Range Hood Ducting Installation: Sizing, Routing, and Sealing a Real Kitchen Run

The Ductwork Problem in a 1,200 CFM Retrofit

When you buy a 1,200 CFM range hood, you commit to moving a lot of air. A common mistake is matching the hood's fan rating to a duct that is too small, which produces noise, grease buildup, and poor capture. For a retrofit in a two-story colonial, the existing 6-inch round duct from a previous 400 CFM hood will not handle the new fan's airflow. The cross-sectional area of a 6-inch duct is about 28 square inches; a 1,200 CFM fan needs at least 8-inch round duct, which has about 50 square inches of area. Using the smaller duct would create static pressure that reduces the fan's actual performance by up to 30 percent.

In this worked example, we plan a run from the hood on the first-floor exterior wall, up through the kitchen ceiling, across the second-floor joist bay, and out through the roof. The total straight-line length is 18 feet, but with two 90-degree elbows and a roof cap, the effective length increases. Each 90-degree elbow adds 5 feet of equivalent duct length, and the roof cap adds 10 feet. The total effective length is 18 + 5 + 5 + 10 = 38 feet. That is under the 50-foot maximum recommended for an 8-inch duct, so the sizing works. We will also replace the existing 6-inch duct with 8-inch galvanized pipe to match the fan outlet.

Before cutting anything, verify the path. In this house, the joist bay above the kitchen runs parallel to the exterior wall, which means we cannot route straight up through the wall. Instead, the duct must go up into the ceiling, turn 90 degrees to run between the joists for 10 feet, then turn 90 degrees again to go through the roof. This routing is typical for retrofits where the vent stack is not directly above the cooktop. We mark the centerline on the drywall, use a stud finder to locate the joists, and confirm that no electrical cables, water lines, or gas pipes run through the joist bay. That last check takes 10 minutes and prevents a costly mistake.

Finally, order the duct pieces. For this job, we need: two 90-degree elbows, one 10-foot straight section of 8-inch galvanized pipe, a roof cap with a built-in damper, and a short section of flexible duct to connect the hood's outlet to the rigid pipe. Avoid using flexible duct for the entire run, because the ribbed interior creates turbulence that reduces airflow and collects grease. Use rigid pipe for the main run and flexible only for the final 12-inch connection from the hood to the rigid pipe, where you need some play to align the pieces.

Also check the hood's outlet orientation. Most hoods have a vertical or horizontal outlet on the top or back. If the hood has a top outlet, the duct goes straight up into the ceiling. If it has a back outlet, you need a 90-degree elbow immediately above the hood to route the pipe up. For this example, the hood has a top outlet, so the first piece is a short vertical section of pipe that connects to the elbow at the ceiling line. That elbow turns the duct 90 degrees to run horizontally between the joists.

Choosing the Right Duct Diameter for Your Fan's CFM

Duct diameter is not a matter of preference; it is a function of the hood's airflow rating. The Home Ventilating Institute (HVI) recommends that a 1,200 CFM hood use at least an 8-inch round duct. A 600 CFM hood can use a 6-inch duct, but if you plan to upgrade the fan later, installing an 8-inch duct now saves you from redoing the work. The rule of thumb is: for every 100 CFM, you need about 1 inch of duct diameter, up to 12 inches for very large commercial-style hoods. This is a starting point, not a substitute for the manufacturer's specifications, which you should always check in the installation manual.

To verify your duct size, calculate the total effective length of the run. Add the straight length to the equivalent length of every fitting. For example, a 45-degree elbow adds 3 feet, a 90-degree elbow adds 5 feet, and a wall cap adds 10 feet. If the total effective length exceeds 50 feet for an 8-inch duct, you either need to reduce the number of turns or step up to a 10-inch duct. For this example, the 38-foot effective length is within the limit, so an 8-inch duct is correct. If the run were longer, you would see a performance drop and might need to increase the diameter.

