A manufactured home has a compact oil fired forced air furnace in one
corner oc a 40x28 manufactured home. A large central duct runs down
the middle of an unfinished basement and perpendicular ducts run in
the joist spaces. The joist spaces in the basement are insulated with
faced fiberglass. The home is in south eastern PA. Seemed like a lot
of heat was wasted because the central duct was not insulated. The
owner of the home complained of cold and one heating contractor
installed a larger central duct (slows down flow further increasing
heat loss?)

Should the duct work be insulated and if so roughly how many years to
payback?

Thank you for any help!