Ventilation Fans + CFM Sizing Reference

Why this matters

Wrong-sized exhaust fan = moisture buildup = mold + rot. Too-small bathroom fan = mirror fog stays + paint peels. Oversized + too-loud = customer turns it off + same result. Right CFM + right sone + right ducting = healthy home. This is the field card.

Bathroom exhaust fans (IRC M1507 local exhaust + ASHRAE 62.2)

Sizing rule: 1 CFM per sq ft (≤ 100 sq ft); larger bathrooms add 50 CFM per fixture (toilet, shower, tub).

Typical sizes:

• Half-bath / powder room: 50 CFM
• Standard bath 50 - 80 sq ft: 80 CFM
• Bath 80 - 100 sq ft: 100 CFM
• Master bath 100 - 200 sq ft: 150 - 200 CFM (often 2 fans)

Sone rating (loudness):

• ≤ 0.5 sones: ultra-quiet ("can't hear it running")
• 0.5 - 1.0: quiet, premium residential
• 1.0 - 2.0: standard, audible but acceptable
• 2.0 - 4.0: loud, builder-grade
• 4.0+: very loud, customer complaints common

Energy Star listing requires a minimum efficacy in CFM per watt, and the threshold has been raised across program versions (and differs between the lower and higher CFM tiers). Pull the current number off the ENERGY STAR spec rather than a remembered one.

Kitchen range hood

Sizing: 100 CFM per 12" of cooktop OR 1 CFM per 100 BTU gas burner output (whichever larger).

Typical:

• 30" cooktop electric: 250 - 300 CFM
• 30" cooktop gas: 300 - 600 CFM (per BTU)
• 36" cooktop: 400 - 600 CFM
• Pro range 48"+: 600 - 1200+ CFM

Make-up air requirement: per IRC M1503, an exhaust hood system capable of moving more than 400 CFM must be provided with make-up air at approximately the exhaust rate (passive vent OR powered). The subsection number has moved between editions; cite M1503 and confirm against the adopted code.

Sone: kitchen range hoods louder than bath; 5 - 8 sone typical at high speed.

Whole-house ventilation (ASHRAE 62.2)

Required ventilation rate: 0.03 x floor area in sq ft, plus 7.5 CFM for each BEDROOM PLUS ONE. The occupant-count version of this formula floats around the trade and it is the wrong variable; 62.2 keys the people term to bedroom count plus one, precisely so the number does not change when the house sells. Example: a 2,000 sq ft three-bedroom is 0.03 x 2000 = 60, plus 7.5 x (3 + 1) = 30, for 90 CFM continuous or the equivalent intermittent rate.

Strategies:

• Exhaust-only: bath fan running continuous at low setting
• Supply-only: fresh-air intake to return duct
• Balanced (HRV/ERV): heat-recovery ventilator brings outside air through heat exchanger

ERV vs HRV

HRV (Heat Recovery Ventilator): transfers heat between exhaust + supply streams. Cold climates.

ERV (Energy Recovery Ventilator): transfers heat + moisture. Humid climates + mixed.

Sizing: HVI Directory of Certified Home Ventilating Products. Typical residential 100 - 200 CFM.

Attic ventilation

Total NFA (net free area): 1 sq ft per 300 sq ft attic floor (with vapor retarder) OR 1 per 150 (without).

50/50 split: half intake (soffit OR eave vents), half exhaust (ridge OR gable OR roof vents).

Power attic ventilator: 600 - 1500 CFM. Use only if passive can't keep up; can pull conditioned air from house if not balanced.

Crawl space ventilation

Vented crawl (older code): 1 sq ft NFA per 150 sq ft floor.

Sealed crawl (2009+ IRC R408): no venting; sealed + conditioned. Better moisture control.

Duct sizing for fan exhaust

Smooth round metal duct best (low resistance).

Flex duct: high resistance; oversize by 1" OR straighten with multiple supports.

Sizing by CFM (typical):

• 50 - 80 CFM: 4" round
• 80 - 200 CFM: 6" round
• 200 - 400 CFM: 8" round
• 400 - 600 CFM: 10" round
• 600+ CFM: per ACCA Manual D

Length penalty: each 90° elbow = 15 - 20 equivalent feet straight duct.

Termination

Roof cap (bathroom + range hood): downdraft, screened, dampered. Stainless, copper, OR aluminum.

Wall cap: side exterior wall; backdraft damper required.

Soffit: discharging INTO a soffit cavity is a code violation, same as the attic; the cavity is inside the envelope. Terminating THROUGH the soffit at a dedicated sealed cap is a different detail that some AHJs accept, and it is still the weakest option because the discharge sits inches from the intake vents feeding the attic.

Crawl OR attic termination: NEVER. Code violation; mold + structural damage.

Backdraft damper

Spring OR gravity-loaded. Prevents reverse flow when fan off. Critical for cold-climate to stop outside air from entering through fan during winter.

Common bathroom fan models

Specific catalog numbers turn over every couple of years, so order off the current HVI listing rather than off a remembered SKU. What does not turn over is the product family and what each one is for.

Builder-grade housing fans (Broan-NuTone base series and the equivalent house-brand units at the big boxes). Around 50 to 80 CFM, 2 to 4 sones, 4 inch outlet, AC motor. Cheap, loud, and the reason half the bathrooms in the country have peeling paint. Fine as a like-for-like replacement in a rental; do not spec one into a remodel a homeowner is paying for.

