Ducts & air
Bathroom and kitchen exhaust: the code minimums and the better numbers
The code sets a floor for exhaust fans. The HVI rules set a better target, and the duct decides whether the fan ever gets there.
Why spot exhaust matters
Bathrooms and kitchens produce most of a house’s moisture, odours and, in the kitchen, combustion by-products and cooking particles. Removing them at the source, before they spread, is far more effective than diluting them with ventilation for the whole house. The International Residential Code (IRC), which most US jurisdictions adopt, sets minimum rates; the Home Ventilating Institute (HVI), the trade body that certifies fan ratings, publishes sizing rules that usually give higher and better figures.
Bathroom fans: the code minimum
IRC section M1507 requires a bathroom exhaust of at least 50 CFM when the fan runs intermittently, switched by the occupant, or 20 CFM when it runs continuously. The minimum applies to every bathroom, whatever its size. It is enough for a small powder room and too little for most full bathrooms with a shower.
Bathroom fans: the HVI rules
| Bathroom | HVI rule | Example |
|---|---|---|
| Up to 100 sq ft | 1 CFM per square foot of floor, minimum 50 | 80 sq ft, 80 CFM |
| Over 100 sq ft | Add up the fixtures | 50 each for toilet, shower and tub; 100 for a jetted tub |
| Ceilings above 8 ft | Use the room volume at 8 air changes per hour | 640 cu ft × 8 ÷ 60 = 85 CFM |
The bathroom exhaust fan calculator applies all three and the code minimum, takes the largest, and rounds up to the next common fan size.
Two bathrooms
A full bathroom of 10 × 8 ft with an 8 ft ceiling, toilet, shower and tub, 10 ft of 4-inch duct with two elbows.
- Code minimum: 50 CFM; area rule: 80 CFM; 8 air changes: 640 × 8 ÷ 60 = 85 CFM
- Required: 85 CFM, so a 110 CFM fan
- Duct: 10 + 2 × 10 + 15 = 45 ft equivalent
A master bathroom of 14 × 11 ft with a toilet, shower, tub and jetted tub.
- Fixture rule: 50 + 50 + 50 + 100 = 250 CFM; 8 air changes: 164 CFM
- Required: 250 CFM, so a 300 CFM fan, on 6-inch duct
The room’s air change rate, which the air changes per hour calculator works out for any room, is a useful check on any fan size.
The duct decides the real airflow
A fan’s rating is measured at a low static pressure, usually 0.1 in wc. A long run of 3-inch flex with three elbows and a louvred cap can halve it. Use the duct size on the fan’s outlet or larger, keep runs short and straight, use smooth metal where you can, and end at a proper cap with a damper. Most 50 to 110 CFM bath fans need 4-inch duct; larger fans need 6-inch. A fan that vents into the attic instead of outdoors is a code violation and a common cause of attic mould. The guide to duct sizing basics explains how fittings add equivalent length.
Noise and controls decide whether a fan is used
A fan that nobody switches on removes nothing. Noise is the main reason fans go unused: HVI-certified fans carry a sone rating, and a fan at 1.0 sone or less is quiet enough that people leave it running. Controls are the second reason. A run-on timer that keeps the fan going for 20 minutes after the light goes off, or a humidity sensor that starts it when the shower steams up the room, does more for moisture than an extra 20 CFM of rating. Where a bathroom fan also serves as the house’s continuous ventilation, a two-speed model that runs quietly on low all day and boosts on demand covers both jobs with one duct.
Range hoods: the code minimum
The same IRC table sets a kitchen exhaust minimum of 100 CFM intermittent or 25 CFM continuous, and section M1503 requires domestic range hoods to discharge outdoors through smooth metal duct with a backdraft damper. The HVI recommends at least 100 CFM for a wall hood, 150 for an island, and more for gas ranges.
Range hoods: sizing by width and by burner output
Two rules of thumb give the airflow. By width, a wall hood needs about 100 CFM per foot of range width and an island hood, which has no wall to contain the plume, about 150. By burner output, a gas range needs about 100 CFM for every 10,000 BTU per hour of burners. The range hood CFM calculator applies both and the HVI minimum, and takes the largest.
A standard gas range and a pro range
A 30-inch wall hood over a gas range with 40,000 BTU/hr of burners.
- Width: 2.5 × 100 = 250 CFM; gas: 40,000 ÷ 10,000 × 100 = 400 CFM
- Required: 400 CFM; makeup air not required
A 36-inch island hood over a pro-style range with 65,000 BTU/hr of burners.
- Width: 3 × 150 = 450 CFM; gas: 650 CFM
- Required: 650 CFM, so a 750 CFM hood, and makeup air is required
Makeup air above 400 CFM
IRC section M1503.6 requires makeup air for a kitchen exhaust above 400 CFM: a way for outdoor air to come in, usually through a motorized damper interlocked with the hood. Without it, a large hood depressurizes the house, pulls combustion gases down the chimneys of a fireplace or a natural-draft water heater, and does not reach its rated airflow anyway. Makeup air ductwork is often the largest cost of a big hood. For many kitchens, a 400 CFM hood that captures well, with a deep canopy and the right height, does more than a 900 CFM hood without makeup air.
Spot exhaust and whole-house ventilation
Bathroom and kitchen fans run for short periods. Whole-house ventilation, sized by the ASHRAE 62.2 formula in the whole-house ventilation calculator, runs continuously or on a schedule and provides the background fresh air. A bathroom fan on a continuous low setting can serve both purposes, which the guide to the ASHRAE 62.2 formula explains. Running the bathroom fan for 20 minutes after a shower is also the easiest moisture control in a house, as the guide to humidity and comfort sets out.