The basic route: cut the hole, run the duct, seal it

Installing a range hood vent through an exterior wall means cutting an opening in the wall, running ductwork from the hood to that opening, and sealing the gap so air and moisture do not leak back inside. The job takes a day or two if you have the right tools and the wall is straightforward. If you hit plumbing, electrical wiring, or a structural beam, the work stops until you reroute around it.

The duct diameter matters: most range hoods use either 6-inch or 8-inch round duct, or a 3.25-by-10-inch rectangular duct. Check your hood's manual to see which one yours needs. The duct must slope slightly downward as it runs toward the wall so condensation drains back into the hood, not pooling inside the duct where it rusts the metal and clogs the vent.

You will need a drill, a drywall saw or reciprocating saw, a level, ductwork (either rigid metal or flexible aluminum), duct tape or mastic sealant, a wall cap (the trim ring that covers the hole on the outside), and caulk. If the wall is brick or stone instead of drywall and wood framing, you will need a masonry bit and more time.

Key Takeaways

  • The duct must slope downward toward the exterior wall so condensation drains back into the hood rather than pooling inside the duct.
  • You need to know your hood's duct diameter (usually 6 or 8 inches round, or 3.25 by 10 inches rectangular) before you buy ductwork or cut the wall opening.
  • Locate the wall opening by drilling a pilot hole from inside, then checking outside to make sure you are not cutting through plumbing, wiring, or a structural beam.
  • Seal the duct connection to the wall cap with mastic sealant or metal-backed duct tape, not cloth tape, so moisture does not seep around the edges.
  • If your wall is brick, stone, or has obstacles like pipes or beams, the job takes longer and may require rerouting the duct or calling a contractor.

Measure and locate the wall opening

Start by deciding where the duct will exit the wall. It should be as close to the range hood as possible to keep the duct run short and straight. Measure from the center of the hood to the exterior wall, then mark that spot on the wall at the same height as the hood's duct collar (the opening where the duct connects). Use a level to make sure the mark is level with the hood.

Before you cut, drill a small pilot hole (about 1/4 inch) through the wall from inside to outside. Go outside and look at the hole. Make sure it is not directly behind a downspout, window, or door, and that there is at least 12 inches of clearance above the ground or any deck. If the spot is bad, drill another pilot hole a few feet away and check again.

Once you have a good spot, use a stud finder to check for wooden framing members, then look for pipes and wiring. If you see any, move the opening or call a contractor. If the wall is clear, you can proceed to cut.

Cut the wall opening and install the wall cap

Use the pilot hole as your center point. If your duct is 6 inches round, you need a 6.5-inch hole (slightly larger than the duct so it slides in). If it is 8 inches round, cut an 8.5-inch hole. For rectangular duct, cut the hole to match the duct width plus 0.5 inches on each side.

From inside, use a drywall saw to cut through the drywall. Once you break through, switch to a reciprocating saw to cut through the rim board (the wooden frame around the edge of the wall). Work slowly so you do not damage the siding on the outside. From outside, use the reciprocating saw to finish cutting through the exterior sheathing and siding.

Slide the wall cap (the trim ring with the damper flap) through the hole from outside. The cap should sit flush against the siding. Caulk around the edges of the cap with exterior-grade silicone caulk to seal out rain and insects. Do not use regular paintable caulk — it cracks in the sun and lets water in.

Run the ductwork from the hood to the wall

Connect the duct to the hood's duct collar using either metal-backed duct tape or mastic sealant. Cloth duct tape (the silver kind) does not hold up to moisture and heat — use only metal-backed or foil tape. Mastic is better because it stays flexible and does not peel. Wrap the tape or explore the sealant around the entire connection so no air leaks out.

Run the duct toward the wall, sloping it slightly downward (about 1/4 inch per foot) so condensation drains back toward the hood. If the duct run is longer than 15 feet, or if you have more than two 90-degree bends, the hood's fan may not push air out strongly enough. In that case, consider installing a booster fan inside the duct.

find the duct to the wall or ceiling with metal straps every 3 to 4 feet so it does not sag or pull away from the hood. Do not use plastic zip ties — they do not support the weight of the duct and can melt from the heat.

Connect the duct to the wall cap and seal everything

Slide the duct through the wall opening and connect it to the wall cap on the outside. The duct should fit snugly inside the cap's collar. Seal this connection with metal-backed duct tape or mastic sealant, wrapping it completely around where the duct meets the cap. This is where most leaks happen, so do not skip this step.

From inside, seal the gap between the duct and the wall opening with caulk or foam sealant. This keeps conditioned air from leaking out around the duct. Let the caulk dry before you turn on the hood.

Check the damper flap on the wall cap to make sure it opens and closes freely. The flap should close when the hood is off so cold air and insects do not come back inside. If it sticks, clean any debris or caulk that is blocking it.

Test the hood and check for leaks

Turn on the hood and hold a piece of tissue paper near the wall cap from outside. The tissue should be pulled toward the cap, showing that air is flowing out. If the tissue does not move, the duct may be kinked, the damper may be stuck, or the connection may be loose.

Go back inside and check the duct connection at the hood. Make sure the tape or sealant is holding and the duct is not pulling away. If everything looks tight, the problem may be a long duct run or too many bends — in that case, a booster fan will help.

After a few days of use, go outside and check the wall cap and caulk for any water dripping or pooling. If you see moisture, the caulk may not be sealing properly, or the duct inside the cap may not be connected tightly. Reapply caulk or reseal the duct connection.

Common obstacles and how to handle them

If you hit a water pipe or drain line while cutting, stop when ready. Do not cut through it. Instead, move the opening a few feet to the side or reroute the duct to avoid the pipe. If the pipe is in the way and you cannot move it, call a plumber to relocate it before you continue.

If you hit electrical wiring, turn off the breaker to that circuit before you touch anything. If the wire is in the path of the duct, call an electrician to reroute it. Do not cut through live wiring or try to move it yourself.

If you hit a structural beam or the wall is solid brick or stone, the job becomes much harder. Cutting through brick requires a masonry bit and a lot of time. If the beam is in the way, you may need to reroute the duct or install the hood in a different location. Call a contractor if you are not sure whether the beam is structural.

Frequently Asked Questions

Can I use flexible duct instead of rigid duct?

Flexible duct works, but rigid metal duct is better because it does not sag, does not trap grease and moisture inside the ridges, and lasts longer. If you use flexible duct, support it every 3 feet and keep the bends as gentle as possible. Never crush or kink flexible duct because it reduces airflow.

What if the duct run is very long or has many bends?

A long duct run or multiple bends reduce the hood's ability to push air outside. If your run is longer than 15 feet or has more than two 90-degree bends, install a booster fan (also called an inline fan) inside the duct. The booster fan sits inside the ductwork and pushes air out, making up for the resistance from the long run.

Do I need a damper flap on the wall cap?

Yes. The damper flap closes when the hood is off, stopping cold air and insects from coming back inside through the duct. Most wall caps come with a damper built in. If yours does not, buy a separate damper and install it inside the duct near the wall cap.

Can I vent the hood into the attic instead of through the wall?

No. Venting into the attic traps moisture and grease in the attic, which rots the wood framing and attracts pests. The hood must vent to the outside through a wall, roof, or soffit. If you cannot vent through a wall, ask a contractor about venting through the roof or soffit instead.

What kind of caulk should I use around the wall cap?

Use exterior-grade silicone caulk, not paintable acrylic caulk. Silicone stays flexible in the sun and cold, and it does not crack or peel. Paintable caulk hardens and cracks within a year, letting water seep in behind the cap.