 To calculate the duct leakage to outside, subtract the leakage with the registers sealed from the total building leakage and multiply by the correction factor.A third test method to determine if ductwork is leaking to the outside is to use a pressure pan, which is a register cover with a pressure tap for a hose connection.With the house pressurized (or depressurized) to 50 Pa (-50 Pa) using a blower door, a pressure gauge is attached to the pressure pan by means of a hose.