Range hood ductwork is one of the most misunderstood components in residential kitchen ventilation. Choose the wrong duct size, use the wrong material, or route the duct with too many elbows, and your expensive range hood will underperform — moving less air than rated, running louder than expected, and potentially failing code inspection. Worse, using inappropriate duct material in a kitchen exhaust application creates a genuine fire hazard.
This guide covers every aspect of kitchen exhaust duct design: how to size your duct based on CFM, which materials are code-compliant (and which are not), routing strategies through walls versus roofs, maximum run lengths with elbow deductions, make-up air requirements under IRC M1503.6, when to insulate, common mistakes that contractors and homeowners make, and a complete cost breakdown. As a duct manufacturer with 30 years of experience operating from our Brookshire, TX warehouse, we see the consequences of poor duct design daily — and this guide reflects real-world knowledge, not just textbook theory.
Range Hood Duct Sizing: Match CFM to Diameter
The single most important decision in range hood duct design is duct size. Undersized ducts restrict airflow, increase static pressure, make the fan work harder (louder), and reduce the effective CFM your hood can move. The hood manufacturer specifies a minimum duct diameter for a reason — undersizing voids the warranty and violates building codes.
The relationship between duct size and CFM capacity is governed by air velocity. For residential kitchen vent duct, the industry standard is to keep air velocity between 700-900 feet per minute (FPM) inside the duct. Higher velocities create excessive noise; lower velocities allow grease to settle and accumulate inside the duct.
| Round Duct Diameter | Equivalent Rectangular | CFM Range | Typical Hood Application | Max Recommended Run |
|---|---|---|---|---|
| 4" | 3.25" x 10" | Up to 100 CFM | Bathroom exhaust only | 25 ft |
| 6" | 3.25" x 14" | 150 - 400 CFM | Standard residential hoods | 30 ft equivalent |
| 8" | 8" x 8" or 6" x 12" | 400 - 900 CFM | Mid-range and pro-style hoods | 30-40 ft equivalent |
| 10" | 8" x 14" | 900 - 1,200 CFM | Professional-grade hoods | 40 ft equivalent |
| 12" | 10" x 16" | 1,200+ CFM | Commercial / high-output hoods | 50 ft equivalent |
Critical rule: Never reduce the duct diameter below the range hood's outlet collar size. If your hood has a 8-inch collar, you must use 8-inch duct for the entire run. Reducing to 6-inch duct will cut your effective airflow by approximately 40-50%, regardless of what the fan is rated for. If anything, you can increase the duct size — going from an 8-inch collar to 10-inch duct reduces velocity, lowers noise, and improves performance on long runs.
For detailed airflow calculations across all duct diameters, see our flex duct sizes guide and CFM chart.
Material Requirements: Why Rigid Metal Is Non-Negotiable
Unlike HVAC supply ducts where flex duct is often acceptable, range hood exhaust ducts have strict material requirements driven by fire safety and grease accumulation concerns.
Kitchen exhaust carries grease. Even with multi-layer baffle filters or mesh filters in the hood, fine grease particles pass through and deposit inside the ductwork over time. In a smooth-wall metal duct, this grease film remains thin and is unlikely to ignite. In a corrugated flex duct, grease accumulates in the ridges and pleats, building up a thick, highly flammable layer that cannot be cleaned. This is why building codes universally require smooth-wall rigid metal duct for range hood exhaust.
| Duct Material | Allowed for Range Hood? | Grease Resistance | Fire Rating | Notes |
|---|---|---|---|---|
| Galvanized steel (rigid) | Yes - Recommended | Excellent | Class 0/1 | Industry standard, most cost-effective |
| Stainless steel (rigid) | Yes - Premium | Excellent | Class 0/1 | Corrosion-proof, 2-3x cost of galvanized |
| Rigid aluminum | Yes | Good | Class 0/1 | Lighter weight, easier to cut |
| Semi-rigid aluminum | Limited | Fair | Class 1 | Short transitions only (6-12" max), some codes prohibit |
| Flexible aluminum duct | No - Code Violation | Poor | Class 1 | Grease traps in corrugations, fire hazard |
| Vinyl/plastic flex duct | Absolutely Not | None | Flammable | Will melt, serious fire hazard |
The only exception for flex: Some hood manufacturers allow a short section (6-12 inches maximum) of semi-rigid aluminum transition duct between the hood outlet and the first section of rigid duct. This transition piece accommodates minor alignment differences and vibration. However, many local jurisdictions prohibit even this — check your local code before using any flexible connector.
For a deeper comparison of flexible and rigid duct applications across HVAC systems, see our exhaust duct guide.