Also consider the duct material. Galvanized steel is the standard because it is rigid, smooth, and fire-resistant. Aluminum flexible duct is acceptable for short runs, but it has a higher friction loss. The smoother the interior surface, the less resistance to airflow. If you use flexible duct, keep it as straight as possible and do not compress it, because each bend or sag reduces airflow. For a retrofit, you might be tempted to use a semi-rigid aluminum duct that can bend around obstacles, but that type is best for dryers, not range hoods, because the spiral ribbing traps grease.

When you buy the duct, check the gauge. Standard 8-inch round duct is typically 26-gauge galvanized steel, which is strong enough for this application. Thinner 30-gauge duct is cheaper but dents easily and can vibrate. The duct sections should overlap by at least 2 inches at the joints, and you should secure them with sheet metal screws at each joint. Do not use duct tape for the connections; it dries out and fails. Use metal foil tape or mastic to seal the joints, which we will cover in the sealing section below.

Routing the Duct Through Joists, Walls, and the Roof

With the 8-inch duct selected, the next step is to cut the openings. For the ceiling, use a hole saw or a drywall saw to cut a 9-inch diameter hole to allow for the duct and a little clearance. For the joist bay, you will cut a rectangular opening in the drywall to access the space, but only if the joists run perpendicular to the duct path. If the duct runs parallel to the joists, you can fit it between them without cutting. In this example, the 10-foot horizontal run goes parallel to the joists, so we only need to cut the drywall at the two ends: one at the ceiling penetration and one at the roof penetration.

At the roof, locate the point where the duct will exit. Go into the attic and measure from the exterior wall to the center of the joist bay. Transfer that measurement to the roof using a plumb bob. Mark the center point, then drill a pilot hole through the roof deck from inside. Go outside and cut a 9-inch diameter hole around the pilot hole using a jigsaw or a reciprocating saw. Be careful not to cut through any rafters; if the hole lands near a rafter, you need to relocate the penetration or use a roof flashing that offsets the pipe. In this house, the hole clears the rafter by 3 inches, so we proceed.

For the vertical run through the roof, you will install a section of 8-inch pipe that goes through the roof deck and extends at least 12 inches above the roof surface. This height is required by most building codes to prevent downdrafts and to keep the exhaust from re-entering through windows or soffits. Before you set the pipe, apply roofing cement around the base of the hole, then slide the pipe through and secure it with a roof flashing. The flashing is a metal or rubber boot that fits over the pipe and under the shingles, creating a watertight seal. Nail the flashing down, then apply a bead of roofing cement over the nails.

Inside the attic, connect the vertical pipe to the horizontal run. Use a 90-degree elbow to turn the duct from vertical to horizontal, then slide the straight section into the elbow. Support the horizontal run with metal strapping or hangers every 4 feet, attaching them to the joists. The duct must have a slight slope toward the roof cap, about 1/4 inch per foot, so that any condensation or grease drips outward and not back into the kitchen. In this example, the horizontal run has a 2.5-inch drop over 10 feet, which is correct.

For the first-floor ceiling, cut a hole for the duct that is slightly larger than the duct diameter, then use a transition piece to connect the ceiling duct to the hood's outlet. If the hood has a back outlet, you would instead cut a hole in the wall behind the hood and route the duct horizontally through the wall cavity. In this example, we have a top outlet, so the duct goes straight up. The transition piece is a rectangular-to-round adapter that fits over the hood's exhaust outlet. Secure it with sheet metal screws, then slide the vertical duct over the adapter and secure it.

Sealing Every Joint to Prevent Leaks and Grease Weep

A range hood duct that leaks at the joints pulls conditioned air from your home and exhausts it, wasting energy and reducing the hood's efficiency. Worse, leaks in the duct can allow grease-laden air to escape into the attic or wall cavity, where it can create a fire hazard. The International Residential Code (IRC) requires that all duct joints be sealed with mastic or metal tape, not duct tape. Duct tape fails within a year due to heat and humidity, so it is not acceptable for this application. Use a high-temperature silicone sealant or a UL-181-rated foil tape.

