AC duct (between ceiling joists)

I'm rerouting some ductwork because of an expanded family room. Basically the only way down to the first floor is thru a new closet on the second floor. I plan on putting 2 8" registers in the first floor family room but running ductwork between the ceiling joists will be tough.
I have heard that it is acceptable to simply use the existing ceiling joists (boxed in on either side) as makeshift ducting. If I were to run 2 8" flex ducts into the cavity between the joists and put the 2 registers somewhere in the cavity - any problems with this? I was thinking I might need to put a vapor barrier at least and maybe foil lined insulation (1/4") on the inside of the cavity - what to you guys think?
TIA, Bob
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I would not use the cavity as an duct.
Metal is the only way for me. I do not like flex duct. Personal opinion
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For return air it is acceptable, but not for suppy air. Greg
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ductwork has never been legal.
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