Question on simple furnace duct installation

I'm going to be installing about 20 feet of new 6" ducts between 2"x12" floor joists spaced 8" apart. It occurs to me that if I install the ducts one by one between the joists, I'm not going to be able to tape around them very well or put any screws in the top. What's the best way to handle this problem? Should I assemble the entire run on the floor and tape it and screw it all together and then lift it all into place in one piece?
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I'm assuming you mean 6"round rigid duct,right? You could do what you said, or instead of tape, use brush on duct sealer. You might be able to get in the tight spots with a brush. Also you can use screws on the bottom by putting a brace between the joists and let the duct rest on the brace. Are these A/C ducts? If they are don't forget to insulate them. They also sell 25' of flexible duct in HD or Lowes already insulated.
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