Choosing between rigid vs flexible duct for a range hood affects three things every day: how much smoke and grease the hood actually pulls out, how loud the fan sounds, and whether the install meets US building code. Rigid duct holds a smooth, fixed shape, so air moves through it with less resistance. Flexible duct compresses into ridges once installed, and those ridges create drag that rigid duct avoids. Whether rigid or flexible duct is better for a range hood also depends on where semi-rigid duct fits in, and semi-rigid does not automatically clear the same code requirement rigid duct does. This guide breaks down the airflow data, the grease and fire safety differences, the exact code requirement, and when semi-rigid duct genuinely works as a middle ground.
Quick Answer
Rigid metal duct is the better choice for a range hood in almost every US kitchen. It moves more air, meets IRC Section M1503, and resists grease buildup better than flexible or semi-rigid duct. Semi-rigid aluminum duct outperforms flexible foil duct on airflow and durability, but it still has a corrugated interior, so confirm with your local building department before assuming it satisfies the same smooth-wall code that rigid duct does.
The Three Duct Types Explained
Three duct types show up in range hood installations: rigid metal duct, semi-rigid aluminum duct, and flexible foil duct. Each one handles airflow, grease, and code compliance differently.
- Rigid duct: Is solid metal pipe, either round or rectangular, with a fixed shape and a completely smooth interior. It comes in galvanized steel, stainless steel, or copper, and it's the only duct type IRC Section M1503 fully approves for range hood exhaust.
- Semi-rigid duct: Is aluminum tubing that holds its shape better than foil duct but still has a corrugated interior underneath the surface. It's more durable than flexible duct and easier to route around tight corners, but that internal corrugation means it does not clear the same smooth-wall code requirement rigid duct does. Some local jurisdictions permit it for short transitions; others don't. Check with your building department before relying on it.
- Flexible duct: Is a compressible tube, usually aluminum foil or a plastic-lined material, with a spiral wire core that lets it stretch and bend. It's the easiest type to install and the cheapest to buy, and it's also the type IRC prohibits for range hood exhaust because of its corrugated interior.
| Duct Type | Interior Surface | Common Material | IRC M1503 Compliant | Best For |
|---|---|---|---|---|
| Rigid | Smooth | Galvanized steel, stainless steel, copper | Yes | The full duct run, hood to exterior |
| Semi-rigid | Corrugated (light) | Aluminum | Varies by jurisdiction | Short transitions where rigid genuinely won't fit, if locally approved |
| Flexible | Corrugated (deep) | Aluminum foil, plastic-lined | No | Not recommended for range hood exhaust |
Does Flexible Duct Reduce Range Hood Airflow?
Yes. Flexible duct creates more resistance to airflow than rigid duct of the same length, which lowers the CFM your range hood actually delivers. That resistance comes from static pressure, and it builds up differently depending on the duct's interior surface, its length, and how many elbows it has to move air through.
What CFM and Static Pressure Actually Measure
CFM (cubic feet per minute) measures how much air a hood moves. Static pressure is the resistance that air pushes against as it travels through the duct. The higher the static pressure, the harder the fan has to work to move the same amount of air.
What the Airflow Data Shows
A laboratory study tested rigid and flexible duct at matched lengths and measured static pressure drop, airflow, and capture efficiency for each.
| Metric | Rigid Duct | Flexible Duct |
|---|---|---|
| Static pressure drop | Lower at every tested length | Higher at every tested length |
| Airflow rate at same fan speed | Higher | Lower |
| Capture efficiency | Higher | Lower |
| Interior surface | Smooth | Ridged, spiral |
The result held at every duct length tested. Flexible duct did not close the gap even on short runs.
Why the Interior Surface Matters
Rigid duct has a smooth wall, so air slides through with minimal drag. Flexible duct has a ridged, spiral interior, and each ridge creates a small pocket of turbulence. Turbulence eats fan power that would otherwise go toward moving air, not toward fighting the duct itself.
