 Depending on the location of the house in the United States, some HVAC systems are completely inside the thermal envelope, some are completely outside the thermal envelope, and some are a combination of the two.Energy conservation is improved mostly by sealing ductwork that is outside the thermal envelope or connected to the outside.With mixed systems, it is possible to determine amount of leakage to the outside by simultaneously pressurizing the house while pressurizing the ductwork, and measuring the amount of flow that is required to equalize the pressure.