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HHI-Pedia Entry

Ventilation and air pressure

By HHI Staff

Ventilation is simply the bringing in of fresh air and the exhausting of stale air. This movement is called exchange of air. The speed at which the exchange takes place is called the exchange rate.


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Exchanging the air in a house is important in order to dilute the concentration of pollutants and moisture found in the indoor air.

When a fan causes air to move directly, it is called active air movement. When air moves indirectly because of a fan, or because of something else causing an air-pressure difference, it is called passive air movement. 

For example, when a window fan actively blows air outdoors through a window, an equal volume of air will passively enter through another window.

For air to move from one place to another, there must be an air-pressure difference to push or pull it. Household air-pressure differences are caused by one or more of the following forces:

  • Natural pressures resulting from natural processes such as the wind
  • Accidental pressures caused by mechanical devices, such as clothes dryers, which move air into or out of a house for a purpose other than ventilation
  • Controlled pressures caused by ventilating fan.
Natural ventilation

Mother Nature has two techniques for moving air into and out of houses—wind and stack effect. Both of these natural phenomena periodically apply pressures to houses and cause air to move into and out of them. When this happens it’s called natural ventilation.




When wind blows against the exterior side of a wall, it exerts more pressure on the outside than exists inside. This difference in air pressure pushes a certain amount of air through the cracks and gaps in the structure. In an old leaky house, you can often feel a draft near a loose-fitting window frame as a winter wind pushes cold air inward.


When wind pushes outside air into a house through windows, leaks or cracks, the change in air pressure causes an equal amount of internal air to be pushed out of the house. In general, wind causes air to move into one side of a house and out the opposite side.


Stack effect


Stack effect results when warm air rises causing an upward pressure of household air. When a house is filled with warm air in the winter, the warm air exerts an upward pressure on the upper half of the building. This causes air to escape through any gaps and holes on higher floors and an equal volume of air to enter through gaps and cracks in the lower half of the house. 

In the summer, stack effect in an air-conditioned house works in reverse. The cool indoor air falls and leaks out through the cracks and gaps in the lower half of the house and warm outdoor air enters through holes in the upper half.

Accidental ventilation


Accidental ventilation can be caused by heating and cooling systems, appliances and chimneys which when operating contribute to a change in air pressure. Clothes dryers and central vacuum cleaners which blow air outdoors when used will cause an equal volume of air to enter somewhere else—through the various cracks and gaps throughout the house..

A chimney is also an accidental ventilating device, although it’s generally not thought of as such. When in use, a chimney operates like an exhaust fan, with warm combustion by-products rising up and out of the house. At the same time, an equal volume of air will be entering the house somewhere else—through the gaps in the walls, floors and ceiling of the house.

Replacing a furnace which uses a chimney with a furnace that doesn’t (e.g., electric heat or sealed-combustion gas), can result in loss of accidental ventilation and sometimes leads to moisture problems in the living space.

Leaky ductwork which is part of forced-air heating or cooling equipment can also contribute to accidental ventilation. Leaky ducts in a crawl space can cause an air-pressure difference between the crawl space and the living space. Accidental ventilation results when the air-pressure difference causes air to move through the cracks in the living space floor. This type of accidental ventilation can be significant because a forced-air heating/cooling fan is quite powerful. Well-sealed ductwork minimizes this effect.

Accidental ventilation can also be caused by closing doors between rooms. Forced air heating can result in an air-pressure a difference between those living spaces which in turn can cause air movement throughout the house.
Controlled ventilation

Controlled ventilation describes mechanical devices or systems designed to control household ventilation. Controlled ventilation systems provide either local or general ventilation.

Local ventilation


A local ventilation system is designed to improve the air quality in one part of a house. Bathroom and kitchen exhaust fans are examples of devices which provide local ventilation. Local ventilation fans are designed to remove polluted or moisture-laden air from one place quickly—before it can travel to other parts of the house.

Bathroom exhaust fans are necessary to move unpleasant toilet odors outdoors, and to rid a bathroom of the excessive moisture generated during bathing or showering. Too much moisture in a bathroom often leads to mold growth.


A kitchen-range hood removes cooking odors, as well as by-products of combustion released by combustion-fired ranges—or burnt food. It also removes moisture generated by cooking and washing dishes.

Local-exhaust fans can also be used in home offices to remove odors from photocopiers and laser printers, as well as in home workshops, and artists’ studios. Sometimes local exhaust fans are installed in closets to keep them fresh. Many healthy houses use a local exhaust fan in the garage.


General ventilation


A general ventilation system is designed to slowly exchange the air in the entire house. This is important because people generally don’t confine themselves to a single room. Fresh air is needed in all living spaces.

While a general ventilation system can be designed to overcome pollutants and moisture-laden air in a house that has no local ventilation, the entire ventilation process will be much more efficient if local exhaust fans are used judiciously.

If a general ventilation system is used in an unhealthy house (one built, maintained, and furnished with unhealthy materials) to dilute the concentration of pollutants in the air, it may need to be very large and powerful. This means it will be expensive to install and to operate, and it will probably be noisy.

Excerpted from:
The Healthy House, 4th Edition
John Bower, Author
Copyright © The Healthy House Institute



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Ventilation and air pressure:  Created on April 1st, 2010.  Last Modified on April 6th, 2010


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