The Basic Path: Ducting from Hood to Outside Air

A range hood vents to the outside by running a metal duct from the back or bottom of the hood, through your wall or ceiling, and out through an exterior wall or roof. The duct carries cooking smoke, steam, and odors away from your kitchen instead of recirculating them back into the room. This is the most effective way to remove heat and moisture from cooking.

The duct itself is usually 6 inches in diameter for residential hoods, though some larger commercial-style hoods use 8-inch ducts. The duct must slope slightly downward as it travels outside — about 1/4 inch per foot of run — so that any condensation drains back toward the hood rather than pooling inside the duct and dripping back into your kitchen.

You will also need an exterior duct cap or damper, which is a vent cover that sits on the outside of your wall or roof. This cap prevents rain, insects, and outside air from entering the duct when the hood is off.

Key Takeaways

  • Metal duct must slope downward toward the hood at about 1/4 inch per foot to prevent condensation from backing up into your kitchen.
  • The shortest, straightest duct path works best — every bend and every foot of length reduces the hood's ability to pull air out.
  • An exterior damper or cap prevents rain and pests from entering the duct when the hood is not running.
  • Flexible duct is easier to install but collects grease and moisture more easily than rigid metal duct, so rigid is preferred for long runs.
  • Most building codes require the duct to terminate at least 10 feet away from windows, doors, and air intakes on your home or a neighbor's home.

Choosing Between Wall and Roof Venting

Wall venting is simpler and faster. You drill or cut a hole through the exterior wall, run the duct straight through, and attach the cap on the outside. The duct run is usually short — often just 1 to 3 feet — which means less resistance and better performance. Wall venting also avoids the complexity of cutting through roof framing and dealing with roof flashing, which can leak if installed incorrectly.

Roof venting is necessary when your hood is on an interior wall with no direct path to an outside wall, or when the exterior wall faces a neighbor's property and codes restrict where you can terminate. Roof venting requires cutting through the roof structure, installing a roof flashing kit to seal around the duct, and ensuring the duct slopes correctly even as it travels upward. A roof vent is more prone to leaks if the flashing is not sealed properly, and snow or debris can block the cap in winter.

Check your local building code before you choose. Some jurisdictions restrict roof venting for range hoods, or require the duct to be insulated if it passes through an unconditioned attic space.

Measuring and Planning the Duct Route

Before you buy materials, trace the path the duct will take from the hood to the outside. Measure the horizontal distance, any vertical rise, and count how many bends you will need. Each 90-degree bend adds resistance equivalent to about 10 feet of straight duct, so minimize turns wherever possible.

Locate the hood's duct outlet — it is usually at the back or bottom of the unit, and the manual will show you which. Then find the best spot on your exterior wall or roof to exit. For wall venting, choose a spot that is as close to the hood as possible and at roughly the same height, so the duct stays level or slopes slightly downward. For roof venting, you will need to run the duct upward through your ceiling and attic.

Measure the duct diameter required by your hood's manual — 6 inches is standard, but confirm this before you buy duct and fittings. Check that your planned exit location meets code: typically at least 10 feet away from windows, doors, property lines, and air intakes (such as a furnace or air conditioning unit intake).

Installing Rigid Metal Duct for Best Performance

Rigid metal duct is the preferred choice for most range hood installations because it resists grease buildup, does not sag over time, and allows the hood to work at full strength. Rigid duct comes in straight sections that connect with slip fittings — one section slides partway into the next, and you find them with sheet metal screws or duct tape rated for high temperature.

Start at the hood outlet and work toward the outside. Attach the first section of duct to the hood's outlet using the hood's own duct collar or a transition fitting. Slide each new section into the previous one by about 1 to 2 inches, then drive three or four sheet metal screws through the overlapping area to lock them together. Do not rely on duct tape alone — screws hold the connection find even if tape degrades over time.

As you work toward the outside, maintain a slight downward slope. If you are running the duct horizontally, place a level on top of the duct and adjust supports so the duct is about 1/4 inch lower at the far end than at the hood. If the duct must rise (as in roof venting), keep the rise as gentle as possible, then slope downward again as it approaches the exterior cap.

Support the duct every 4 to 5 feet with metal brackets or straps. Do not let the duct sag between supports, because sagging creates low spots where condensation pools and grease collects.

