Why can a large building have a lower n50?
A large building can have a lower n50 even when its leakage per square metre of envelope is no lower. The reason is geometric: n50 divides the airflow at 50 Pa by the test volume, while q50 divides that same airflow by envelope area. A building with less envelope area per cubic metre therefore gets a lower air-change rate for the same area-normalized leakage. The number describes the chosen denominator as well as the envelope’s leakage. [1] [2]
- At 50 Pa, n50 = q50 × (A/V), where q50 is airflow per envelope area, A is envelope area and V is internal test volume. [1] [2]
- For the same q50 of 0.6 m³/(h·m²), A/V values of 1.2 m⁻¹ and 0.3 m⁻¹ give n50 values of 0.72 h⁻¹ and 0.18 h⁻¹. [1]
- A lower n50 can reflect a lower envelope-area-to-volume ratio; it does not by itself show lower leakage per square metre. [1] [2]
- Compare results only after checking the test pressure, the tested boundary, and how envelope area and internal volume were defined. [1] [2]
How area-based leakage becomes n50
At 50 Pa, let q50 be the measured air leakage flow divided by envelope area A, and let V be the internal test volume. Total airflow is q50 × A; dividing by V gives n50 = q50 × (A/V). The units confirm the relationship: m³/(h·m²) × m² ÷ m³ = h⁻¹. This equation converts between two ways of expressing the same airflow. Neither value alone describes every aspect of envelope quality. [1] [2]
A worked comparison with the same leakage per area
Consider two explicitly hypothetical buildings, both tested at 50 Pa and both with q50 = 0.6 m³/(h·m²). Building A has envelope area A = 600 m² and test volume V = 500 m³, so A/V = 1.2 m⁻¹. Its implied flow is 0.6 × 600 = 360 m³/h and n50 = 360 ÷ 500 = 0.72 h⁻¹. Building B has A = 6,000 m² and V = 20,000 m³, so A/V = 0.3 m⁻¹. Its implied flow is 0.6 × 6,000 = 3,600 m³/h, but n50 = 3,600 ÷ 20,000 = 0.18 h⁻¹. Building B has ten times the total airflow at 50 Pa, yet one quarter of Building A’s n50, because its volume is forty times larger while its envelope area is ten times larger. These are arithmetic examples, not measured building results. [1]
What to compare in a test report
A fair comparison needs the same pressure reference and clearly identified test boundary, envelope area and internal volume. The ASIEPI report explains that results from separate zones can include leakage to adjoining heated or cooled parts, and that testing only selected zones can miss leakier areas. A building-wide rating therefore depends on which spaces were tested and how their results were combined. The report is a 2009 technical paper discussing practices and proposals at that time; its examples should not be read as current regulatory requirements. [1]
Use the metric that answers the question
Use n50 when the question concerns air changes per hour for the defined test volume; use q50 to compare leakage flow per envelope area. Record both when interpreting a large-building result, along with the pressure, test boundary, area and volume definitions. If a report gives only n50, request the 50 Pa airflow, envelope area and test volume before concluding that the larger building has a tighter envelope per square metre. [1] [2]