Forget AC! Cool Your Home Naturally

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Now let’s examine some of the most common forms of natural ventilation:

Cross ventilation. When air flows into a room from one side and out the other, you’ve got cross ventilation. As with any air movement, cross ventilation can cool your body by speeding up the evaporation of sweat, and it can cool your home by removing hot air, especially at night, if the incoming outdoor air is cool.

Any room with openings on opposite sides can be cross-ventilated if the openings are large enough. But most homes are at least two rooms deep in many places, so you also need to look at airflow through the whole house in order to ventilate the rooms effectively. Think of your house as a system of corridors and doorways that can channel air from one end to the other.

Looking at the air motion notes you’ve made, see if the combination of prevailing breezes, house orientation, room layout and openings (windows and other vents) could be improved to make the best use of those breezes. Are there air inlets on the windward side and outlets on the leeward side? If your home isn’t oriented to face the prevailing breeze, you can use landscaping or wing walls on the windward side of your house to encourage the wind to approach your inlets head-on.

As you plan to make changes in your house’s air inlets and outlets, consider these guidelines:

• Openings in opposite walls allow maximum air movement.
• Openings in adjacent walls create air turbulence, increasing the cooling effect.
• A combination of low inlets and high outlets can achieve the greatest scouring of room air. This strategy is especially useful for night cooling of thermal mass floors.
• If you install new openings, make sure the air moves around the people in the room in order to best cool them. Having either some low and some high openings or all openings at a mid-height should achieve the desired effect.

Finally, you can enhance the effectiveness of cross ventilation by naturally cooling the air before it enters your home. Shade, plantings or water (in arid climates), in the form of a pond, fountain or mister, all can remove heat from the air. When located on the windward side of your home, these features will increase your indoor comfort in hot weather.

The chimney effect. If you live where breezes are rare, or just a bit too gentle, you can use the “chimney” or “stack” effect to enhance air movement. The chimney effect is driven by the rising of warm air; when air is heated, it expands, becomes lighter and rises. If that rising warm air is allowed to escape high in a structure, it will be replaced by cooler (heavier) air entering lower in the structure.

The rate of air movement is affected by the vertical distance between inlets and outlets, the size of the openings, and the difference in air temperature from the bottom to the top of the chimney; the greater each of these features, the faster the air movement. One advantage of this strategy is that it doesn’t require any particular orientation to the prevailing breezes; it drives itself.

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