Good ventilation is important to your comfort and safety, as are a reliable heating system, smoke alarms and a dry basement. It will help to protect the occupants from irritating pollutants and potentially dangerous gases like carbon monoxide. Well-planned ventilation also helps to manage moisture and reduce the likelihood of mold and mildew growth. Air-Pollution Dynamics The concentration of a particular air contaminant in the air we breathe inside a home is primarily dependent upon how quickly (the rate) the material is released in the home and how fast it leaves via dilution, exhaust or air cleaning. How fast it is being released in a home depends on what the source is. The contaminant could be something like moisture from showering or a damp basement floor or wall; odors from burning something in the oven, candles or tobacco; or even solvents slowly evaporating from furniture, building components or finishes. The rate at which any of these materials or stale air is diluted or removed from the home is determined by local exhaust fans, general dilution with outdoor air via air leakage or an open window, or some type of air-cleaning device. Building Tightness and Ventilation Older homes often have three to five times more ventilation than they need for good health, and this leads to high heating or cooling energy costs and excess winter dryness. For a specific home, the lower the general dilution rate from building leakage (the natural ventilation rate) the higher you might expect contaminant concentrations to be if the contaminant source is located within the home or in the earth under the home. However, it is important to note that large-scale measurement of contaminant levels in homes over the past 20 years has not turned up a statistical connection of pollutant exposures to building tightness or natural leakage. This observation confirms that, in most cases, how fast a material is being released inside the home is at least as important as the ventilation rate from air leakage or fan-powered ventilation. If a specific home were designed to leak just the right amount of ventilation air at temperatures below zero, the leakage would not provide enough fresh air when the temperatures were at or above freezing. In other words, the rate of building leakage is strongly inversely proportional to temperature difference: The colder it is, the more air leakage you get through a given hole. In heating climates, too much air leakage means greater heating bills and very dry indoor conditions. In a cooling climate, too much air leakage means greater cooling bills and possible humidity problems. These are the reasons that "build tight and ventilate right" is the way most homes are now constructed and have been for much of the past 25 years. Air-tight, well-insulated homes with proper foundation moisture-management features for the climate zone they are built in, as a class, are no more likely to have an air-quality problem than old poorly insulated homes with high, uncontrolled air-leakage rates or poor moisture management. This is why it is recommended today to construct energy-efficient homes (both well-insulated and airtight), and to provide the correct amount of planned ventilation air with mechanical means (fans), instead of relying on uncontrolled natural ventilation (building air leakage). For some mild climates, an alternative to the approach of using fans to move ventilation air is to have multiple manually controlled air-leakage sites that are opened or closed as the weather and the occupants' needs change. Additionally, in some areas of the country, regardless of home tightness, special exhaust systems are needed for dealing with pollutants under homes, such as radon from soil gases. Radon-removal systems use a small long-life fan installed to slightly depressurize the soil under the home to capture radon gas from the soil and move it outdoors before it enters a home or crawlspace. These same long-life, low-power-consumption radon fans are also used to provide quiet, reliable, low-level, whole-house ventilation. Even with adequate ventilation, it is important to recognize that keeping unusually strong pollutant sources, such as strong cleaning compounds or liquid fuels, out of a home is a vital part of good home management. Any hazardous liquid material that evaporates from a container should be stored outside of the home or in a fire-safe space where the air is continuously exhausted. Ventilation ducts that move air to a home should not be located in attached garages. Garages should be isolated from the home as much as possible. HOW MUCH VENTILATION IS ENOUGH?
Fans, Operable Windows or Both There are two basic approaches to ventilation: Exhaust ventilation for localized pollutant sources, and general ventilation for the whole home. Local spot exhaust ventilation: In most home ventilation-system designs today, wall-switch-controlled mechanical ventilation (small exhaust fans) is used to remove locally generated contaminants at their source. Typical sources include moisture from showers and tubs, and odors from bathrooms, kitchens or hobbies. These exhaust systems are designed to provide air-movement rates that meet building codes and are capable of providing reliable intermittent exhaust flow that moves pollutants out of the home and at the same time pulls fresh, outdoor air in for the occupants. Some of these can be left on for long time periods to provide general ventilation if they are specified to be long-life and quiet. There is a variety of bath and kitchen exhaust fans available, and not all are designed for continuous low-level use. General ventilation systems: When the windows and doors of a home are closed, the entry points for the outdoor air moved by exhaust fans can either be planned openings or uncontrolled air-leakage sites. There are varieties of general ventilation systems available to move ventilation air. They can be as simple as installing a long-life quiet fan attached to multiple bathrooms and a kitchen that runs nearly continuously; or they can be more complicated, incorporating an energy-recovery or dehumidifying device. Fresh-air supply can also be incorporated into a central furnace or air-conditioning unit. In a few climates in the United States, the outdoor air is both comfortable and clean. In these areas, open windows may reduce the need for mechanical fans and air filters to move air in or out of a home. Predominant Heating or Predominant Cooling Climates In climates where the home is heated or air-conditioned for long periods of time, a general ventilation system that provides both supply and exhaust air and which also recovers energy likely makes sense. These energy-recovery devices also incorporate particle air filters that will remove most pollen and mold spores. These devices can also be used to move contaminated air out of rooms that have sources of contaminants and to provide clean, filtered and preconditioned outdoor air to rooms where people spend a lot of their time. In some coastal climates or during certain swing seasons, where excess outdoor humidity is a concern, a dehumidifier that also brings in and dehumidifies outdoor air may be a very wise choice. This type of device may be of particular use in basements or in older homes, where cool, damp basements are prone to mold growth during certain times