Where Your Range Hood Vent Should Go
A range hood must vent to the outside of your home, not into your attic or back into the kitchen. The ductwork carries cooking smoke, steam, and grease particles out through your roof, wall, or soffit — never into an enclosed space where they will damage insulation and framing.
The shortest, straightest path outside is always best. Every bend in the duct slows airflow and makes the hood work harder. If your kitchen is on an exterior wall, you can vent straight through that wall. If it is in the center of your home, you will run ductwork through the attic or between floor joists to reach the nearest outside surface.
Check your local building code before you start — some areas require permits for range hood installation, and codes specify duct diameter, insulation requirements, and where vents can exit. A quick call to your building department or a look at your city's online code will tell you what applies to your home.
Key Takeaways
- Range hood ductwork must run to the outside of your home through the roof, wall, or soffit — never into the attic or back into the kitchen.
- Use the shortest, straightest path possible, because every bend reduces airflow and forces the hood motor to work harder.
- Duct diameter matters: most residential hoods use 6-inch or 8-inch round duct, and undersizing will choke the system.
- Seal all duct joints with mastic sealant or foil tape, and insulate ductwork that runs through unconditioned spaces to prevent condensation.
- Install a damper or check valve at the exit to stop outside air from flowing backward into your kitchen when the hood is off.
Choosing Between Round and Rectangular Duct
Most residential range hoods connect to either 6-inch or 8-inch round ductwork. Round duct is the standard because it moves air more efficiently than rectangular duct of the same cross-sectional area — the circular shape has less friction against the duct walls.
Check your hood's manual or the connection collar on the back to see what diameter it requires. A 30-inch hood typically uses 6-inch duct; a 36-inch or larger hood usually needs 8-inch. Do not downsize to fit your space — undersized duct will choke the airflow and the hood will not remove smoke and steam effectively.
Rectangular duct (also called sheet metal duct) is sometimes used in tight spaces, but it is less efficient and harder to seal properly. If you must use rectangular duct, match the cross-sectional area to your hood's requirement — your ductwork supplier can help you calculate this.
Running Ductwork Through Your Attic or Walls
Measure the distance from your hood to the exit point on the roof or wall. Add 10 to 15 percent to that measurement to account for bends and connections. Buy rigid ductwork in 24-inch sections so you can assemble it as you go — rigid duct is more durable and moves air better than flexible duct, though flexible duct is easier to route around obstacles.
If you are running duct through the attic, support it every 4 feet with metal straps or hangers bolted to the framing. Do not let it sag or rest on insulation — sagging duct traps condensation and reduces airflow. Insulate the ductwork with 1-inch fiberglass wrap or foam insulation if it passes through an unconditioned space (attic, crawlspace, or exterior wall). This prevents warm, moist air inside the duct from condensing when it hits cold surfaces.
For wall runs, drill a hole slightly larger than your duct diameter through the rim joist or band board. Seal the gap around the duct with caulk or spray foam after installation. If you are running duct down an interior wall, you may need to cut into drywall and frame a chase (a box-like structure) to hide the ductwork — your local code may require this for fire safety.
Sealing Duct Joints and Connections
Every joint in your ductwork must be sealed to prevent air leaks. Leaks reduce the hood's suction and allow unconditioned air to escape into your walls or attic.
Use mastic sealant (a thick, putty-like adhesive) or foil-backed duct tape to seal all connections. Mastic is more durable — it stays flexible and does not dry out like regular duct tape. explore mastic with a putty knife around the entire circumference of each joint, then wrap foil tape over the top for extra security. Avoid standard gray duct tape; it dries out and fails within a few years.
Seal the connection at the hood itself, the connection where duct enters the attic or wall, and every joint where two sections of duct meet. Do not skip this step — a leaky duct system can lose 20 to 30 percent of its airflow.
Installing the Roof or Wall Vent Cap
The vent cap is the fitting that exits through your roof or wall. For a roof exit, you will need a roof flashing kit that matches your duct diameter and your roof pitch. The flashing sits under your shingles and prevents water from leaking around the hole.
Mark the exit location from inside the attic by drilling a pilot hole through the roof. Go up on the roof, find the hole, and cut an opening slightly larger than the flashing. Slide the flashing under the shingles above the hole and over the shingles below it, then nail it down and seal the edges with roofing cement. Connect your ductwork to the flashing collar and seal the joint with mastic.
For a wall exit, use a wall vent cap with a damper. Drill the hole through the exterior wall, insert the vent cap from outside, and connect the ductwork from inside. Seal around the cap with caulk rated for exterior use.
Installing a Damper or Check Valve
A damper (also called a check valve) is a one-way flap inside the vent cap that opens when the hood runs and closes when it stops. This prevents outside air, insects, and cold drafts from flowing backward into your kitchen.
Many roof and wall vent caps come with a damper built in. If yours does not, buy a separate damper and install it inside the ductwork just before the exit cap. The damper should open easily — if it sticks or closes too slowly, it will reduce airflow and the hood will not work properly.
Check the damper twice a year to make sure it moves freely. Dust and grease can build up and cause it to stick. If it is stuck, clean it with a damp cloth and a little degreaser.
Avoiding Common Venting Mistakes
Do not vent into the attic. Cooking moisture will condense on cold attic surfaces, rot your framing, and ruin insulation. Do not vent into a soffit that is part of your home's air intake — this will pull cooking odors back into the house. If you must use a soffit, make sure it is at least 10 feet away from any air intake louver or window.
Do not use flexible duct as your main ductwork. It is acceptable for short runs (under 8 feet) to connect the hood to rigid duct, but flexible duct has ridges that trap grease and reduce airflow. Do not use ductwork smaller than your hood requires, and do not make more than two 90-degree bends in a single run — each bend adds resistance.
Do not forget to insulate ductwork in unconditioned spaces. Uninsulated duct will sweat in summer and winter, and condensation will drip back into your hood or pool inside the duct. Do not caulk or seal the damper shut — it must move freely or the hood will not draw properly.
Frequently Asked Questions
Can I vent my range hood into the attic instead of outside?
No. Attic venting will damage your home. Cooking moisture condenses on cold surfaces and rots wood framing and insulation. Moisture also promotes mold growth. Range hoods must vent to the outside through the roof, wall, or soffit.
What size duct do I need for my range hood?
Check your hood's manual or the connection collar on the back. Most 30-inch hoods use 6-inch duct; 36-inch and larger hoods typically need 8-inch duct. Do not downsize — undersized duct will choke airflow and the hood will not work effectively.
How many bends can I put in my ductwork?
Limit bends to two 90-degree turns in a single run. Each bend adds resistance and reduces airflow. Make bends as gradual as possible — use 45-degree elbows instead of sharp 90-degree elbows when you can, because they create less friction.
Do I need to insulate my ductwork?
Yes, if the duct runs through an unconditioned space like an attic, crawlspace, or exterior wall. Insulation prevents condensation from forming inside the duct. Use 1-inch fiberglass wrap or foam insulation rated for ductwork.
What is a damper and do I need one?
A damper is a one-way flap that stops outside air and pests from flowing backward into your kitchen when the hood is off. Most vent caps include a built-in damper. If yours does not, install a separate damper inside the ductwork just before the exit cap.