Duct Routing Options: Through Wall vs. Through Roof
Every range hood duct must terminate outdoors — recirculating hoods with charcoal filters are a separate category and do not require ductwork. For ducted installations, there are two primary range hood venting options: through an exterior wall or through the ceiling and roof.
| Factor | Through Wall | Through Ceiling / Roof |
|---|---|---|
| Best for | Hoods on exterior wall, single-story, rangetop against wall | Island hoods, interior walls, multi-story buildings |
| Typical duct length | 5-15 ft (shortest path) | 10-30 ft (up through attic to roof) |
| Number of elbows | 1-2 (up from hood + through wall) | 2-3 (up from hood + horizontal run + through roof) |
| Installation cost | $800-1,500 | $1,200-2,500 |
| Roof penetration | No (lower leak risk) | Yes (requires proper flashing) |
| Airflow performance | Better (shorter run, fewer elbows) | Good (if properly sized and routed) |
| Termination cap | Wall cap with backdraft damper | Roof cap with storm collar and flashing |
| Aesthetics (exterior) | Visible wall cap on exterior | Roof cap (less visible from ground) |
| Condensation risk | Low (short run, minimal temp exposure) | Moderate (attic section may condense) |
Our recommendation: Choose the shortest possible route with the fewest elbows. In most cases, that means through the wall for hoods mounted on an exterior wall. For island hoods or hoods on interior walls, ceiling/roof routing is necessary — just ensure the total equivalent duct length stays within the manufacturer's specification.
Maximum Run Length and Elbow Deductions
Every elbow and turn in a range hood duct creates turbulence that restricts airflow. The HVAC industry quantifies this by converting elbows to equivalent feet of straight duct:
90-degree elbow: Adds 10 feet equivalent length.
45-degree elbow: Adds 5 feet equivalent length.
Straight duct: 1 foot = 1 foot equivalent.
Most residential range hoods specify a maximum equivalent duct length of 30 feet. Here is how that works in practice:
Example 1 — Simple wall venting: Hood on exterior wall. Duct goes straight up 2 feet from the hood, one 90-degree elbow, then 4 feet horizontal through the wall. Total equivalent length = 2 + 10 + 4 = 16 feet. Well within limits.
Example 2 — Roof venting with attic run: Hood on interior wall. Duct goes up 3 feet, one 90-degree elbow into the wall cavity, up 8 feet through the wall and attic, one 90-degree elbow to go horizontal to the roof jack, 2 feet horizontal to the roof penetration. Total equivalent length = 3 + 10 + 8 + 10 + 2 = 33 feet. This exceeds 30 feet — you would need to upsize the duct by one diameter or use 45-degree elbows instead (5 feet each, saving 10 feet of equivalent length).
Design tip: If your layout requires more than two 90-degree elbows, consider using 45-degree elbows instead. Two 45-degree elbows make a 90-degree turn with only 10 feet of equivalent length instead of 10 feet for a single 90-degree elbow — and the gentler turn angle reduces noise significantly. For long runs, also consider upsizing the duct one diameter (e.g., 8-inch instead of 6-inch) to reduce static pressure and compensate for the added friction. Our duct fittings guide covers elbow types and their pressure loss characteristics in detail.
Make-Up Air Requirements (IRC M1503.6)
This is the code requirement that surprises most homeowners — and many contractors. IRC Section M1503.6 states: "Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within the dwelling unit, each exhaust system capable of exhausting in excess of 400 CFM shall be provided with a make-up air system."
In plain language: if your range hood moves more than 400 CFM, you must provide a way to replace that air. A hood pulling 600 CFM out of the kitchen creates a 600 CFM deficit inside the house. In a tightly sealed modern home, this negative pressure can:
Backdraft gas appliances. A water heater or furnace with an atmospheric (natural-draft) vent relies on positive stack pressure to exhaust combustion gases up the flue. Negative pressure from a range hood can reverse this flow, pulling carbon monoxide into the house. This is a life-safety issue.
Make doors hard to open. Strong negative pressure creates a pressure differential across exterior doors. Homeowners with 1,200 CFM hoods report front doors being difficult to open while the hood is running.
Reduce hood performance. Without make-up air, the fan works against the house envelope. Actual airflow drops well below the rated CFM as the fan struggles against negative pressure.
Make-Up Air Solutions by CFM
Under 400 CFM: No make-up air required by code. The house's natural infiltration (leakage through walls, windows, doors) is sufficient.
400-600 CFM: A passive make-up air damper (motorized damper that opens when the hood runs) is usually sufficient. Cost: $200-500 installed. Mount it in an exterior wall near the kitchen, ideally on the opposite side of the room from the hood to create cross-flow.