of the year. These devices can also incorporate particle air filters for removing most pollen and mold spores. These dehumidifying air cleaners are very efficient and economical to operate and can remove up to 100 pints a day of excess humidity from the air in a home. Ventilation Guidelines How much air: Most building codes now reference the American Society of Heating, Refrigeration and Air-Conditioning Engineers standards for ventilation in residential facilities. These standards specify the option of intermittent ventilation in multiple areas or continuous low-level ventilation for the whole home. Ventilation systems that meet this standard consist of either a central fan or fans, or multiple fans that are used to create pressure differences to move air out of a specific room and out of the home. Ventilation systems can also include supply fans to move air into the home directly or into a furnace or AC unit. Controls: Ventilation fans have some type of electrical control. The control can be as simple as a fan tied into a bathroom light switch, a bathroom or basement humidistat, or more sophisticated instruments, such as a programmable intermittent controller. Air entry/exits: The ventilation system may have planned air intakes, air exhausts or both. If it has both, they would normally be kept a specified distance apart. Particulate air filters: Often a coarse, disposable fiberglass filter, or reusable metal or plastic filter (that is easy to see through) is incorporated into ventilation systems to keep insects out and the fan or cooling coil from clogging. These course filters will not clean the air, and they should be replaced with at least a moderate-efficiency filter that is effective for removing pollen and mold spores. In general, an extended-surface pleated filter will provide superior air filtration and a long life. A good rule of thumb is: The harder it is to see through the filter, the deeper the pleats and the more there are, the better the filter will work and the longer it will last. HEPA (high-efficiency particulate air) or near-HEPA filters are available from several manufactures for retrofit into a heating, cooling, dehumidifying or ventilation system. Always follow the manufacturer's installation guidelines. Most central-type electronic air cleaners work well, but they must be cleaned frequently to keep them working as designed. Air cleaners that release materials into the air of the home to remove pollutants during their operation are not likely a wise investment. If it sounds too good, it probably is. Check for consumer or government guidelines on use of certain devices. Viable and non-viable organisms (mold and other materials of organic origin) are often found in the indoor air. These include dust mites, viruses, bacteria, molds, mildews, fungi, and animal dander and fur. Many people have severe allergic reactions to these organisms. Exposures to some organisms are associated with fatal diseases (e.g., legionnaire's disease, Hantavirus, drug-resistant tuberculosis). As noted above, good particle air filters will capture much of the material after it is released in a home. Keeping the relative humidity in the house between 30 and 50 percent will minimize the growth of many of the living organisms. However, homes in heating climates should seldom be humidified above 30 percent in the winter, or excess mold growth may occur in or on building components. If humidity is added, it must be added with clean steam or a vigorously maintained device. Air-conditioned homes should be kept at 60 percent relative humidity or lower during the cooling season, or excess mold growth is likely to occur on any cold surface in the home. It is recommended that tightly constructed houses have outside combustion air supplied directly to all combustion devices in the building and that a qualified technician do a backdrafting check to make sure that any naturally vented combustion appliances do not release fumes into the home when exhaust fans are operated. This is especially true for large downdraft kitchen exhausts that are part of a cooking surface (such as the Jenn-Air range/grill), as they move large quantities of air to attempt to stop hot gases and odors from rising. Using or Disturbing Hazardous Materials in a Building VOC aerosols and liquids: When using odiferous or hazardous aerosols, or materials that are applied in liquid form, it is imperative that you follow the manufacturer's directions. "Well-ventilated area" may mean an open window on one end and a 20-inch window fan sealed into a window on the other to suck out fumes. This level of ventilation is often needed during home remodeling, even if it means leaving the thermostat at 72° with an open window. If it can be arranged, the safest place to use such a product is probably outdoors. Less noxious or hazardous materials that accomplish the same task are increasingly available. Mold, lead paint, asbestos: If potentially hazardous or irritating materials - such as mold, lead paint, asbestos or fibrous insulation - are to be disturbed inside a home, it should only be done following recommended procedures of containment and isolation to avoid exposure to the occupants and spreading the materials to other locations. How Can You Tell if There Is a Ventilation Problem? With some types of contaminants, it is very easy to identify a problem. Anything that has a noticeable odor when you enter a home can be obvious - for example, sewer gas from a leaky fitting, propane or natural gas from a leaking gas fixture or pilot light, or mold smells from microbial growth sites located indoors. If formaldehyde or other volatile organic compounds are present in high enough concentrations to be smelled, it may be strong enough to pose problems to sensitive people. Moisture in a heating climate can sometimes be seen condensing on windows, and if this situation lasts for any length of time, mold will likely show up on cold Sheetrock surfaces behind furniture and in closets. Fungi or mold in closets or other low-ventilation places indicate too much moisture, or too little wall insulation or air movement. An inexpensive hygrometer can indicate the relative humidity inside the home or crawlspace. Digital units often cost less than $40 and are typically very accurate from 20 to 80 percent relative humidity. The likely presence of other pollutants can be guessed by the presence of sources. A frequently operated unvented heater is likely giving off combustion byproducts at potentially unhealthy levels unless it is located in a well-ventilated area. An entire room of new particleboard shelving is likely outgassing some formaldehyde. Unvented gas ranges and ovens inappropriately used for space heating, cigarette smoking or burning large, sooty candles are obvious culprits. If you are concerned about pollutant levels in your home, there are passive monitoring devices available for radon, mold, nitrogen dioxide, carbon monoxide and formaldehyde. Active monitors are readily available for carbon monoxide and combustible gases. William A. Turner, MS, PE is president of Turner Building Science LLC, a subsidiary of the H.L. Turner Group Inc. in Concord, N.H. He has more than 25 years of experience in IAQ/HVAC evaluation and solutions. You can reach him at 207-583-4571 ext. 11, or by email at firstname.lastname@example.org.