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

Yes, you can use flexible duct for a range hood, but it comes with real trade-offs. Flexible duct is easier to install than rigid metal ductwork, costs less, and fits into tight spaces where rigid pipe won't bend. The catch: flexible duct reduces airflow, traps grease and moisture inside its ridges, and degrades faster than rigid alternatives. Most building codes allow it, but many contractors and manufacturers recommend against it for kitchens where you cook frequently.

The decision depends on three things: how often you cook, how much grease your cooking produces, and whether you have space for rigid duct. A range hood venting through flexible duct to an exterior wall works fine for light cooking. The same setup fails quickly if you fry food several times a week.

Key Takeaways

  • Flexible duct reduces airflow by 10 to 20 percent compared to rigid metal duct of the same diameter, making your range hood less effective.
  • Grease and moisture collect inside the ridges of flexible duct and are difficult to clean, leading to buildup that further reduces airflow and creates a fire hazard.
  • Most building codes permit flexible duct for range hoods, but many manufacturers void the warranty if you use it instead of rigid duct.
  • Flexible duct typically lasts 5 to 10 years before the inner lining deteriorates, while rigid metal duct lasts 20 years or longer.
  • If you use flexible duct, keep the run as short as possible, use the largest diameter your hood supports, and clean it every three to six months.

How flexible duct reduces range hood performance

Flexible duct has a corrugated inner surface that creates friction as air moves through it. This friction slows the airflow your range hood can produce. A 6-inch flexible duct loses roughly 10 to 20 percent of the airflow that a 6-inch rigid metal duct would deliver, depending on the duct length and how tightly it is coiled.

The longer the flexible duct run, the worse the performance. A 10-foot straight flexible duct loses less airflow than a 20-foot run with bends. If your range hood is rated for 400 cubic feet per minute (CFM) and you install it with 30 feet of flexible duct, you may only get 300 CFM at the hood opening. That means cooking odors linger longer, and steam takes longer to clear.

Rigid metal duct — either round or rectangular — has smooth interior walls that let air move freely. This is why professional kitchens and high-end residential installations use rigid duct almost exclusively.

Grease buildup and cleaning challenges with flexible duct

Range hood exhaust carries grease particles, especially when you cook with oil or butter. Rigid duct has smooth walls that grease slides down and exits through the vent. Flexible duct's ridged interior traps grease in the grooves, where it accumulates and hardens over time.

Once grease builds up inside flexible duct, cleaning it is nearly impossible without removing the entire duct section. Rigid duct can be cleaned with a brush or by a professional duct cleaning service. Flexible duct cannot be brushed effectively because the brush gets stuck in the ridges. Many homeowners end up replacing the entire duct rather than trying to clean it.

Grease buildup also reduces airflow further and creates a fire hazard. If grease ignites inside the duct — which can happen if a flame from the stovetop enters the hood — the fire spreads faster through a duct clogged with grease. This is why fire codes in some jurisdictions restrict flexible duct in kitchens with gas ranges.

What building codes and manufacturers say about flexible duct

Most local building codes allow flexible duct for range hood venting, provided it meets minimum standards. The duct must be UL-listed (tested by Underwriters Laboratories), have a fire-resistant inner lining, and be properly supported. The duct cannot be crushed, kinked, or coiled tightly, because these deformations block airflow.

However, many range hood manufacturers specify rigid duct in their installation instructions and will not honor the warranty if you use flexible duct instead. Before you buy a range hood, check the manual or contact the manufacturer to see what they require. Some brands allow flexible duct only for short runs under 10 feet.

Local building inspectors may also have stricter rules than the general code. In some areas, flexible duct is prohibited for gas ranges or for ductwork that passes through walls or attics. Call your local building department before you install anything.

When flexible duct is acceptable and when it is not

Flexible duct works reasonably well in these situations: you have a short run (under 15 feet) to an exterior wall, you cook with electric heat, you cook light meals without much oil or butter, and you can access the duct for cleaning every few months. Under these conditions, flexible duct will last 5 to 10 years and keep odors and steam under control.

Flexible duct is a poor choice if you have a gas range, cook frequently with oil or butter, have a long ductwork run (over 20 feet), or cannot easily access the duct for cleaning. In these cases, the reduced airflow, grease buildup, and short lifespan will frustrate you within a year or two.

If your range hood is rated above 400 CFM, rigid duct is strongly recommended. High-powered hoods need the smooth airflow that only rigid duct provides. Flexible duct will choke the performance of a powerful hood.

Installation tips if you choose flexible duct

If you decide to use flexible duct, follow these steps to get the best results. First, use the largest diameter duct your range hood supports — usually 6 inches for residential hoods. Larger diameter means less friction and better airflow. Second, keep the run as short as possible and avoid unnecessary bends. Every bend adds friction and reduces airflow.

Third, support the duct every 3 to 4 feet with metal straps or hangers to prevent sagging. A sagging duct traps moisture and grease in the low spots. Fourth, do not crush or coil the duct tightly. If you must coil it temporarily during installation, straighten it out before you seal it. Fifth, use metal duct tape or mastic sealant at all connections to prevent air leaks.

Finally, plan to clean the duct every three to six months if you cook regularly. Use a duct brush designed for flexible duct, or hire a professional duct cleaning service. Mark your calendar so you do not forget.

Rigid duct as the better long-term choice

Rigid metal duct costs more upfront — typically $2 to $5 per foot compared to $0.50 to $2 per foot for flexible duct — but it lasts 20 years or longer. Rigid duct is easier to clean, maintains airflow, and resists grease buildup. If you plan to stay in your home for more than 10 years, rigid duct is the better investment.

Rigid duct also comes in two types: round and rectangular. Round duct is most common and fits standard range hood outlets. Rectangular duct (also called trunk duct) takes up less space in walls and ceilings and is useful if you have tight clearances. Both types are smooth-walled and perform equally well.

If you have limited space and cannot fit rigid duct, a hybrid approach works: use rigid duct for the main run and a short section of flexible duct (under 5 feet) only at the final connection to the exterior vent. This gives you most of the benefits of rigid duct while keeping installation straightforward.

Frequently Asked Questions

Can I use flexible duct if my range hood is over a gas stove?

Most building codes and manufacturers discourage flexible duct for gas ranges because of fire risk. Gas flames can enter the hood and ignite grease buildup inside the duct. If you have a gas range, check your local code and your hood's manual. Rigid duct is the safer choice.

How often do I need to clean flexible ductwork?

If you cook regularly with oil or butter, clean the duct every three to six months. If you cook light meals without much grease, every six to twelve months is acceptable. The more you cook, the more often you clean. Rigid duct requires less frequent cleaning because grease does not stick to smooth walls.

Will my range hood warranty be void if I use flexible duct?

Many manufacturers require rigid duct and will not cover warranty claims related to reduced airflow or performance if you use flexible duct. Check your hood's manual before installation. Some brands allow flexible duct for short runs but not long ones.

What is the maximum length of flexible duct I can use?

Most manufacturers recommend keeping flexible duct runs under 15 to 20 feet. Longer runs cause significant airflow loss. If your ductwork must be longer, use rigid duct for the main run and flexible duct only for the final short section.

Can I use flexible duct inside my walls or attic?

Check your local building code. Some jurisdictions prohibit flexible duct inside walls or attics because it can be crushed or damaged during future renovations. Rigid duct is safer for concealed runs. If flexible duct is allowed, it must be properly supported and protected from punctures.