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 conflict with the size and location which produce the best ventilation.
Principles of air movement as applied to houses are:
Air moves because of differences in temperature or differences in pressure.
In single-storey houses, the movement of air because
of differences in temperature is negligible. The placement
of windows in most houses, 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 house 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 house because of differences in pressure. It moves from high pressure areas inside the house through
openings to areas of lower pressure outside
the house.
Air moves through the house because of differences in pressure. Partitions slow down the movement of air and divert its path.
To speed movement of air within a room, the openings through which the air leaves the house 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 held to a minimum.
The angle at which the air enters and leaves the room is the controlling influence on the pattern of air movement within the house. This angle depends on the location and type of window.
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.
Use the following recommendations as a guide in selecting and locating windows for ventilation:
Provide ventilation openings in excess of 10 percent of the floor area of a
room. This is a general rule-of-thumb. Most building codes have
established minimums of 4 to 5 percent of the floor area, but just as
large glass areas provide daylight for cloudy days, sufficient ventilation
openings can offer relief on warm, sultry ones.
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.
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.
Windows 
