The basic setup: where the ductwork goes

A range hood vents to the outside by running metal ductwork from the hood itself through your wall, ceiling, or roof to an opening in your home's exterior. The duct carries cooking smoke, steam, and odors out of your kitchen instead of recirculating them back into the room. The duct must slope slightly downward as it travels away from the hood—even a quarter-inch drop per foot helps moisture drain back toward the hood rather than pooling inside the pipe.

The duct exits through a wall cap or roof cap, which is a metal fitting with a flapper or damper that opens when the hood runs and closes when it stops. This prevents outside air, insects, and rain from entering your kitchen when the hood is off. The shortest, straightest path from hood to outside is always the best choice—every bend and every foot of duct adds resistance that makes the fan work harder and move less air.

Key Takeaways

  • Ductwork must run from the hood to an exterior wall cap or roof cap, sloping downward slightly so moisture drains back toward the hood.
  • Rigid metal duct (galvanized steel or aluminum) moves air more efficiently than flexible duct and is less likely to trap grease and moisture.
  • The duct diameter must match your hood's outlet—typically 6 inches for residential hoods—and you cannot reduce it to fit through walls.
  • The exterior cap must have a damper or flapper that closes when the hood is off to prevent backflow and pest entry.
  • If venting through a wall is impossible, a roof cap works but requires careful flashing to prevent leaks around the opening.

Choosing between wall venting and roof venting

Wall venting is the simpler and more common choice. You drill or cut a hole through the exterior wall at or near the hood location, insert a wall cap, and run ductwork from the hood to that cap. The hole is typically 6 to 8 inches in diameter (depending on your duct size), and the wall cap sits flush against the siding. This method is faster to install, easier to maintain, and less likely to leak because there is no roof penetration.

Roof venting is necessary when your hood is far from an exterior wall or when the wall location would be awkward—for example, on the side of your house that faces the street. A roof cap is larger than a wall cap and requires flashing (a metal collar that slides under the roofing shingles) to prevent water from running down into your attic. Roof venting is more complex to install and more prone to leaks if the flashing is not sealed correctly, so many homeowners hire a roofer for this work.

If your kitchen is in the center of your home and no exterior wall is nearby, roof venting may be your only option. However, if you have a choice, wall venting is the lower-risk route.

Duct material and sizing

Rigid metal duct is the best choice for range hood venting. Galvanized steel or aluminum duct resists corrosion from moisture and cooking fumes, does not trap grease inside ridges the way flexible duct does, and moves air more efficiently. Rigid duct is also easier to clean if grease buildup occurs. Most residential range hoods have a 6-inch outlet, so your duct should be 6 inches in diameter throughout the run to the exterior cap.

Flexible duct (also called flex duct or dryer vent hose) is cheaper and easier to route around obstacles, but it is not ideal for range hoods. The ridges inside trap grease, the material can sag and hold water, and it is harder to clean. If you must use flexible duct, use the smooth-interior type rather than the ribbed kind, keep runs as short as possible, and support it every 3 feet with a strap or clamp so it does not sag.

Never reduce the duct diameter to fit through a wall or ceiling. A 6-inch hood outlet connected to 5-inch duct creates a bottleneck that reduces airflow and makes the fan work harder. If the duct must pass through a tight space, use rigid duct and route it carefully—do not squeeze it down.

Installing a wall cap and running ductwork

Start by locating the exterior wall where you want the cap to exit. Mark the center point on both the inside and outside of the wall, then drill a small pilot hole through to confirm the location. From outside, use a drywall saw or hole saw (sized to match your duct diameter, usually 6 or 7 inches) to cut the opening. Remove any insulation or blocking inside the wall cavity.

Insert the wall cap from outside, pushing it through the hole until the flange (the flat part) sits flush against the siding. find it with the fasteners provided—usually screws or nails through the flange into the wall framing. Seal around the cap with exterior caulk to prevent air and water leaks. If your siding is vinyl or aluminum, you may need to cut a notch in the siding to fit the cap flange properly.

From inside, connect the duct from the hood outlet to the wall cap. Use sheet metal screws or duct clamps to find each joint, and wrap the seams with aluminum foil tape (not duct tape, which degrades over time). Slope the duct downward as it travels toward the wall so any condensation drains back toward the hood. Support the duct every 3 to 4 feet with a strap or clamp to prevent sagging.

