The basic path: hood to wall or roof, then outside

A range hood vents to the outside by running metal ductwork from the hood itself through your wall or ceiling to open air. The duct carries cooking smoke, steam, and odours away from your kitchen instead of recirculating them back into the room. The shortest, most direct route is almost always the best one — a straight path to an exterior wall beats a long run through your attic or crawlspace, because longer ducts lose suction and collect grease.

Most hoods connect to either 6-inch or 8-inch round duct (measured by diameter). Your hood manual will specify which size it needs. The duct runs from the hood outlet, through walls or ceilings, and terminates outside with a wall cap or roof flashing that prevents rain and pests from entering.

Key Takeaways

  • Ductwork should run as straight as possible from the hood to the outside wall or roof, because bends and length reduce airflow and trap grease.
  • Use rigid metal duct or insulated flex duct rated for kitchen exhaust — never plastic dryer vent hose, which is a fire hazard in this process.
  • The duct must terminate outside with a wall cap or roof flashing, not inside your attic or crawlspace.
  • Ductwork should slope slightly downward toward the hood to prevent condensation and grease from pooling inside the duct.
  • Dampers (one-way flaps) prevent outside air from flowing backward into your kitchen when the hood is off.

Choosing between wall and roof venting

Wall venting is simpler and cheaper. You drill or cut a hole through the exterior wall closest to the hood, run the duct straight through, and attach a wall cap on the outside. This works well for kitchens on the ground floor or any kitchen where an exterior wall is nearby. The duct run is short, airflow stays strong, and grease does not collect as easily.

Roof venting requires running ductwork up through your attic or ceiling cavity and out through the roof. This path is longer, which means more resistance to airflow and more places for grease to accumulate inside the duct. Roof venting makes sense only when no exterior wall is close enough, or when the hood is on an upper floor and a wall vent would be awkward. If you choose roof venting, the duct must slope downward back toward the hood so condensation drains toward the hood, not pooling inside the duct.

Either way, the duct should be as short and straight as possible. Every bend, every foot of extra length, and every change in direction reduces the hood's suction power.

Ductwork materials and sizing

Use rigid metal duct (galvanized steel or aluminum) or insulated flex duct designed for kitchen exhaust. Rigid duct is more durable and easier to clean; flex duct is easier to route around obstacles. Never use plastic dryer vent hose or uninsulated flex duct — both are fire hazards because grease-laden air can ignite inside them, and they do not withstand the heat and moisture of kitchen exhaust.

Match the duct diameter to your hood outlet. Most residential hoods use 6-inch duct; some larger or commercial-style hoods need 8-inch. Stepping down to a smaller duct (for example, 6-inch duct on an 8-inch hood outlet) creates a bottleneck and kills airflow. Stepping up is wasteful but does not hurt performance.

Seal all duct joints with metal foil tape or duct mastic (a sticky sealant made for this purpose). Do not use cloth duct tape — it degrades in the heat and humidity of kitchen exhaust. Sealed joints prevent air leaks that reduce suction and allow grease to escape into your walls.

Routing the duct through walls and ceilings

Before you cut, locate studs, joists, electrical wiring, and plumbing using a stud finder and by checking your home's framing layout if you have it. Cutting through a live wire or water line is dangerous and expensive. If obstacles are in the way, route the duct around them rather than crushing or kinking it.

For wall venting, drill or cut a hole through the exterior wall at the same height as the hood outlet, or slightly higher. Run the duct straight through and find it to the hood outlet with a hose clamp or the connector provided by the hood manufacturer. On the outside, attach the wall cap with the included fasteners and seal around it with exterior caulk to keep water out.

For roof venting, run the duct up through the attic or ceiling cavity and out through the roof. You will need roof flashing (a metal collar that sits under the shingles and over the duct opening) to prevent leaks. Flashing is installed by sliding it under the upper shingles, securing it with roofing nails, and sealing the edges with roofing cement. If you are not comfortable working on a roof, hire a roofer to install the flashing.

Installing dampers and preventing backflow

A damper is a one-way flap inside the duct that opens when the hood runs and closes when it stops. This prevents cold outside air from flowing backward into your kitchen and prevents pests from entering through the duct. Most wall caps and roof flashing come with a built-in damper. If yours does not, install a separate damper inside the duct near the hood outlet.

Check that the damper moves freely — a stuck damper defeats its purpose and reduces airflow. Over time, grease buildup can jam a damper, so inspect it every few months and clean it if needed. A damper that will not close is a sign that the duct needs cleaning.

Insulating ductwork to reduce condensation

If your duct runs through an unheated space (an attic, crawlspace, or exterior wall cavity), condensation will form inside the duct on cold days. This moisture mixes with grease, drips back into your hood, and can damage the motor. Insulated flex duct or rigid duct wrapped with fibreglass insulation prevents this. The insulation keeps the warm exhaust air warm as it travels outside, so it does not cool enough to condense.

If you use rigid duct, wrap it with 1-inch fibreglass duct wrap and find it with foil tape. If you use flex duct, buy the insulated version from the start — it costs only slightly more than uninsulated and saves you the wrapping step.

Termination caps and preventing rain entry

The duct must end outside with a cap that prevents rain from entering and allows air to flow out freely. A wall cap is a straightforward box with a damper flap that mounts flush to your exterior wall. A roof cap is a cone or dome that sits on top of the roof flashing. Both should have a damper that closes when the hood is off.

Position the cap so the damper flap opens downward and outward — this helps rain run off rather than pooling. Make sure the cap is not blocked by plants, debris, or snow. A blocked cap forces the hood to work harder and can cause backflow into your kitchen.

Inspect the cap and damper twice a year, especially before winter. Clear any lint, grease, or debris. If the damper sticks, clean it or replace the cap.

Frequently Asked Questions

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

No. Venting into the attic, basement, or crawlspace fills those spaces with moisture and grease, which damages insulation, promotes mold, and creates a fire hazard. The duct must terminate outside with an open cap.

What if I cannot reach an exterior wall or roof without running a very long duct?

A long duct run loses suction and traps grease, but it is still better than venting indoors. If the run is longer than 25 feet or has more than four bends, consider upgrading to a more powerful hood or relocating the hood closer to an exterior wall. Some commercial-grade hoods are designed to handle longer runs.

Do I need to hire a professional to install the ductwork?

Wall venting is straightforward enough for a handy homeowner — you need a drill, a hole saw, and basic tools. Roof venting is more complex because it involves working at height and installing flashing correctly to prevent leaks. If you are not comfortable on a roof or cutting through framing, hire a contractor or roofer.

What size hole do I drill for the ductwork?

For 6-inch duct, use a 6.5-inch hole saw to leave a small gap for the wall cap flange. For 8-inch duct, use an 8.5-inch hole saw. The gap allows the cap to sit flush against the wall and be sealed with caulk.

How often should I clean the ductwork?

Inspect the duct and damper every three to six months. If you cook frequently or use high heat often, grease will accumulate faster. Most residential ducts need a thorough cleaning once a year. A professional duct cleaning service can remove buildup that you cannot reach yourself.