A quantitative measure of how easily air passes through a building envelope or component under a specified pressure difference, usually reported as volumetric flow per unit area at a reference pressure (e.g., m³·h⁻¹·m⁻² at 50 Pa) or as a whole‑building normalized leakage (m³·h⁻¹·m⁻² or n50). It characterizes uncontrolled leakage paths, not designed ventilation.

Principle

Air flow through cracks and porous assemblies at low pressure differences follows pressure‑driven flow relationships (often approximated by power laws); higher measured permeability implies larger uncontrolled infiltration/exfiltration under driving forces (wind, stack effect, mechanical systems), increasing uncontrolled heat and moisture transport.

Demonstration

Illustrative procedure — Situation: A blower‑door test pressurizes a dwelling to 50 Pa and measures volume flow Q_50. Recognition: The technician normalizes leakage by envelope area to report q_50 = Q_50 / A_env (m³·h⁻¹·m⁻² @ 50 Pa). Action: Designers use q_50 to estimate typical infiltration at operating pressures via established correlations and to size remediation (air‑sealing) efforts. Consequence: Lower permeability reduces uncontrolled energy loss but may require provision of controlled ventilation.

Misapplication

Interpreting a single-pressure air permeability result as the actual in‑use infiltration rate without accounting for differing driving pressures, wind exposure, stack effect, or the distribution of leaks; the error is treating a laboratory-like test point as equal to dynamic, site‑specific conditions.

Consequence

Underestimating permeability leads to unexpected energy losses, moisture transport and discomfort; overestimating it can lead to unnecessary sealing cost or overdosing mechanical ventilation if designers compensate incorrectly.

Reversal

At very low driving pressures, leakiness may be dominated by micro‑porosity with different flow exponents; at high pressures or with localized large openings behaviour departs from simple power laws. Also, a very low measured test leakage does not eliminate localized transient flows through intended openings (vents, chimneys) that affect occupants.

Boundary

Clearly within: pressure‑normalized volumetric leakage through an envelope or element measured under defined test conditions (e.g., 50 Pa). Boundary case: air permeability of a component (window) specified per metre of joint — aggregation to whole building depends on installation. Clearly outside: designed ventilation system duct leakage or purposefully operable vents — these are not uncontrolled envelope permeability.

Semantic Tension

Air permeability ↔ Ventilation strategy: improving airtightness (lower permeability) reduces uncontrolled losses but increases dependence on controlled ventilation for IAQ; tension exists between energy efficiency and the need for intentional, balanced ventilation.

Synthesis

Air permeability is the measurable indicator of uncontrolled leakage propensity and a primary control on infiltration-driven energy and moisture flows; effective building performance requires integrating airtightness targets with ventilation design and accounting for variable driving forces and leak distribution.