One of the tenets of green building is “Build tight, ventilate right.” By sealing leaks in the enclosure, we reduce heating and cooling bills, improve comfort, and prevent condensation in building assemblies. By using mechanical ventilation, rather than relying on a loose enclosure and weather-dependent forces like wind and the stack effect, we can provide consistent levels of air exchange. In principle, this approach allows for high levels of energy efficiency while maintaining healthy indoor air.

When I started in green building in the early 2000s, “Build tight, ventilate right” was a best practice embraced by progressive builders. Since then, this concept has made its way into code. In 2015, the IRC—the model residential code on which most state building codes are based—began requiring blower-door testing for new construction and set a maximum allowable air-leakage rate based on home volume. (The code air-leakage targets, 5.0 ACH50 in Climate Zones 1 and 2 and 3.0 ACH50 in Climate Zones 3–8, represent relatively low bars, and high-performance builders are routinely sealing homes to well below 1.5 ACH50.)

Since 2012, the IRC has required that homes tighter than 5.0 ACH50 (which, following the 2015 provisions above, now means all new homes) have whole-house mechanical ventilation. Ventilation can be continuous or intermittent so long as an average airflow target (in cubic feet per minute, or cfm) is met. The airflow target, which is based on the ASHRAE 62.2-2010 standard, is calculated as:

cfm = (0.01 × total square foot area of the house) + [7.5 × (number of bedrooms + 1)]

The targets are also provided in table form (taken here from the 2021 IRC):

The code allows for several types of whole-house ventilation: