Airtightness needs a continuous boundary
Uncontrolled air movement through gaps carries heat and can transport moisture into an assembly. Airtightness means a continuous air-control layer, not a house without ventilation. Plaster, membranes or another designed system may provide that layer. Each building section should show a connected route from the floor through the walls to the roof, including transitions between materials.
Transitions require preparation
Window joints, cable penetrations and changes of substrate need accessible surfaces and compatible seals. Adhesive tape will not work reliably on every dusty or wet surface. Specify preparation, movement allowance and installation sequence before procurement. Later electrical work must not silently puncture connections that have already been inspected. Service zones can reduce the number of penetrations but still need coordinated detailing.
Understand the test result
A fan pressurisation test measures airflow at a specified pressure difference. The n50 result is expressed in h⁻¹ and also depends on the adopted internal volume. It is neither the everyday ventilation rate nor proof of indoor air quality. A useful report records the method, opening preparation, volume and test conditions. Compare results only when their definitions and preparation are compatible.
Allow an opportunity to repair
An intermediate test before finishes helps locate leakage while junctions can still be repaired. A final measurement describes the building at the time of testing; later penetrations may alter that condition. The construction brief should name responsibility for the air-control layer, inspection stages and how repairs are recorded. Begin by tracing the boundary on your drawings, then use the blower-door and ventilation guides to separate leakage assessment from the provision of controlled fresh air.