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Indoor Air Quality basics:

Building scientists have found that there are several important steps to ensure good indoor air quality and they are generally consistent with energy conservation. These basic steps are:

1. Create a good, tight boundary between indoor and outdoor air.

2. Minimize or eliminate pollutant sources within the house.

3. Maintain indoor humidity within a healthy range.

4. Provide adequate fresh air ventilation for people in the home.

This approach cuts heating and cooling energy losses, reduces uncomfortable drafts, permits better moisture control, and maintains an indoor environment that is healthier and more energy-efficient.

Perhaps the most widely misunderstood aspect of the house-system 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 organ isms, so I prefer not to use the term “breathing” for a house. There is still some underlying wisdom behind that statement, however. As we have already seen, it is vitally important to allow wall assemblies the opportunity to dry; this is done by using building materials that are permeable to water vapor. But this is not breathing; vapor diffusion does not depend on air movement. To understand the dynamics of air quality, let’s first look at air barriers.

Treat Air Like Water

We work hard to keep water out of our homes. Foundations, siding, windows, doors, roofing, and flashing represent a substantial investment in keeping out water. Then we purposefully bring water into the house through plumbing supply and drainage systems, which are another large price tag in our homes. Clean water is essential to our health, so we control it carefully. Fresh air is also essential, but in most residential buildings, we pay no attention to controlling it. If we treated air as we do water, by keeping out unwanted air and intentionally introducing fresh air, we would provide much healthier indoor environments for our families. FIG 21-0

Attic and Crawl Space Venting

Most building codes have minimum requirements for attic, cathedral-ceiling, and crawl space 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 a thermal boundary between the conditioned spaces and the attic or crawl space. Ideally, there should be no air exchange at all between those spaces and indoors.

Basically, attic, roof, and crawl space venting are “Band-Aids” that attempt 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 crawl spaces (with vapor retarders and footing drainage) are more important to a building’s health. Some building codes are just beginning to recognize the importance of these concepts, and venting requirements are beginning to change.

FIG 21-1-2: (above) Passive roof ventilation is designed to transport moisture out of attics, but in certain circum stances, it can also be a point of entry for water or moisture-laden outside air. (below) Crawl space vents do little to reduce moisture loads in the living space, and they can introduce warm humid air into the crawl space during the summer.

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 last decade 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. The next section details ways to create good air barriers in existing homes, and there is more discussion of housewrap in here. With this information, you should be well equipped to create a very good air barrier in your house.

Controlling Humidity

In winter, excessive dryness can be controlled by a tight building envelope. People often think their heating system is what dries out the air in winter, but actually it is the dry outdoor air constantly seeping in and needing to be heated that lowers the humidity indoors. Many people use humidifiers, but these devices need a lot of maintenance, and some types may create health problems of their own. Excess humidity in winter is best handled by mechanical ventilation, particularly by fans that exhaust moisture at its source: in bathrooms and kitchens.

In the summer, air leakage tends to add humidity to the indoor environment. The best way to control this is with a tight building envelope and air-conditioning or other mechanical dehumidification. Air-conditioning dehumidifies better when it is sized properly, and it will work better if the building envelope is tight, because excessive air leakage in humid weather brings moisture into the house. Of course, spot ventilation in bathrooms and kitchens is also important in the summer.

FIG 22-0: It is important that bathroom fans work properly and exhaust air to the outside, not just into the attic.

FIG 22-1: Kitchens are another source of moisture; range hoods should always be vented directly to the outside.


The biggest air leaks occur in places where housewrap is not installed.


One example of misguided building science is the application of a vapor retarder on the inside of houses in a hot, humid climate. Although most model codes have been updated to discourage this practice, it is not unusual to see vinyl wallpaper—an excellent vapor retarder—in hot, humid areas. If the exterior sheathing is vapor permeable, this can lead to big problems as out door moisture condenses on the inside surface of the vinyl. In general, vapor retarders should be installed on the outside of houses in these climates, and they must be avoided on the interior of out side walls. Such climates include Florida and the southern half of other Gulf Coast states, including southeastern Texas, and the eastern coast north to Maryland.


In hot, humid climates, vapor retarders do not need to be separately applied; instead, they may be integrated with the wall sheathing or exterior insulation materials by using foil-faced rigid-foam insulation or low-permeability, thin structural sheathing, such as foil-backed Thermo-ply.

