A Primer on Ventilation and Indoor Air Quality

Healthy indoor air quality requires some fresh outdoor air. Without this fresh outdoor air, stale indoor air will build up contaminates from cooking smoke, outgassing of chemicals from objects in the home, odors, infiltration from attics and crawlspaces/basements/garages, etc. It is well documented that indoor air is more polluted than outdoor air.

When air leakage allows unintended infiltration through holes, gaps, and cracks some of it is relatively direct from outdoors and thus fairly fresh. An example would be leaks around windows or doors. However, studies have shown that the majority of unintended infiltration comes via undesirable pathways such as attics and (especially) crawlspaces. This ongoing flow of fresh and not-so-fresh air occurs naturally. I call this random air. Industry standards call instead for a systematic, intentional, whole-house approach to providing enough fresh air.

It may seem counter-intuitive to spend resources to air seal a home, then turn around and purposefully bring in outdoor air and/or to exhaust indoor air. To a degree, the goals of energy efficiency and indoor air quality are at odds here. The key is to reach a healthy balance. You need to breathe fresh, not random, air. This is even more important than energy efficiency. To help you find that healthy balance, Carolina Sustainability applies the standard created by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). Under this standard, for example, a 2000 square foot house with three bedrooms would require 23cfm of continuous airflow. This is not enough to create a big energy penalty.

There are three broad approaches to achieve this whole-house ventilation. A supply-only approach calls for introducing fresh air into the house and relying on outward air leakage (exfiltration) to balance the pressure. In our humid climate, this has to be done carefully; it is usually done by bringing the air directly into the return side of the heating/cooling system, so that unit performs dehumidification. An exhaust-only approach uses one or more vent fans to take air out of the house. This is often done by leaving a quiet, high efficient bathroom exhaust fan running at a slow speed. This approach assumes that infiltration will make up for the vacuum created by the vented air. This infiltration is, of course, moist random air.

The third approach is called balanced. In calls for a device that brings air in and also takes an equal amount of air out of the home. One such device is called a Heat Recovery Ventilator (HRV). An ERV device includes a heat exchanger; although the ingoing and outgoing streams of air remain separate, heat is exchanged so that the incoming air comes close to matching the temperature of the outgoing air. If you add a mechanism to dehumidify the incoming air, you have an Energy Recovery Ventilator (ERV). In general, HRVs are more suited to northern climates, and ERVs are more suited to the humid south. As you might expect, HRVs and ERVs are more expensive, and require more power than a simple fan. Relatively inexpensive units designed for single rooms (often installed in bedrooms, so you benefit about eight hours a day) are available, but the efficiencies of some of these small models are less than stellar.

Within these three broad approaches there are numerous strategies, and some combine approaches. For example, a common option is to duct outdoor air into the return side of the heating/cooling system and pair this with a bathroom exhaust fan. To the extent that the airflows are matched, this is a fairly balanced system, and it costs less than an ERV.  There are many factors to consider with such a setup, and a comprehensive home performance review is the way to get the information needed to sort it out.