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.
"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle." "I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them." Barry Goldwater