Because Mother Nature is partially responsible for the air-exchange rate in all houses, it is important that we understand how she does her work. Naturally occurring pressures cause an exchange of air in houses. Keep in mind that positive and negative pressures themselves are neither good nor bad, but they can have positive or negative effects. For example, they can cause fresh air to enter a house (a positive effect) or they can cause pollution- or moisture-related problems (a negative effect).
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Air-exchange rate - measure of the speed of the exchange of air between indoors and outdoors.Wind and stack effect are the most significant natural pressures that cause air to move through houses. In contrast, diffusion is insignificant. Builders often use plastic sheeting in walls as a diffusion retarder, slowing the movement of water vapor through the wall. Such a retarder will also slow down considerably the movement of all the different components of air—the oxygen, nitrogen, and various other gases. Because natural pressures are not continuous or predictable, they will only move air through a house in an equally unpredictable manner.
As with natural pressures, accidental pressures can cause pollution- or moisture-related problems. It isn’t unusual for there to be a great deal of infiltration and exfiltration whenever the forced-air heating or cooling system is running. Leaky ducts provide accidental ventilation that can be excessive, resulting in high heating/cooling bills.
Diffusion - the scattering of air particles through a material, or from one area to another.In addition to all the ways air enters and leaves a house naturally and accidentally (e.g. wind, stack effect, clothes dryers, chimneys, leaky ducts, etc.), there are mechanical systems specifically designed to ventilate houses using controlled pressures. This is controlled ventilation—in other words, ventilation on purpose—and it is the only way to exchange the air in houses that is consistent, reliable, and predictable. Progressive building codes advocate that houses be built purposefully airtight—for comfort, energy efficiency, and moisture control —and incorporate a mechanical ventilation system that will supply fresh air. Actually, a mechanical ventilation system is a good idea in all houses, tight or loose, because the pressures induced by wind and stack effect are very erratic and unpredictable. We need fresh air continually, not randomly.
Take control of your ventilation with HVI-Certified Products to supply clean, fresh, healthy air throughout your home. Whole house ventilation continuously expels stale indoor air, and consistently provides fresh, oxygen-rich air. Products with the HVI-Certified Label are guaranteed to perform to specification; giving you the peace of mind that you are receiving enough fresh air for a healthy, comfortable environment.
Adapted from: Understanding Ventilation: How to design, select, and install residential ventilation systems by John Bower © 2010 The Home Ventilating Institute (HVI).
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