Q. How Much Ventilation Is Needed?
A. Ventilation requirements are outlined in ASHRAE Residential Ventilation Standard 62.2. Whole house ventilation requirements for a typical house ranges between 45 to 75 cubic feet per minute (cfm); larger houses have a higher rate. The chart to the left is a rough guideline for whole house ventilation requirements. You can also use a simple formula for determining ventilation rate: CFM = [(0.01 x total square footage) + 7.5 x (# of bedrooms + 1)]. For a 3200 square foot home with four bedrooms, that would mean (0.01 x 3200) + (7.5 x 5) = 69.5 cfm. If you can determine the natural air changes per hour (ACH) through air infiltration, then you can reduce the ventilation requirement by that amount. A blower door test is required to assess a home’s natural ACH.
There are several methods you can use to provide the necessary ventilation, such as a supply fan, exhaust fan, or a combination. Simple exhaust or supply fans can be installed and sized to provide the necessary quantity of fresh air. However, these basic fans can result in a significant energy loss and can lead to unwanted humidity levels. HRV’s (heat recovery ventilators) or ERV’s (energy recovery ventilators) are a more sophisticated method combining both exhaust and supply air without a significant energy penalty. These systems recapture the energy of the outgoing air and diffuse it back into the incoming air. In addition, ERV’s can help to reduce the amount of moisture gain associated with incoming fresh air. Moreover, these ventilation systems can be attached to an air filter, adding greater control and quality to the incoming fresh air.