The stack effect happens when warm, dry air from basements and crawlspaces migrates to upper levels in a home and eventually escapes, while cool, wet air enters lower areas. This is often the cause of humidity within a space.
The stack effect often leads to excess moisture that migrates upwards through the house, where it then evaporates into drier air, resulting in an even level of humidity throughout all rooms. The stack effect can cause damage if not corrected soon enough.