Range hood ductwork carries cooking smoke and steam outside your home, and the path it takes matters more than most homeowners realize

The ductwork is the pipe system that moves air from your range hood to the outside of your house. It needs to slope slightly downward as it runs, stay as short and straight as possible, and connect to the hood with the right size pipe — usually 6 inches in diameter for most residential hoods. If the ductwork is too long, has too many bends, or slopes the wrong way, your hood will work harder, move less air, and may push moisture back into your kitchen instead of outside.

Installing ductwork yourself is possible if you have basic tools and are comfortable working in your attic or crawlspace, but the work requires planning before you cut any holes. You need to know where your exterior wall is, whether you can run the duct straight there, and whether your hood is vented to the outside or recirculates air inside. If your hood is a recirculating model with a filter instead of a duct, you do not need ductwork at all.

Key Takeaways

  • Ductwork diameter must match your hood's outlet — usually 6 inches — and the duct should run as straight as possible to the nearest exterior wall.
  • The duct must slope downward toward the outside at a pitch of at least 1/4 inch per foot to prevent moisture from running back into the hood.
  • Flexible ductwork is easier to install than rigid metal duct, but rigid duct moves air more efficiently and resists crushing better.
  • You will need to cut a hole through your exterior wall, install a wall cap or roof flashing, and seal all connections with mastic or metal tape — never use duct tape alone.
  • If your ductwork runs more than 25 feet or has more than four bends, the hood's fan may not move enough air, and you may need a booster fan.

Measure your hood outlet and choose your duct diameter

Before you buy any ductwork, look at the outlet on the back or top of your range hood. Most residential hoods have a 6-inch round outlet, but some have 7-inch outlets or rectangular openings. Measure the opening or check your hood's manual — the duct diameter must match this outlet exactly, or you will have air leaks and poor performance.

Write down the outlet size and whether it is round or rectangular. If your hood has a damper (a flapper that closes when the fan is off), note that too — you will need to connect your ductwork to the damper, not directly to the hood. Take a photo of the outlet with your phone so you have it when you shop for ductwork.

Plan the duct route from hood to exterior wall

The shortest path outside is always the best path. Stand at your range hood and look up into the cabinet or soffit above it. Trace an imaginary line straight up and outward toward the nearest exterior wall. If you have an attic, you will run the duct through the attic and out through the roof or a gable wall. If you have a crawlspace or basement, you may be able to run the duct horizontally along the rim joist and out through a basement wall.

Measure the distance from the hood outlet to the exterior wall. If the distance is under 10 feet with no bends, you have an ideal situation. If it is 15 to 25 feet or requires two or three bends to avoid joists and wiring, your hood will still work but will be less efficient. If the distance is over 25 feet or requires more than four bends, talk to a professional — you may need a booster fan to move enough air.

Check what is in your path: electrical wiring, plumbing, HVAC ducts, and structural members like joists and beams. You can route around most of these, but you cannot cut through a structural beam. If a beam is in your way, you will need to run the duct a different direction or hire a contractor to assess whether the beam can be notched.

Select flexible or rigid ductwork

Flexible ductwork is easier to install because it bends without fittings and takes up less space. It comes in 25-foot lengths and costs less than rigid duct. The downside is that it crushes more easily if something leans on it, and the inner ridges create more friction, so air moves through it less efficiently. Flexible duct also sags over time if it is not supported every 3 to 4 feet.

Rigid ductwork is made of galvanized steel or aluminum and moves air more efficiently because the inside is smooth. It requires fittings to connect straight sections and to make bends, which takes more time and creates more joints to seal. Rigid duct is heavier and harder to route through tight spaces, but it lasts longer and does not sag.

For most homeowners, flexible ductwork is the practical choice if your run is under 15 feet and straight. Use rigid duct if you have a longer run, multiple bends, or want the best long-term performance. Never use both types in the same run — pick one and stick with it.

Cut the exterior wall opening and install the wall cap

Once you have planned your route, you need to cut a hole through your exterior wall where the duct will exit. If you are venting through the roof, you will use roof flashing instead — skip to the next section. For a wall exit, mark the center point on the outside of your wall where the duct will come through. The hole should be the same diameter as your ductwork (usually 6 inches) plus 1/2 inch for clearance.

