Establish a means for measuring the air pressure difference across the test specimen which takes into account the difference that may exist between the static pressure in the interior of the building and the static pressure on the exterior. In the case of an interior mounted test chamber, it may be possible simply to open a window in a room to balance the pressure.Apressure tap to the exterior for a pressure measuring device on an interior-mounted chamber or a pressure tap to the interior for an exterior-mounted test chamber would properly account for any difference.