Quiet residential ceiling fans (Panasonic WhisperCeiling and WhisperFit, Broan Roomside and ULTRA series, Delta Breez Signature and SLM, Aero Pure). This is the working default: DC or ECM motor, sub-1.0 sone, 4 or 6 inch outlet, Energy Star listed. Panasonic's Whisper line is the one most trades reach for by reflex, and several models are field-selectable across two or three CFM settings from a switch on the housing, which lets you stock one fan for a range of bathrooms.

Fan-light and fan-light-heater combinations. Convenient, but read the electrical load. A resistance-heater unit is a dedicated 20A circuit in most cases, and the heater side draws far more than the fan side. Sell the customer on the heat, wire it for the heat.

Humidity-sensing and continuous-run units. Broan SensAire and the Panasonic WhisperGreen family with the humidity module. These run at a low continuous rate and boost on humidity or motion, which is how a bath fan does double duty as the ASHRAE 62.2 whole-house exhaust point.

Remote and inline blowers. The blower sits in the attic or a joist bay with a plain grille in the ceiling. This is how you get very high CFM at very low room noise, and how you serve two grilles (toilet compartment plus shower) from one blower. More duct work, more install time, and the blower needs to stay accessible for service.

Retrofit housings. Several manufacturers make a housing that installs from below through the existing ceiling opening, so you are not in the attic and not patching drywall. Worth knowing about on any second-story bath with a finished ceiling above.

Selection order that works: required CFM first, then sone rating (target 1.0 or less on anything residential), then outlet size matched to the duct you actually have, then features. A quiet fan the customer leaves running beats a powerful fan they switch off.

Common range hood models

Hoods sort by mounting style first, because that decides the rough-in. Brand matters less than whether the thing is ducted and whether it is sized to the cooktop.

Over-the-range microwave. The most common installation in American kitchens and the weakest ventilation in the category. Low CFM, shallow capture area, and very often shipped recirculating (no duct, charcoal filter only). If a customer complains about grease and smoke and has one of these, the honest answer is that the appliance is the problem.

Under-cabinet hood (Broan, Zephyr, Best, and every appliance house brand). Mounts to the bottom of the upper cabinet, ducts up through the cabinet or out the back. Straightforward retrofit, moderate CFM, available ducted or convertible to recirculating.

Wall-mount chimney hood. No cabinet above; the stack conceals the duct. Bigger capture area, higher CFM options, and the one most remodels move to when they delete the upper cabinet over the range.

Island hood. Ducts straight up through the ceiling and roof. Needs more CFM than the equivalent wall unit because there are no adjacent walls helping contain the plume, and it is the least forgiving of a long or elbowed duct run.

Insert or liner (Vent-A-Hood, Zephyr, Best and similar, dropped into a carpenter-built or custom hood shell). The blower and baffles are the product; the shell is millwork. Sized by insert width, which must still cover the cooktop.

Downdraft. Rises behind or beside the cooktop and pulls down. Physics is working against it, so it needs high CFM to do a mediocre job and it usually needs a longer duct run under the floor. Spec it when the kitchen design leaves no other option, and set expectations.

Blower location. Integral (in the hood), inline (in the duct run), or exterior (on the roof or wall). Moving the blower away from the hood is the single most effective way to cut kitchen noise at high CFM, and an exterior blower keeps grease-laden air out of the attic duct joints.

Filters. Baffle filters (stainless, dishwasher-safe) outperform mesh on grease capture and are what you want over a gas range. Charcoal filters only exist in recirculating mode and do nothing for moisture or combustion products.

Two things to check before you quote: the hood must be at least as wide as the cooktop (wider is better on a pro range), and anything over the IRC threshold of 400 CFM drags a make-up air requirement into the job. That make-up air provision is a real scope item, not a footnote.

Install considerations

Wiring: bath fan = 15A circuit; range hood = 15A OR 20A per load.

Switch options: standard on/off; humidity-sensing (Broan SensAire, Panasonic FV-08VRL2 self-running on humidity); timer; motion-sensing.

Continuous operation (ASHRAE 62.2): low-speed continuous bath fan.

Make-up air: required > 400 CFM exhaust per code; passive vent OR powered.

Common ventilation mistakes

Undersized bath fan (moisture stays; mold develops). Loud fan (customer turns off; defeats purpose). Soffit termination of bath exhaust (moisture into attic). Flex duct too long / crushed / sagging (no airflow). Missing backdraft damper (cold air in winter). Range hood ductless recirculating (doesn't remove moisture OR grease). Skipping make-up air on large range hood (negative pressure pulls flue gas back from atmospheric water heater). Exhaust running into attic (mold; insulation damage).

Testing

Test airflow: tissue paper OR flow meter at register. Should pull tissue toward intake firmly.

Sone test: turn on; should be barely audible OR low whisper. If "loud", check for crushed duct, restriction, OR oversized fan in small space.

Moisture test: run hot shower 10 min; mirror should NOT fog OR should clear within 5 min after.

References

• ASHRAE 62.2 (Ventilation for Acceptable Indoor Air Quality in Residential)
• IRC M1503 (Range Hoods)
• HVI Certified Home Ventilating Products Directory
• ACCA Manual D (Residential Duct Systems)
• Manuall internal: HVAC Common Error Codes, Duct Sealing Reference