Drill a pilot hole from inside to outside so you can see where the hole will be on the exterior. Go outside and mark a circle around the pilot hole. Use a drywall saw or hole saw to cut the opening from the inside, or use a reciprocating saw from the outside if you are cutting through siding and sheathing. Wear a dust mask — cutting through insulation and exterior materials creates a lot of dust.

Once the hole is open, install a wall cap (also called a vent hood or termination cap) on the outside. The wall cap has a flange that covers the hole and a damper that closes when the fan is off, preventing cold air and pests from entering. Slide the duct into the wall cap from inside, then find the cap to the exterior wall with screws or caulk, depending on the cap design. Seal around the cap with exterior caulk to prevent water from leaking in.

Install ductwork from hood to exterior opening

Start at the range hood outlet. If your hood has a damper, connect the ductwork to the damper outlet. If not, connect directly to the hood outlet. Slide the duct onto the outlet and find it with a hose clamp or metal band clamp — tighten it firmly so the connection does not slip.

Run the ductwork along your planned route, supporting it every 3 to 4 feet with metal straps or brackets. The duct must slope downward toward the exterior at a pitch of at least 1/4 inch per foot — this means for every 4 feet of horizontal run, the duct should drop 1 inch. This slope prevents condensation from running back into the hood. Use a level to check the slope as you go.

If you are using flexible ductwork, avoid kinks and sharp bends. If you need to change direction, use a smooth 90-degree elbow fitting rather than crimping the duct. If you are using rigid ductwork, connect sections with slip fittings and find them with sheet metal screws. Seal every connection with mastic (a sticky sealant) or metal foil tape — never use standard duct tape, which degrades over time and fails in warm, humid conditions.

Connect the far end of the ductwork to your wall cap or roof flashing. Make sure the connection is tight and sealed. If you have a roof vent, the flashing should be installed before the ductwork so the duct slides into the flashing from inside the attic.

Seal all connections and test airflow

Every joint in your ductwork is a place where air can leak out, reducing the hood's performance and wasting energy. Go through the entire run and check every connection. Use mastic or metal foil tape to seal any gaps. Mastic is better than tape because it stays flexible and does not peel off, but metal foil tape works if applied carefully with no wrinkles.

Once all connections are sealed, turn on your range hood and feel the airflow at the wall cap or roof vent. You should feel strong air movement. If the airflow is weak, check that the damper is opening fully and that nothing is blocking the ductwork. If the ductwork is very long or has many bends, weak airflow may mean you need a booster fan installed in the ductwork to increase air movement.

Look for any visible dust or debris around the ductwork connections inside your attic or crawlspace. If you see dust, that connection is leaking and needs to be resealed. Check again after a few weeks of use — sometimes connections settle and need a second seal.

Frequently Asked Questions

Can I use PVC pipe or dryer vent hose for range hood ductwork?

No. PVC is not rated for the heat and moisture from cooking, and dryer vent hose is too small and not designed for the airflow. Use ductwork rated for range hoods — flexible aluminum or rigid galvanized steel. Check the ductwork packaging to confirm it is rated for kitchen ventilation.

What if I cannot run the duct straight outside and need to make multiple bends?

Each 90-degree bend reduces airflow, so limit bends to four or fewer. If you need more bends, use smooth elbows rather than sharp crimps, and consider a booster fan to maintain airflow. A booster fan installs inside the ductwork and pushes air along when the main hood fan is running.

Do I need to insulate the ductwork?

Insulation is not required but is helpful if the ductwork runs through an unheated attic or crawlspace. Insulation reduces condensation buildup inside the duct, which can drip back into the hood. Wrap the ductwork with fiberglass insulation rated for ducts and find it with foil tape.

What is the difference between a damper and a wall cap?

A damper is a flapper inside the ductwork or at the hood outlet that closes when the fan is off, preventing backflow. A wall cap is the exterior fitting where the duct exits the wall — it usually has a built-in damper. Some hoods have a damper built in, so you do not need a separate one.

Can I vent the range hood into my attic instead of outside?

No. Venting into the attic pushes moisture and cooking odors into the space above your ceiling, which can damage insulation, promote mold, and rot wood framing. Range hoods must vent to the outside — either through a wall or the roof.