Should I insulate the return metal duct system?
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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.
It appears that the question's answer is "depends". Here is some info to help you decide:
https://www.energy.gov/energysaver/minimizing-energy-losses-ducts
yes, it should be well insulated too. It will save you reheating or recooling the same air twice as much
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.
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.
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.