How Length and Elbows Compound the Problem
| Factor | Effect on Rigid Duct | Effect on Flexible Duct |
|---|---|---|
| Longer duct run | Resistance increases gradually | Resistance increases faster |
| Each additional elbow | Adds moderate resistance | Adds greater resistance |
| Real-world CFM delivered | Closer to rated CFM | Often noticeably below rated CFM |
What This Means for Your Hood's Rated CFM
A range hood rated at 750 CFM on rigid duct rarely delivers its full rated CFM once it's routed through several feet of flexible duct. The fan motor doesn't change. The path it's pushing air through does.
Flexible Duct and Grease: The Safety Side
Grease is a normal byproduct of cooking, and every range hood duct has to handle it. The question is how well each duct type sheds that grease instead of holding onto it.
How Grease Builds Up Differently in Each Duct Type
Rigid duct's smooth interior lets grease vapor condense and drain back toward the hood or run straight out through the termination cap. Flexible duct's ridges work against that. Grease settles into the low points of each ridge, and because the surface isn't smooth, it doesn't drain out on its own. Over time, that buildup narrows the airway and adds fuel exactly where fuel shouldn't collect.
| Factor | Rigid Duct | Flexible Duct |
|---|---|---|
| Grease drainage | Flows back or out | Collects in ridges |
| Ease of cleaning | Can be wiped or brushed | Cannot be practically cleaned |
| Buildup location | Along the duct wall | Trapped in ridge pockets |
| Inspection access | Straightforward | Difficult, ridges hide buildup |
Is Flexible Duct a Fire Hazard for a Range Hood?
Yes, trapped grease inside flexible duct is a documented fire hazard. Fire investigators regularly cite grease-filled flexible duct as a contributing factor in kitchen exhaust fires. Beyond the trapped grease itself, most flexible duct is thin aluminum foil or foil with a plastic coating. Foil resists ignition, but it has a low melting point, so sustained heat from a grease fire can melt or breach it faster than solid steel. A plastic coating or liner adds genuine fuel of its own. Rigid steel duct holds its shape and contains heat far longer.
What Regular Cleaning Can and Can't Fix
Routine filter cleaning removes grease before it ever reaches the duct, and that habit matters regardless of duct type. But once grease does get past the filter, rigid duct gives you a real way to remove it. Flexible duct largely doesn't. A homeowner or technician can run a brush or degreaser through rigid pipe. Reaching inside the ridges of flexible duct to clean out trapped grease isn't practical, which means the buildup tends to stay there for the life of the duct.
Is Flexible Duct Against Code for a Range Hood?
Yes. IRC Section M1503.1 requires range hood exhaust duct to have a smooth interior surface, and flexible duct's corrugated interior does not meet that standard.
What IRC Section M1503.1 Actually Requires
The International Residential Code governs one- and two-family homes across most of the US, and Section M1503.1 states that the duct serving a range hood must have a smooth interior surface, be airtight, include a backdraft damper, and stay independent of any other exhaust system. Section M1503.2 limits the approved materials to noncombustible metal, most commonly galvanized steel, stainless steel, or copper; some state-adopted versions of the mechanical code also permit aluminum. Check your local code for the exact list. Flexible duct, with its ridged interior, fails the smooth-surface requirement outright regardless of which metal list applies.
Does NFPA 96 Apply to a Home Kitchen?
No. NFPA 96 governs commercial cooking operations, like restaurants, not residential kitchens. Homeowners sometimes see NFPA 96 cited online as the reason flexible duct is banned, but the rule that actually applies to a house is IRC M1503.1, not NFPA 96. The two codes reach a similar conclusion on smooth-wall duct, but they cover different buildings entirely.
Do Manufacturer Instructions Matter Too?
Yes, often as much as the code itself. Most range hood manufacturers specify smooth rigid metal duct in their installation manual, and a local inspector can fail an installation for not following the manufacturer's instructions even in areas where local officials enforce the written code loosely. Manufacturer instructions can also affect a warranty claim if a duct-related issue comes up later.
What Happens If an Inspector Finds Flexible Duct?
