What is the impact of wind on building airtightness test?

Wind creates a heterogeneous pressure distribution around the building and induces a pressure difference between indoor and outdoor even when the fan is turned off. The impact of wind can be partly compensated by measuring and subtracting this zero-flow pressure and by making the average between pressurization and depressurization test.

When we test a building for air tightness, a two-channel manometer is used to measure both the fan pressure on the blower door system and the pressure difference between indoor and outdoor.  The building is either pressurized or depressurized by the blower door fan to create a measurable pressure differential that generates air flow through building’s leaks. This pressure differential is measured in units of Pascals (Pa).The impact of wind can be observed with the two-channel manometer and can sometimes make it difficult to get a stable, reliable reading since the wind causes the conditions outside to constantly fluctuate.

On windy days, leaks located downwind and upwind won’t have the same pressure difference. Therefore the flowrate through each leak will depend on its location on the building envelope: the impact of some leaks will be overestimated and some others will be underestimated. 

Performing the test both in pressurization and depressurisation helps to limit the impact of wind on the result of the airtightness test.

Additionally, to account for windy conditions, a bias pressure (zero-flow pressure) is collected before the airtightness test. This is where the two-channel manometer collects a sample reading of the outdoor conditions for a given amount of time, usually 60 seconds. Some test standards require an additional bias pressure to be collected at the end of the test as well. The average pressure differential is then applied to the blower door results to remove the measured impact of wind, similar to the way we zero a scale before we weigh an object. Nevertheless, as the relation between flowrate and pressure is not linear, this subtraction is not sufficient to cancel the error.

Other recommendations exist but as stressed in [1] depend on standards, for example some texts recommend to smooth windy conditions with mechanical wind dampening where the outdoor reference tubing is branched into multiple sections with 90 degree turns.

This subject has been discussed in [1] and has been the subject of much research.

References

[1] Hurel N. & Leprince V., 2021. VIP 41: Impact of wind on the airtightness test results. AIVC, 2021

See also

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