What term describes air that leaks into a structure?

The term that describes air leaking into a structure is infiltration. Infiltration is a natural process where outside air enters a building through various openings, such as cracks, gaps, or unnoticed openings in the building envelope. This can occur due to differences in air pressure inside and outside the building, often exacerbated by wind or thermal dynamics.

Understanding infiltration is critical for HVAC professionals, as it directly impacts energy efficiency and indoor air quality. Excess air leakage can lead to increased heating and cooling loads, making it more challenging to maintain comfortable indoor temperatures and leading to higher energy costs. Proper sealing and insulation help minimize infiltration, improving a building's overall energy performance.

The other terms relate to air movement in ways that do not specifically describe this process of outside air entering a space. Ventilation typically refers to the intentional process of bringing in outside air for the purpose of improving air quality or thermal comfort. Exfiltration describes air that escapes from a building to the outside, while recirculation refers to the process of reusing indoor air rather than bringing in fresh air from outside. Understanding these distinctions is crucial for effective HVAC system design and operation.