Is this level of the house below grade? If so, you have drainage problems around the house, moisture is getting in from foundation walls and/or from below the subfloor.

If this is above ground level, you have possible roof leaks or pipe leaks seeping water through walls.

First thing to do is get a de-humidifier or two to combat the humidity retained inside. Then inspect exterior foundation for low spots, water drainage issues, cracks in foundation, etc.