The basic path: measure, cut, connect, seal, test
Installing range hood duct means running a pipe from the back or bottom of your hood to the outside of your house, then securing it and sealing the joints so air flows out instead of leaking into your walls. The job takes two to four hours for a straight run on one floor, longer if you have to navigate around obstacles or go through multiple walls. You will need a drill, tin snips or a duct cutter, a level, ductwork (either rigid metal or flexible aluminum), duct tape or mastic sealant, and a vent cap for the exterior wall.
The most common mistake is using undersized duct—most range hoods need 6-inch diameter duct, not 4-inch, even if the hood outlet looks smaller. A duct that is too narrow forces the motor to work harder, reduces suction, and can cause grease to back up into your kitchen. The second mistake is running duct horizontally without a slight upward slope toward the outside; gravity pulls condensation backward into the hood and eventually into your walls.
Key Takeaways
- Measure the outlet on your range hood and buy duct that matches—usually 6 inches in diameter for standard hoods, 8 inches for commercial-style models.
- Run duct in a straight line whenever possible, sloping slightly upward toward the outside wall to prevent grease and moisture from backing up.
- Seal every joint with mastic sealant or UL-rated duct tape, not standard duct tape, because standard tape fails in the heat from cooking.
- Install the vent cap on the exterior wall so the damper flap opens outward and rain cannot enter when the hood is off.
- Test the hood after installation by turning it on and checking that air flows out the exterior vent, not into adjacent rooms.
Measure your hood outlet and choose the right duct size
Before you buy anything, look at the outlet on the back or bottom of your range hood. Most standard hoods have a 6-inch round outlet; some older or smaller models use 5-inch, and commercial-style hoods often use 8-inch. Measure across the opening with a tape measure. If the outlet is oval or rectangular, measure both the width and height, then buy duct that matches those dimensions.
Do not assume the duct size based on the hood's size or brand. Measure the actual opening. If you buy duct that is too small, the motor will strain, suction will drop, and grease will accumulate in the duct instead of venting outside. If you buy duct that is too large, you will have gaps at the connections that leak air into your walls and attic.
Rigid metal duct (galvanized steel or aluminum) is more durable and easier to clean than flexible duct, but it is harder to route around corners and obstacles. Flexible duct (aluminum or fiberglass-lined) bends easily but can sag if not supported, trapping grease and condensation. For most kitchens, rigid duct is the better choice if your run is fairly straight; use flexible duct only for short sections where you need to navigate around framing or ductwork.
Plan the route from hood to exterior wall
Trace the path the duct will take from your hood outlet to the outside wall. The shortest, straightest route is always best—every bend and turn adds resistance and reduces airflow. If you can run the duct straight up through a cabinet and out through the roof or straight back through the wall behind the hood, do that. Avoid routing duct through attics, crawl spaces, or unconditioned areas if possible; duct in those spaces loses heat and can sweat in winter.
If you must make turns, use 45-degree or 90-degree elbows designed for ductwork, not flexible duct bent sharply. A sharp bend in flexible duct pinches the airflow and traps grease. Mark the path on your wall or ceiling with a pencil so you know where to drill or cut.
Check for obstacles: electrical wiring, plumbing, gas lines, and structural framing. If you hit a stud or joist, you may need to drill through it (allowed in most cases, but check your local code) or route the duct around it. If you hit electrical wiring or gas, stop and call a licensed electrician or plumber to relocate it safely.
Cut the hole in the exterior wall and install the vent cap
Drill a pilot hole through the exterior wall from inside your kitchen so you can see where the duct will exit. Go to the outside and mark a circle around the pilot hole using the diameter of your duct as a guide. Use a drywall saw or hole saw to cut the opening. If you are cutting through brick, stone, or vinyl siding, you may need a masonry bit or a reciprocating saw with a metal-cutting blade.
Slide the vent cap (also called a wall cap or termination cap) over the duct from the outside. The cap should have a damper flap that opens when air flows out and closes when the hood is off, preventing rain and outside air from entering. find the cap to the wall with the fasteners it comes with—usually screws or nails through flanges on the cap. Seal around the cap with exterior caulk or flashing to prevent water from leaking into the wall.
