Calculation of Insulation Layer for Air Ducts (Rubber Insulation Board): During on-site construction, the rubber insulation layer of galvanized air ducts requires a much larger amount of material due to the outer layer size of the insulation material being 5cm (δ=25mm) larger than the size of the air duct, as well as the insulation strip at the flange of the air duct (L=120mm). 1. Taking a 3.6-meter-long 1000 * 500 air duct (with a length of 1.2m per section) as an example, the area of the duct is 10.8m2, and without loss, the material required for the δ=25mm rubber insulation board is 12.744m2. 12.348 (1.143%)