Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. These three types of natural ventilation effects are further described below. Similarly, buoyancy caused by differences in humidity can allow a pressurized column of dense, evaporatively cooled air to supply a space, and lighter, warmer, humid air to exhaust near the top. Temperature differences between warm air inside and cool air outside can cause the air in the room to rise and exit at the ceiling or ridge, and enter via lower openings in the wall. Wind can blow air through openings in the wall on the windward side of the building, and suck air out of openings on the leeward side and the roof. This places a limit on the application of natural ventilation in humid climates. However, unlike true air-conditioning, natural ventilation is ineffective at reducing the humidity of incoming air. At interior air velocities of 160 feet per minute (fpm), the perceived interior temperature can be reduced by as much as 5☏. Fresh air is required in buildings to alleviate odors, to provide oxygen for respiration, and to increase thermal comfort. Natural ventilation, unlike fan-forced ventilation, uses the natural forces of wind and buoyancy to deliver fresh air into buildings. For example, historic buildings used the stairway as the exhaust stack, a technique now prevented by code requirements in many cases. Code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system. Openings between rooms such as transom windows, louvers, grills, or open plans are techniques to complete the airflow circuit through a building. It is useful to think of a natural ventilation system as a circuit, with equal consideration given to supply and exhaust. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. Pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. Natural ventilation systems rely on pressure differences to move fresh air through buildings. In favorable climates and buildings types, natural ventilation can be used as an alternative to air-conditioning plants, saving 10%–30% of total energy consumption. With an increased awareness of the cost and environmental impacts of energy use, natural ventilation has become an increasingly attractive method for reducing energy use and cost and for providing acceptable indoor environmental quality and maintaining a healthy, comfortable, and productive indoor climate rather than the more prevailing approach of using mechanical ventilation. Almost all historic buildings were ventilated naturally, although many of these have been compromised by the addition of partition walls and mechanical systems.
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