Natural Ventilation Systems: Cooling Design Strategies for Buildings

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Natural ventilation systems rely on pressure and temperature differences to move fresh air through buildings. Air movement by pressure differences can be caused by wind and movement by temperature differences is caused by the buoyancy effect in the air.  The amount of ventilation air that passes through the building depends on the size and placement of inlet and outlet openings.

Fresh air from natural ventilation increases thermal comfort, provides oxygen for building occupants, and relieves odors.  Cooling with natural ventilation works better in dry climates than in humid ones due to the fact that dense, humid air is more difficult to remove from a building by only using natural drafts.  Mechanical and fan-induced cooling would work better in humid climates.

Natural Ventilation Design Techniques

The following guidelines are commonly used in the design of natural ventilation cooling systems:

• Place large enough air inlet and outlet openings to permit the air to flow freely throughout the building.
• Use windows with low solar heat gain coefficients (SHGC) and place overhangs over all east and west-facing windows.
• Place ridge vents, which is an opening at the highest point on the roof, to allow warm, humid air to escape the building’s attic.
• Use two separate supply and exhaust openings for each room.
• Locate exhaust openings as high as possible as an outlet for warm air.
• Locate supply air opening as low as possible as an inlet for cool air to maximize ventilation air mixing inside the room.
• Air openings can be either louvers or windows.
• Maximize wind induction and cross ventilation by sitting the ridge of a building perpendicular to summer winds.  You can find out wind directions in your area by using a weather service online.
• Buildings should be placed where summer wind obstructions are minimal.
• Naturally ventilated buildings should be narrow.  Wide buildings make it difficult to distribute ventilation air throughout.
• Consider providing high louvers in between rooms inside the building to aid with ventilation airflow.
• Staircases in multiple-story buildings provide a great path for draft ventilation.

Cross and Draft Ventilation

Cross ventilation happens when you have a positive, wind-induced, pressured ventilation at one side of a building where the air enters and negative ventilation at the other side of the building where the air exits.  Cross ventilation keeps a building pressurized and has a high air change rate, since it uses natural wind forces to keep the building cool.

Draft ventilation consists of cool air entering a building through openings placed on lower sections of the building and warm air exiting the building through openings located in the higher sections.  Draft ventilation works on the principle of temperature differences between cold and warm air to create a stack effect inside the building.  Since the warm inside the building is less dense than the cool air outside, warm air will exit through the openings on top and cool air will enter through openings on the bottom.