Airtightness
Convert air changes n50 to air permeability q50
- where
- air permeability at 50 Pa (m³/(h·m²))
- air changes per hour at 50 Pa
- internal volume (m³)
- envelope area (m²)
Explain this equation
Converts air changes at 50 Pa (n50) into air permeability (q50), the figure used in UK Building Regulations and SAP.
n50 times the volume gives the leakage in m³/h; dividing by the envelope area gives leakage per square metre of envelope.
The reverse of the q50 to n50 conversion.
Sense check: 3 air changes in a 300 m³ house with 250 m² of envelope is 3.6 m³/(h·m²).
Watch out: Passivhaus 0.6 n50 targets are far tighter than typical UK q50 values; compare like with like.
Converts an air change rate at 50 Pa into the UK air permeability figure used by Building Regulations and SAP.
Enter your values to see the result.
Good to know
Useful when a target is set in n50 (for example EnerPHit 1.0 h⁻¹ or Passivhaus 0.6 h⁻¹) and you need the equivalent q50 for the SAP or PAS 2035 paperwork.
Common mistake
Using external volume. V is the internal heated volume the test is based on.
Limits
Relies on the same AE and V the tester used.
Sources
- BS EN ISO 9972:2015 Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method (air change rate n50 = V50 ÷ internal volume; air permeability q50 = V50 ÷ envelope area)
- ATTMA Technical Standard L1: Measuring the air permeability of building envelopes (dwellings) — air permeability q50 = leakage at 50 Pa ÷ envelope area, m³/(h·m²)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.