What is infiltration in the context of housing?

Infiltration refers specifically to the unintentional movement of outdoor air into a building, primarily through cracks, gaps, and openings in the building envelope. This can occur around windows, doors, and other penetrations in the structure, allowing outside air to flow into the conditioned spaces of a home without being properly managed or treated.

Understanding infiltration is critical for evaluating and improving building performance because it can lead to higher energy costs, as heating or cooling systems must work harder to maintain desired indoor temperatures due to the unwanted exchange of air. Effective air sealing and insulation are key strategies used to minimize infiltration, enhance energy efficiency, and improve indoor air quality by controlling the overall airflow within the home.

The other options describe different phenomena: air flowing out describes exfiltration, controlled airflow involves mechanical ventilation systems, and heating of air within walls does not directly pertain to the concept of infiltration. Thus, the focus on unintentional outdoor air entering the home accurately captures the essence of infiltration in building science.