I am trying to fit a 7" flex duct between some floor joists and it would fit better without the insulation. Can I remove the insuluation and just use the plastic part?
Remove the insulation from flex duct
ductshvac
Related Solutions
It's a butterfly style diffuser damper. Home depot sells them, but I don't see one that has the long collar that you would want to attaching to the flex tube.
Your best bet is finding an online retailer that supplies parts for the flex duct systems. I see this one from a google search that includes the grille and damper box:
Considering how much worse flexible duct is for airflow .vs. smoothwall, I think you'll be fine with 8"; better than it was, for certain.
Assuming real-world installation, this paper is quite enlightening (figure 2 also appears to show that 8" is slightly more than twice as good as 6" even under ideal conditions, where there's less difference between flex and smoothwall) but figure 3 is very enlightening, and that's for what's considered a "good, nominal" flex install.
Texas A&M airflow & pressure in flex ducts
Figures 4, 5 & 6 are downright scary.
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Best Answer
Technically I suppose you could remove the insulation around flex. Why would you want to though? I understand that it fits tight through there but in my installation of commercial duct work (on the side) we have just compressed the insulation. It is after all flexible. One thing that insulation around duct work does is prevent condensation on your ducts when you run the air conditioning in the summer. You may think no big deal but that slight bit of moisture and a couple of drops can start other issues. It's good to keep your ducts insulated. I would say either pipe it through the (and insulate the pipe) joists and put flex on after that or just squish the flex with the understanding that it may affect the airflow to that room (depending on the squish factor)