[header] How to cut duct board [title] Start with a full-sized sheet of duct board. [step] Standard duct board comes in sheets that are typically 48 inches (120 cm) wide by 96 inches (240 cm) long. On one side of the sheet is a thick layer of fiberglass insulation. 
On the other is a thin foil backing which serves as the outside of the duct. [substeps] Duct board is manufactured in three different thicknesses : 1 inch (2.5 cm), 1.5 inches (3.8 cm), and 2 inches (5.1 cm).