Building airtightness tests have become very common in several countries, either to comply with minimum requirements of regulations or programs, or to justify input values in calculation methods. This raises increasing concerns for the reliability of those tests. Despite the extensive debates about how the building pressurization test standard ISO 9972 should address sources of uncertainties, no change has been implemented. According to the current standard, the zero-flow pressure shall not exceed 5 Pa for the test to be valid. Consequently, in moderately windy conditions, it may be impossible to perform a pressurization test in accordance with the standard, even using precautions with a careful uncertainty analysis. This study investigates numerically, with the use of a commercial CFD code, the impact of unsteady wind on fan pressurization tests. Two test houses, built in cross-laminated-timber (CLT), are modelled and used as case study. Various leakage location, fan placement and wind profiles are used as input in a set of Scenarios.