Air Barriers and Indoor Air Quality
Perhaps the most widely misunderstood aspect of the whole-house approach is the relationship between air-leakage control and healthy indoor air. Conventional wisdom says “a house has to breathe,” but what exactly does that mean? People and animals need to breathe. Houses are complex systems, but they are not living organisms, so I prefer not to use the term “breathing” for a house. There is some underlying wisdom behind that statement, however. It is vitally important to allow buildings to dry; this is aided by using building materials that are permeable to water vapor. Venting of attic spaces and siding also helps, provided the vented spaces are separated from the thermal boundary by a good air barrier. To understand the dynamics of air quality, let’s first look at air barriers.
What is an air barrier?
Although it may seem obvious, an air barrier stops air. We’ve already seen that most types of insulation don’t stop air movement. Another myth that has taken root over the years is the idea that housewrap makes a house airtight. Most house wraps are good air barriers, but the biggest air leaks occur in places where housewrap is not installed, where it is installed improperly, or where it is not detailed well.
Can a house be “too tight”?
Let’s go back to the axiom “a house has to breathe.” We certainly need fresh air in our homes, but where does that fresh air come from? In most homes, the fresh air supply enters through random air leaks in the enclosure—which may include gaps and holes in dirty, damp, moldy, pesticide-treated basements, crawlspaces, or attics. Most people, if given a choice, would prefer to have an intentional provision for fresh air in the form of a mechanical ventilation system. Creating a tight thermal boundary that minimizes air leakage has many benefits beyond energy savings: It increases comfort, reduces the chance of moisture damage in the building structure, discourages mold growth in wall and ceiling cavities, and reduces the risk of ice. Most important, it allows the operation of mechanical ventilation systems to control the indoor environment for better health as weather conditions vary. The forces that move air through a house—the stack effect, wind, and mechanical systems—move the most air when outdoor weather conditions are extreme. This is also the time when it costs the most to heat or cool that air. On the other hand, even a leaky house tends to be under-ventilated in mild weather, when those forces are minimal and windows and doors are closed. Ideally, a house should be tight enough to avoid over-ventilating when outdoor conditions are the most extreme; then in milder conditions, the difference can be made up with mechanical ventilation.
Mechanical ventilation, as opposed to random air leaks, ensures the right amount of air exchange year-round,. Mechanical ventilation (is) ventilation of indoor spaces to provide fresh air and remove unwanted moisture. A mechanical ventilation system consists of one or more fans, usually with ductwork, which bring fresh outdoor air into the living space, exhaust indoor air to the outside, or do a combination of both. It can also include filtration of incoming air. This is very different from passive (or fan-induced) venting of the roof, attic, or crawlspace, but people often assume (incorrectly) that the purpose of those attic and crawlspace vents is to ventilate the entire building (i.e., “Let the house breathe”).
Indoor Air Quality
Controlling the indoor environment through tight building and a mechanical ventilation system offers health benefits as well. The best way to control indoor relative humidity is to control air exchange with a combination of an air-tight thermal boundary, air-tight ductwork, and controlled mechanical ventilation. Contain the air, then you can control it. There are, of course, many sources of indoor air pollution besides moisture and its related effects. Volatile organic compounds (VOCs) can be found in paints, stains, cleaners, solvents, wood preservatives, and carpeting. Formaldehydes are found in manufactured wood products, such as interior-grade plywood, medium-density fiberboard (MDF), carpets, and furniture. Fuel, automotive products, and hobby supplies stored in the home or garage can also be toxic, as well as many common household chemicals: cleaning products, aerosol sprays, moth repellents, pesticides, and herbicides, for example. Pesticides, herbicides, and radon gas can be drawn into the house from underground (yes, air can move underground, too). Running automobiles and improperly vented (or unvented) combustion appliances can send deadly carbon monoxide (CO) into the home. How can you deal with these pollutants? Isn’t it bad to tighten a house with these substances present? The answer is, “It depends.” Of course, if you tighten a house that already has a significant source of toxic fumes, the concentration of pollutants will likely increase. Source reduction is always the first priority, and separating unavoidable toxins from the living space is critical. At the same time, unintended airflow can also contribute to indoor pollution. For example, sealing air leaks between a house and garage, and sealing leaky ducts in the garage can cut paths for auto exhaust to migrate indoors. Meanwhile, a mechanical ventilation system that introduces fresh air helps dilute any toxins that remain. There’s a slogan in the building-science industry that sums up these ideas: “Build tight, and ventilate right!”
Although outdoor air pollution is significant in some parts of the country, research done by the U.S. Environmental Protection Agency indicates that indoor air is typically 2 to 5 times more polluted than outdoor air. In some cases, research shows that indoor air can be as much as 100 times more polluted than outdoor air. Bringing in less polluted outdoor air in a controlled way is one important way to improve indoor air quality.
Treat Air Like Water
Attic and Crawlspace Venting
Most building codes have minimum requirements for attic, cathedral-ceiling, and crawlspace venting. It is important to remember that this passive venting is different from fresh-air ventilation. Venting is designed to carry excess moisture out of those spaces so it does not damage the building structure. These venting strategies actually have nothing to do with indoor air, but they are often called “ventilation,” which leads to confusion. Remember that there is (usually) an intentional thermal boundary between the conditioned spaces and the attic or crawlspace. Ideally, there should be no air exchange at all between those spaces and indoors; when air flows through the thermal boundary materials, there is much more potential for condensation and damage. Basically, attic, roof, and crawlspace venting attempts to carry away moisture that should not be there in the first place. Good air sealing at the thermal boundary and proper water management of basements and crawlspaces (with good drainage; see “Insulating a House” and “Renovations”) are more important to a building’s health.