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Ventilation Fans + CFM Sizing Reference

Reference Equipment Updated Jun 19, 2026 · HVAC · Plumbing · Roofing

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 (NEC 503.7 + 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 rating bathroom fans 1.4 CFM/watt minimum.

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 M1503.4, exhaust > 400 CFM = make-up air required (passive vent OR powered).

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

Whole-house ventilation (ASHRAE 62.2)

Required ventilation rate: 7.5 CFM per occupant + 0.03 × floor area sq ft. Example: 2,000 sq ft + 4 occupants = 30 + 60 = 90 CFM continuous OR equivalent intermittent.

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
  • 100 - 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 termination: NOT RECOMMENDED for bath exhaust (moisture returns to 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

Common range hood models

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.

The single most-impactful bathroom fan habit is the DUCT-LENGTH CHECK before quoting a job. A 50 CFM fan rated for 4 ft duct delivers 20 CFM through 25 ft of crushed flex duct around 3 corners. Customer's "fan that doesn't work" is usually a fan being suffocated by its own ducting. Upgrade duct (smooth metal, shortest path, fewest bends, properly sized) before upgrading the fan. Often the same fan + better duct moves 2x the air + costs less than a fan replacement.

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

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