I have an 8" duct that comes up through the kitchen floor (runs in subfloor between basement and 1st floor and vents outside) and through a kitchen cabinet bottom that will house a pop up vent. I can't access the 90 elbow in the floor (given the size of the cut out in the floor and sub floor). How do I seal the 8" duct coming up from the floor to the 90 degree elbow? Should I just smear mastic on the inside of the 90 elbow and jam in the vertical piece?

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