Before you connect any two pieces of duct, apply a bead of mastic to the male end of the pipe. Slide the female end over it, then secure the joint with three sheet metal screws placed 120 degrees apart. Wipe away any excess mastic that squeezes out, then cover the joint with foil tape, pressing it down firmly. For the elbows, apply mastic to the inside of the elbow before sliding it over the pipe. The goal is to create a continuous airtight seal along the entire run. Do this for every joint, including the connection to the hood's adapter and the roof cap.

Pay special attention to the seams along the length of the duct. If you use spiral duct, the spiral seam is already crimped and relatively airtight, but you should still seal it with foil tape. If you use snap-lock pipe, there is a longitudinal seam that runs the length of the pipe; seal that seam with mastic as well. For the roof cap, apply a bead of mastic around the base of the cap before you secure it to the roof flashing. This prevents water from getting under the cap and also prevents exhaust from leaking out at the roof line.

Once all the joints are sealed, turn on the hood and check for leaks. Use a smoke pencil or a piece of incense and hold it near each joint while the fan runs. If the smoke is drawn into the joint, there is a leak. If it blows away, the seal is good. You can also feel for air movement with your hand. Fix any leaks by applying more mastic or adding foil tape. Do not rely on the duct being 'good enough'; a small leak at a joint can reduce the system's efficiency by 10 percent or more. After the leak test, insulate the duct in unconditioned spaces, such as the attic, to prevent condensation and heat loss.

The Final Hookup: Connecting the Hood and Testing Airflow

The last step is connecting the duct to the hood itself. Most hoods have a rectangular outlet that is 3.25 x 10 inches or 8 inches round. If your hood has a round outlet, you can slide the duct directly over it and secure it with a hose clamp. If it has a rectangular outlet, you need a transition piece that fits over the outlet and reduces to the round duct. In this example, the hood has an 8-inch round outlet, so we slide the vertical duct over it. The duct should overlap the outlet by at least 1 inch. Secure it with a hose clamp, then seal the connection with foil tape.

Before you turn on the hood, verify that the damper on the roof cap opens freely. The damper is a spring-loaded flap that closes when the fan is off to prevent outside air from coming in. If it is stuck, the fan will struggle to push air out, and you will get poor performance. Test it by reaching into the duct from the roof and pushing the flap. It should move easily and close on its own. If it does not, adjust the spring or replace the cap.

Turn on the hood at full speed and measure the airflow. A simple way to check is to hold a piece of paper under the hood; it should stay stuck to the filter. For a more precise measurement, use a manometer to measure static pressure, but for most homeowners, the paper test is enough. If the paper stays up, the duct is working. If it falls, there is a blockage or a leak. Check the duct for any crushed sections or loose connections, and recheck the joints.

Finally, test for backdrafts. Close all windows and doors in the kitchen, then turn on the hood. Go outside and check the roof cap; you should feel air blowing out. If you feel air being sucked in, the damper is not opening, or the duct is blocked. Also, check that the make-up air damper, if you have one, is open. For a 1,200 CFM hood, most building codes require a make-up air system that introduces fresh air to replace the exhausted air. If you skip this, the hood will pull air through the nearest cracks, which can cause backdrafting from water heaters or furnaces. In this example, the house has a make-up air damper wired to the hood, which opens when the fan is on. Test that it opens by looking at the damper while someone turns on the hood.

After the test, clean up any debris and install the duct cover or canopy that came with the hood. The cover hides the duct where it meets the ceiling or wall. If you have a telescoping duct cover, adjust it to fit the height of your ceiling. For this example, the duct goes through the ceiling, so you need a ceiling trim ring that covers the gap between the duct and the drywall. Apply a bead of caulk around the ring to finish the installation. Turn the power back on and let the hood run for a few minutes to clear any residual construction dust.