Hi I recently posted as I've not long moved house and there were some wet patches on some underlay in an upstairs bedroom. I was worried there was a leak but think that's rules out now. Anyway I moved the underlay to a different bedroom, it completely dried... In the other room it was fine for 2 weeks. I then moved the underlay back into the original room and placed it in a different spot, on top of the carpet and the stains/ wet patches are reappearing in the same place. SO WEIRD! Now this room is cold, and condensation was another thought but I mean this is exactly the same patches. When I first felt the patches it felt kind of oily. Could something be in the underlay that's causing it to show the wet damp spots again when it's cold or what on earth could it be?
Warm room below, cold bedroom above. Warm air full of water vapour condenses on the underlay. If your room below is say 22 degrees and 70% humidity the dew point can be as high as 12-14 degrees, so a cold bedroom can easily be cold enough to encourage condensation. Cure - ventilate better downstairs, and keep the bedroom slightly warmer.