[AIVC, Air infiltration and ventilation glossary, 1992]:a blower door is fitted to a building, induced air flow through the fan creates an artificial, uniform static pressure within the building. Internal and external pressure taps are made, and a manometer is used to measure the air flow required to produce a given pressure difference. The higher the flow rate required to produce a given pressure difference, the less airtight the building.pressurization, internal fan[AIVC, Air infiltration and ventilation glossary, 1992]:the building’s own mechanical ventilation system can be used to provide the required pressure differential from within. The supply fans are operated while al1 return and exhaust fans are turned off, and all return dampers are closed, so that air can only leave through the doors, windows and other leakage sites.