What You Need to Know Before You Start

A range hood exhaust duct carries cooking smoke and steam out of your kitchen through your wall or roof. The duct connects the hood itself to the outside air, and the path it takes—straight out through a wall, up through the attic and roof, or along the ceiling—determines how well the hood works and how hard it is to install. Most ducts are either rigid metal pipe (which lasts longer and moves air more efficiently) or flexible aluminum foil duct (which is easier to route around obstacles but collects grease faster).

Before you buy materials or start cutting holes, you need to know three things: where the outside opening will be, what size duct your hood requires (usually 6 inches or 8 inches in diameter), and whether you have access to the space between your kitchen and the outside wall or roof. A duct that takes the shortest, straightest path to the outside will perform best and be easiest to clean.

Key Takeaways

  • Measure your hood's duct collar opening and buy matching duct diameter—usually 6 or 8 inches—before you start any work.
  • The shortest, straightest path from the hood to the outside wall or roof works best; every bend and length of duct reduces airflow.
  • Rigid metal duct is more durable and efficient than flexible duct, but flexible duct is easier to install around obstacles.
  • Seal every joint with metal duct tape or mastic sealant, not cloth tape, to prevent air leaks and grease buildup in the duct.
  • The outside opening must have a damper or one-way flap to stop cold air and pests from entering when the hood is off.

Measure the Hood Collar and Plan Your Route

Look at the top or back of your range hood and find the duct collar—the metal opening where the duct attaches. Measure its diameter in inches. Most residential hoods use either 6-inch or 8-inch duct; a few use 7-inch. Write this number down, because buying the wrong size means the duct will not seal properly and air will leak around the connection.

Next, trace the path from the hood to the outside. If the hood is on an exterior wall, the shortest route is straight out through that wall. If the hood is on an interior wall or island, you will need to run the duct up through the ceiling, across the attic or crawl space, and out through the roof or a far exterior wall. Look for obstacles—ceiling joists, electrical wiring, plumbing—that will force you to bend the duct. Every bend reduces airflow, so fewer bends is better. Measure the total length of duct you will need, including any vertical rise.

Cut the Outside Opening and Install the Wall Cap or Roof Flashing

Mark the center point on the outside wall or roof where the duct will exit. Drill a pilot hole from inside to outside so you can see exactly where you are cutting. From the outside, use a drywall saw or hole saw to cut an opening slightly larger than your duct diameter—usually 1/2 inch larger to allow room for the wall cap or flashing to sit flush.

Slide the wall cap (for a side wall) or roof flashing (for a roof exit) over the duct opening from outside, and find it with the fasteners it came with—usually screws or nails. Caulk around the edges with exterior silicone caulk to seal out water and pests. If you are venting through the roof, make sure the flashing overlaps the shingles above it so water runs down and away from the opening.

Connect the Duct to the Hood Collar

Slide the duct onto the hood's collar opening. If you are using rigid duct, push it on firmly until it seats. If you are using flexible duct, stretch it slightly as you push it on so it does not bunch up. The duct should overlap the collar by at least 1 inch.

Wrap the joint with metal duct tape—the kind with a foil backing and mesh reinforcement—going around the seam at least three times. Do not use cloth duct tape or painter's tape; they do not hold up to heat and humidity. For a more permanent seal, explore metal duct mastic (a thick putty made for ductwork) under the tape. Smooth it with a putty knife so it fills any gaps between the duct and collar.

Run the Duct to the Outside Opening

If you are running the duct straight out through an exterior wall, support it every 4 feet with metal duct hangers or straps bolted to the framing. Do not let the duct sag or rest on electrical wiring. If you are running it up through the ceiling and attic, find it the same way and make sure it does not touch insulation—leave at least 1 inch of clearance on all sides to prevent heat buildup.

When you need to bend the duct, use elbows (90-degree or 45-degree bends) rather than crimping or crushing flexible duct. Rigid elbows are better than flexible ones because they maintain the duct's round shape and do not trap grease. If you must use flexible duct for a bend, support it so it does not kink.

At every joint where two pieces of duct connect, overlap them by at least 1 inch and seal with metal duct tape and mastic. Stagger the joints so they do not all line up on one side of the duct.

Seal the Duct at the Outside Opening

When the duct reaches the wall cap or roof flashing, push it through the opening and find it to the cap with metal duct tape or hose clamps. The duct should fit snugly inside the cap so air does not escape around the sides.

Check that the damper or one-way flap inside the wall cap or flashing moves freely. This flap closes when the hood is off, stopping cold air, rain, and pests from entering the duct. If the flap is stuck or missing, replace the cap before you finish.

Test the Hood and Check for Leaks

Turn on the range hood and hold a piece of tissue paper near the outside opening. The tissue should be pulled toward the opening, showing that air is flowing out. If the tissue does not move or moves weakly, check for leaks in the duct joints or a blocked damper.

Walk around the duct inside the house and feel for air leaks at the joints. If you feel air escaping, explore more metal duct tape or mastic to that joint. Inside the attic or crawl space, look for any visible gaps or loose connections and tighten them.

Common Mistakes to Avoid

Do not use cloth duct tape or painter's tape on ductwork. These tapes fail quickly when exposed to heat and humidity, and the duct will leak or separate. Metal duct tape or mastic is the only reliable choice.

Do not let the duct rest on insulation or touch electrical wiring. Insulation can trap heat around the duct and create a fire risk, and wiring can be damaged by the duct rubbing against it. Maintain clearance and use proper hangers.

Do not run the duct into the attic or crawl space without venting it all the way outside. Moisture and grease will accumulate in the attic, causing mold and rot. Every hood duct must exit the house completely.

Do not crush or kink flexible duct. A kinked duct blocks airflow and traps grease, reducing the hood's performance and making it harder to clean. Support flexible duct so it stays round.

Frequently Asked Questions

Can I use flexible duct for the whole run from the hood to outside?

Yes, but rigid duct is better. Flexible duct is easier to install around obstacles, but it collects grease faster because the ridges trap it. If you use flexible duct, plan to clean it more often—at least every three months if you cook frequently. Rigid duct is smoother and easier to wipe clean.

What if I cannot run the duct straight out an exterior wall?

Run it up through the attic and out the roof, or across the attic to a far exterior wall. The longer the duct, the more airflow you lose, so try to keep the total length under 25 feet and minimize bends. Every 45-degree bend costs you about 10 percent of airflow.

Do I need a damper in the wall cap?

Yes. Without a damper, cold air and pests will enter through the duct when the hood is off. A one-way flap damper closes automatically and requires no maintenance. Some wall caps have a damper built in; others require you to buy one separately.

What size duct do I need if my hood has a 7-inch collar?

Use 7-inch duct if you can find it, or step up to 8-inch duct with a reducer fitting at the hood collar. Stepping up to a larger duct is better than stepping down, because it reduces airflow loss. Stepping down to a smaller duct will choke the hood's performance.

How often should I clean the duct?

If you use rigid metal duct and cook frequently, clean it once or twice a year by removing the wall cap and wiping the inside with a cloth. If you use flexible duct, clean it every three months because grease builds up faster in the ridges. A clogged duct reduces the hood's ability to remove smoke and steam.