High indoor humidity can spur mold growth. High humidity may result from poor construction/rehabilitation, site design that does not properly manage water, and/or inadequate air exchange. A reasonable target for relative humidity is 30-60 percent. A low cost hygrometer, available at hardware stores, can be used to measure relative humidity. In cool climates, inadequate ventilation in the winter can contribute to excessive moisture and humidity because normal activities create moisture (cooking, bathing, breathing), and there is insufficient natural ventilation (opening windows) or mechanical ventilation (fans, exhaust systems) to remove the moisture. In warmer climates, the heating, ventilation, and air conditioning (HVAC) system can pull warmer, humid air inside. In this case, the ventilation system may help create indoor humidity problems unless the system also dehumidifies the air.

Common sources of airborne contaminants include:

• Indoor contaminants. These include chemicals used in the construction or renovation of buildings (e.g., glues, off-gassing from carpets, emissions from particle board, cleaning compounds). In addition, appliances that burn gas can produce particulates and carbon monoxide. Incomplete combustion and poor ventilation of these appliances (cook stoves, gas furnaces, gas boilers, and gas water heaters) can contribute to indoor contaminants. Gas cook tops should be used with fans that send exhaust outside. Gas-fired heating appliances should be sealed and power-vented systems installed to remove products of incomplete combustion. Wood-burning stoves can also create particulates and must be vented outside.

• Outdoor contaminants. Outdoor particulates can be drawn inside when the heating or cooling system draws air into a home. Particulates and allergens found in outdoor air can be asthma triggers. Filtering incoming air for HVAC systems effectively filters particulates. Experts recommend using filters with a MERV 6-8, but higher MERV levels trap smaller particles and generally are more appropriate for those with allergies or where the indoor environment has a high concentration of mold spores, dust particles, or other allergens.

Two types of ventilation can help control harmful air contaminants and humidity: spot ventilation and dilution ventilation. Spot ventilation draws air from a particular location (e.g., bathroom, kitchen) and exhausts it to the outside. Dilution ventilation address low-level contamination throughout the home.

Spot Ventilation. Exterior exhaust fans should be installed in all bathrooms and kitchens. These fans remove humidity and carbon monoxide. The most effective fans are quiet and durable. Use fans that operate at one sone or less and exhaust to the outdoors. Fans equipped with timers or de-humidistat controls are useful to ensure the fans run for a sufficient period of time. A good rule of thumb is to run a bathroom fan for about 45 minutes after a shower.

Dilution Ventilation. Dilution ventilation addresses the entire living space. Air changes (exchanging indoor air with outdoor air) and air cleaning help determine the effectiveness of dilution. Air changes result from a combination of natural ventilation (infiltration; leakage; windows) and mechanical (controlled) ventilation. Air cleaning occurs when particulates are filtered and when air is dehumidified to remove moisture. The goal is to provide sufficient changes to ensure a healthy environment. There are several types of heating and cooling systems with filtration that can be installed to accomplish this. A common element necessary in all systems is duct sealing, particularly on the return side (side drawing in the air). The Air Conditioning Contractors Association (ACCA) provides guidance on duct sealing in its Manual D: Duct Design.

Sizing HVAC Systems

It is important not to oversize a system. Oversizing can contribute to poor air distribution and insufficient dehumidification, creating an environment that promotes mold growth. Oversized heating systems can “short cycle,” meaning that the system does not run long enough to turn the fan on for a sufficient period to distribute new air. Systems that short cycle during air conditioning will deliver cold air in short bursts but not necessarily dehumidify the air. The resulting cold, clammy environment can encourage mold growth. Some contractors oversize HVAC systems to compensate for duct leakage and to minimize complaints about heating or cooling delivery. The ACCA provides guidance on system sizing in its Manual J.

HVAC Systems Can Contribute to Air Quality Problems

HVAC systems can also exacerbate indoor air quality problems. The HVAC system may be contaminated (because of mold in duct lining or bacteria on coil or filters, for example), and the system may spread these pollutants throughout the home. Second, the HVAC duct distribution system can spread pollutants from one portion of the home to another. Regular maintenance and duct sealing can help minimize these problems.

Sources and Additional Information:

Air Conditioning Contractors Association (ACCA) - www.acca.org

American Society of Heating and Refrigerating and Air Conditioning Engineers, Inc - www.ashrae.org

Building Science Corporation - www.buildingscience.com/resources/mechanical/default.htm

Home Energy Magazine - http://homeenergy.org

Minnesota State Regulations - Ventilation - http://www.revisor.leg.state.mn.us/arule/7672/1000.html

Washington State Regulations - Ventilation - www.doh.wa.gov/hsqa/fsl/CRS/resources.htm