The stack effect is the movement of warm, dry air rising and escaping through the upper levels of the home and cooler, wet air finding its way into the lower levels in order for the home to equalize. This happens in summer and winter but is most pronounced in the winter because indoor-outdoor temperature differences are the greatest.

Excess moisture in crawl spaces and basements often migrates to the upper levels of the home through the stack effect. In essence, whatever air is below the house is also inside the house.