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 opening and how far they are opened.
The difficulty in using a window for both admissions of light and air is that its size and location for the best daylight often conflict with the size and location that produce the best ventilation.
Principles of air movement as applied to houses are:
- Air moves because of differences in temperature and differences in pressure. In single storey houses, the movement of air because of differences in temperature is negligible. Movement of air due to pressure differences, therefore, should govern the placement of windows in most houses.
- 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 the differences in pressure. It moves from high-pressure areas inside the house through openings to areas of lower pressure outside the house.
- 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, ie: 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 that 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.
Use the following recommendations as a guide in selecting windows for 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, humid 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 that 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, a) across the room and b) within the level that occupants sit and stand. Do this by placing windows away from exterior corners and placing double hung, sliding, casement and awning windows in the lower part of the wall since such windows cannot direct the movement of air downward. Venetian blinds and similar interior control help to deflect air movement. 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 from the house; avoid setting trees, shrubbery and fences in the path of the breeze; plan interior partitions and furnishings so that they do not obstruct air flow within the house.