The Basic Route: Straight Through the Wall
The simplest way to vent a range hood outside is to run ductwork straight through an exterior wall. You drill or cut a hole from inside your kitchen to the outside, insert a duct that matches your hood's outlet size (usually 6 or 8 inches in diameter), and attach a wall cap on the outside to keep rain and pests out. This method works best if your range hood is on an exterior wall and you have a clear path without major obstacles like studs, pipes, or electrical wiring.
Before you cut, locate what's inside your wall using a stud finder and by checking your home's layout. If you hit a stud or pipe, you'll need to reroute slightly or call a contractor. The wall cap itself—a metal or plastic fitting with a damper flap—costs $15 to $40 and is the only part that shows on the outside of your house.
Key Takeaways
- Venting through an exterior wall is the shortest and most efficient route, requiring a hole, ductwork, and a wall cap with a damper to prevent backflow.
- Roof venting works when your hood is directly below the roof, but it requires flashing to seal the hole and prevent leaks, and it's harder to install than wall venting.
- Ductwork must slope slightly downward toward the outside to prevent grease and condensation from pooling inside the pipe.
- Dampers are essential—they close when the hood is off to stop cold air, pests, and odors from entering your kitchen.
- Flexible duct is easier to install than rigid metal duct, but rigid duct moves air more efficiently and lasts longer.
Measuring and Choosing Your Duct Size
Check your range hood's manual or the outlet opening itself to find the duct diameter—most residential hoods use 6-inch or 8-inch ducts. Measure twice. An undersized duct will choke airflow and make the hood noisy and ineffective. An oversized duct wastes money and reduces the hood's suction power.
Once you know the size, buy ductwork to match. You have two main choices: rigid metal duct (aluminum or galvanized steel) or flexible duct (a wire spiral wrapped in plastic or foil). Rigid duct is more efficient, quieter, and lasts 15+ years. Flexible duct is cheaper and easier to bend around obstacles, but it's noisier, collects grease inside the ridges, and may need replacing in 5 to 10 years. For a straight run through a wall, rigid duct is the better choice.
Installing Ductwork Through an Exterior Wall
Start by marking your hole on the inside wall at the height of your hood's outlet. Use a drill with a long bit to poke a pilot hole all the way through to the outside—this marks your target. Go outside and verify the hole location is clear of trim, siding seams, or obstacles. Mark a circle around the pilot hole using the duct diameter as your guide.
Cut the hole using a reciprocating saw or drywall saw on the inside, then cut through the siding and sheathing on the outside. You may need to remove a section of siding to fit the wall cap properly. Slide the duct through from inside, then attach the wall cap on the outside, sealing the gap between the cap and siding with caulk rated for exterior use. Inside, connect the duct to your hood's outlet using duct clamps or a collar, and seal any gaps with metal duct tape or mastic sealant.
Slope the duct slightly downward as it runs toward the outside—even a 1/4-inch drop per foot helps grease and condensation drain out instead of pooling inside the pipe. A horizontal duct that sags will trap moisture and eventually clog.
Venting Through the Roof
If your hood is directly below the roof and a wall vent is not practical, you can run ductwork up through the attic and out through the roof. This route is longer, requires flashing to prevent leaks, and is harder to seal properly—but it works when the hood is far from an exterior wall.
Measure from the hood outlet to the roof, accounting for the slope of the roof itself. Run rigid or flexible duct up through the attic, keeping it as straight as possible. Where the duct exits the roof, install a roof flashing kit designed for your duct size. The flashing sits under the shingles above the hole and over the shingles below, creating a watertight seal. Caulk around the flashing with roofing cement. Attach a roof cap on top to keep rain and debris out.
Roof venting requires more materials and labor than wall venting, and any mistake in the flashing can lead to leaks inside your attic. If you're not comfortable working on a roof, hire a roofer or HVAC contractor for this part.