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? Leaving your fresh air supply to enter through random air leaks in the house—which may include gaps and holes in dirty; damp, moldy, pesticide-treated basements, crawl spaces, and attics—has been com pared to pulling air into our lungs through holes punched through our abdomens. Most people would prefer to breathe through the openings — the nose and mouth—that were intended for that purpose. Similarly, every home should have an intentional provision for fresh air in the form of a mechanical ventilation system.

Creating a tight building envelope 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 likelihood of ice dams in snow country. Most important, it allows the operation of mechanical ventilation systems to control the indoor environment for better health and under varying weather conditions.

Remember that the forces that move air through a house—the stack effect, wind, and mechanical systems (especially combustion equipment and duct leakage)—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. The ideal house is tight enough that it has just enough fresh air leaking in when outdoor conditions are the most extreme; then the difference can be made up with mechanical ventilation for the rest of the year.

Minimizing Pollutants in the Home

• Source reduction is the most important way to deal with toxins; keep them out of the house whenever possible! Learn which household products are the most toxic, get rid of any you don’t use, and substitute less toxic products whenever you can.

• Separation of toxic materials from everyday living spaces is another useful strategy. Don’t just put dangerous chemicals in a childproof cupboard; keep them in a metal cabinet in the garage, and make sure the garage is separated from the rest of the house. Better yet, store them in a locked, outdoor storage shed.

• Dilution of toxins is helpful, too. Installing a mechanical ventilation system helps bring in fresh air regardless of the weather conditions; such a system helps reduce concentrations of harmful substances.

• Combustion appliances must be vented properly. Don’t use unvented heaters of any kind, including “vent-free” fireplaces.

• Humidity control reduces mold sources by keeping indoor surfaces warm and maintaining relative humidity.

• Radon testing can detect this invisible, cancer-causing underground soil gas, which can occur anywhere. The only way to tell whether you have dangerous levels in your house is to test for it. Use an EPA-certified laboratory and follow the directions carefully for an accurate test.

Fig 23-0: Whether you have a tight or a leaky house, any natural-draft combustion appliance may produce deadly carbon monoxide. The only way to know is to have a professional test the flue gases.

Mechanical ventilation

Mechanical ventilation, as opposed to random air leaks, ensures the right amount of air exchange year round, under all conditions. When I talk about mechanical ventilation in this book, it is very important to understand that I am referring to ventilation of indoor spaces to provide fresh air and remove unwanted moisture. A mechanical ventilation system always consists of one or more fans, usually with ductwork, which either bring fresh outdoor air into the living space, or 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 crawl space, but people often assume that the purpose of those attic and crawl space vents is to ventilate the entire building (i.e., “let the house breathe”). See discussion of installing basic mechanical ventilation systems.

Indoor air quality

Controlling the indoor environment through tight building and a mechanical ventilation system offers other health benefits as well. One of these is control of the indoor relative humidity. How do you control humidity? The simple answer is by controlling air exchange.

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 and automotive products stored in the home or garage can also be toxic.

Many common household chemicals, such as cleaning products, aerosol sprays, moth repellents, pesticides, and herbicides, can be toxic. Pesticides, herbicides, and radon gas can be drawn into the house from underground (yes, air moves under ground in many places). 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 definitely “yes” but, also, “not at all.” If you tighten a house that already has a significant source of toxic fumes, the concentration of pollutants will probably increase. Therefore, source reduction is important, and separating unavoidable toxins from the living space is criti cal. At the same time, unintended airflow can also contribute to indoor pollution. Sealing air leaks between the house and the garage, for example, can improve air quality by cutting off the path 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!”


An important rule of thumb: To maintain good indoor air quality, provide about 15 Cu. ft. per minute (CFM) of fresh air per person.


Unlike water molecules, air can’t leak through solid materials. Air leaks through cracks where different parts and materials of the building are connected together. Gaps between window and door frames and rough framing, places where exterior walls make a jog or are interrupted by a cantilever, and connections between dormer walls and roofs are areas often missed by house wrap. And the biggest leaks are often hidden in attics, basements, and crawl spaces where housewrap is never installed.


Although outdoor air pollution is significant in many 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. This includes most urban areas, most of the time. In some cases, research shows that indoor air can be as much as IOU times more polluted than outdoor air. Bringing in less polluted outdoor air in a controlled way is important for improving indoor air quality

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