I hear ya not code hear on residential....commercial sometimes depends on the engineer..... i don't disagree with you just not common practice here...
It is required in states enforcing the energy code. Of course if the duct work is inside the building envelop and residential not commercial then it isn't required.
Mastic is the best way to go but give it time to dry before running the system. I presume Winnipeg doesn't use the IECC since you don't.
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