Infiltration rate measures the volume of uncontrolled outside air entering a building through unintended openings like cracks, gaps, and leaks in the building envelope. Expressed in cubic feet per minute (CFM) or air changes per hour (ACH), this metric affects both indoor air quality and energy efficiency. Reducing infiltration through improved sealing and construction quality minimizes heating and cooling losses while improving ventilation predictability.
Measurement and Standards
Blower door testing quantifies infiltration by pressurizing buildings to 50 Pascals and measuring required airflow to maintain that pressure (CFM50 rating). Modern commercial construction aims for infiltration rates below 3 ACH50, while older buildings may exceed 10 ACH50. Residential standards typically target 7 ACH50 or better. The ASHRAE 90.1 standard requires envelope commissioning to verify performance meets design specifications.
Impact on System Design
High infiltration rates complicate HVAC design by introducing unpredictable outdoor air quantities, reducing heating/cooling efficiency, and potentially compromising humidity control. Uncontrolled infiltration can account for 20 to 40 percent of annual energy losses in poorly sealed buildings. Properly designed mechanical ventilation systems can better control air quality and energy use compared to reliance on infiltration. Building commissioning and air sealing during construction prevent costly operational issues.