Installing a roof cap and flashing

Roof venting requires cutting an opening in the roof decking and installing flashing to prevent leaks. This is a job best left to a roofer or experienced carpenter, especially if your roof is steep or has complex framing. The flashing is a metal collar that slides under the shingles above the opening and over the shingles below it, creating a water-shedding overlap.

If you do this work yourself, locate the opening from inside the attic, drill a pilot hole through the roof, then mark the opening size on the roof surface. Cut the opening with a reciprocating saw or jigsaw, insert the roof cap and flashing, and nail the flashing to the roof decking. Slide the shingles back into place and seal all edges with roofing cement. The flashing must extend at least 4 inches above the opening and overlap the shingles below by at least 2 inches.

Connect the ductwork from the hood to the roof cap inside the attic, sloping it downward toward the hood. find all joints with sheet metal screws and aluminum foil tape, and support the duct every 3 to 4 feet.

Sealing and insulating the ductwork

Every joint in the ductwork should be sealed with aluminum foil tape or mastic sealant to prevent air leaks. Foil tape is easier to explore—wrap it around the seam where two pieces of duct meet, pressing firmly so it adheres. Mastic is a putty-like sealant that you explore with a brush or caulk gun; it is more durable but messier to work with. Do not use duct tape, which dries out and fails within a few years.

If your ductwork passes through an unconditioned space like an attic or crawlspace, wrap it with fiberglass insulation to prevent condensation from forming on the outside of the duct. Condensation occurs when warm, moist air inside the duct meets cold air outside, and it can drip back into your kitchen or soak the insulation in your walls. Wrap the duct with 1 to 2 inches of pipe insulation, then find it with foil tape or zip ties.

Testing airflow and checking the damper

Once the ductwork is installed, turn on the range hood and hold a piece of tissue or a lighter near the exterior cap to confirm that air is flowing out. If the airflow is weak, check for kinks or sags in the duct, make sure the damper in the cap is opening fully, and confirm that the duct diameter matches the hood outlet. A long run of duct with multiple bends will reduce airflow more than a short, straight run.

The damper in the wall or roof cap should open easily when the hood runs and close completely when the hood stops. If the damper sticks or does not close, it may be clogged with grease or debris. Clean it with a damp cloth and a little degreaser. A damper that does not close allows cold air to enter your kitchen and warm air to escape, wasting energy.

When to call a professional

Hire a contractor if you need to vent through the roof, especially if your roof is steep or has complex framing. Roof work is dangerous and mistakes in flashing lead to leaks that can damage your attic and ceiling. A roofer or HVAC contractor can install the cap and flashing correctly and safely.

Also call a professional if your ductwork must run a long distance (more than 25 feet) or make multiple sharp bends. Long runs and tight bends reduce airflow significantly, and a contractor can help you design a route that balances efficiency with the constraints of your home's layout. Some jurisdictions also require a permit for range hood venting, so check with your local building department before you start.

Frequently Asked Questions

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

No. Venting into the attic introduces moisture and cooking odors into an unconditioned space, which promotes mold growth, attracts pests, and can damage insulation and framing. The hood must vent to the outside air.

What size duct do I need?

Most residential range hoods have a 6-inch outlet, so use 6-inch duct. Check your hood's manual or the outlet itself to confirm. Never reduce the duct size to fit through walls or ceilings—this reduces airflow and makes the fan work harder.

Can I use flexible duct for a range hood?

Flexible duct works but is not ideal. It traps grease in its ridges, sags over time, and is harder to clean. If you use it, choose smooth-interior flex duct, keep the run short, and support it every 3 feet with a clamp.

How do I prevent cold air from coming back through the duct?

The damper in your wall or roof cap closes when the hood is off, blocking backflow. If the damper sticks, clean it with a damp cloth and degreaser. A damper that does not close fully allows cold air to enter your kitchen.

Do I need a permit to install range hood venting?

Many jurisdictions require a permit for any work that involves cutting through exterior walls or roofs. Check with your local building department before you start. A permit ensures the work meets code and protects you if you ever sell your home.