What a Gap Between Positive and Negative Pressure Results Reveals
A gap between pressurisation and depressurisation results can reveal that airflow through the tested envelope differs by pressure direction, or that test conditions and measurement variation contributed to the difference. Field and research reports describe direction-dependent airflow through leakage paths and around door and window seals, while measurement guidance explains why repeated readings can vary. The gap alone cannot distinguish these explanations or identify a component. Compare the paired pressure-flow curves, baseline readings and site observations before treating a component explanation as more than a hypothesis. [4] [2] [1] [3]
- ISO 9972:2015 covers fan tests using pressurisation or depressurisation. A UK housing study reported an average difference of 4.1%, with observed differences from under 1% to over 14%; those figures describe that study, not a universal range. [1] [4]
- A UK housing study describes how airflow direction can affect leakage paths and how pressure can push windows and doors against or away from their seals. A whole-building test does not identify which component caused a difference. [4] [1]
- Repeated readings can differ because of natural variation or instrument instability. Wind, temperature differences and test apparatus also affect measurement accuracy. [2] [1]
- Compare raw pressure and airflow readings, baseline pressure, setup and environmental conditions in both directions before proposing a cause. Moment-to-moment pressure variation is normal, according to the US Department of Energy's Building America Solution Center. [3] [1]
What do the two results measure?
A fan pressurisation test measures the airflow needed to maintain indoor-to-outdoor pressure differences. Pressurisation puts indoor pressure above outdoor pressure; depressurisation puts it below. ISO 9972:2015 describes testing in either mode over a range of pressure differences and says measurement accuracy depends on the apparatus, instruments and ambient conditions. This article refers to ISO 9972:2015, the third edition. [1]
How can airflow differ by direction?
Airflow through a leakage path can respond differently when the pressure direction reverses. A UK housing report explains that leakage paths have aerodynamic characteristics that depend on airflow direction, and that pressurisation or depressurisation can push windows and doors against or away from their seals. This makes directional airflow a possible explanation for a gap, but it does not establish that a particular opening or junction in a tested building is responsible. ISO 9972 determines air permeability for the envelope or a defined part of it; it does not evaluate individual components. [4] [1]
What pattern points towards measurement variability?
A gap that changes between repeated runs, alongside shifting baseline pressure or changing environmental conditions, makes measurement variability a relevant possibility to investigate. The National Physical Laboratory explains that repeated measurements may differ because of natural variation or instrument instability, even when the operator has done nothing wrong. ISO 9972 also identifies ambient conditions and apparatus as factors affecting accuracy. These observations help guide a repeat test; they do not by themselves establish the cause of a difference. [2] [1]
What records make the comparison useful?
Keep raw airflow and pressure readings for each direction, baseline pressure before the fan runs, test setup and preparation notes. Record indoor and outdoor temperatures, wind conditions, the tested boundary, temporary seals and the state of doors, windows and adjustable openings. The US Department of Energy's Building America Solution Center recommends documenting as-found conditions and checking baseline pressure and equipment setup; it notes that minor moment-to-moment pressure variation is normal. A UK housing study reported an average directional difference of 4.1% and differences from less than 1% to more than 14% among its tested dwellings. The report discusses historical ATTMA Technical Standard 1 (2006); its figures show that differences occurred in that study, not current general guidance or a prediction for another building. [3] [4]
What should you conclude from the gap?
Use paired curves to decide what to inspect and retest, not to assign a leak to a junction or product. First check whether the readings cover comparable pressure ranges, whether the baseline and setup were stable, and whether wind or temperature conditions changed. If a gap persists under repeatable conditions, inspect suspect areas in both pressure directions and report the observations separately from the measured results. A component-level explanation requires supporting site evidence; the whole-building result alone cannot supply it. [1] [2] [3]