I just had central A/c installed. Ducting for the 2nd floor runs through the attic. Someone told me I should consider insulating the ductwork to help cut down my energy costs and make the system more efficient. The ducts themselves seem like they have some kind of insulation around them compared to the bathroom vent ducts (not insulated). The insulation is thin relative to insulation found in walls. I saw a post about bathroom exhaust fans and insulating the ducts...I'm guessing that's a similar concept.
Is this common practice? Would there be a noticeable difference in cost and temperature ?