Exfiltration is the uncontrolled leakage of conditioned indoor air escaping through building envelope penetrations, gaps, and cracks to the exterior. Complementary to infiltration, exfiltration occurs when interior pressurization forces air outward, typically resulting from oversized or improperly balanced HVAC systems. This air loss directly increases heating and cooling costs while reducing system effectiveness.
Causes and Effects
Exfiltration develops when mechanical ventilation introduces more outdoor air than return ductwork removes, creating positive interior pressure. Leakage paths include around electrical outlets, through rim joists, at window and door frames, and via penetrations for pipes and conduits. A building losing 15 percent of supply airflow through exfiltration must run equipment 15 percent longer to maintain indoor conditions, increasing energy consumption and operating costs. Moisture-laden exfiltrated air can condense in wall cavities, causing structural damage.
Prevention and Control
Proper HVAC system balancing ensures exhaust airflow matches or slightly exceeds supply airflow, maintaining slightly negative building pressure. Comprehensive air sealing during construction prevents exfiltration pathways. Duct sealing and pressure testing verify system integrity. Buildings with controlled exfiltration rates below 2 CFM50 per 100 square feet of envelope area maintain better energy performance and indoor environmental quality than poorly sealed structures.