Range Hood Installation: Ducted vs Ductless, Sizing, and Venting Through Walls or Roof
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A range hood removes cooking smoke, grease, moisture, and combustion byproducts from your kitchen. The difference between a hood that works and one that just makes noise comes down to three things: adequate airflow for your cooktop, a properly sized and routed duct, and makeup air so the hood can actually pull. Most range hoods underperform not because the fan is too small, but because the ductwork restricts airflow or the house is too tight for the hood to exhaust at its rated capacity.
Sizing the Hood and Fan
The hood should be at least as wide as the cooktop — wider is better. For a 30-inch range, use a 30 or 36-inch hood. For a 36-inch cooktop, a 42-inch hood provides better capture, especially on the front burners.
Fan capacity is measured in CFM (cubic feet per minute). For electric cooktops, the minimum recommendation is 100 CFM per linear foot of cooktop width — so 300 CFM for a 30-inch range. Gas cooktops need more: add together the BTU ratings of all burners, divide by 100, and use that as your minimum CFM. A 60,000 BTU gas range needs at least 600 CFM.
Mounting height matters. Most manufacturers specify 24 to 30 inches above an electric cooktop and 30 to 36 inches above gas. Mounting too high reduces capture effectiveness dramatically — every inch above the recommended range loses performance. Mounting too low blocks sight lines and may create a fire hazard.
Professional-style ranges with high-output burners (15,000+ BTU per burner) need commercial-grade ventilation. Budget accordingly — the hood and installation for a pro range can easily cost more than the range itself.
Ducted vs Ductless
Ducted hoods exhaust air outside. They remove heat, moisture, grease, and combustion gases from the kitchen. This is the only type that actually ventilates. Every building code for new construction requires ducted ventilation for gas cooktops.
Ductless (recirculating) hoods filter the air through charcoal filters and blow it back into the kitchen. They remove some grease and odor but cannot remove heat, moisture, or combustion byproducts. Think of them as a last resort for situations where running a duct to the exterior is impossible.
The charcoal filters in ductless hoods need replacement every 3 to 6 months (or sooner with heavy cooking). They cannot be cleaned — only replaced. Factor this ongoing cost into the decision. A ducted hood with washable aluminum mesh filters has essentially zero ongoing filter cost.
If you currently have a ductless hood and want to upgrade, the biggest project cost is not the hood itself — it is running the ductwork through the wall or ceiling to the exterior. This is the part most people underestimate.
Ductwork Sizing and Routing
Use rigid smooth-wall duct — never flexible duct for range hood exhaust. Flex duct has corrugated interior surfaces that trap grease and dramatically increase air resistance. Grease buildup in flex duct is also a fire hazard.
Match the duct diameter to the hood outlet. Most residential hoods use 6-inch round or 3.25 x 10-inch rectangular duct. Never reduce the duct size below the hood outlet — this chokes the airflow. You can transition from round to rectangular to fit through wall cavities, but maintain equivalent cross-sectional area.
Keep duct runs as short and straight as possible. Every 90-degree elbow adds roughly 5 feet of equivalent length. Every foot of duct adds friction. Most hoods specify a maximum equivalent duct length (typically 25 to 40 feet). A straight shot through the wall behind the hood is ideal — a run through the ceiling, across the attic, and out through the roof adds 15 to 20 feet of equivalent length plus multiple elbows.
Install a wall cap or roof cap with a backdraft damper at the termination point. This prevents cold air, wind, and animals from entering through the duct when the hood is off. Clean the cap annually — grease accumulates and can prevent the damper from closing fully.
Makeup Air Requirements
Any hood exhausting more than 400 CFM typically requires a makeup air system — a powered damper that opens when the hood turns on, allowing outside air to enter the house to replace the air being exhausted. Without makeup air, a powerful hood depressurizes the house, which can back-draft combustion appliances, make doors hard to open, and cause the hood itself to perform poorly.
Check your local building code for the exact threshold. The International Mechanical Code (IMC) and International Residential Code (IRC) set the threshold at 400 CFM for range hoods in tight homes. Some jurisdictions set it lower.
