Natural ventilation of a house is primarily a summer problem - one of comfort. The number, size, and placement of ventilation openings are all important elements in planning windows for ventilation. The effectiveness of windows in achieving desired ventilation depends also on which windows are opened and on how far they are opened.

The difficulty in using a window for both admission of light and air is that its size and location for the best day lighting often conflict with the size and location which produce the best ventilation.

The principles of air movement as applied to houses are:

• Air moves because of differences in temperature or differences in pressure. In single storey homes, the movement of air because of differences in temperature is negligible. The placement of windows in these homes therefore, should be governed by movement of air due to pressure differences.
• A high pressure area is created when air strikes a building. Low pressure areas are created as the air moves over and around the building. Air flows into a home through openings in the wall against which the wind blows. The wall acts as a dam, causing the air pressure to build up. Air flows out of a home because of differences in pressure. It moves from high pressure areas inside the home, through openings to areas of lower pressure outside the home.
• To speed the movement of air within a room, the openings through which the air leaves the home should be larger than those through which it enters.
• Obstacles in the path of moving air cause it to change direction, thus slowing it down, i.e. trees, shrubbery or fences on the outside; partitions, walls, or furniture on the inside. Because the cooling effect of air in summer depends on its speed, obstructions which slow the movement of air should be kept to a minimum.

Obstacles in the path of moving air slow it down and divert its flow away from the windows. Full advantage of breeze is obtained when there are no obstructions.

The angle at which the air enters and leaves the room is the controlling influence on the pattern of air movement within the home. This angle depends on the location and type of window. Refer to the following tips to select and locate windows for the best ventilation.

• Provide ventilation openings in excess of 10% of the floor area of a room. This is a general rule of thumb. Most building codes have established minimums of 4-5% of the floor area, but just as large glass areas provide daylight for cloudy days, sufficient ventilation openings can offer relief on warm, sultry days.
• Locate the house and the ventilation openings to take full advantage of prevailing breezes. Do this by determining the high and low pressure areas as defined by the shape of the house - the walls which the breeze will strike and the walls around which the air moves. Allow for changing wind directions.
• Locate windows so as to effect the best movement of air across the room and within the level that occupants sit and stand. Do this by placing windows away from exterior corners and placing horizontal sliding, casement and awning windows in the lower part of the wall since these window styles cannot direct the movement of air downward.
• Plan landscaping, interior partitions, and furniture so they do not interfere with air movement. If possible, place the house so that existing buildings and hills do not divert the wind away from the house. Avoid setting trees, shrubbery and fences in the path of the breeze.

A high pressure area is created when air strikes a building. Low pressure areas are created as air moves over and around a building.

Air moves through the house because of differences in pressure. Partitions slow down the movement of air and divert its path.

Place windows away from exterior corners to achieve the best air movement across room. The flow of air when corner windows are used is indicated by the dotted arrow.

For summer comfort, air should flow across the room at the level of occupancy.

If the location and type of windows are such that they cause the air to flow along the ceiling, the room can be uncomfortable for occupants.

The position of a window in a wall depends on the type of window. Some windows can deflect air downward and do not have to be placed low in the wall.