With an efficient furnace for heating the air that is then blown and distributed through out your home, if your air ducts are not insulated, you are losing heat through the walls of the duct work, especially if the ducts are going through an unheated basement or outside wall of your home. Insulating your ducts will keep your moving air at the temperature when it left the blower, and save you a lot in heating costs during the year.
Most hardware stores, like Home Depot and Lowes, carries duct insulation. To determine just how much you will need, measure the length of the ducts you will be covering. Typically, duct insulation comes in rolls of 4 feet by 100 feet. You’ll be covering your ducts in 4 foot sections at a time.
Measure the perimeter of the duct and add 6 more inches to allow for the insulation to overlap. Then cut the insulation, wrap the duct and overlap the insulation. Staple the overlap together, then use insulation tape along the seam to give the insulation an air tight fit.
For joints, you’ll need to cut slits into the measure insulation piece to help it to fit tight.
You only need to insulate duct work that is going through an unheated room or garage. For safety purposes, wear a mask to keep from breathing insulation fibers.