Moisture under your mattressby Tom Jones
Have moisture on the underside of your mattress? Here's something I learned from an experience on a boat.
If the underside of the mattress is next to a cold surface, with hot bodies on top giving off moisture as we humans do, the underside of the mattress is likely to be moist due to condensation. It may even develop mildew.
One simple fix is to put cardboard under the mattress. Several boxes being thrown out by a local store, cut to shape, will do fine. The cardboard provides sufficient insulation between the bottom of the mattress and the cold(er) surface below to prevent condensation. A single layer of cardboard will probably be sufficient, but the corougated type of cardboard will provide greater insulation.