Exhaust duct carries contaminated, humid, or heated air from inside a building to the outside. Every bathroom fan, kitchen range hood, dryer vent, and industrial exhaust system requires properly sized and installed exhaust ducting to function safely and effectively. Improper exhaust ducting causes moisture damage, mold growth, grease fire hazards, indoor air quality problems, and code violations.
This guide covers the three most common exhaust duct applications — bathroom, kitchen, and industrial — with sizing tables, material selection, insulation requirements, and installation best practices. As a manufacturer of flexible ducts and industrial exhaust hoses, we build duct for all three applications at our Brookshire, TX facility.
Bathroom Exhaust Duct
Bathroom exhaust fans remove moisture, odors, and airborne contaminants from bathrooms, preventing mold growth and maintaining indoor air quality. The exhaust duct connects the fan housing to an exterior termination point, typically through the attic and out the roof or gable wall.
Sizing
| Fan CFM | Duct Diameter | Max Run (rigid) | Max Run (flex) |
|---|---|---|---|
| 50-80 CFM | 4" (100mm) | 25-35 ft | 15-25 ft |
| 90-110 CFM | 4" or 6" | 30-50 ft | 20-35 ft |
| 150+ CFM | 6" (150mm) | 50+ ft | 35+ ft |
Material Selection
Rigid smooth-wall metal provides the best airflow and is easiest to clean. Use galvanized steel or aluminum pipe in 4" or 6" diameter. For attic runs, insulated flex duct is the most popular choice because it prevents condensation, is lightweight, and routes easily around attic obstructions. The insulation (R4.2 minimum) keeps the warm exhaust air above dew point, preventing moisture from condensing inside the duct and dripping back toward the fan.
Installation Tips
Slope the duct slightly downward toward the exterior termination (1/4" per foot minimum) so any condensation drains outside rather than back into the bathroom. Use foil tape or mastic at all joints — cloth duct tape will fail. Keep the run as short and straight as possible. Every 90° elbow reduces effective length by 5 feet and increases the chance of condensation problems.
Kitchen Exhaust Duct
Kitchen range hood exhaust requires rigid smooth-wall metal duct only. Flexible duct is prohibited for kitchen exhaust in virtually all building codes because grease-laden air deposits residue on any ridged or corrugated surface, creating a fire hazard that cannot be cleaned effectively.
Sizing
| Hood CFM | Duct Size | Notes |
|---|---|---|
| 150-400 CFM | 6" round or 3.25×10" rect | Standard residential |
| 400-900 CFM | 8" round or 3.25×14" rect | Pro-style residential |
| 900+ CFM | 10" round | Commercial, may require MUA |
Make-up air (MUA): Range hoods rated above 400 CFM in many jurisdictions (and above 300 CFM in some, including Washington state) require a make-up air system to replace the exhausted air. Without MUA, a powerful range hood depressurizes the house, causing backdrafting of gas appliances (water heaters, furnaces), difficulty opening doors, and poor exhaust performance. Check IRC M1503.6 and local amendments.
Code Requirements (IRC M1503)
Kitchen exhaust ducts must be constructed of galvanized steel, stainless steel, or copper with a minimum thickness of 0.019 inch (26 gauge). Joints must be secured with sheet metal screws and sealed with high-temperature aluminum foil tape. The duct must terminate outside through a wall cap or roof cap with a backdraft damper. Kitchen exhaust must not terminate in an attic, crawl space, or garage.
Industrial Exhaust Duct
Industrial exhaust systems handle a wide range of contaminants: welding fumes, paint overspray, sawdust, chemical vapors, heat, and smoke. The duct material must be selected based on the exhaust temperature, chemical composition, and particulate loading.
| Application | Temperature | Recommended Duct | Product |
|---|---|---|---|
| Dust collection (wood/metal) | Ambient | PU or PVC hose | PU Hose |
| Welding fume extraction | Up to 200°C | Nylon or canvas hose | Canvas Hose |
| Engine exhaust / hot air | Up to 300°C | Silicone fabric hose | Silicone Hose |
| Chemical fume extraction | Varies | PVC or PVDF rigid duct | Consult engineer |
| Paint booth exhaust | Ambient | Galvanized steel spiral | Spiral Duct |
For portable ventilation and temporary exhaust setups, nylon fabric duct hoses are the standard — they are lightweight, collapsible for storage, and available in diameters up to 24 inches. For permanent industrial exhaust installations, spiral duct is preferred for its low air leakage and smooth airflow characteristics.
When to Insulate Exhaust Duct
Insulate exhaust duct whenever it passes through a space that is colder than the exhaust air — which is nearly always the case for bathroom exhaust running through attics in any climate. Without insulation, warm moist exhaust air condenses inside the duct, drips back toward the fan, stains ceilings, and promotes mold growth in the attic.
Use pre-insulated flex duct (R4.2 minimum, R6 recommended for cold climates) or wrap rigid exhaust duct with duct wrap insulation. Kitchen exhaust duct does not typically need insulation because the grease-laden air should not condense under normal cooking temperatures — but check local code for specific requirements.
5 Common Exhaust Duct Mistakes
1. Terminating in the attic. The most common and most damaging mistake. Exhausting warm, moist air into a cold attic causes condensation on roof sheathing, leading to wood rot, mold, and eventually structural damage. Every exhaust duct must terminate outside the building envelope.
2. Using uninsulated duct through the attic. Even when the duct terminates outside, an uninsulated run through a cold attic condenses moisture inside the duct. The water drips back and stains the ceiling around the fan. Use insulated duct or wrap with R4.2+ insulation.
3. Too many elbows. Each 90° elbow is equivalent to 5 feet of straight duct in terms of airflow resistance. A bathroom fan duct with three 90° elbows in a 15-foot run has an effective length of 30 feet — likely exceeding the fan's rated maximum. Plan routes to minimize elbows; use 45° elbows (2.5 ft equivalent) where possible.
4. Using flex duct for kitchen exhaust. Flexible duct traps grease in its corrugations. Grease buildup in flex duct is a fire hazard that cannot be effectively cleaned. Kitchen range hood exhaust must use rigid smooth-wall metal duct.
5. Missing the backdraft damper. Without a backdraft damper at the exterior termination, outside air, insects, and rain can enter the duct when the fan is off. In cold climates, the cold air backdraft can freeze condensation inside the duct, creating ice blockages.
Need exhaust ducting for your project? Browse our flexible ducts for bathroom exhaust, industrial hoses for fume extraction, or contact us for sizing assistance.