An inspector checking a range hood install traces the duct from the cabinet opening to the exterior termination and verifies the material is smooth rigid metal. If an inspector finds flexible duct during that check, the installation typically fails, and the installer must replace the duct before the inspector signs off on the job. This applies at rough-in and can resurface at resale if a home inspector flags it.
| Code Reference | Applies To | Duct Requirement |
|---|---|---|
| IRC Section M1503.1 | Residential kitchens | Smooth interior surface, airtight, backdraft damper |
| IRC Section M1503.2 | Residential kitchens | Noncombustible metal; galvanized steel, stainless steel, or copper (some states also permit aluminum) |
| NFPA 96 | Commercial kitchens | Not applicable to homes |
Is Rigid or Flexible Duct Quieter for a Range Hood?
Properly installed rigid duct is usually quieter overall, but the real answer depends on two separate noise sources, and each duct type handles them differently.
The Two Kinds of Duct Noise
| Noise Source | What Causes It |
|---|---|
| Turbulence noise | Air moving past ridges or rough interior surfaces, heard as whooshing or whistling |
| Vibration noise | Blower motor vibration traveling through the duct material, heard as humming or rattling |
Why Flexible Duct Can Sound Louder
| Cause | Effect |
|---|---|
| Ridged interior surface | Creates turbulence as air passes over each ridge |
| Rising static pressure | Turbulence noise increases as resistance builds |
| Kinked or compressed sections | Narrows the airway, raising both resistance and noise together |
Why Rigid Duct Can Also Sound Louder If Installed Wrong
| Cause | Effect |
|---|---|
| Metal duct touching framing directly | Transmits blower vibration into the structure as a hum or rattle |
| Missing vibration-isolation connector at the blower | Lets motor vibration travel straight into the duct |
| Quiet airflow, noisy structure | Explains why some installers report rigid duct as noisier |
What Actually Determines the Final Noise Level
| Factor | Effect on Noise |
|---|---|
| Duct material (rigid vs flexible) | Rigid lowers turbulence noise; flexible raises it |
| Vibration isolation at the blower | Missing isolation raises noise on rigid duct |
| Duct sizing | Undersized duct raises noise on both types |
| Elbow count | More elbows raise turbulence noise on both types |
| Installation quality | Determines whether rigid duct's quiet potential is realized |
Rigid duct installed correctly, sized to the hood's CFM rating and isolated from the blower with a vibration-dampening connector, is quieter than flexible duct in most kitchens. The installer discussions that call rigid duct noisier almost always trace back to a missing isolation connector or an undersized duct, not the material itself.
Cleaning and Maintenance
Duct maintenance is one of the clearest practical differences in the rigid vs flexible duct for range hood decision, since you can actually clean one type and largely cannot clean the other.
How to Clean Rigid Duct
| Step | What It Involves |
|---|---|
| Remove and wash the baffle or mesh filters | Monthly for regular cooking, more often for heavy or frequent frying |
| Wipe the accessible duct interior | A degreaser and a long-handled brush reach several feet into rigid duct |
| Inspect the termination cap | Grease and debris often collect where the duct meets the exterior cap |
| Schedule a full duct cleaning | Every 1 to 3 years depending on cooking volume, more often for heavy grease or wok cooking |
Why Flexible Duct Resists Cleaning
| Obstacle | Why It Matters |
|---|---|
| Ridged interior | Brushes and cleaning tools can't reach into the low points of each ridge |
| Trapped grease | Builds up in the ridges over time with no practical way to remove it |
| Reduced airway over time | Buildup narrows the duct further, adding to the airflow loss already caused by the ridges |
| No real fix short of replacement | Once grease accumulates, you usually must replace the flexible duct |
A Simple Maintenance Comparison
| Maintenance Task | Rigid Duct | Flexible Duct |
|---|---|---|
| Filter cleaning | Same for both types | Same for both types |
| Duct interior cleaning | Practical with a brush and degreaser | Not practical |
| Grease removal | Removable | Largely permanent |
| Long-term airflow | Stays closer to rated CFM | Degrades further as buildup adds to ridge resistance |
| Typical fix for heavy buildup | Clean the duct | Replace the duct |
Rigid vs Flexible Duct Cost: What to Expect
Rigid duct costs more per foot than flexible duct, but a range hood duct run is short, so the real dollar difference stays modest for most kitchens.
