Airtightness
Air permeability q50 and air changes n50 from a blower-door test
- where
- air permeability at 50 Pa (m³/(h·m²))
- air leakage rate at 50 Pa (m³/h)
- envelope area (m²)
- air changes per hour at 50 Pa
- internal volume (m³)
Explain this equation
Turns the leakage rate from a blower-door test into the two common airtightness figures: air permeability q50 and air changes n50.
The fan pressurises the house to 50 pascals and measures how much air it has to push in to hold that pressure; that equals the air leaking out, V₅₀. Dividing by the area of the envelope (walls, roof and ground floor) gives q50, the figure used in UK Building Regulations. Dividing by the internal volume gives n50, the figure used by Passivhaus and many European schemes.
Both are the same leakage, normalised by area or by volume.
Sense check: 1200 m³/h through 250 m² of envelope is 4.8 m³/(h·m²); for a 300 m³ house that is 4 air changes an hour at 50 Pa. Older homes are often 10 to 15; good new builds 3 to 5.
Watch out: if retrofit makes a home much tighter, make sure ventilation is adequate. At or below 3 natural ventilation alone is not suitable.
Turns the leakage flow measured by a fan pressurisation test at 50 Pa into the two figures used in the UK: air permeability per m² of envelope (q50) and air changes per hour (n50).
Enter your values to see the result.
Good to know
Testers report the 50 Pa flow as the average of the pressurisation and depressurisation results. Envelope area AE is the total internal area of every surface separating heated space from outside or unheated space, including the ground floor. Volume V is the internal heated volume. Building Regulations and SAP use q50; Passivhaus and many European schemes quote n50.
Common mistake
Leaving the ground floor out of the envelope area, or using external dimensions for volume. Both make the dwelling look leakier or tighter than it is.
Limits
Arithmetic only. The test itself must follow ATTMA TSL1 / BS EN ISO 9972 (building preparation, wind, temperature and flow-pressure regression).
Sources
- ATTMA Technical Standard L1: Measuring the air permeability of building envelopes (dwellings) — air permeability q50 = leakage at 50 Pa ÷ envelope area, m³/(h·m²)
- 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)
- HM Government, Approved Document F Volume 1: Dwellings, 2021 edition (para 1.24 and Table 1.3 whole dwelling ventilation rates; Table 1.7 background ventilators; para 1.64 continuous extract; paras 1.45–1.46 airtightness; Section 3 existing dwellings)
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.