it in a tent or car for the day.

Valve or puncture: when air really is escaping#

The valve or pump seal. Intex's manual lists leakage between the internal pump and its holder frame, with four causes: pump not locked in, a dirty O-ring, a twisted or damaged O-ring, and internal beam threads caught in the seal. It also warns that turning the dial away from the closed centre position opens the valve "even if pump is not running". Most pad valves can be replaced at home with a kit, according to Cascade Designs, though its TwinLock valves go to its repair shop. Valves are the cheapest suspect, so rule them out first, as the field repair kit procedure also advises.

A puncture. Cascade's method is to listen first, then wipe the pad with a wet, soapy cloth or submerge it and watch for bubbles. A pinhole that takes all night to leak "may take a while for bubbles to form, in some cases ten or more minutes". Folding the pad in half raises the pressure in one section. The company is blunt about the odds: "the vast majority of air leaks are caused by punctures".

The decision tree#

Work down the list and stop at the first yes.

1. Is the airbed in its first few days of use? Probably stretch. Inflate it 2 or 3 times and rest it a few hours each time, indoors, before anything else.
2. Was it inflated warmer than the night got? Indoors, in a sunny tent, from a warm car or by mouth all count. Soft but not flat by morning, and firm again as the day warms, means cold. Top up as described below.
3. Does it lose firmness unloaded at a steady temperature? If not, there is no leak. If it does, go on.
4. Is the valve closed, clean and seated? Check the dial or cap, the O-ring and the seat for grit, then wipe soapy water around the valve and wait. Bubbles here end the search.
5. Do bubbles appear anywhere else? Soap or submerge it in sections, give each section the full ten minutes, and mark a hole the moment you find it. Patch it as in the repair kit guide, or send it for repair.

Check this yourself: the steady-temperature hold test#

The one test that separates cold from a leak needs no instrument, only temperature control, because one degree C of drift moves pressure by 0.35 kPa.

1. Inflate the bed or pad with a pump, not your breath, where it will be tested, and leave it an hour to reach room temperature.
2. Top it up to firm and note the room temperature.
3. Leave it unloaded for 8 to 12 hours somewhere the temperature does not swing: an interior room, not a conservatory, garage or car.
4. Press the same spot with the same hand when the room is back at the starting temperature.

If it feels the same, there is no leak worth chasing and the problem was cold, stretch or both. If it is clearly softer, go to step 4 of the decision tree.

How to stop a sound bed going soft#

• Inflate at the temperature you will sleep at. Set the bed up in the tent an hour before bed, not in the living room or the warm car, and top it up just before you get in. The car camping checklist already puts the pump and patch kit in the sleep crate, where they need to be.
• Top up once early in the night if the forecast drop is large, to restore what the first hours of cooling took.
• Use a pump sack, not your breath, on pads. The vapour in breath costs an extra 1.5 to 3.5 percentage points of volume on top of the larger loss from cooling down from 34 degrees.
• Insulate under an airbed. An airbed on cold ground keeps cooling all night. A blanket or foam mat underneath slows that and stops the bed making you cold, as covered in why you sleep cold.
• Let a little air out before leaving it in heat. Both Intex and Cascade Designs warn about bursting.

Common mistakes#

Running a leak test overnight in a tent. The tent follows the night air down, so a sound bed fails. Test indoors at a steady temperature.

Hunting for a puncture on a new airbed. In its first days, stretch looks identical to a slow leak. Do the inflate-and-rest cycles first.

Pumping it rock hard to compensate. It buys a little reserve and raises the risk of bursting in daytime heat. Inflate to firm and top up later.

Blaming the fabric before the valve. A dial left slightly open, a dirty O-ring or grit in a valve seat will never show up in a surface search.

Storing a mouth-inflated pad rolled and sealed. The water from your breath stays inside it. Store it unrolled with the valve open, as gear storage sets out.

Frequently asked questions#

Why is my air mattress losing air but there is no leak?#

Usually because the air cooled. Each degree C of cooling at constant volume takes about 0.35 kPa off the pressure, and the pressure holding a 75 kg sleeper up is only about 1.6 kPa. A night 5 degrees colder than the room you inflated in removes the equivalent of all of it, so the bed gives up 2 to 5 percent of its volume and you sink into it. On a new bed, vinyl stretch adds to that.

Is it normal for a new air mattress to lose air?#

Yes, for the first few days. The material stretches to its working shape, giving the same air more room, and that feels exactly like a leak. Intex recommends filling a new airbed at least 2 or 3 times and letting it rest a few hours each time, after which the effect becomes smaller. Do this at home before a trip, so stretch is not confused with cold or a puncture.

Why does my sleeping pad lose air at night?#

If you inflated it by mouth, mostly because the breath cooled. Breath leaves the mouth at about 31 to 35 degrees C and carries water vapour, much of which condenses as the pad cools. On a night near freezing that can take 11 to 16 percent of the volume. Inflate with a pump sack at tent temperature and top up before bed. If it still goes flat, test for a leak.

How do I find a slow leak in an air mattress or pad?#

Check the valve first: dial or cap fully closed, O-ring clean and seated, no grit in the seat. Then inflate firmly, listen, and wipe the surface in sections with a soapy cloth, or submerge it in calm water. A pinhole can take ten or more minutes to show bubbles. Folding a pad in half raises the pressure in one section and makes a small hole easier to find.

Should I inflate an air mattress less in a cold tent?#

No. Inflate it fully, but in the tent at the temperature you will sleep at, then top up just before getting in and once early in the night if a big drop is forecast. Starting soft leaves no reserve for further cooling. Let air out only before a hot day.

Can an air mattress pop from heat?#

It can. Warming raises pressure by the same 0.35 kPa per degree that cooling removes, so a bed sealed firm in the morning that warms by 10 degrees in a sunny tent or car gains about 3.5 kPa, twice the pressure that holds a sleeper up. Intex warns that in heat the air will expand significantly and the bed may burst, and Cascade Designs tells owners to partially deflate pads before leaving them.