Airtightness
Infiltration rate estimate from air permeability (÷ 20 rule, SAP 10.2)
- where
- infiltration before shelter (per hour)
- air permeability (m³/(h·m²))
- infiltration from chimneys, flues and fans (per hour)
- shelter factor
- number of sheltered sides
- infiltration after shelter (per hour)
Explain this equation
Estimates the everyday air leakage of a home from its blower-door result, using the rule of thumb in SAP.
A test at 50 Pa is far harsher than normal weather, which creates only a few pascals. Dividing q50 by 20 is the long-standing rule for turning the test result into an average natural infiltration rate. Open chimneys, flues and extract fans add their own allowance. Each side of the house sheltered by neighbouring buildings or trees cuts infiltration by 7.5%.
An empirical rule linking test pressure leakage to typical in-use leakage.
Sense check: q50 of 5 gives about 0.25 air changes an hour; with two sheltered sides about 0.21.
Watch out: this is a rough average. Real infiltration varies a lot with wind, temperature and how leaks are distributed.
Estimates the everyday air infiltration rate (air changes per hour at normal pressures) from a 50 Pa test result, the way SAP does.
Enter your values to see the result.
Good to know
SAP 10.2 divides AP50 by 20 to estimate infiltration at typical pressures and adds the openings allowance (chimneys, flues, extract fans). This is the long-standing rule of thumb n ≈ n50/20 published by M. H. Sherman (LBNL, 1987), crediting Kronvall and Persily. SAP then reduces it by 7.5% for each sheltered side and applies a monthly wind factor, not included here.
Common mistake
Using the 50 Pa air change rate as the everyday ventilation rate. It is around twenty times too high.
Limits
An average-climate estimate. The true divisor varies roughly from 10 to 30 with exposure, height and leak distribution. No monthly wind factor.
Sources
- BRE, SAP 10.2 The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (AP50 ÷ 20 infiltration estimate; shelter factor (20) = 1 − 0.075 × sheltered sides; (38) = 0.33 × n × V; (36) = 0.20 × exposed area where thermal bridging details are unknown; (39) HTC; (40) HLP = HTC ÷ floor area; MVHR effective air change n_adj + n_mech × (1 − η))
- 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.