Why dividing n50 by a fixed number is only an estimate
Dividing n50 by about 20 can provide a rough annual infiltration estimate for some low-rise homes, but it does not turn a pressure-test result into a measured natural air-change rate. n50 is the air-change rate under a fan-induced 50 Pa pressure difference. Natural infiltration varies as wind and temperature create changing pressures across the envelope. Sherman’s simplified method makes that difference visible by combining the leakage result with a climate ratio and correction factors for building height, shielding and leakage type. Its output is an estimate for a stated scope, not a fixed conversion that applies to every building. [1] [3]
- n50 is the air-change rate under a fan-induced 50 Pa pressure difference; it is an indicator of envelope leakage, not the natural infiltration rate. [1]
- Sherman’s simplified method uses a climate-specific leakage-infiltration ratio. Across more than 200 US and Canadian weather sites, the ratio ranged from about 15 to 30; most North American values were 17 to 23. [1]
- For a two-storey, exposed house near Washington, DC, Sherman’s correction factors change the ratio from 21 to 15. With n50 of 10 h⁻¹, that gives about 0.67 h⁻¹, compared with 0.5 h⁻¹ from dividing by 20. [1]
- Sherman describes the shortcut as suitable for large ensembles of annual averages or rule-of-thumb estimates, and advises against it for short-term or highly building-specific estimates. [1]
What n50 measures
A fan pressurisation test measures leakage by imposing a pressure difference and recording the airflow needed to maintain it. The resulting n50 expresses air changes per hour at 50 Pa. That is a repeatable indicator of envelope leakage under the test condition; it is not the airflow the building experiences in ordinary weather. Sherman cautions that ACH50 should not be confused with an infiltration rate because the test uses an artificially induced condition. [1]
What a fixed divisor leaves out
The familiar shortcut divides the 50 Pa air-change rate by approximately 20. ASHRAE describes this as a simple method that does not account for the mechanisms driving infiltration. Sherman’s method shows what is compressed into that single divisor: a climate-dependent leakage-infiltration ratio, then adjustments for height, wind shielding and leakage type. The ratio is not identical across climates, and the correction factors vary with the building and its exposure. [1] [2]
How climate and exposure change one example
Sherman calculated a leakage-infiltration ratio, N, for more than 200 weather sites in the United States and Canada. The reported values range from about 15 to 30, while most of North America falls between 17 and 23. For a typical US case, the ratio is about 19, which helps explain the rule of 20. The climate input matters: applying 20 everywhere can move the estimate away from a climate-specific value even before building corrections are made. [1]
Sherman’s worked example holds the pressure-test result at 10 h⁻¹ at 50 Pa and changes the house and site inputs. For a two-storey house near Washington, DC, the base climate ratio is 21. The two-storey height factor is 0.8; the exposed-site shielding factor is 0.9; and the leakage-type factor remains 1.0 because the example does not identify the leak type. The adjusted ratio is 21 × 0.8 × 0.9 × 1.0 = 15. The estimated annual infiltration is therefore 10 ÷ 15, or about 0.67 h⁻¹. A flat divisor of 20 gives 10 ÷ 20 = 0.5 h⁻¹ for the same n50. These are estimates from Sherman’s method, not measurements of a particular house. [1]
The factors also show why exposure descriptions matter. In Sherman’s table, the shielding factor is 1.2 for a well-shielded site, 1.0 for normal shielding and 0.9 for an exposed site. The height factors are 1.0 for one storey, 0.9 for one and a half storeys, 0.8 for two storeys and 0.7 for three storeys. Leak type adds another correction: 1.4 for small cracks, 1.0 for normal leakage and 0.7 for large holes. As arithmetic using the Washington base ratio of 21 and two-storey height factor of 0.8, the listed factors produce ratios from about 10.6 (exposed site, large holes: 21 × 0.8 × 0.9 × 0.7) to about 28.2 (well-shielded site, small cracks: 21 × 0.8 × 1.2 × 1.4). For n50 = 10 h⁻¹, those ratios correspond to about 0.94 to 0.35 h⁻¹. This is arithmetic from Sherman’s tables, not a prediction for a particular building; selecting factors requires evidence about its site and leakage assumptions. [1]
Wind and temperature vary over time
Natural airflow changes because wind and temperature create pressure differences across the envelope. Wind pressure varies with speed and direction, terrain and shielding; buoyancy pressure depends on indoor-outdoor temperature difference and building height. NIST’s infiltration guidance lists temperature difference, wind speed and direction, fan and window operation, envelope airtightness, opening distribution, height, footprint and orientation among the factors that affect infiltration. A single n50 result contains none of that weather history or the pressures acting on different faces. [2] [3]
Choose the estimate for the question
Sherman states that the simplified method is appropriate “for large ensembles of annual averages or for rule-of-thumb estimates.” He also concludes that “any time short-term or highly specific building specific estimates are required, the approach described herein is unadvisable.” For a building-specific model, report the model, climate or weather period, terrain and shielding, geometry, leakage distribution, and relevant fan or window operation. If the question concerns infiltration on a particular test day, a tracer-gas measurement addresses that condition more directly, but NIST notes that the result remains limited to the day’s weather and building configuration. [1] [3]