My comprehensive article on residential ventilation systems, “Designing a Good Ventilation System,” was published back in 2009. A few things have changed in the last eight years, so it’s time to revisit the topic.
Most building scientists aren’t willing to provide a simple answer to the question, “At what point is a home so tight that the home requires a mechanical ventilation system?” A typical answer is, “It depends — but unless your house is very leaky, it’s better to err on the side of caution and install a mechanical ventilation system.”
Building codes aren’t so vague, however. According to the 2012 and 2015 versions of the International Residential Code (IRC), any new home with a blower-door test result of less than 5.0 ach50 is required to have a whole-house ventilation system. This code requirement can be found in Chapter 3, section R303.4, and in Chapter 15, section M1507.1 of the IRC.
Since the new IRC code requires homes in all zones except Zones 1 and 2 to achieve an airtightness result of no more than 3 ach50, the code effectively mandates a whole-house mechanical ventilation system for homes in Zones 3 through 8. If you live in Zones 1 or 2, and if your blower door test came in at less than 5.0 ach50, your home is also required to have a whole house ventilation system.
The bottom line: If you’re getting your advice from GBA, you’ll be building a tight house — so your house needs a mechanical ventilation system.
The building code is vague concerning the details of a mechanical ventilation system; it doesn’t really tell builders what type of equipment is needed to comply with the code. The code is specific, however, about ventilation rates. Here are the minimum airflow rate requirements according to the 2015 IRC, section M1507.1:
These code-mandated ventilation rates are…