Material Cost Per Linear Foot
- Flexible duct: $1 to $4 per linear foot
- Rigid metal duct: $7 to $13 per linear foot
What This Looks Like for a Real Range Hood Run
A typical range hood duct run is short, often 5 to 20 feet from the hood collar to the exterior termination. On a 15-foot run, rigid duct might cost roughly $45 to $75 more in material than flexible duct. A typical range hood and installation often costs $800 or more, so that material gap represents a small fraction of the total project.
What the Lower Upfront Cost of Flexible Duct Doesn't Include
| Hidden Cost | Flexible Duct | Rigid Duct |
|---|---|---|
| Airflow performance | You pay for rated CFM but may not get it | Delivers closer to rated CFM |
| Cleaning option | Heavy buildup usually means full replacement | Practical to clean with a brush |
| Inspection risk | May require paying for labor twice if it fails | Nearly always passes on the first check |
| Functional lifespan | Needs replacing sooner due to trapped grease | Lasts the life of the hood with basic maintenance |
The Value Comparison
Rigid duct costs more upfront but avoids the airflow loss, cleaning limitations, and potential rework that come with flexible duct. For the small material cost difference on a typical run, rigid duct is the better long-term value in nearly every kitchen.
When Is Semi-Rigid Duct Okay?
Semi-rigid duct works as a short, unavoidable transition piece, not as a substitute for a full rigid duct run.
What "Short Transition" Actually Means
A short transition is typically a foot or two of duct where rigid pipe genuinely cannot bend or fit, usually right where the duct exits the hood collar before connecting to the main rigid run. It is not a way to route an entire duct through a tight wall cavity or around multiple obstacles.
Why Semi-Rigid Still Isn't a Clean Code Pass
| Consideration | What It Means |
|---|---|
| Interior surface | Still corrugated, even though the exterior looks more rigid than flexible duct |
| IRC M1503.1 | Requires a smooth interior surface, which semi-rigid does not fully meet |
| Local enforcement | Some jurisdictions allow it for short transitions; others don't |
| Safest approach | Confirm with your local building department before installing any semi-rigid section |
When Semi-Rigid Makes Practical Sense
- A tight cabinet cutout where rigid elbow fittings won't physically clear the space
- A brief offset that aligns the hood collar with the main duct run
- A repair situation where a short section needs replacing without opening a wall
When Semi-Rigid Is the Wrong Choice
- As the main duct run from the hood to the exterior termination
- As a way to avoid the cost or labor of installing rigid duct
- In any jurisdiction where the local building department does not permit it
What to Do If Flexible Duct Is Already Installed
Finding flexible duct already in place, whether a contractor installed it or it shipped inside the hood's box, is a common situation with a clear path forward.
Start With an Inspection, Not a Full Replacement
Before assuming the worst, check the actual run. A short section of flexible duct connecting to a mostly rigid run is a different problem than an entire flexible run from the hood to the exterior.
| What You Find | What It Means |
|---|---|
| Full flexible run, hood to exterior | Full replacement with rigid duct is the right fix |
| Mostly rigid with one flexible section | Replacing just that section often solves the problem |
| Flexible duct that's visibly sagging or crushed | Priority fix regardless of length, since airflow loss is worse here |
| Flexible duct still in the box, unused | Simplest case. Use rigid duct instead before installing anything |
Why This Is Worth Fixing, Not Just Living With
Flexible duct in place doesn't fail all at once. It reduces airflow gradually, traps grease over time, and may not meet code if a home inspector or future buyer's inspector checks it during resale. Fixing it now is a smaller job than fixing it later, especially before drywall or cabinetry closes the duct run in.
