How to Insulate a Cooker Hood Flue

Cooker hood installers cover the hood's flue pipe with fire-rated duct insulation, often called a duct blanket. The insulation has a woven fiberglass or mineral wool core that acts as a thermal barrier, and a foil outer liner, which serves as a vapor barrier. The insulation's core prevents any combustible materials surrounding the flue from igniting and the vapor barrier stops moisture from condensing on the flue pipe's exterior surface. If moisture condenses on the flue, the inner core absorbs the water and loses its thermal resistance.

Tape measure
Utility knife
Metal-backed duct tape
Plastic squeegee
1/2-inch-wide duct bands
Slotted screwdriver

Instructions

- 1
  Unroll a section of fire-rated duct insulation onto a clean floor, keeping the foil backing facing the floor. A duct blanket comes from the manufacturer in 2-, 3- or 4-foot-wide rolls and measures about 25 feet when unrolled.
- 2
  Measure the circumference of the cooker hood's flue using a tape measure. Add 3 inches to the measurement, and place a mark on duct insulation so that it is as long as your modified measurement. The extra three inches creates the insulation's vertical overlap.
- 3
  Slice through the insulation's woven inner layer with a utility knife. Spread open the incision in the woven inner layer with one hand and cut the interior side of the exposed foil backing with the utility knife. Attempting to cut the foil backing and the woven inner layer at the same time usually results in a ripped or damaged foil backing.
- 4
  Wrap the insulation around cooker hood's flue, keeping the foil backing facing out. Position the bottom of the first piece against the top of the cooker hood. Secure the foil backing on the vertical overlap to the exposed foil backing next to the overlap, using metal-backed duct tape to hold the overlap in place. Metal-backed duct tape resists heat damage.
- 5
  Cut each additional piece of insulation as needed using the same methods.
- 6
  Mount the second piece of insulation on the flue pipe using the same method to install the first piece. Pull down on the bottom edge of the second piece of insulation until it creates a 3-inch-wide horizontal overlap over the top of the first piece. Repeat this process until the top of the last piece of insulation butts against the bottom of the roof deck.
- 7
  Seal the exposed edge of each horizontal and vertical overlap with metal-backed duct tape. Press the duct tape onto the insulation's foil backing with a plastic squeegee.
- 8
  Slip a 1/2-inch-wide duct band around the flue pipe's insulation. Position the duct band about 1-1/2 inches away from the bottom edge of the insulation that butts against the cooker hood. Tighten the duct band by turning the screw counterclockwise using a slotted screwdriver. Install an additional duct band every 8 to 10 inches up the flue pipe. Place the last band about 1 1/2 inches away from the roof deck.

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