Range Hood Ducting: Duct Size, Runs and Makeup Air

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A range hood is only as good as the duct behind it. Homeowners often spend time comparing hood styles, finishes and blower ratings, then accept whatever ducting the installer can fit between the cabinets and the roof. The result can be a powerful hood that barely clears smoke, rattles loudly, drips condensation in winter, or quietly dumps grease-laden air into an attic. Vent hood ducting decides how much air actually leaves the kitchen. This guide explains how duct size relates to hood airflow, why smooth rigid metal is preferred over flexible duct, how run length and elbows affect performance, where the duct should terminate, when high-airflow hoods raise makeup air questions, and why venting matters more in kitchens with gas cooking. It closes with what inspectors look for and rough cost ranges for Front Range homes.

Part of our Home Ventilation guide — start there for the full picture →

What Vent Hood Ducting Needs to Do

A ducted range hood captures heat, moisture, grease, odors and combustion byproducts above the cooktop and pushes them outdoors. The duct is the path between the hood’s blower and an exterior cap. For that path to work, it needs to be:

• Large enough to carry the hood’s rated airflow without excessive resistance.
• Smooth and airtight so air and grease travel outside rather than leaking into cabinets, walls or attics.
• Short and direct with as few turns as practical.
• Terminated outdoors through a proper wall or roof cap with a backdraft damper.

The U.S. Environmental Protection Agency’s A Brief Guide to Mold, Moisture and Your Home advises venting moisture-producing appliances such as stoves to the outside where possible and using exhaust fans when cooking. A recirculating hood, which filters air and blows it back into the kitchen, can catch some grease and odor but does not remove moisture or combustion gases from the house. When a ducted path to the outdoors is feasible, it is generally the better choice.

Matching Duct Size to Hood CFM

Range hoods are rated in cubic feet per minute (CFM). Smaller under-cabinet hoods may move a few hundred CFM, while professional-style hoods above large gas ranges can exceed 1,000 CFM. As airflow rises, the duct has to grow with it.

Every hood manufacturer publishes a required duct size in its installation instructions, and that document is the authority for a given model. Typical patterns look something like this, though specific products vary:

• Lower-airflow under-cabinet hoods often use a 3¼ by 10 inch rectangular duct or a round duct in the 6 inch range.
• Mid-range hoods commonly call for 6 or 7 inch round duct.
• High-airflow hoods often require 8, 10 inch or larger round duct.

Reducing the duct size below the manufacturer’s specification, sometimes done to fit through a joist bay or reuse an old duct, increases static pressure and noise and reduces the airflow the hood can deliver. It may also void a warranty. A larger hood connected to an old, undersized duct is a common source of disappointment after a remodel.

ENERGY STAR’s key product criteria for ventilating fans include range hoods up to 75 watts, with a minimum efficacy of 2.8 CFM per watt and a maximum sound level of 2.0 sones, and they require range hoods to meet the specification at each possible configuration. Larger, more powerful hoods fall outside that category, but the criteria are a useful reminder that efficiency and noise ratings depend on how the fan is tested and installed.

Best Range Hood: our top 3 picks

Product	Best for	Key features	Price	Buy
Cosmo COS-63175S	Wall chimney for most kitchens	380 CFM, 6-inch duct Backdraft damper named Baffle filters, 3-prong plug	$250.99as of Nov 6, 2026 2:33 pm GMT+0000	Buy now →Check current price
IKTCH IKP02-30	High-output wall chimney	900 CFM rating 4 fan speeds 5-year warranty	$429.00as of Nov 6, 2026 2:33 pm GMT+0000	Buy now →Check current price
VIKIO HOME VK-RP10-30	Dual-intake wall chimney	580 CFM, dual side intake 1 to 9 minute delay off 6-inch flex duct included	$249.99as of Nov 6, 2026 2:33 pm GMT+0000	Buy now →Check current price

Prices are Amazon’s listed prices as of the time shown and change often; check Amazon for today’s price. As an Amazon Associate we earn from qualifying purchases.

See every pick compared: Best Range Hood →

Rigid Smooth Metal vs Flexible Duct

Most range hood manufacturers and many local codes call for smooth-wall metal duct for kitchen exhaust, and many specifically prohibit plastic or vinyl flexible duct. The reasons are practical:

• Airflow. The corrugated interior of flexible duct creates much more friction than smooth metal, cutting effective airflow, especially on longer runs.
• Grease. Grease-laden air leaves deposits in corrugations that are hard to clean and can become a fire concern over time.
• Durability. Flexible duct sags, kinks and tears, and foil-type products can be crushed during later work.