Steps to Correct an Existing Flexible Duct Run
- Step 1: Confirm the duct path and material from the hood collar to the exterior termination
- Step 2: Measure the flexible section's length and note how many elbows or bends it passes through
- Step 3: Get a rigid duct kit sized to match the hood's rated duct diameter, not a smaller size
- Step 4: Replace the flexible section with rigid duct and seal joints according to the manufacturer's instructions
- Step 5: Confirm the termination cap has a working backdraft damper before closing up the work
If a Contractor Installed the Flexible Duct
Ask the contractor directly whether the installation meets IRC Section M1503.1 and your local code. If it doesn't, request a correction as part of the original job rather than treating it as a separate expense, since a code-compliant install was the original agreement.
How to Install Rigid Duct for a Range Hood
Installing rigid duct correctly comes down to matching the duct size to the hood, minimizing elbows, and sealing every joint so the run stays airtight.
Step 1: Confirm Your Hood's Required Duct Size
Check the hood's installation manual for its rated duct diameter, commonly 6, 8, or 10 inches depending on the CFM rating. Never reduce to a smaller duct than the hood requires. A smaller duct raises static pressure and cuts airflow, even with rigid material.
Step 2: Plan the Shortest, Straightest Path
Route the duct from the hood collar to the exterior with as few turns as possible. Every elbow adds resistance, so a shorter run with fewer bends outperforms a longer run with more of them, even in rigid duct.
What If Rigid Duct Won't Fit Through the Cabinets?
Rigid pipe does not thread through tight cabinet framing the way flexible duct does. If the cabinet layout blocks a straight rigid run, you have three options: modify the cabinet interior to make room, offset the duct run with rigid elbows to clear the obstacle, or use a short section of semi-rigid duct strictly as a transition piece where rigid physically will not clear the space. Do not route an entire flexible run through a blocked cabinet just to avoid modifying the framing.
Step 3: Choose the Right Rigid Duct Material
Use galvanized steel, stainless steel, or copper, matching what your local code allows under IRC Section M1503.2. Confirm the exact approved materials with your local building department before purchasing.
Step 4: Connect and Seal Every Joint
Point crimped ends in the direction of airflow, away from the hood and toward the exterior termination. This keeps grease condensate from pooling at the joints or leaking through the seams. Secure sections with short self-tapping metal screws, around 3/8 inch, so fasteners don't extend into the airstream and snag grease or lint. Seal every joint with UL 181AP-approved aluminum foil tape or duct mastic, not standard cloth duct tape, which dries out and fails under heat and grease over time.
Step 5: Install the Backdraft Damper
Confirm the termination cap includes a working backdraft damper. This keeps outside air, pests, and cold drafts from entering through the duct when the hood isn't running, and IRC M1503.1 requires it.
Step 6: Support the Duct Properly
Secure rigid duct to framing at regular intervals so it doesn't sag, vibrate, or place stress on the joints. Avoid resting the duct directly against wood framing without a standoff, since that direct contact can transmit blower vibration as noise.
Step 7: Test Airflow Before Closing the Wall
Run the hood on its highest setting and check for unusual noise, airflow at the termination cap, and any air leaking from the joints. Fix any problems now. Accessing the duct later, after drywall or cabinetry closes the run in, takes far more work.
| Step | Key Check |
|---|---|
| 1. Confirm duct size | Match the hood's rated diameter, never reduce |
| 2. Plan the path | Fewest elbows, shortest run |
| 3. Choose material | Galvanized steel, stainless steel, or copper per local code |
| 4. Seal joints | UL 181AP foil tape or mastic, crimp ends facing airflow direction |
| 5. Install damper | Confirms IRC M1503.1 compliance |
| 6. Support the duct | Prevents sagging and vibration noise |
| 7. Test before closing up | Catch problems while they're still easy to fix |
Rigid vs Flexible Duct at a Glance
| Factor | Rigid Duct | Semi-Rigid Duct | Flexible Duct |
|---|---|---|---|
| Interior surface | Smooth | Corrugated (light) | Corrugated (deep) |
| Common material | Galvanized steel, stainless steel, copper | Aluminum | Aluminum foil, plastic-lined |
| IRC M1503.1 compliant | Yes | Varies by jurisdiction | No |
| Airflow (CFM delivered) | Closest to rated CFM | Better than flexible, still reduced | Lowest of the three |
| Grease drainage | Flows back or out | Partial, some ridge collection | Collects heavily in ridges |
| Fire safety | Best, holds shape and contains heat | Better than flexible, still corrugated | Lowest fire resistance: thin foil can melt, plastic liner adds fuel |
| Noise (installed correctly) | Quietest when isolated from the blower | Moderate | Highest, turbulence from ridges |
| Cleaning | Practical with a brush and degreaser | Difficult | Not practical |
| Material cost per linear foot | $7 to $13 | Mid-range | $1 to $4 |
| Best use | Full duct run, hood to exterior | Short, unavoidable transitions only | Not recommended for range hood exhaust |
Where Victory Fits
Duct choice matters most on higher-CFM hoods, since a larger fan has more airflow to lose to a poor duct path. The Victory Twister-36 is a useful example of how duct size and type directly change delivered performance.
