Airtight homes are like a double-edged sword. Although energy-efficient, they tend to have poor indoor air quality, because polluted air doesn’t escape and fresh air isn’t introduced. In fact, according to the U.S. Environmental Protection Agency (EPA), indoor air quality is 4-5 times worse than outdoor air quality.
Indoor air has many sources of pollution: off-gassing of building materials such as cabinets, carpeting, and adhesives; everyday living including cooking and bathing; and remodeling projects such as sanding, painting, and furniture refinishing. Outdoor pollutants near the home and pollution moving through the HVAC system can cause buildups of bacteria, mold spores, pollen, and dust indoors. In some climates, inside and outside humidity levels may cause mildew or excessive condensation problems.
Best Practices®
Probably the most effective type of ventilation for a home is a mechanical, ducted, whole-house system that provides a steady supply of fresh, filtered, conditioned air from the outside while exhausting stale interior air. Because the same amount of air is drawn into the home as is exhausted, the pressure within the home remains balanced, which helps prevent backdrafting of combustion appliances. Mechanical ventilation systems can automatically maintain 0.35 air changes per hour, the rate usually recommended to maintain good air quality. Many sizes and configurations of mechanical ventilation are available; however, the most robust systems include filtering, pre-conditioning, and booster fans.
Filtering. Fresh air passes through filters as it enters the system. This helps control contaminants that would otherwise re-circulate through the home. In some cases, homeowners need to clean or replace filters periodically.
Pre-conditioning. A heat recovery ventilator (HRV) uses heat exchangers to recapture 60-80% of the conditioned temperatures from the outgoing air to heat or cool incoming fresh air. Incoming and outgoing airflows pass through different sides of the unit, but are not mixed, allowing conditioned exhaust air to raise or lower the temperature of incoming fresh air without contaminating it. In climates with extreme differences between indoor and outdoor humidity, energy recovery ventilators (ERVs) are recommended over HRVs because ERVs also exchange moisture between the two air streams to control humidity. In hot, humid climates, ERVs are critical for drying out incoming fresh air; in cold, heating-dominated climates, ERVs can help hold in what little humidity may be indoors to control wintertime window condensation.
Booster fans. Small, separately switched booster fans can be located in bathrooms and kitchens to control moisture or heat generated by showering and cooking. Odors and pollutants are quickly removed, while the energy used to condition the air is recovered through the HRV/ERV. Some codes may still require stoves to be separately vented to remove grease or gas fumes.
Potential drawbacks
The biggest drawback to installing mechanical ventilation is the higher upfront cost to homeowners. An HRV-equipped system costs about $2,000 installed, with variables such as home size and the design of existing or planned HVAC systems being major factors. ERV-equipped systems cost more. Homeowners must also understand the additional mechanical complexities of the systems, and be responsible for maintenance, particularly in systems with filters.
More Best Practices® for heating, ventilation and air conditioning can be found in BuildIQ University’s online training course, Mechanicals: HVAC.
