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How to cut duct board
Start with a full-sized sheet of duct board.
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. 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).