Measuring Duct Leakage To Outside

Measuring the duct leakage to the outside requires us to use a blower door and a duct blaster. The thermal envelope should be -25 Pascals in reference to the outdoor pressure, and all registers and return grilles should be masked. The test creates something of an air barrier due to the neutralization of pressures; some of the duct leakage stays in the thermal envelope, and not all leakage goes outdoors. Duct leakage that stays inside the thermal envelope leaves occupants with discomfort, but leakage to the outdoors leaves occupants feeling uncomfortable and leaves the building vulnerable to moisture problems.

To get the house down to -25 Pascals, you must start the blower door under all of the correct parameters. For this video, the blower door must be set to depressurize inside the building and be configured for a model-3 fan. Once you hit “play” the first time, you want to start the baseline and change the flow to 25 CFM before hitting “play” again.

You’ll also want a gauge on the duct blaster; you can set the cruise to 0 Pascals and turn on your speed controller. That selection ramps up the duct blaster to pull the duct system under -25 Pascals of pressure to match the building envelope pressure. When the pressure reaches 0, the CFM reading will show you the leakage outside the thermal boundary (leakage to the outdoors). In this case, we’re losing 60 CFM to the outdoors.