Airtightness
Convert air permeability q50 to air changes n50
- where
- air changes per hour at 50 Pa
- air permeability at 50 Pa (m³/(h·m²))
- envelope area (m²)
- internal volume (m³)
Explain this equation
Converts an air permeability result (q50) to air changes per hour at 50 Pa (n50), for comparing with schemes that use n50.
q50 is leakage per square metre of envelope; multiplying by the envelope area gives the total leakage in m³/h. Dividing by the internal volume expresses that as how many times the house's air is swapped each hour at 50 Pa.
Same leakage, different normalisation.
Sense check: q50 of 5 with 250 m² of envelope and 300 m³ volume is about 4.2 air changes. Small homes have more envelope per volume, so their n50 is higher for the same q50.
Watch out: use the same envelope area and volume definitions as the test report.
Converts a UK air permeability result into air changes per hour at 50 Pa, for comparison with Passivhaus or EnerPHit targets.
Enter your values to see the result.
Good to know
Both figures come from the same leakage flow V50 = q50 × AE = n50 × V, so the conversion only needs the envelope area and volume. For a typical two-storey house AE/V is about 0.8–0.9, so n50 is a little below q50.
Common mistake
Treating q50 and n50 as interchangeable. They are equal only when envelope area equals volume numerically.
Limits
Use the same AE and V the tester used. Flats and bungalows have very different AE/V ratios.
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.