How Duct Choice Changes This Hood's Real Output
| Spec | 6" Duct | 8" Duct |
|---|---|---|
| Delivered CFM | 600 | 750 |
| Noise level | Higher at the same airflow demand | Lower, quieter operation |
The Twister-36 ships with both a 6-inch and an 8-inch duct transition included, so the choice comes down to what the kitchen's existing duct path supports. The hood's own airflow numbers show the same principle covered earlier in this guide: a duct with less resistance delivers more of the fan's rated CFM and runs quieter doing it. That's true when comparing 6-inch to 8-inch rigid duct, and it holds even more when comparing rigid to flexible duct at any size.
Confirmed Specs
- 750 CFM (600 CFM on a 6-inch duct)
- 0.7 to 4.5 sones
- 19-gauge, 304-grade stainless steel
- Dual 6-inch and 8-inch duct transitions included
- Lifetime motor warranty
What This Means for a Duct Decision
A hood in this CFM range is exactly where duct choice has the most to gain or lose. Running it through rigid duct, sized correctly and matched to the included transition, lets the fan deliver close to its full rated airflow. Running the same hood through flexible duct works against the fan from the first foot, regardless of the hood's own build quality.
Conclusion
Rigid duct is the right choice for the full duct run in almost every US kitchen. It delivers more CFM, meets IRC Section M1503.1, sheds grease instead of trapping it, and lasts the life of the hood.
Semi-rigid duct only fits short, unavoidable transitions, and only after the local building department confirms it. It is not a substitute for a full rigid run.
Flexible duct is not the right choice for range hood exhaust. It loses airflow, traps grease, and fails the smooth-interior code requirement, all to save a small amount on a typical project's total cost.
Already have flexible duct installed? Replacing it with rigid duct is a straightforward fix, worth doing before the wall or cabinet closes up.
Frequently Asked Questions
What type of duct is best for a range hood?
Rigid metal duct, in galvanized steel, stainless steel, or copper, is the best type for a range hood. It has a smooth interior that maximizes airflow, sheds grease instead of trapping it, and meets IRC Section M1503.1.
Is it okay to use flexible duct for a range hood?
No, flexible duct is not okay to use for a range hood exhaust run. Its corrugated interior fails the IRC smooth-surface requirement, traps grease, and reduces the hood's airflow at every duct length tested.
Does a range hood duct need to be rigid?
Yes, IRC Section M1503.1 requires the full duct run from a range hood to have a smooth interior surface, which only rigid duct reliably meets. Semi-rigid duct may work for a short transition if the local building department allows it, but it is not a substitute for rigid duct.
Is semi-rigid ducting okay for a range hood?
Semi-rigid ducting works only as a short, unavoidable transition, not as the main duct run. Its interior is still corrugated, so it does not automatically meet the same smooth-wall code that rigid duct does, and local approval varies by jurisdiction.
What are the disadvantages of flexible duct for a range hood?
Flexible duct reduces airflow, traps grease with no practical way to clean it out, and doesn't meet US building code for range hood exhaust. It can also sag or kink over time, which narrows the airway further and adds to the noise and airflow loss already caused by its ridged interior.