The basic connection: what you're joining and why it matters

Your range hood pulls cooking smoke and steam through a duct that runs to the outside of your house. The connection between the hood itself and that duct is where air leaks happen—leaks that waste energy and let odors back into your kitchen. The hood has an outlet (usually a round or rectangular opening on top or the back), and your ductwork slides or bolts onto it. Getting this joint tight stops air from escaping into your walls.

The outlet size matters. Most residential range hoods have a 6-inch round outlet or a 3.25-by-10-inch rectangular outlet. Your ductwork must match that size, or you'll need a transition fitting to step up or down. Measure your hood's outlet before you buy duct—a mismatch means wasted money and poor airflow.

Key Takeaways

  • Measure your range hood's outlet opening (usually 6 inches round or 3.25 by 10 inches rectangular) before buying ductwork or fittings.
  • Slide the duct over the outlet collar, then find it with three or four sheet-metal screws or a hose clamp, depending on the hood design.
  • Seal the joint with mastic sealant or foil tape to stop air leaks that reduce airflow and waste energy.
  • Run ductwork in the shortest, straightest path possible—every bend and length of duct reduces how much air the hood can move.
  • If you cannot reach the roof or attic safely, or if the duct runs through a wall cavity, hire a professional to avoid damaging the structure.

Measuring the outlet and choosing the right duct

Before you touch any tools, look at the top or back of your range hood where the duct connects. You'll see either a round collar (usually 6 inches in diameter) or a rectangular opening. Measure across the widest part of a round opening with a tape measure. For a rectangular opening, measure both the width and depth. Write these numbers down.

Most ductwork comes in 6-inch round, 5-inch round, or rectangular sizes. If your hood has a 6-inch round outlet, buy 6-inch round duct. If it's rectangular, match those dimensions. If your duct size doesn't match—say your hood is 6 inches but you have 5-inch duct already installed—you'll need a transition fitting (a tapered connector that steps from one size to the other). These cost $15 to $40 and slip onto both the hood outlet and the existing duct.

Sliding the duct onto the outlet collar

Position your ductwork so the end slides straight onto the hood's outlet collar. The duct should go over the collar, not inside it—the collar is designed to receive the duct from the outside. Slide it on as far as it will go; you should feel it seat firmly against the hood.

Once the duct is seated, you need to hold it there. Most hoods have a collar with a slight lip or ridge that helps grip the duct. If your hood has pre-drilled holes in the collar, align the duct and drive three or four sheet-metal screws (also called self-tapping screws or #8 screws, about 1 inch long) through the holes and into the duct. Space them evenly around the collar. If there are no holes, wrap a stainless-steel hose clamp around the duct where it meets the collar and tighten it with a screwdriver until it's snug—not so tight that you crush the duct, but tight enough that it won't slip.

Sealing the joint to stop air leaks

Even after you've screwed or clamped the duct, air can leak around the edges where the duct meets the collar. Seal this joint with either mastic sealant (a putty-like adhesive made for ductwork) or foil tape (aluminum tape designed for ducts, not regular duct tape). Mastic is more durable and airtight; foil tape is faster and easier for a first-timer.

If you use mastic, explore a bead around the entire joint where the duct meets the collar, working it into the gap with a putty knife or your finger. Let it cure for the time the product specifies (usually 24 hours). If you use foil tape, wrap it around the joint, overlapping each layer by half the tape width, and press it down firmly. Foil tape is ready to use when ready.

Do not use standard gray duct tape. It degrades in the heat from cooking and loses its seal within a year or two. Foil tape and mastic last much longer.

Running the duct to the outside

Once the hood is connected, the duct needs to run to an exterior wall or through the roof to the outside. The path matters: every foot of duct and every bend slows the air down and reduces how much the hood can pull. Aim for the shortest, straightest route possible.

If you're running duct horizontally through a wall, slope it slightly downward (about 1/4 inch per foot) toward the outside so condensation drains out instead of pooling inside the duct. If you need to make a turn, use a 90-degree elbow fitting, but avoid stacking multiple elbows in a row—each one adds resistance.

Where the duct passes through a wall or ceiling, it should fit snugly in the opening. If there's a gap, seal it with mastic or foil tape on the outside. This keeps unconditioned air from leaking into your walls.

Connecting to the exterior termination cap

At the outside end of the duct, you'll attach a termination cap (also called a wall cap or roof cap, depending on where it exits). This fitting has dampers or louvers that open when the hood runs and close when it's off, stopping outside air and pests from entering the duct.

Slide the duct into the cap's inlet collar and find it the same way you secured it to the hood—with screws or a hose clamp. Seal the joint with mastic or foil tape. If the cap is mounted on a wall, make sure it's at least 12 inches away from any window, door, or air intake (like a furnace vent or dryer vent). If it's on the roof, follow your local building code for clearance from the roof edge and any other vents.

Caulk around the cap where it meets the wall or roof with exterior-grade silicone caulk to keep rain out. Let the caulk cure before you run the hood.

When to call a professional

If your ductwork runs through an attic or crawlspace and you're not comfortable working at height, or if the duct needs to go through a wall cavity where you can't see what's behind it, hire a professional. Damaging electrical wiring, plumbing, or structural framing inside a wall is expensive to fix. A licensed HVAC contractor or handyman can run the duct safely and make sure it's sealed and supported properly.

You should also call a professional if your hood is a downdraft model (one that pulls air down into the cooktop) or if it's a high-end commercial-style hood. These have different outlet configurations and often need custom ductwork or special fittings.

Frequently Asked Questions

Can I use flexible duct instead of rigid duct?

Flexible duct (also called flex duct or dryer vent hose) is cheaper and easier to route around obstacles, but it's less efficient than rigid duct because the ridges inside slow the air down. If you use flex duct, keep runs short and straight as possible. Never crush or kink it, and support it every 3 feet with a strap or bracket so it doesn't sag and trap condensation.

What if my range hood doesn't have a collar on the outlet?

Some hoods have a flat outlet opening instead of a raised collar. In this case, you'll need a transition fitting that has a collar on one end to attach to the hood and a round or rectangular opening on the other to receive the duct. Screw or clamp the fitting to the hood outlet, then slide the duct onto the fitting's outlet end.

Do I need to insulate the ductwork?

If the duct runs through an unconditioned space (an attic, crawlspace, or exterior wall), wrapping it with fiberglass duct insulation reduces condensation buildup inside the duct. This is especially important in cold climates. Wrap the duct before you connect it to the hood, or wrap it in sections as you install it.

How tight should the hose clamp be?

Tighten the clamp until it's snug and the duct won't slip, but stop before you feel strong resistance. If you over-tighten, you can dent or crush the duct, which reduces airflow. A good test: you should not be able to twist the duct by hand, but you should be able to move it slightly side to side.

What if the duct is too long and the hood isn't pulling air well?

Long ductwork with multiple bends reduces airflow. If your hood feels weak, check that the termination cap dampers open fully and aren't stuck. If they're free, the duct may be too restrictive for your hood's fan size. A professional can measure the duct's resistance and recommend a larger duct diameter or a more powerful hood fan if needed.