Installing a Damper to Stop Backflow
A damper is a hinged flap inside or at the end of your ductwork that closes when the hood is off, preventing outside air, pests, and odors from flowing backward into your kitchen. Most wall caps and roof caps have a built-in damper. Check that it moves freely and closes completely when you push on it by hand.
If your duct runs a long distance (more than 25 feet), consider adding an inline damper inside the duct as well. This gives you a backup seal and reduces the load on the hood's motor. Dampers cost $20 to $60 and are straightforward to install between duct sections using clamps.
Common Mistakes and How to Avoid Them
The most frequent error is running ductwork horizontally or upward without slope, which traps grease and water inside the pipe. Over time, this buildup clogs the duct and reduces airflow. Always slope toward the outside, even slightly.
Another mistake is using flexible duct for the entire run. While it's tempting because it's straightforward to bend, flexible duct is noisier, collects grease in its ridges, and degrades faster. Use it only for short sections (under 3 feet) where you need to bend around an obstacle, and use rigid duct for the main run.
Undersizing the duct or using too many bends and elbows also chokes airflow. Each 90-degree elbow reduces the hood's effectiveness. Minimize turns and keep the duct as straight as possible. If you must use elbows, use long-radius elbows instead of sharp 90-degree ones—they create less resistance.
Finally, forgetting to seal gaps between the duct and the hood outlet, or between the wall cap and the siding, lets air leak out and reduces suction. Use metal duct tape, mastic sealant, or both. Caulk the outside seams with exterior-grade caulk to keep water out.
Tools and Materials You'll Need
| Item | Purpose | Typical Cost |
|---|---|---|
| Stud finder | Locate studs and pipes inside the wall before cutting | $15–$40 |
| Drill with long bit | Poke a pilot hole through the wall | $10–$30 (if you don't own one) |
| Reciprocating saw or drywall saw | Cut the hole through drywall and siding | $20–$100 (if you don't own one) |
| Rigid or flexible ductwork (6 or 8 inch) | Carries air from hood to outside | $1–$3 per foot |
| Wall cap or roof cap with damper | Seals the hole and prevents backflow | $15–$60 |
| Duct clamps and connectors | Joins duct sections and attaches to hood | $5–$15 |
| Metal duct tape or mastic sealant | Seals gaps in ductwork | $5–$15 |
| Exterior caulk | Seals the cap to siding or roof | $5–$10 |
| Roof flashing (if venting through roof) | Prevents leaks where duct exits roof | $20–$50 |
Frequently Asked Questions
Can I vent my range hood into the attic instead of outside?
No. Venting into the attic dumps hot, moist air and grease vapor into an unvented space, which causes mold, rot, and damage to insulation and framing. Building codes require range hoods to vent completely outside. The only exception is a recirculating hood with a charcoal filter, which pulls air through the filter and back into the kitchen—but these are less effective at removing heat and moisture.
What if my ductwork is longer than 25 feet?
Long runs lose efficiency because friction inside the duct slows airflow. If your duct is longer than 25 feet, use the largest duct size your hood supports (usually 8 inches instead of 6), minimize bends, and consider upgrading to a more powerful hood. You may also add an inline booster fan inside the duct to push air farther, though this adds cost and noise.
Do I need to insulate the ductwork?
Insulation is not required, but it helps in cold climates. Uninsulated duct can sweat on the inside when warm, moist air from cooking meets cold outside air, and that condensation drips back into the hood. Wrapping the duct with foam insulation (R-6 or higher) prevents this. It also reduces noise slightly. Insulation costs $1 to $2 per foot.
What size hole do I cut for a 6-inch duct?
Cut a hole slightly larger than the duct diameter—about 6.5 to 7 inches for a 6-inch duct. This gives you room to slide the duct through and fit the wall cap. The cap itself covers the gap between the duct and the hole edge.
Can I use PVC pipe instead of metal ductwork?
No. PVC is not rated for the heat and grease vapor from a range hood. Metal duct (aluminum or galvanized steel) is required by code. PVC can warp, melt, or catch fire if exposed to high temperatures. Stick with metal duct or approved flexible duct.