The term airtight is often used to describe houses with very little natural air leakage. Unfortunately, the word can have two meanings: 1) it can mean there is no air at all flowing through the house by any means (not a good idea), or 2) it can refer to a lack of infiltration and exfiltration as a result of a tight structure (a good idea). By the second definition, you can have an airtight house with plenty of fresh air flowing through it—if it has a mechanical ventilation system.
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How to measure tightness
Even though houses, in general, are tighter than they used to be, there is still a wide variation from house to house. Some new houses are extremely tight, some aren’t quite as tight, and some are surprisingly “loose”. Because airtightness is a function of how many random or planned holes there are, and because random holes are often hidden, you simply can’t tell how tight a house is by its looks. Rather than guessing, researchers and contractors use proven techniques to measure house tightness. Once you know how tight a house is, you can estimate how much infiltration and exfiltration there will be on an average basis, and how much mechanical ventilation you will need.
Blower-door testing
A blower door (often called a fan door) is a popular diagnostic device that can be used to assess airflows and pressures in a house. With a blower door, you can estimate average infiltration rates based on the tightness of the house, then gauge the factors to help you select the proper controlled ventilation system.
A blower door contains a large fan, a fan speed controller, and one or more pressure gauges mounted in an adjustable framework that can be temporarily placed in an exterior door opening of a house. These devices are used regularly by weatherization contractors and builders specializing in green houses or retrofits.
To use a blower door, the entry door is opened, the blower door frame is installed in the opening then expanded to form a seal around all four sides of the opening. When the fan is turned on, it will blow air out of the house, causing the house to be depressurized. (Blower doors can also be installed to blow air into a house to pressurize it.) A pressure gauge, with both an indoor and an outdoor sensor, will tell you how much negative pressure the interior of the house is experiencing in relation to the outdoors. By knowing the quantity of air that is leaving the house (as read on an airflow gauge) when the entire house is subjected to a certain pressure (as read on a pressure gauge), you will know how much air is moving through the house at a given pressure. This is a measure of how tight the house is.
Tightness estimates using methods such as blower doors can give you a better idea of how much air is moving into and out of a house than simply guessing.
A mechanical ventilation system is a more effective way of ventilating a house than relying on natural and accidental ventilation. A tightness estimate will help you evaluate how much fresh air your mechanical ventilation system should provide to optimize the health of your home.
For help in choosing the right ventilation system for your home, visit the Publications Section of the HVI website at http://www.hvi.org/publications/index.cfm.
Adapted from: Understanding Ventilation: How to design, select, and install residential ventilation systems by John Bower © 2011 The Home Ventilating Institute (HVI).
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