[AIVC, Air infiltration and ventilation glossary, 1992]:this technique allows building airtightness to be examined at small ( Pa) pressure differentials with minimal interference from climatic forces. Tue air flow through the building envelope can be evaluated by using a piston assembly to vary the effective volume of the structure. Measuring the amplitude of the pressure response inside the building and phase relationship between this pressure and the velocity of the piston, enables the air flow through the building to be determined.(See DC Pressurization)