Theatrical smoke is a very effective way to find leaks in a duct system.  A quick check will point out smoke as it spills from pressurized leak locations.  Leaks in attics and crawlspaces that can be hard-to-detect are no problem with this method. This type of detection is called theatrical for a reason - it is quite a sight to see for homeowners and builders.

Caution: theatrical smoke injected through the Model 300 test fan inlet may cause damage and void warranty. Risk increases with frequency. Instead, inject non-toxic chemical smoke into the flex-duct then reconnect the DucTester fan so it blows that smoke into the duct system, as shown in the video above.

Duct leakage to outdoors tests are needed, even if duct work is within the conditioned space

If ducts are installed completely within the conditioned space do we need to test them? The International Energy Conservation Code (IECC) requires we only test duct work if it is outside the conditioned space, such as an un-insulated attic or basement.  If we do test them, is total duct leakage enough, or are these leaks still finding their way to the outside? These questions are always popping up on online HVAC communities.

Looking at the surface of the issue, it might appear to be pointless to do a duct leakage to outdoors test if the ducts are completely contained within the envelope. This would be the case if the ducts themselves were on the “surface” but most are hidden in wall cavities, which often connect to the outdoors to some degree.  Any duct leakage will then be partially to the outdoors (via the walls) as well as to the indoors.