Makeup air kits are available from most hood manufacturers. They typically include a motorized damper that installs in the wall or ductwork, a relay that ties into the hood's fan switch, and a filter. The damper opens when the hood turns on and closes when it turns off.
A simpler (but less comfortable) approach is a passive makeup air inlet — a duct with a manually operated damper that you open when cooking. The downside is that it lets in unconditioned outside air, which is cold in winter and hot in summer. A powered system with tempered (heated) makeup air is more comfortable but significantly more expensive.
Installation Steps (Wall-Vent Under-Cabinet Hood)
Turn off power to the existing hood or the circuit you will connect to. Verify with a non-contact voltage tester.
Hold the new hood in position (or use the mounting template included with most hoods) and mark the duct outlet location on the wall. Also mark the mounting screw locations. If the duct goes through a cabinet above, mark the cabinet bottom for the duct opening as well.
Cut the duct opening through the wall. For an exterior wall, this means cutting through drywall, insulation, sheathing, and siding. A 6-inch duct needs a 6-1/4 to 6-1/2-inch hole for clearance. Use a reciprocating saw to cut through wood sheathing and a hole saw or jigsaw for siding, depending on the material.
Install the wall cap from outside. Apply exterior-grade caulk around the flange before screwing it to the siding. Connect the duct from the wall cap to the hood outlet using rigid duct and aluminum foil tape (not standard duct tape, which dries out and fails in heat). Support horizontal duct runs every 4 feet with hanging straps.
Mount the hood to the cabinet bottom or wall brackets. Connect the electrical wiring (typically a standard 120V plug or hardwired connection to a junction box). Restore power and test all fan speeds and the light.
Common Installation Mistakes
Using flex duct: the single most common mistake. It traps grease, restricts airflow by 30 to 50 percent compared to smooth rigid duct, and is a fire risk. Replace any existing flex duct with rigid when installing a new hood.
Venting into the attic: never exhaust a range hood into the attic. The moisture and grease cause mold, wood rot, and fire hazard. The duct must terminate at the building exterior through a wall or roof cap.
Undersizing the duct: transitioning a 6-inch hood outlet to 4-inch duct (to fit through an existing wall opening) cuts the effective airflow roughly in half. Enlarge the opening instead.
Forgetting the backdraft damper: without one, cold air pours through the duct in winter, and you may hear wind noise on breezy days. Most wall and roof caps include a built-in damper, but check — some cheap caps do not.
Ignoring makeup air: a 600+ CFM hood in a tight house creates enough negative pressure to pull combustion gases from a gas water heater or furnace back into the house. This is a carbon monoxide risk. If you hear a whooshing sound when you open an exterior door with the hood running, you need makeup air.
Frequently Asked Questions
Do I need a permit to install a range hood?
It depends on your jurisdiction and the scope of work. Replacing an existing hood with a similar one (same location, existing duct) typically does not require a permit. New duct penetrations through exterior walls, new electrical circuits, or hoods over 400 CFM (which trigger makeup air requirements) often do require a building or mechanical permit. Call your local building department — the answer takes two minutes and avoids potential issues when selling the house.
Can I vent through the roof instead of the wall?
Yes, but wall venting is preferred when possible. Roof venting requires a longer duct run (adding friction and reducing performance), creates a roof penetration that needs to be properly flashed and sealed, and makes grease cleanup of the duct termination more difficult. If you must vent through the roof, use a proper roof cap designed for range exhaust — not a standard plumbing vent cap or attic ventilator.
How loud should a range hood be?
Hood noise is measured in sones. Under 3 sones on low speed is generally considered acceptable for open-plan kitchens where you might hold a conversation. Under 6 sones on high speed is tolerable for heavy cooking. High-end hoods with external or inline blowers (mounted in the duct run rather than in the hood body) are the quietest option, often under 1 sone on low. If noise matters, check the sone rating at each speed — not just the maximum CFM, which is usually the loudest setting.