What is building infiltration?
Building infiltration is the uncontrolled entry of outdoor air through cracks, joints, penetrations, gaps and other leakage paths in the building envelope.
Why infiltration matters during wildfire smoke
Outdoor smoke can enter even when doors and windows are closed. Indoor PM2.5 therefore depends not only on outdoor concentration but also on envelope leakage, wind, pressure differences, HVAC operation, deposition and filtration.
Infiltration is not the same as ventilation
- Infiltration: uncontrolled outdoor-air entry through leakage paths.
- Ventilation: intentional outdoor-air exchange through natural or mechanical systems.
- Recirculation: indoor air moved through HVAC or a portable cleaner without necessarily adding outdoor air.
Drivers
- Wind pressure on the building envelope.
- Stack effect from indoor–outdoor temperature differences.
- Exhaust fans and mechanical pressure imbalance.
- Envelope leakage and open doors/windows.
Indoor smoke model
A simple single-zone mass-balance model treats infiltration as an outdoor-to-indoor particle input and CADR as one of the removal terms. Real buildings may have multiple zones and time-varying leakage, so the model should be treated as a planning abstraction.
Related concepts and tools
PM2.5 · CADR · ACH · Indoor Wildfire Smoke Calculator · Smoke & Indoor Air Quality
Tecnobosque Reference Concept · reviewed September 2026. Infiltration rates are building- and weather-dependent and should not be treated as fixed universal values.