Three quite different faults produce the same three-in-the-morning symptom: a valve that is not fully seated, a puncture, and air that has only cooled. Working out which one you have takes ten minutes at home and saves a lot of guessing on a hillside.
Start with the valve
The valve is the first suspect because it is the easiest fault and the easiest to miss. Modern mats use a range of designs — flat one-way valves, twist valves, multi-part inflate and deflate arrangements — and several of them can look shut while sitting a fraction open.
Check that any cap is seated square and fully home, that no grit or sand is trapped in the seal face, and that the valve body has not worked loose from the mat. On a two-stage valve, confirm you have closed the right one.
Wetting the closed valve and watching for bubbles is the simplest test there is, and it costs nothing.
Then rule out temperature
Air contracts as it cools. A mat inflated hard in a warm tent at dusk will be noticeably softer by dawn, with nothing wrong with it at all. On a cold night the difference is enough to have people hunting for a puncture that does not exist.
The tell is the pattern. Cooling produces a mat that is softer and then stays where it is. A leak keeps going: soft at midnight, flat at three, on the ground by six.
The practical response to cooling is to inflate slightly less than hard at bedtime and top it up if you wake, rather than to start patching.
Finding a real leak
Inflate the mat firmly and start by listening — a hiss is faster than any other method. Then feel along the seams with a wet hand or a damp cloth, since evaporating water is cold where air is escaping.
If that fails, the reliable method is water. In a bath or a large tub, section by section, press the mat under and watch for a bubble stream. Mark the spot immediately with a permanent marker or a piece of tape, because a wet mark disappears as it dries.
The places to look first: the base fabric, which is the face against grit, the area around the valve, and the fold lines where the mat is packed the same way every time. On a self-inflating mat, a delaminated internal bond feels like a soft lump rather than a leak, and it is a different fault.
Patching, per the maker’s instructions
Most mats are supplied with a repair kit, and the maker publishes instructions for it — including which adhesive suits the mat’s fabric, whether the surface needs preparing, and how long the repair must cure before the mat is inflated. Follow those, because coatings differ between mats and a patch applied over an incompatible surface lifts.
Where the kit has been lost, the maker generally states what the fabric is so that a compatible kit can be bought. Ordinary household tapes are a way to finish a trip, not a repair.
Cure time is where field repairs fail. A patch inflated too soon peels along one edge, and you get to do it again.
Stopping it happening again
Clear the pitch before anything is laid down: stones, sticks, thorns, the tent peg someone dropped. A footprint cut slightly smaller than the tent floor protects the floor, and a sit mat or the tent’s own floor protects the mat.
Vary how you fold it. A mat rolled from the same end with the same creases every time eventually cracks along them.
Store it at home unrolled with the valve open, so the inside dries and nothing is held compressed. And read the R-value the maker publishes for the mat and the standard behind it — ASTM F3340 made those figures comparable between makers — because a mat that keeps deflating is one you will end up replacing, and that is the number to replace it on.
What people ask before buying
My mat is softer in the morning but not flat. Is that a leak?
Often not. Air contracts as it cools, so a mat inflated in a warm tent at dusk sits lower by dawn without losing any air at all. A genuine slow leak usually keeps going and ends with the mat on the ground rather than merely softer.
Where do most punctures actually happen?
On the base, from grit and small stones under the tent floor, and around the valve where the mat is folded repeatedly. Checking the pitch before laying anything down and using a groundsheet cut slightly smaller than the tent floor removes most of the first cause.
Last reviewed 6 September 2026