Galvanized steel is the usual choice, with stainless steel sometimes used in exposed or high-end installations. Joints are typically fastened and sealed with metal foil tape rated for the purpose rather than ordinary cloth duct tape, and crimped ends should point in the direction of airflow so grease and condensation do not catch on the seams. Some manufacturers allow a short section of semi-rigid metal duct to make a final connection; their instructions should be followed closely.

Shortest Run and Minimal Elbows

Duct length and fittings add resistance. Manufacturers usually express this as an equivalent length: each elbow, transition and wall cap counts as a certain number of feet of straight duct. Exceeding the maximum equivalent length listed for a hood reduces performance and increases noise.

Good practice includes:

• Choosing the most direct route to an exterior wall or roof, which often favors a wall cap behind or beside the range.
• Using gentle, long-radius elbows rather than sharp 90-degree turns where possible.
• Avoiding back-to-back elbows close to the hood outlet, which can cause turbulence.
• Keeping transitions from rectangular to round, or between sizes, to a minimum.
• Sloping horizontal runs slightly toward the exterior, when manufacturer instructions permit, so condensation drains outdoors rather than back to the hood.

Island hoods are the most challenging layouts, since the duct must climb through the ceiling and travel through a joist bay or attic before reaching the outside. In cold Front Range attics, any duct section passing through unconditioned space is commonly insulated to reduce condensation, and joints need to be well sealed to keep warm, greasy air out of the insulation.

Exterior Termination: Never Into the Attic

A kitchen exhaust duct should end at a dedicated wall or roof cap that discharges outdoors. Installations found during inspections that fall short include ducts ending in attics, ducts pointed at soffit vents with no connection, and ducts tied into bathroom fan lines or chimneys. Each of those can deposit grease and moisture inside the building.

A soffit termination is a particular trap. Because soffits normally serve as attic air intakes, warm, moist, greasy air released near them can be drawn back into the attic. Some codes and manufacturers allow specific soffit terminations with purpose-built fittings, but a duct simply aimed at a perforated soffit panel is not the same thing. The guide to attic ventilation explains why exhaust air should never mix with attic intake airflow.

A backdraft damper in the cap or near the hood keeps cold outdoor air, insects and wind-driven snow from entering when the hood is off. Dampers stuck open by grease or ice are a common winter complaint in Colorado, along with frost forming around wall caps on cold nights. Periodic cleaning keeps the damper moving freely.

Makeup Air for High-CFM Hoods

When a hood exhausts a large volume of air, that air has to be replaced. In a leaky older house, replacement air seeps in through gaps. In a tighter modern home, a powerful hood can depressurize the house, starving itself of airflow and potentially pulling air backward through other openings, including the vents of fuel-burning appliances such as water heaters, furnaces and fireplaces.

For that reason, many jurisdictions require makeup air for kitchen exhaust above a certain airflow. A widely cited threshold in model residential codes is 400 CFM, but the exact rule, the adopted code edition and the accepted methods vary among Colorado cities and counties. Makeup air approaches range from a motorized damper on a fresh-air duct that opens when the hood runs to tempered systems that heat incoming air before it reaches the kitchen, which matters a great deal during Front Range winters.

Homeowners planning a high-airflow hood should ask the local building department which code applies, what makeup air is required, and whether a permit is needed. A licensed HVAC contractor can then evaluate interactions with existing combustion appliances.

Gas Cooking Pollutants and Why Venting Matters

Gas ranges add combustion byproducts to the usual cooking smoke and moisture. The EPA lists unvented combustion appliances such as gas stoves among the primary indoor sources of nitrogen dioxide (NO2). It explains that NO2 acts mainly as an irritant affecting the eyes, nose, throat and respiratory tract, and that in homes with gas stoves, kerosene heaters or unvented gas space heaters, indoor levels often exceed outdoor levels. Among its steps to reduce exposure, the EPA recommends installing and using an exhaust fan vented to the outdoors over gas stoves.

The EPA’s carbon monoxide page makes a similar point. It states that average CO levels in homes without gas stoves vary from 0.5 to 5 parts per million, that levels near properly adjusted gas stoves are often 5 to 15 ppm, and that levels near poorly adjusted stoves may be 30 ppm or higher. It also recommends keeping gas appliances properly adjusted and using an exhaust fan vented to outdoors over gas stoves.

Those points make a strong case for a well-ducted hood in any kitchen with gas cooking, and for actually running it, ideally on the back burners where hoods capture best. A hood that vents into an attic or recirculates air does not remove these gases from the home. Carbon monoxide alarms installed according to manufacturer instructions and local requirements add another layer of protection. For connection, clearance and shutoff questions, see the guide to gas stove installation.

