What is best described as leakage out of a material or structure?

Exfiltration is leakage out of a material or structure. In building science, the term usually describes indoor air escaping through cracks, joints, gaps, or other unintended openings in a building envelope.

Exfiltration is the outward counterpart to infiltration, which is outside air entering a building. Pressure differences drive both processes. Wind, the stack effect caused by temperature differences, and mechanical ventilation equipment can make air move through openings in either direction.

The distinction matters because escaping indoor air can carry heat and moisture into walls, roofs, or other parts of the structure. In cold conditions, that moisture may condense within the building envelope, contributing to damp insulation, mold, corrosion, or frost-related damage. Exfiltration can also waste energy by allowing conditioned air to escape.

The word has other technical meanings. In cybersecurity, exfiltration usually means the unauthorized removal of data from a computer system, while in military usage it can mean the extraction of personnel from a dangerous area. In the context of material or structural leakage, however, the building-science meaning is the intended one.