Old and drafty houses are not good enough
Over the years, ventilation in houses and apartment buildings was thought to good enough when fresh air slipped in through windows and random cracks and holes unintentionally left in the building envelope during construction. But that meant ventilation design was essentially random. Air movement wasn't planned or controlled; builders often said, "buildings need to breathe" and let it go at that.
Then, health officials began to focus on home ventilation as way to help prevent the spread of airborne disease. By the late 1800s, proper ventilation in the U.S. was defined as 30 CFM/person, and by 1925 this ventilation rate was set by law in 22 states, according to researchers at Lawrence Berkeley National Laboratory.
However, the random ventilation strategy means a home may be under-ventilated or over-ventilated depending on things the weather, which resulted in potential unhealthy indoor air quality, wasted energy, drafty buildings, and moisture in wall assemblies.
For many years, windows that opened were considered an acceptable ventilation approach. But in 2005, the California Energy Commission researched indoor air quality and ventilation in a sample of homes built in 2002 or later. The study found that Californians did not open windows often enough to provide adequate ventilation to keep indoor pollutants at acceptable levels.
So mechanical ventilation is now required in California to improve indoor air quality in all new residential construction, when alterations are made to a residence, and in residential additions of 1,000 square feet or greater.
Now building codes, including ASHRAE 62.2, require greater levels of ventilation based on the size of the home and expected number of occupants, based on the number of bedrooms.
The 2013 version of the ASHRAE 62.2 standard takes into account the fact that many homes built today have a much tighter building envelope. Builders can't count on random airflow to provide ventilation. Instead, builders must measure the actual air tightness of the home.
That means ventilation is more important than ever. Homes built to tighter standards, such as Energy Star, EarthCraft, LEED, or Passive House, won't leak enough air to perform well. Although tighter homes save energy and reduce the impact on the environment, people still need air to breathe, and ventilation can remove or dilute indoor pollutants for better indoor air quality.
If you're building a new home or doing a deep green remodel, work with a qualified HVAC contractor or designer to ensure your home has adequate ventilation under the current codes. The old rules of thumbs just don't apply any more.