“Stack Effect” is the common term used to describe the effect of excessive air leaks in the upper portions of a home, which causes underpressure in the lower areas. Because warm air rises, air mass from the lower floors replaces the air that escaped into the attic, leading to lower pressure downstairs. Cold air is being drawn into the house from outside. Chimney downdrafts, and air rushing into the house when a door or window is opened downstairs are two indicators of the stack effect. Often there is not one single large air leak, but the effect is rather caused by the sum of many small leaks, and this can be difficult to resolve.
This situation can lead to dangerous ventilation issues.