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One of the most important ideas to emerge in recent years is the concept that a house is much more than an assemblage of materials. Instead, building scientists and researchers now view a house as an interactive system. This means all the different components and sub-systems of a house actually work in conjunction with each other to create an indoor environment for the occupants. A change in one part of a house can easily have repercussions elsewhere in the building. For example, adding insulation and weatherizing a house usually means the heating system runs less in the winter, so utility bills are lower. But weatherizing a house can also affect the way moisture migrates through walls and ceilings. Some newly-tightened houses begin experiencing a higher relative humidity indoors—and mold growth. Weatherization also slows down the natural air-exchange rate, resulting in indoor air that’s more stale and polluted. Tightening a house can also have a negative effect on how a chimney functions. So, unless you understand how the various components of a house interact, you can accidentally create some very real health problems.
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We routinely see the human body as an integrated system consisting of several sub-systems (circulatory system, nervous system, etc.), and an automobile as a system consisting of various integrated components (engine, transmission, steering, etc.). While houses have always acted as systems, an understanding of those systems is more important with today’s houses than with those built a hundred years ago. That’s because tight, well-insulated, energy-efficient houses are less forgiving than the loose, drafty, energy hogs built in the past. It’s even been suggested that a tight house be viewed as more than a system, but “as a spacecraft.”
In some ways, it’s easier to create a serious health problem in a tight house than a loose house. In Canada, moldy houses are likely to have a low natural air-exchange rate—in other words, they are tightly built without adequate ventilation. The answer isn’t to build loose houses. The answer is to understand how houses function like systems, and then build them accordingly.
The health of a house is in a constant state of change. The indoor air quality at any particular time is affected by a number of factors related to the occupants and the structure itself. The structural factors include the temperature of materials, the tightness of the house, the age of the materials or house, the type of foundation, the layout of the rooms, and the type of building materials used. The occupant factors include the number of people in a house, their ages, their activities (cooking, cleaning, hobbies, bathing, etc.), as well as the products they use to furnish the house. All of these factors are in constant flux, so the interactions are different at different times of day, month, or year. The goal is to understand the interactions, and minimize the factors that result in poor indoor air quality. Because there are a wide range of occupant lifestyles and habits, the same house could be healthy for one family, but unhealthy for another.
Viewing a house as a system is being widely promoted at cutting-edge builder’s conferences, and the system approach is becoming more widely accepted by engineers, researchers, architects, designers, and utilities. For example, in New England the Northeast Utilities published a booklet titled "The House as a System" to better educate their customers. In the northwestern part of the U.S., the Bonneville Power Administration has an excellent "Super Good Cents" program that relies heavily on the system approach, as does Canada Mortgage and Housing Corp.’s R-2000 energy-efficiency program, and the Canadian Home Builders’ Association’s Builders’ Manual. In the southeastern U.S., the Advanced Energy Builder’s Field Guide is an excellent example of how to apply the system approach to house building.
(This article is from the archives of the original Healthy House Institute, and the information was believed accurate at the time of writing.)
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