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). The thickness of the board you use will be determined by the type of installation you’re performing.
A red-handled duct knife slices a V-shaped groove into the insulation so that the individual sections can be folded neatly over on themselves with no wasted space at the corners. An orange-handled knife creates an interlocking “shiplap” edge. When folded, these grooves create an airtight seal that safeguards against air leaks, heat or cool air loss, moisture drips, and other airflow complications.
Always position the guide square so that the longer vertical arm is on the right. It will serve as a straight edge when you begin cutting.
Make sure the side with the foil backing is facing down. When working with duct board, you’ll be making your measurements and cuts to the inside of the duct. For best results, use a tilting table or some other surface that will give you a little elevation on one side. This will make it easier to guide the duct knives all the way through the opposite end of the duct board.
The metal blade guard on the gray duct knife will wrap around the outer edge of the board and rest flat against your work surface. [4] X Research source
Be careful not to cut through the foil backing, or you’ll have no way of conjoining the top and side panels. The staple flap will form a connection with the closure joint you just cut with the gray duct knife. [5] X Research source
Once they’ve been folded, the first and third panels will form the sides of the duct, while the second and fourth panels will serve as the top and bottom. You’ll need to add a few extra inches to the width of each panel in order for them to align properly. 1 inch (2. 5 cm) duct board will require an additional 1. 75 inches (4. 4 cm), 1. 5 inches (3. 8 cm) duct board will need 2. 75 inches (7. 0 cm), and 2 inches (5. 1 cm) duct board calls for 3. 75 inches (9. 5 cm). [7] X Research source
Press the lefthand side of the knife up against the outer edge of the guide square. It will help guide the movement of the knife to ensure that every cut is straight and precise. [9] X Research source It can help to use a guide square with some grip on the backside. That way, you won’t have to press down as hard to hold it in place or worry about it slipping as you move the duct knife along.
If you’re using an orange-handled duct knife to cut shiplap edges, you’ll need to turn the blade around the other way when making the second cut to allow the corner folds to fit into place properly. [11] X Research source Remember to start each cut on the righthand side of the guide square to keep them consistent.
Be sure to turn orange-handled knives back around (the way you were holding them originally) before making the final pass.
