Ventilation Standards

Indoor Air Quality
Chip Henderson, AIA, CEM, LEED Faculty
July 19, 2012
Ventilation Standards

Standard 62.1 provides a specific formula to calculate the ventilation rate for residential dwelling units and the corridors serving residential units. ASHRAE Standard 62.2 (specifically crafted for residential occupancy in three levels or less) also provides a specific formula to calculate the ventilation rate for residential dwelling units. The EPA, DOE, and LEED for Homes have all embraced Standard 62.2 as the appropriate “ventilation rate” for mid-rise residential projects with 4-5 floors.  Calculating the ventilation flow rates following Standard 62.2 (rather than 62.1) will result in a slightly lower volume of outside air. This is a good thing in hot humid climates.

Regardless of which standard or formula is selected, the words used in the 2009 International Mechanical Code;  “0.35 ACH [air changes per hour] but not less than 15 cfm/person” best describes the ventilation intent. What is less obvious is that there are two components to ventilation; a controlled component plus an uncontrolled component. The controlled component is of course the outdoor air we are adding to the rooms via the mechanical ventilation system. The uncontrolled component is the infiltration of outdoor air created by wind, stack effect and the pressure effects from the operation of the mechanical system(s). The uncontrolled infiltration of outdoor air in most buildings is larger than commonly assumed and typically larger than the desired controlled component. Adding mechanical ventilation to an already ‘leaky’ building would only make an already bad situation worse.Blower Door

It is very difficult for the mechanical engineer to design an effective ventilation system without knowing how much of the “0.35 ACH” goal is already provided via uncontrolled infiltration. As we move forward refining and defining IAQ performance goals, setting and verifying air leakage limits will be imperative to help building owners, occupants, and project teams (especially the mechanical engineer) ensure the installed mechanical ventilation system is functioning as intended.