Stack Effect
From Swikipedia
Introduction
Stack effect is the phenomenon in a building or building component caused by wind pressure and temperature differentials which results in air being drawn through some components of a building and out others creating a continuous pattern of air flow. In simple words, stack effect is a natural phenomenon of air movement driven by the difference in air density between the exterior and the interior of a building. It is also referred to as the "chimney effect", and helps drive natural ventilation and infiltration.
Brief Description
The "stack effect" is when warm air moves upwards in a building. This happens in summer and winter season. In summers, warm air rises because it's lighter than cold air. So when it rises, it escapes out of the upper levels of the building, through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. The stack effect basically causes air infiltration.
Stack effect ventilation is an especially effective strategy in winter, when indoor/outdoor temperature difference is at a maximum. A chimney heated by solar energy can be used to drive the stack effect without increasing room temperature, and solar chimneys are very widely used to ventilate composting toilets in parks.
In a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation. Stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it. Especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke.
References:
http://www.greenbuildingcookbook.info/Glossary.html
http://www.silverstonetek.com/tech/wh08_0266.php
http://en.wikipedia.org/wiki/Stack_effect
http://www.basementsystems.com/crawl-space/stack-effect.html
