Not properly ventilated (so you have an extractor fan?). You should have one. And if you do, is it venting correctly?
Is there a leaking pipe either under the floor or in the cavity walls? Central heating or water. Get your water system pressure checked by a plumber.
Are you just drying all your damp clothes in there? Or are you cooking without proper air circulation?
Are your walls built with proper engineering block and damp course?