Cutting Through Wall or Roof and Installing the Cap

For wall venting, locate your exit hole on the exterior wall. Drill a pilot hole from inside, then go outside and mark the center point. Use a drywall saw or hole saw to cut a hole slightly larger than your duct diameter — usually 6.5 inches for a 6-inch duct. Cut through the exterior sheathing and siding carefully to avoid damaging the wall structure.

Slide the duct through the hole and attach the exterior duct cap or damper. Most caps have a flange that sits flat against the exterior wall and is secured with screws or caulk. Seal around the cap with exterior-grade caulk to prevent water and air leaks. If the duct passes through an insulated wall cavity, wrap the duct with insulation to prevent condensation from forming on the outside of the duct.

For roof venting, the process is similar but requires a roof flashing kit designed for your duct size. The flashing is a metal boot that sits under the roof shingles and directs water away from the duct opening. Cut the roof hole, slide the flashing under the shingles above the hole, nail it down, and seal all edges with roofing cement. Then attach the duct and cap. Roof venting is more involved and is best done by someone experienced with roof work, because improper flashing is a common source of leaks.

Using Flexible Duct When Rigid Is Not Practical

Flexible duct (also called flex duct or dryer vent hose) is easier to route around obstacles and requires fewer fittings, so it is tempting to use for range hoods. However, flexible duct has a ribbed interior that traps grease and moisture, making it harder to clean and more prone to blockages. If you use flexible duct, keep the run as short as possible — ideally under 10 feet — and plan to clean it more often than you would rigid duct.

Connect flexible duct to the hood outlet and to the exterior cap using hose clamps or duct tape rated for high temperature. Make sure the duct does not kink or pinch, because this restricts airflow and creates a spot where grease accumulates. Support the duct every 3 feet to prevent sagging. Never use standard duct tape — it fails in the heat from cooking. Use foil tape or aluminum-backed tape rated for temperatures above 200 degrees Fahrenheit.

If you do use flexible duct, inspect it every few months for blockages or grease buildup. If the hood's suction weakens noticeably, the duct is likely clogged and needs cleaning.

Avoiding Common Installation Mistakes

The most frequent error is running the duct level or uphill toward the outside. This causes condensation to pool inside the duct, which then drips back into the hood or kitchen. Always slope the duct downward — even a slight slope of 1/4 inch per foot makes a difference.

Another common mistake is using too many bends or running the duct too long. Every bend and every extra foot of duct reduces the hood's suction. If the hood feels weak after installation, the duct path is likely too restrictive. Straighten the route if possible, or consider upgrading to a more powerful hood.

Do not terminate the duct directly under a soffit, eave, or overhang, because the exhaust will blow back onto your roof or siding. Terminate at least 12 inches away from any obstruction, and aim the cap downward and away from your home.

Finally, do not skip the exterior cap or damper. Without one, outside air and pests enter the duct when the hood is off, and rain can drip into your kitchen. A damper with a spring-loaded flap is ideal because it opens only when the hood runs.

Frequently Asked Questions

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

No. Venting into an attic or crawlspace fills those areas with moisture and grease, which damages insulation, promotes mold growth, and creates a fire hazard. Building codes prohibit this. The duct must terminate outside your home's envelope.

What size duct do I need?

Check your range hood's manual — most residential hoods use 6-inch diameter duct, but some use 5-inch or 8-inch. Using the wrong size reduces performance and may void the warranty. Match the duct size to the hood's outlet.

How often should I clean the duct?

With rigid metal duct, cleaning is rarely necessary if the hood has a good grease filter. With flexible duct, inspect every few months and clean if you notice reduced suction. If you cook frequently with high heat or fry often, plan to clean rigid duct once or twice a year.

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

No. PVC pipe melts at the temperatures range hood exhaust reaches. Dryer vent hose is not rated for the heat and grease from cooking. Use only duct rated for range hoods — typically 26-gauge galvanized steel or aluminum rigid duct, or high-temperature flexible duct.

What if my duct cap keeps getting blocked by snow or debris?

Install a cap with a larger hood or guard, or choose a location that is more exposed to wind. Some caps have a screen to keep pests out while allowing air to flow — these are less prone to blockage than solid caps. Avoid locations directly under trees or in low spots where debris collects.