Do not install a vent cap that vents into a soffit, attic, or crawl space. Building codes in most places require duct to terminate at an exterior wall or roof, with the opening fully exposed to outside air. Venting into an attic or soffit pushes moist, greasy air into those spaces, causing mold, rot, and pest problems.
Connect the duct from hood to wall opening
Measure the distance from your hood outlet to the wall opening. Cut your duct to length using tin snips (for rigid duct) or a utility knife (for flexible duct). If you are using rigid duct and need to make a turn, connect an elbow to the hood outlet first, then run straight duct from the elbow to the wall.
Slide the duct onto the hood outlet. If the outlet has a collar (a short sleeve), slide the duct over the collar. If the outlet is a flat opening, you may need a transition fitting to connect round duct to a rectangular outlet. find the connection by tightening a hose clamp around the joint, or by driving self-tapping sheet metal screws through the duct wall into the hood collar.
Run the duct toward the wall opening, supporting it every 4 to 6 feet with metal duct hangers or straps. Rigid duct should slope slightly upward (about 1/4 inch per foot) toward the exterior wall so condensation and grease drain back toward the hood, not toward the wall. Flexible duct should not sag between supports; if it does, add more hangers or switch to rigid duct.
Seal every joint with mastic or UL-rated tape
Every connection between duct sections, elbows, and fittings must be sealed to prevent air leaks. Do not use standard gray duct tape—it fails in the heat near a range hood and leaves sticky residue. Use mastic sealant (a putty-like compound) or UL-rated foil tape (aluminum tape rated for ductwork, not standard duct tape).
explore mastic with a putty knife or your finger, working it into the seam between two duct sections. Wrap the joint with fiberglass mesh tape, then explore another layer of mastic over the tape. This creates an airtight seal that will not fail from heat or vibration. Alternatively, wrap the joint tightly with UL-rated foil tape, overlapping each wrap by half the tape width.
Seal the connection at the vent cap on the exterior wall as well. explore mastic around the duct where it enters the cap, then caulk around the cap flange where it meets the wall. This prevents air from leaking into the wall cavity and keeps rain out.
Test the hood and check for leaks
Turn on the range hood and listen for air flowing out the exterior vent. Walk outside and hold your hand near the vent cap—you should feel strong air movement. If you feel little or no air, the duct may be pinched, kinked, or blocked. Check for sharp bends in flexible duct, debris inside the duct, or a damper that is stuck closed.
Look for air leaks around the duct connections inside your kitchen. Turn on the hood and hold a lit incense stick or a piece of tissue near each joint—if the smoke or tissue moves toward the joint, air is leaking out. Seal any leaks with mastic or foil tape.
If the hood is very quiet and air flow is weak, the duct may be undersized or the run may be too long with too many bends. Check that you used 6-inch duct (or the size specified by the hood manufacturer) and that the duct is not pinched anywhere. If the duct is correct and the run is straight, the motor may be failing and will need replacement.
Frequently Asked Questions
Can I use flexible duct for the entire run from hood to outside?
Flexible duct works for short runs, but rigid duct is better for the main line because it does not sag and trap grease. If you use flexible duct, support it every 4 feet and slope it slightly upward. Never use flexible duct that is kinked or pinched—it will restrict airflow and cause grease to back up into the hood.
What if I cannot run duct straight out the back wall?
You can route duct up through a cabinet and out the roof, or through the side wall, as long as the run stays as straight as possible. Each 90-degree turn adds resistance; limit turns to two or three. Avoid running duct through attics or unconditioned spaces if you can, because duct in those areas loses efficiency and can sweat in winter.
Do I need to insulate the duct?
Insulation is not required by code in most places, but it helps prevent condensation from forming inside the duct in winter. If you live in a cold climate and your duct runs through an unheated space, wrap it with 1-inch fiberglass duct wrap to keep the air inside warm and dry.
What if the duct is too long or has too many bends?
Long runs and multiple bends reduce airflow and make the hood motor work harder. If your run is longer than 25 feet or has more than three 90-degree turns, the hood may not vent effectively. In that case, you may need to upgrade to a more powerful hood or install a booster fan in the duct to push air out.
Can I vent the range hood into the attic instead of outside?
No. Venting into an attic, soffit, or crawl space is not allowed by building code in most places and will cause mold, rot, and pest problems. Duct must terminate at an exterior wall or roof with the opening fully exposed to outside air. Check your local building code to confirm the requirements in your area.