When the testing equipment is not small enough to accurately measure the leakage - then it is called zero - usually below 10 CFM of leakage.
Essentially every seam of all the metal and every inside connection of the inner sleeves of flex duct has to have duct mastic at least the thickness of a nickel. This is actually per the Building America Practices Guides.
I have not had enough time to put together the video footage of this type of work being done in action yet - but will do so for the series.
Yes this is possible to consistently get zero duct leakage and there are contractors in Nevada and California that install these systems one after the next. They even have a club called the "Ring 4 Club" or something like that - meaning the duct blaster needs a 4th ring to actually measure how low the leakage is - if it even could. --There is no such thing as a 4th ring by the way.
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