a 8 x 30 inch x 20 ft long cold air return line, hung from the floor joices... will insulateing it help..
it s in a non heated basement... tks... richard

There is little benefit to insulating a return duct. It contains cold air to begin with so what you might save in minimal heat return would be well offset by the cost of the insulation. If this was a crawl space in a very cold environment it might pay but for a regular basement probably not. You should, however, consider sealing all of the joints in the return. You can get duct sealer mastic at many hardware stores. Brush it on the joints. I noticed a change in the house air flow, particularly in the summer-in Atlanta.

Tks

Please check "YES", if my answer was helpful! Thanks, Brian

The greater the difference between the air temperatures inside and outside the duct, the greater value of insulation. If the duct is already made from fiberglas ductboard, it already has some insulating value. If it is uninsulated sheet metal and the basement is more than 20 degrees below the house temperature, insulation will make a difference. How about the supply ducts?

Im building a 400 square foot apartment with two 8" heating duct feeding it.
What size return line do I need?
Rob