600-1,200 CFM: A powered make-up air unit with a tempering coil is recommended. In cold climates, introducing 1,000 CFM of 20°F outside air into the kitchen is uncomfortable and increases heating costs. A tempering coil heats the incoming air to 55-65°F. Cost: $1,500-3,000 installed.
Over 1,200 CFM: Commercial-grade make-up air handling unit required. Typically ceiling-mounted or integrated into the HVAC system. Cost: $3,000-8,000+. At this point, consult an HVAC engineer for system design.
Range Hood Duct Insulation: When You Need It
Unlike HVAC supply ducts which almost always require insulation, range hood ducts only need insulation in specific situations. Understanding the difference saves money and avoids unnecessary work.
When to insulate:
Insulate the duct wherever it passes through an unconditioned space — attics, unheated garages, crawl spaces, or exterior wall cavities. When you cook, the exhaust air is warm and humid (steam, grease vapor). As this warm air travels through a cold duct section, it can condense on the duct walls, drip back down into the hood, and eventually cause grease buildup and corrosion. In cold climates, this condensation can even freeze inside the duct.
For attic runs, use R6 fiberglass duct wrap as the standard. In extreme cold climates (IECC zones 6-8), consider R8. Make sure the vapor barrier faces outward. For sealing duct joints before insulating, follow our duct seal guide.
When insulation is not needed:
Ducts running through interior walls, between floor joists in conditioned space, or in climate-controlled basements do not need insulation. The temperature differential between the exhaust air and the surrounding space is minimal, so condensation is not a concern. Adding insulation in these locations wastes money without providing benefit.
7 Common Range Hood Duct Mistakes
After 30 years in the duct manufacturing business, we see the same mistakes repeated on jobsites across the country. Avoid these, and your range hood installation will perform as designed.
1. Using flexible duct for the exhaust run. This is the most dangerous mistake and the most common one we encounter. Flex duct — whether aluminum, mylar, or vinyl — is never acceptable for range hood exhaust. The corrugated interior traps grease, creating a fire hazard. It also has 3-4x the friction loss of smooth-wall duct, killing your hood's airflow. If an inspector catches it, the entire run must be replaced with rigid metal.
2. Undersizing the duct. A 600 CFM hood connected to a 4-inch duct will move maybe 200 CFM. The fan motor overheats, the noise is unbearable, and the hood cannot clear smoke from the cooking surface. Always match duct diameter to CFM — see the sizing table above.
3. Too many elbows. Every 90-degree elbow costs you 10 feet of equivalent duct length. Three elbows in a short run means 30 feet of equivalent length before any straight duct is added. If your layout requires more than two 90-degree elbows, redesign the route or upsize the duct. Use 45-degree elbows where possible — they cost half the equivalent length.
4. No backdraft damper. Without a backdraft damper at the wall or roof cap, outside air enters the duct when the hood is off — bringing cold drafts in winter, hot humid air in summer, insects, and odors. Most quality wall caps and roof caps include a built-in damper, but verify before installation. Some hoods also have an internal damper, but relying on two dampers (hood + cap) is best practice.
5. Terminating in the attic or crawl space. This violates every building code in the US. Range hood exhaust must terminate outdoors, period. Dumping grease-laden, humid air into an attic causes wood rot, mold, fire hazard, and will fail any home inspection. We have seen attics with visible grease stains on roof sheathing from this mistake.
6. Using screws inside the duct. Sheet metal screws protruding into the duct interior catch grease and create accumulation points. Use foil tape, mastic sealant, or draw bands to connect duct sections. If screws are unavoidable (for structural integrity on vertical runs), use them on the outside only and seal the penetrations with high-temperature sealant.
7. Ignoring make-up air requirements. Installing a 900 CFM hood without make-up air provision is a code violation (IRC M1503.6 for hoods over 400 CFM) and creates real safety hazards — backdrafting gas appliances, difficulty opening doors, and reduced hood performance. Budget for make-up air at the planning stage, not as an afterthought.
Code Requirements: IRC M1503 Overview
The International Residential Code (IRC) Chapter 15, Section M1503 governs residential kitchen exhaust systems. Here are the key provisions every homeowner and contractor should know:
M1503.1 General. Range hoods must discharge to the outdoors through a duct. The duct must be airtight, constructed of galvanized steel, stainless steel, or copper, with smooth interior surfaces. The duct must not terminate in an attic, crawl space, or other enclosed space.
