##  [Air Permeability](/air-permeability-0) 

 Definition

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

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

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

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

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

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

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

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

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.