Range Hood Duct: A Complete Guide for Homeowners

You can buy the most powerful range hood on the market and still end up with a smoky kitchen if the ductwork behind it is undersized or poorly routed. The range hood duct is the part nobody sees, yet it decides whether all that airflow the fan promises actually leaves your house. Too small, too long, or too twisty, and the blower chokes. This guide covers duct sizing, materials, routing, and the venting rules that make the difference between a hood that clears the air and one that just makes noise.

Why the Duct Matters More Than the Hood

Range hood performance is rated in CFM, cubic feet per minute of air moved. But that number is measured under ideal conditions. Every foot of duct, every elbow, and every transition adds resistance, called static pressure, that the blower has to overcome. Undersize the duct or add too many turns, and a 600 CFM hood might deliver a fraction of that at the cooktop. The blower simply can’t pull air through a restrictive path.

That’s why matching the duct to the hood is not optional. The manufacturer specifies a minimum duct diameter for a reason, and reducing it to fit an existing hole is one of the most common and costly mistakes homeowners make.

Sizing the Duct Diameter

Duct size scales with the hood’s airflow. As a general rule, follow the manufacturer’s spec, but these ranges are typical:

• Up to 400 CFM: 6-inch round duct
• 400 to 600 CFM: 7-inch round duct
• 600 to 900 CFM: 8-inch round duct
• 900 to 1,200 CFM: 10-inch round duct or larger

Round duct flows more efficiently than rectangular for the same cross-sectional area, so it’s the preferred choice when you have the space. If you must use rectangular duct in a tight wall cavity, size up to keep the equivalent area, and use a proper transition fitting rather than crushing round pipe into a slot.

Never step down the duct size to match an old vent. If your new hood calls for 8-inch and the existing wall cap is 6-inch, replace the cap and enlarge the run. Necking down to a smaller pipe strangles the airflow and defeats the purpose of a stronger hood.

Choosing the Right Duct Material

Rigid metal duct is the standard for range hoods, and for good reason. Smooth galvanized steel or stainless steel offers the least resistance and won’t sag or trap grease the way flexible duct does. Rigid duct is also easier to clean and far less of a fire hazard, since kitchen exhaust carries grease-laden air.

Avoid flexible foil or plastic duct entirely. Its corrugated interior creates enormous resistance, its ridges collect grease, and many building codes prohibit it for range hoods precisely because of the fire risk. If an installer suggests flex duct to save time, push back and insist on rigid pipe. Seal every joint with foil tape or approved duct sealant, and secure connections mechanically so they can’t pull apart over time.

Keeping the Run Short and Straight

Airflow loves a straight, short path. Every elbow you add is roughly equivalent to adding several feet of straight duct in terms of resistance. A common guideline is to treat each 90-degree elbow as about 5 to 10 feet of equivalent straight run, depending on the fitting. That’s why the best installations route the duct as directly as possible to the exterior.

When you must turn, use gentle, smooth-radius elbows rather than sharp bends, and never place two elbows right next to each other if you can avoid it. Many manufacturers cap the total equivalent length at a specific number of feet for a given CFM; exceed it and you may need a larger duct or an inline booster fan. Plan the route before you cut a single hole.

Venting Options: Wall, Roof, and the Recirculating Trap

A range hood duct should terminate outdoors, either through an exterior wall or up through the roof. Wall venting is usually the shortest and simplest, especially for a hood on an exterior wall. Roof venting is common for island hoods or interior walls, but it means a longer vertical run and roof penetration that must be flashed and sealed properly to prevent leaks.

Both terminations need an exterior cap with a backdraft damper to stop cold air, pests, and rain from entering when the hood is off. Choose a cap sized to your duct, and avoid caps with fine mesh screens that clog with grease and choke airflow. Never vent a range hood into an attic, crawlspace, or soffit; dumping greasy, moist air into those spaces invites mold, structural damage, and fire hazards.

Ductless or recirculating hoods skip the ductwork entirely and pass air through a charcoal filter before returning it to the room. They’re a fallback when running a duct is truly impossible, but they don’t remove heat or moisture and they capture far less grease and odor. If you cook regularly, a properly ducted hood is worth the extra effort.

Makeup Air: The Rule People Forget

Powerful hoods, generally those rated 400 CFM and above, pull so much air out of the house that they can create negative pressure. In a tightly sealed modern home, that negative pressure can backdraft combustion appliances like a furnace, gas water heater, or fireplace, pulling dangerous flue gases including carbon monoxide back into the living space. Many building codes now require a dedicated makeup air system for high-CFM hoods.

Because this involves combustion safety, it’s not a detail to guess at. Confirm your hood’s CFM against your local code’s makeup air threshold, and have a licensed HVAC contractor or your building official evaluate whether your home needs a makeup air kit. This is exactly the kind of planning decision that protects your family rather than just your cooktop.

Insulating and Sealing the Duct

In colder climates, a duct running through an unheated attic or an exterior wall can create condensation problems. Warm, moist kitchen air hits the cold metal, water condenses, and it can drip back down toward the hood or pool inside the run. Insulating the duct where it passes through unconditioned space helps keep the metal warmer and reduces condensation. It’s an easy thing to overlook, but a duct sweating inside a wall causes hidden moisture damage over time.

Sealing the joints properly serves two purposes. First, it keeps greasy air from leaking into wall and ceiling cavities, which is both a mess and a fire concern. Second, a well-sealed duct maintains the airflow the blower was designed to move, since air escaping through gaps is air that never makes it outside. Use foil tape rated for the application or an approved duct mastic, and secure each connection mechanically so vibration can’t work it loose over the years.

Matching Duct Length to Blower Power

Every blower has a limit to how much resistance it can overcome, and long or convoluted duct runs eat into that budget fast. If your kitchen layout forces a long run or several turns, you have a few options. You can size the duct up a diameter to reduce resistance, choose a hood with a more powerful blower rated for longer runs, or add an inline or remote blower mounted closer to the exterior termination, which pulls rather than pushes the air and handles long runs better.

Remote and inline blowers have another benefit: they move the noisy motor away from the cooktop, so the hood is quieter at the source. If you’re planning a kitchen where the range sits on an interior island far from any exterior wall, this is often the smart approach. Plan the blower choice and the duct route together rather than buying the hood first and hoping the ductwork cooperates.

Installation and Maintenance Tips

Mount the hood at the height the manufacturer specifies, typically 24 to 30 inches above an electric cooktop and 24 to 36 inches above gas, balancing capture efficiency against clearance. Support the duct at regular intervals so it can’t sag, and keep the interior as smooth as possible. Once installed, clean the mesh or baffle filters every few weeks depending on how much you cook, since a grease-clogged filter also strangles airflow.

Have the duct itself inspected and cleaned periodically, especially if you fry often, because grease buildup inside the run is both an efficiency and a fire concern. A well-designed range hood duct paired with routine cleaning keeps your kitchen air clear, your walls free of grease film, and your blower running the way it was engineered to. Spend your effort on sizing and routing the duct correctly, and the hood will finally deliver the performance you paid for.