Editor’s note: This article is the fourth in a series of occasional stories on green building by Carl Seville.
All buildings need local, or spot, ventilation to remove pollutants, including moisture from bathrooms and cooking odors from kitchens. But they also need whole-house or individual-unit ventilation to dilute pollutants that can’t easily be removed and keep the air fresh for occupants.
Yet, although municipal codes require local ventilation, they don’t typically require whole-house ventilation. Most green building certification programs do, however, and such ventilation is highly recommended to maintain comfort and high indoor air-quality standards.
The most common ventilation standards, developed by ASHRAE, include 62.2 – Ventilation and Acceptable Indoor Air Quality Guidelines in Low-Rise Residential Buildings and 62.1 – Ventilation for Acceptable Indoor Air Quality for commercial buildings, including residential common areas. A new version of 62.2 was released in 2013, although most green building programs reference the 2010 or 2007 versions. There is much dispute among industry experts over ventilation system types and ventilation rates, but these standards are the best the industry has to work with for the time being.
Balanced Ventilation Is Best
As buildings have become better insulated and air sealed, the need for ventilation increases to maintain good indoor air quality. Old, leaky buildings could get away with less ventilation, but those leaks also allow heat, moisture, and pollutants to move in and out uncontrolled, leading to higher energy use and inconsistent indoor air quality. Whole-house ventilation, in contrast, can be provided with supply-only, exhaust-only, or balanced systems.
Supply-only ventilation is most commonly provided with an air intake connected to the HVAC return plenum, distributing outside air to the house through the duct system. Green building programs require a mechanical damper on the intake duct and timed controls that manage the total amount of outside air introduced, ensuring that it meets the minimum rates noted in ASHRAE62.2.
Exhaust-only ventilation commonly uses a continuously running exhaust fan, often in a bathroom, relying on infiltration to make up for the air being removed. In humid and mixed climates (most of the eastern half of the United States), exhaust-only ventilation can draw moisture into wall cavities, where it can condense and cause moisture problems, including mold and wood rot. Although exhaust-only ventilation isn’t prohibited in humid climates, it’s not generally considered a good long-term strategy, except in dry climates.
Balanced ventilation is most commonly provided with an energy recovery ventilator (ERV) or a heat recovery ventilator (HRV). These ventilators bring in outside air, transferring heat to or from the outgoing indoor air to conserve energy, while avoiding bringing in excessive heat, cold, or humidity. The heat-exchanging properties of ERVs and HRVs make them more efficient than supply or exhaust ventilation.
Balanced ventilation can be provided with a single-point ERV or HRV, or the air can be ducted throughout a house or apartment. With supply-only and balanced ventilation, outside air intakes must be located away from possible sources of pollutants, such as vehicle-idling areas, and clothes dryer or bathroom exhausts. Intakes are normally placed in an exterior or open corridor wall.
Resident Comfort Matters Too
Ventilation systems should be designed to meet or exceed the ventilation rate specified in ASHRAE 62.2, but occupants or management should be able to adjust the rate as necessary, based on comfort and air quality, within individual units. Ventilation systems can be designed to run continuously or for a specified number of minutes per hour, with manual overrides to increase or decrease the rate to maintain indoor air quality. Some residents may find they need a higher ventilation rate because of their cooking or bathing habits, while others may be comfortable with a lower rate, saving energy in the process.
Installing a well-designed and -functioning whole-house ventilation system, and instructing tenants on how to manage them, helps ensure that their indoor air is healthful and comfortable.
Carl Seville is a green builder, educator, and consultant on sustainability to the building industry. He is a HERS rater and holds the LEED AP Homes and Green Rater designations. During his 25 years as a remodeling contractor, Seville led the development of the EarthCraft Renovation green building certification program and supervised its pilot projects. He certifies single and multifamily buildings under all green programs. Visit his website at www.sevilleconsulting.com.