I was ready to trash my $180 air pad until a guy at a trailhead in Bend asked one question
I had packed up my sleeping pad to return it after two miserable nights near Sisters, Oregon where I woke up on the ground both times feeling like I'd slept on concrete. Before I shipped it back I ran into this older guy at the Bend trailhead who watched me cram it into the stuff sack and just asked, "are you blowing it up with your breath?" I admitted I was, and he explained that my breath adds moisture inside, which freezes overnight in cold air and makes the pad go flat, since the foam inside soaks it up and the valves get icy. I tried his trick of using the pump sack it came with and sealing the valve right after, and I slept through the whole next night at 28 degrees without sinking an inch. Now I want to know if anyone has dealt with a pad that still deflates even when you pump it up dry, because I have a second older pad doing that and I can't figure out if it's a leak or something else.
Man, that pump sack trick is a game changer. I had the same thing happen with my first pad and could not figure out why it kept going flat on cold nights until someone told me the same thing about breath moisture.
For your older pad, the first thing I would do is fill up a bathtub or sink with water and dunk the whole inflated pad in there. Any leak will show up as bubbles, even a tiny pinhole. Check the valve area first because that is where most leaks hide, and sometimes it is just a dirty valve that needs a rinse and a little silicone lube. If it holds air fine in the tub but still goes flat in the cold, it might just be the temp drop making the air inside shrink, which is normal and not a leak at all. If it is a slow leak at a seam or a baffle, that is usually not worth fixing on an older pad and I would just use it as a loaner.