Should I insulate the return metal duct system?

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I have the floor removed, so I have easy access to the ductwork. The supply duct is insulated on the inside of the metal duct, but the return is not insulated.

  6 answers
  • Dianacirce70 Dianacirce70 on Nov 30, 2017
    If you just make sure all seams are sealed, that should be fine. Even in a unfinished basement it shouldn't get below 50 or 60 degrees, and will cycle through the furnace again before it gets into the house.
  • Cynthia H Cynthia H on Nov 30, 2017
    It appears that the question's answer is "depends". Here is some info to help you decide:
    • Jim Jim on Nov 30, 2017
      Thank you Cynthia for the web link. There is a allot of good information on this link.
  • DesertRose DesertRose on Nov 30, 2017
    yes, it should be well insulated too. It will save you reheating or recooling the same air twice as much
  • William William on Nov 30, 2017
    No! Return ducts do not need to be insulated. The air moves so fast through the ducts it won't make a difference. What you could do is insulate the supply duct on the outside. The inside insulation helps but really doesn't do as much as outside insulation. It's old technology. It's called duct liner. 1/2" black fibrous product covered with a mesh adhered to the duct. Over time it does deteriorate and you may get fine black dust in the house coming out of the supply vents.
  • William William on Nov 30, 2017
    Hello Jajsacres. The liner shreds in chunks and fine dust. There is no mesh small enough that can trap the fine dust without blocking the air flow. Similar to the fine particles fiberglass insulation releases into the air when disturbed or the fine dust from sanding drywall. It will settle everywhere and breathing it isn't great either.
    • Jim Jim on Nov 30, 2017
      Hello William, so the best thing I could do is replace the old supply duct with new duct and insulate it on the outside. Is it better to use the flexible insulated duct or the solid metal ducts to the vents? Thank you, Jim
  • William William on Dec 01, 2017
    In your situation with the floor opened up and easy access to the ducts replacing and insulating the outside would be the best solution to prevent any future problems. I have always used solid pipe for the runs to the vents except where a direct run was not possible. You should also insulate the runs. The flexible pipe is corrugated and has a tendency to trap dirt/dust in the corrugations. Also it can sag creating a restriction for air flow. One thing to remember: for every 90 degree elbow/bend is equal to 10' of straight run. That is quite a restriction and slowing of air flow. Elbows are adjustable so we would rotate them to form 45 degree angles.