Measure the distance around the outside of the duct (perimeter). Write this measurement down.
Measure the thickness of the duct insulation you will be using. Multiply this thickness by four (to account for a layer on each side of the duct). Add this measurement to the perimeter measurement above.
Cut a piece of duct insulation at the measurement you calculated in Step 2. Wrap it around the duct; pull it tight.
Tape the entire length of the seam with foil tape. Press the tape down firmly to ensure it does not pull off. This will secure the duct insulation to the duct.
Cut the next piece of insulation and install in the same manner. Tape the seam where the two pieces of insulation meet with foil tape. This ensures a seamless length of insulation. Continue until all the ducts are insulated.