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Design Considerations: Provide Fresh Air

Windows provide the primary means to control air flow in most homes. People open windows to provide fresh air, ventilate odors and smoke, dissipate heat and moisture, and create air movement on hot days. While exhaust fans and central air systems can mechanically ventilate a room, opening a room to the outdoors is perceived as more direct and natural.

Connecting Indoor and Outdoor Space View to outdoors

In order to ensure that all residences have access to the healthful aspects of natural ventilation, state or local building codes commonly regulate the minimum size of the ventilation opening in a window and the egress opening. Typically, codes require that about 5% of the floor area of a "living area," such as a bedroom or living room, be provided in ventilation area. These regulated areas should be carefully checked before sizing or replacing a window.

The potential value of natural ventilation as an energy efficiency strategy depends on climate and lifestyle. In a mild climate, there may be many hours during the year when outdoor air can be used to improve comfort and save energy for air conditioning. In climates with severe summers and/or winters or in dusty, noisy, or humid locations, the value of natural ventilation may be limited. To the extent that natural ventilation requires occupants to open and close windows, the lifestyle of the occupant may also be a factor. Finally, security concerns may limit the opportunities to provide natural ventilation.

An alternative approach that provides a steady amount of outdoor air is to incorporate a small ventilation element into the frame of the window. This passive approach has been used in the northwest United States to meet state code requirements for a minimum amount of outdoor air in new, tightly built houses that do not use mechanical ventilation. These "trickle ventilators" have been widely used in European houses. The slots go through the window frame, normally on the top or bottom, with screens and flaps that keep out bugs and rain. They can be adjusted by occupants to control the amount of air flow. The peak amount of air exchange can be controlled in each room by properly sizing the ventilators in each window.

Positive and Negative Pressure Improving Ventilation
Ventilation is maximized by providing for cross-ventilation of as many spaces in the house as is practical. In normal wind conditions, the side of a building facing the wind will have a zone of positive pressure and the opposite side will have a zone of negative pressure. By providing adequate ventilation openings on these two sides of the house, a positive flow of air through the interior, from positive to negative pressure, is encouraged. Of course, the interior layout of the house must permit the air to flow through, and interior doors in the ventilation path must remain open.

Ventilation effectiveness depends on wind speed, the angle at which the wind strikes the window, and the location and size of the window. A room with a single opening will have only 12–23% of the wind velocity. This improves up to 51% if windows are located on adjacent walls and as much as 65% of the outside air velocity can be reached with windows on opposite walls.

Ventilation Openings

opening height as a fraction of wall height 1/3
opening width as a fraction of wall width 1/3 2/3 3/3
single opening 12-14% 13-17% 16-23%
two openings in same wall 22% 23%
two openings in adjacent walls 37-45% 37-45% 40-51%
two openings in opposite walls 35-42% 37-51% 47-65%

range = wind 45° perpendicular to opening
Source: Sun, Wind & Light, G.Z. Brown and Mark DeKay, 2001.

If windows cannot be located on opposing walls, high- and low-pressure areas can be induced with the use of casements. Of all window types, casements provide the most control of ventilation direction and intensity. Because the sash can be opened into an air stream, breezes that would otherwise pass by can be directed into the room. Window types in which the sash remains flush with the wall ventilate well only with direct pressure differences across the window. In addition, as noted previously, virtually the entire window area of casement units can be opened, while sliders are limited to less than half of the area.

Operable Skylights Even without external winds, double-hung windows can sometimes provide natural ventilation caused by stratified air flow within a room. Cooler fresh air enters at the bottom opening, while hotter air at the ceiling level is allowed to exit through the top opening. The taller the windows and the higher the ceiling, the more pronounced is this effect.

Operable skylights or roof windows can aid overall ventilation in a house significantly by creating a similar thermal chimney effect, letting hot air escape from the ceiling level where it accumulates and causing cooler air to be drawn in through lower windows.

Trees, shrubs, exterior walls, and earth berms can all divert wind patterns. These elements can be used to some extent to funnel or direct breezes through a house. Conversely, if these elements are placed without regard for prevailing wind patterns, they can act as obstructions to natural ventilation.