Installation Considerations and Related Projects

Most range hood projects happen alongside larger kitchen work, and planning the duct route early saves money. Questions worth settling before cabinets go in include:

• Which wall or roof location gives the shortest run?
• Do joists, beams or plumbing block the preferred path?
• Will the duct pass through unconditioned space that requires insulation?
• Is a roof penetration in a location where snow and ice can be managed?

The guide to range hood installation walks through mounting heights, wiring and choosing between hood types. Over-the-range microwaves often include a built-in vent fan with limited airflow, and their ducting rules differ by model; the guide to over-the-range microwave installation covers those details. For budgeting, the overview of kitchen remodel cost shows how ventilation fits into a broader project. When the hood exhausts through a long or awkward path, a remote blower may help, an approach explained in the guide to the inline exhaust fan.

What a Home Inspector Looks For

The American Society of Home Inspectors (ASHI) Standard of Practice requires inspectors to inspect kitchen, bathroom, laundry and similar exhaust systems. During a typical inspection, that often means operating the hood, checking for visible ducting, looking for an exterior termination, and noting signs of grease or moisture problems. Common report items include recirculating hoods where a ducted hood was expected, flexible plastic duct in a cabinet, ducts ending in attics, missing or stuck dampers, and grease staining around wall caps.

Inspectors generally do not measure airflow or evaluate makeup air design, which falls outside a standard inspection. Buyers who want a deeper look can arrange a follow-up with an HVAC contractor. The heating and cooling inspection hub covers related mechanical items, and the main guide to hiring a home inspector outlines how to compare scopes.

Common Ducting Problems in Older Front Range Kitchens

Kitchens in Denver-area homes built from the 1950s through the 1990s often carry ventilation choices that made sense for smaller hoods but fall short today. Patterns that show up repeatedly include:

• Undersized rectangular duct left in place after a larger hood was installed, so the new blower runs loud while moving less air than its rating suggests.
• Ducts that turn several times inside a soffit above the cabinets before reaching an exterior wall, adding equivalent length that the original installer never calculated.
• Old wall caps with missing or painted-shut dampers, which let cold air into the kitchen in winter and invite birds or insects in spring.
• Recirculating hoods replacing ducted ones during a cosmetic remodel, sometimes leaving an abandoned duct open to the outdoors behind the cabinets.
• Microwave-hood combinations set to recirculate when a duct actually exists, or ducted through a cabinet with gaps around the connection.

Most of these are fixable when a kitchen is opened up for other work. Addressing them before new cabinets and backsplash tile are installed is far easier and cheaper than cutting into finished surfaces later.

Cleaning and Maintaining the Duct System

Kitchen exhaust collects grease even with good filters. Basic maintenance keeps airflow up and reduces the chance of grease buildup in places that are hard to reach:

1. Clean or replace hood filters on the schedule in the owner’s manual. Baffle and mesh filters can often go in a dishwasher; charcoal filters in recirculating hoods are replaced rather than washed.
2. Check the exterior cap a couple of times a year. The damper should open when the hood runs and close when it stops, and the screen or louvers should be free of lint, nests and grease.
3. Listen for changes. A new rattle, whistle or drop in airflow can indicate a loose duct section, a sticking damper or a failing blower.
4. Look for drips. Water or brown residue around the hood in cold weather often points to condensation in an uninsulated section of duct.

Homes with heavy frying or wok cooking may benefit from periodic professional cleaning of the duct itself, particularly on long runs.

Range Hood Ducting Costs

Costs depend on duct size, material, route length, roof or wall penetrations, and local labor rates, so these are only rough ranges. Straight galvanized duct, elbows and a basic wall cap for a short run are usually a modest materials expense. Larger diameters, stainless steel, insulated duct and roof jacks add cost.

Labor varies widely. Replacing a hood on an existing, correctly sized duct is often a fairly simple job. Running a new duct through an exterior wall, cutting a roof penetration, rerouting through ceiling joists for an island hood, or adding makeup air can push a project into the high hundreds or several thousand dollars along the Front Range. Getting written quotes that list duct diameter, material, route and termination makes bids easier to compare.

References

• Nitrogen Dioxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency
• Carbon Monoxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency
• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
• Ventilation Fans Key Product Criteria — ENERGY STAR (U.S. EPA)
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Planning an offer on a Front Range home with a new kitchen or a big gas range? Reach out through our contact page and we can connect you with an inspector who will trace where that hood really exhausts.