Stack effect refers to the pressure differential that is imposed on a building exposed to a steady wind. This variance influences the movement of air through the building envelope. On the side of the building facing the wind, the building pressure is lower, so untreated outside air infiltrates. On the opposite side, where the outside pressure is lower, the conditioned air exfiltrates, or leaves the building. In both cases, the HVAC equipment has to work harder to maintain comfortable conditions in the building. Generally, the taller the building, the more pronounced the stack effect. Lower floors are not usually exposed to the full force of winds, because of nearby structures and trees.
This is another advantage of screening devices, which can be as elegant as a line of trees or as functional as a privacy fence. Any number of landscape features can interrupt the airflow on lower floors but still cannot mitigate high infiltration losses on upper floors. Well placed windbreaks can protect service entrances, lobbies, and other public passageways by minimizing outside air infiltration there. However, better sealing of the envelope on the upper floors is the only remedy against infiltration losses.
Alternately, wind barriers that are too extensive can cause an increase in HVAC loading in the summer. That is, many locations have a prevailing wind in the summer, such as down a hill or toward a body of water, which can be used to help cool the building.
Landscaping can be used to channel the breeze toward the structure. Careful design of landscaping can take advantage of both effects or concentrate on the one which will have the most pronounced effect on the building load.