UPDATED on June 25, 2018
Most green builders include some type of mechanical ventilation system in every home they build. That’s good. Since green buildings usually have very low levels of air leakage, mechanical ventilation is usually essential.
Unfortunately, several research studies have shown that a high number of mechanical ventilation systems are poorly designed or installed. Among the common problems:
- Ventilation fans with low airflow because of ducts that are undersized, crimped, convoluted, or excessively long.
- Ventilation systems that ventilate at too high a rate, or for too many hours per day, resulting in a severe energy penalty.
- Ventilation systems that waste energy because they depend on inappropriate fans (for example, 800-watt furnace blowers).
It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned.
If you’re unfamiliar with residential ventilation systems, it’s a good idea to review the ventilation information in the GreenBuildingAdvisor encyclopedia.
The ASHRAE standard
ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.
The residential ventilation requirements in the 2018 International Residential Code (IRC) differ from the requirements of the ASHRAE 62.2 standard, however. According to the 2018 IRC, the minimum ventilation rate is 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. (For more information on code requirements for ventilation, see “An Update on the Residential Ventilation Debate.”)
Systems complying with ASHRAE 62.2 have ventilation rates that are relatively low; for example, a 2,000-square-foot house with three occupants requires 83 cfm of mechanical ventilation. That’s about as much airflow as is provided by a typical bath exhaust fan. (Of course, systems complying with the minimum requirements of the 2018 IRC have even lower ventilation rates.)
Since ventilation airflows…