M1503.2 Duct material. Ducts serving domestic cooking appliances must be of galvanized steel, stainless steel, or copper. Duct joints must be made in the direction of airflow (male end pointing downstream) and sealed with approved methods. No screws or fasteners shall penetrate the duct interior where grease can accumulate.
M1503.3 Duct length. Duct length must not exceed the limits specified by the range hood manufacturer's installation instructions. Where manufacturer instructions are not available, ducts shall not exceed 30 feet equivalent length with allowances for elbows.
M1503.4 Makeup air. Exhaust hood systems capable of exhausting in excess of 400 CFM shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Makeup air must be provided by a listed and labeled device and be automatically controlled to start and operate simultaneously with the exhaust system.
M1503.6 Clearances. Ductwork must maintain required clearances from combustible materials. For Type I hoods (commercial-style), clearance requirements are more stringent. Standard residential hoods typically require 18 inches minimum clearance from the cooking surface to the bottom of the hood.
For spiral duct applications in commercial kitchen exhaust, see our spiral duct guide.
Range Hood Duct Installation Cost Breakdown
Costs vary significantly based on routing complexity, duct length, and whether you are adding ductwork to an existing kitchen or installing during new construction. Here is a realistic breakdown based on 2026 pricing:
| Component | Material Cost | Professional Install | Notes |
|---|---|---|---|
| 6" rigid galvanized duct (per ft) | $3-6 | $8-15 | 26-gauge snap-lock |
| 8" rigid galvanized duct (per ft) | $5-9 | $10-20 | 26-gauge snap-lock |
| 90-degree elbow (6" or 8") | $8-18 | $15-30 | Adjustable preferred over fixed |
| Wall cap with backdraft damper | $20-60 | $75-200 | Includes exterior cutting and sealing |
| Roof cap with flashing | $40-120 | $200-500 | Flashing + storm collar + sealant |
| Foil tape + mastic sealant | $15-30 | Included | UL 181B-FX listed tape required |
| R6 duct insulation wrap (per ft) | $2-5 | $5-10 | Only for unconditioned space sections |
| Make-up air damper (passive) | $100-250 | $200-500 | Required for hoods > 400 CFM |
| Make-up air unit (powered + tempered) | $800-2,000 | $1,500-3,000 | For hoods 600+ CFM in cold climates |
Total Project Cost Estimates
Simple wall venting (existing exterior wall): $800-1,500 installed. Short duct run (5-10 feet), 1-2 elbows, wall cap. Most common residential installation.
Roof venting through attic: $1,200-2,500 installed. 10-20 feet of duct, 2-3 elbows, roof cap with flashing, duct insulation in attic. Higher cost due to roof penetration and attic work.
New ductwork from scratch (no existing path): $1,500-3,500 installed. May require opening walls/ceilings, framing modifications for duct chase, drywall repair and painting after. Add $200-500 for make-up air if hood exceeds 400 CFM.
Professional-grade hood with make-up air: $3,000-6,000+ installed. 8-10" duct, powered make-up air unit with tempered air, possibly dedicated electrical circuit for make-up air blower. HVAC engineer design fee may add $500-1,000.
Range Hood Duct Installation Checklist
Before you start (or before you hire a contractor), verify every item on this list:
1. Duct size matches hood CFM. Check the manufacturer's installation manual for minimum duct diameter. Do not reduce below the collar size.
2. Rigid smooth-wall metal duct only. Galvanized steel, stainless, or rigid aluminum. No flex duct for the exhaust run.
3. Calculate total equivalent duct length. Straight feet + (90-degree elbows x 10) + (45-degree elbows x 5). Must be under manufacturer's maximum (typically 30 feet).
4. Route is the shortest path to exterior. Through wall is always preferred over through roof if both options are available.
5. All joints sealed. Use foil tape (UL 181B-FX) or mastic on every joint. Joints oriented in direction of airflow (male end downstream).
6. Backdraft damper at termination. Wall cap or roof cap must include a functioning backdraft damper.
7. Insulation on unconditioned sections. R6 wrap on any duct running through attics, garages, or exterior wall cavities.
8. Make-up air provision. Required for hoods over 400 CFM per IRC M1503.6. Plan and budget for this during design, not after installation.
9. Proper termination cap. Duct terminates outdoors with a proper wall or roof cap. Never in attic, crawl space, soffit, or garage.
10. Clearances from combustibles. Maintain required clearances per manufacturer instructions and local code, typically 18 inches from cooking surface to hood bottom.
Need help selecting the right duct components for your range hood installation? Contact our engineering team — we stock rigid and flexible ducts, fittings, wall caps, and insulation materials at our Brookshire, TX warehouse with fast shipping across the US.