The suction is greatest at the base, decreasing as the building rises to a neutral pressure plane somewhere between the ground floor and the roof. Figure 1 Stack effect Wind , like stack effect, is a natural phenomenon. Wind increases the positive air pressure acting against a building on the windward side, and produces a negative pressure on the leeward side and on the walls parallel to the wind direction. Since the wind in most regions of the country does not blow strongly all that often, its importance does not reside in the increased air leakage that it causes, but rather in the large pressures it can exert on those components in a building envelope that otherwise offer a high resistance to air leakage. If the difference in air pressure across a component exceeds its strength, that component may cease to be useful as an air barrier.
Most buildings rely on some form of ventilation system to exhaust contaminated air.
This system may run continuously, or may be operated automatically or manually. Even in a balanced system, the amount of air supplied may be increased or decreased relative to the amount being exhausted, increasing or decreasing the inside pressure accordingly. Figure 3 Influence of fan operation on internal pressures In a highrise building, for example, the pressure may be increased to reduce the suction at street level and facilitate the opening of exterior doors. On the other hand, in buildings where occupancy requires or generates high humidity levels, such as art galleries and swimming pools, the inside pressure may he reduced so as to decrease the flow of warm moist air into the walls and roof. An air barrier may consist of a single material or of two or more materials which when assembled together make up an air impermeable, structurally adequate barrier.