Why must the interior spaces around a test zone be at the same pressure during zonal testing?

The requirement for interior spaces around a test zone to be at the same pressure during zonal testing is fundamentally about minimizing the impact of surrounding airflows. When conducting an airtightness test, it’s crucial that any differences in pressure between the test zone and adjacent spaces do not influence the results. If there are pressure differentials, air may flow from the surrounding areas into or out of the test zone, which could lead to inaccuracies in the measurement of the zone’s airtightness.

Maintaining equal pressure helps to ensure that the test accurately reflects only the air leakage characteristics of the test zone itself, without interference from external airflows. This is essential for obtaining reliable data that can be used to assess the performance of the building's envelope and its overall energy efficiency.