Flex duct works for range hoods, but only in specific situations

You can use flexible ductwork for a range hood, but it is not the best choice in most cases. Flex duct is cheaper and easier to install than rigid metal duct, which is why homeowners are tempted to use it. However, flex duct has real drawbacks for range hoods: it restricts airflow more than rigid duct, it collects grease inside the corrugations, and it degrades faster when exposed to heat and moisture from cooking.

The National Fire Protection Association (NFPA) and most building codes allow flex duct for range hoods only when it is used as a short connector between the hood and a rigid duct run, not as the entire ductwork. If you are thinking about running flex duct all the way to your exterior wall or roof, building codes in most jurisdictions will not permit it.

Key Takeaways

  • Flex duct can connect a range hood to rigid ductwork, but cannot be the main duct run under most building codes.
  • Flex duct collects grease inside its ridges, making it harder to clean and more prone to blockages than smooth metal duct.
  • Smooth-wall rigid metal duct (aluminum or galvanized steel) moves air more efficiently and resists heat and grease buildup better than flex duct.
  • If flex duct is used at all, it should be no longer than 8 feet, kept as straight as possible, and inspected regularly for sagging or damage.

Why flex duct is problematic for range hoods

Flex duct has a corrugated inner surface that creates friction as air moves through it. This friction reduces the volume of air your range hood can move, which means it will not pull smoke and steam as effectively. A range hood that cannot move air efficiently defeats its purpose—you end up with lingering odors, grease in your kitchen, and moisture that can damage walls and cabinets.

The corrugations also trap grease. When you cook, grease particles travel through the ductwork. In rigid metal duct, they slide along a smooth surface and can be wiped out during cleaning. In flex duct, grease collects in the ridges and hardens there. Over time, this buildup narrows the duct opening and can become a fire hazard. Cleaning flex duct properly requires taking it apart, which is labor-intensive and often leads homeowners to skip maintenance altogether.

Flex duct also deteriorates faster than metal. The plastic inner liner breaks down when exposed to the heat and moisture that range hoods produce. After a few years, the liner can crack or separate from the outer coil, reducing airflow and allowing grease to escape into your walls.

What building codes actually say about flex duct and range hoods

The International Residential Code (IRC), which most jurisdictions adopt or base their codes on, permits flex duct for range hoods only as a transition duct—a short connector between the hood itself and the main ductwork. The IRC specifies that this transition should be no longer than 8 feet and should be as straight as possible to minimize airflow restriction.

Some local codes are stricter. Check with your city or county building department before you install anything. If you are planning a renovation or new construction, you will need a permit, and the inspector will verify that your ductwork meets local code. If you are replacing an existing range hood and the current ductwork is flex, you are generally allowed to keep it as-is under a grandfather clause, but you cannot expand or replace it with more flex duct.

When flex duct is acceptable and how to use it correctly

Flex duct is acceptable when it serves as a short connector between the range hood and a rigid duct that runs to the exterior. For example, if your hood is mounted under a cabinet and the main duct runs horizontally through the wall to the outside, you might use 3 to 5 feet of flex duct to bridge the gap between the hood outlet and the rigid duct. This is practical because it allows the hood to move slightly without stressing the connection, and the rigid duct handles the bulk of the airflow.

If you do use flex duct in this way, follow these rules: keep it as short as possible (under 8 feet), avoid kinks and sharp bends, support it every 18 inches with a strap or clamp so it does not sag, and use duct tape or metal clamps to seal all connections. Do not use duct tape alone—it fails over time. Seal with mastic sealant or metal clamps, then tape over the seam for extra security.

Inspect the flex duct every six months. Look for cracks in the outer coil, separation of the inner liner, or sagging sections. If you see damage, replace that section when ready. Clean the inside of the flex duct annually by disconnecting it and wiping out accumulated grease with a damp cloth.

The better choice: rigid metal ductwork

Rigid metal duct is the standard for range hoods because it moves air efficiently, resists grease buildup, and lasts longer. You have two options: smooth-wall aluminum duct or galvanized steel duct. Both are smooth on the inside, which means air flows freely and grease does not stick. Aluminum is lighter and easier to work with; galvanized steel is more durable and slightly cheaper.

Rigid duct does cost more upfront and takes longer to install because you have to cut and fit each section precisely. However, the payoff is a range hood that works properly, requires less maintenance, and will not need replacement for 15 to 20 years. If you are hiring a contractor, the labor cost is similar whether you use flex or rigid duct, so the extra material cost is often worth it.

What happens if your existing ductwork is flex

If your range hood is already connected to flex duct and it is working adequately, you do not have to replace it when ready. However, you should inspect it regularly and plan to upgrade it when you replace the hood or if you notice reduced airflow, grease buildup, or visible damage.

If you are replacing the hood itself, this is the right time to upgrade the ductwork. You will have the hood disconnected anyway, and the cost to replace the duct at that point is minimal compared to doing it separately later. Many contractors will recommend this upgrade, and it is worth the investment.

Frequently Asked Questions

Can I use flex duct if I insulate it well?

Insulation does not solve the core problems with flex duct for range hoods: the corrugated surface still restricts airflow and traps grease. Insulation is useful for preventing condensation in cold climates, but it does not make flex duct suitable as a main duct run. If you need insulation, use rigid duct with an insulated wrap.

How often should I clean flex duct if I use it?

Clean flex duct annually, or every six months if you cook frequently. Disconnect it from the hood and wipe the inside with a damp cloth to remove grease. If grease has hardened in the corrugations, you may need to use a soft brush or soak it in warm soapy water. Rigid duct is easier to clean because grease does not stick to the smooth surface.

Will my range hood work if the duct is kinked or sagging?

Yes, but not well. Kinks and sags restrict airflow and trap grease. Support flex duct every 18 inches with straps or clamps and keep it as straight as possible. If you notice sagging, tighten the supports or replace that section of duct. Sagging also allows water to pool inside, which can back up into the hood.

Do I need a permit to replace my range hood ductwork?

Most jurisdictions require a permit for any ductwork installation or replacement. Contact your local building department before you start. A permit ensures the work meets code and protects you if there is ever a problem. The inspection fee is usually small, and the inspector can catch issues you might miss.

Can I use flex duct if I install a damper or backdraft preventer?

A damper or backdraft preventer does not change the limitations of flex duct. These devices prevent outside air from flowing back into your kitchen when the hood is off, but they do not improve airflow or reduce grease buildup. Use them in addition to proper ductwork, not as a substitute for upgrading from flex to rigid duct.