Random air movement into and out of a house caused by accidental pressures.

Houses can be pressurized or depressurized in a variety of ways. For example, “naturally” occurring pressures—the result of Mother Nature—are quite common. They result in natural ventilation, something that varies considerably from day to day. This is primarily the result of pressures caused by the wind and by temperature differences (warm air exerts a small upward pressure as it rises up into cooler air).

Pressures in houses can also be caused by mechanical equipment. Some mechanical devices aren’t specifically designed to cause air to move into and out of a house for the purpose of ventilating the house, but they do so anyway. For instance, the fan in a clothes dryer blows air out of a house, so it has an effect on the air pressure in a house. A clothes dryer’s main purpose isn’t to ventilate—it is to dry clothes—so it contributes to accidental ventilation. Accidental pressures only cause air movement when the mechanical device is operating. Because these pressures are rarely continuous, they are sporadic in causing air movement through a house. Thus a number of different factors cause accidental ventilation.

Houses also contain mechanical devices, such as window fans, that are deliberately designed to exchange air in a building for the purpose of supplying fresh air or expelling stale air. This is what ventilation is really all about. This is called controlled ventilation—ventilation that is created “on purpose.”

So, the air pressures that push air through the holes (either random or deliberate holes) in houses fall into three categories: natural, accidental, and controlled. The direction the air moves (outdoors to indoors, or indoors to outdoors) depends on which way the pressure pushes. Air will always move from an area of high pressure, through a hole, to an area of low pressure.

Source: Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems - published by The Healthy House Institute