Waking up on a flat thermarest is never nice, and the most minute of punctures can turn your trip into misery.
First of all, obvious I know but, always carry a repair kit. This usually contains Seamgrip, some patches, and piece of chalk to mark where the hole is.
To find the hole, hold the thermarest right up next your mouth and stick you tongue out. Slowly move the thermarest in front of your mouth (like in the photo), until you feel a tiny movement of air on your tongue. Mark the hole with the chalk and either fix the leak straight away or continue to look for more leaks (there’s nearly always more than one). This method will avoid you having to hold your thermarest under water and get it soaking wet.
About the Author
Alex Palmer is the co-owner and founder of Cold Mountain Kit and started climbing back in 1989. He's climbed extensively throughout the UK, French Alps and has been as far afield as Yosemite, Northern Patagonia and South Africa. A total trad climbing snob.