By The Home Ventilating Institute (HVI)
In order for air to move into or out of a house, there are two basic requirements. First, there must be a path through which the air travels, and second, there must be an air pressure difference to push the air molecules through the pathway. The relationship between air pressure and house tightness is integral to an understanding of how ventilation systems work.
Holes in houses Even when all the windows are closed, there are still holes in houses—they just aren’t as noticeable. There are hidden gaps between the window or door frames and the 2x4s holding up the wall. There are even narrower gaps between the floor and the walls, and even smaller gaps around electrical outlets. There are also many hidden holes inside the structure that were cut through studs, floor joists, and rafters by plumbers, electricians, or heating/cooling contractors.
If it were possible to combine all of the small holes and gaps into one single hole, the result would be an opening of several square inches in a very tight house, or several square feet in a very loose house.
The holes are called random holes, because they weren’t created for the purpose of supplying the occupants with fresh air. If a hole is created on purpose, specifically to provide a pathway for air to travel through, it is called a deliberate hole. The installation of a controlled ventilation system requires one or more deliberate holes.
Pressures in houses The pressures measured in houses are usually less than 50 Pascals (Pa). This may not seem like very much pressure, but it is enough to cause air to move through the random holes in a house and potentially cause some serious problems. For example, 3 Pa of negative pressure in the vicinity of a chimney is often enough to cause backdrafting (combustion gases flowing down a chimney instead of up).
Natural pressures caused by the wind and by temperature differences vary considerably day-to-day. Warm air exerts a small upward pressure as it rises up into cooler air.
Accidental pressures can be caused by leaky ducts, chimneys, or by mechanical equipment not specifically designed to ventilate a house, such as a clothes dryer. As with natural pressures, these positive and negative accidental pressures themselves are neither good nor bad, but sometimes they can cause pollution or moisture-related problems.
Pressure is also affected by mechanical equipment that is deliberately designed to exchange air in a building for the purpose of supplying fresh air or expelling stale air. This is what ventilation is really all about. This is called controlled ventilation—ventilation that is created “on purpose.”
Changing the tightness of a house If you weatherize or tighten up a house to make it more energy-efficient, you will affect the amount of air being exchanged, as well as the pressures the house experiences. A window fan blowing into a tightened house will move less air, but you will also be able to measure an increase in pressure. Care must be taken so that the house is not under-ventilated, resulting in poor air quality. The answer is not loose houses—it is tight, energy-efficient, comfortable houses with mechanical ventilation systems.
Today, some energy-efficient builders are purposefully using special techniques to build houses that are almost hermetically sealed. Naturally occurring pressures don’t provide enough air to supply the needs of the occupants. A tight house must have a controlled mechanical ventilation system to supply it with fresh air and to remove stale air. Ventilation products certified by the Home Ventilating Institute (HVI) have been tested and proven to help homeowners maximize indoor air quality.
• John P. Lapotaire, CIEC • Certified Indoor Environmental Consultant • Microshield Environmental Services, LLC • www.Microshield-ES.com
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