Reference

Ventilation and Exhaust Minimums

The code minimums and the industry sizing rules for fresh air, exhaust fans, range hoods, air changes, purifiers and indoor humidity.

Last updated

Whole-house ventilation, ASHRAE 62.2

required rate (CFM) = 0.03 × floor area (sq ft) + 7.5 × (bedrooms + 1) fan size when running part time = required rate ÷ fraction of time running

The 2013 and later editions use 0.03 per square foot; the 2010 edition used 0.01. Count actual occupants instead of bedrooms + 1 where there are more. An infiltration credit is allowed only with a blower-door test.

House Required rate
1,000 sq ft, 2 bedrooms 52.5 CFM
1,500 sq ft, 3 bedrooms 75 CFM
2,000 sq ft, 3 bedrooms 90 CFM
2,500 sq ft, 4 bedrooms 112.5 CFM
2,800 sq ft, 4 bedrooms, 5 occupants 121.5 CFM

ERV and HRV units are chosen at about 1.25 times the continuous rate, from common sizes of 70, 100, 130, 150, 200, 250 and 300 CFM; typical sensible recovery is about 70%.

Bathroom exhaust

Rule Rate Source
Code minimum, intermittent 50 CFM IRC M1507
Code minimum, continuous 20 CFM IRC M1507
Bathrooms up to 100 sq ft 1 CFM per sq ft, minimum 50 HVI
Bathrooms over 100 sq ft 50 CFM each for toilet, shower, tub; 100 CFM for a jetted tub HVI
High ceilings 8 air changes per hour HVI

Exhaust must discharge outdoors, never into an attic or soffit.

Kitchen exhaust and makeup air

Rule Rate Source
Code minimum, intermittent 100 CFM IRC M1507
Code minimum, continuous 25 CFM IRC M1507
Wall hood by width 100 CFM per ft of range width HVI
Island hood by width 150 CFM per ft of range width HVI
Gas range 100 CFM per 10,000 BTU/hr of burners Common rule
Makeup air required Hoods above 400 CFM IRC M1503.6

Air change targets

Space or purpose Air changes per hour
Whole-house outdoor air, typical house under ASHRAE 62.2 about 0.35
Bedroom supply air from a central system 4 to 6
Living room supply air about 7
Bathroom exhaust 8
Kitchen exhaust while cooking about 15
Air purifier by the AHAM ⅔ rule, 8 ft ceiling 4.8
air changes per hour = CFM × 60 ÷ room volume (cu ft)

Air purifiers, fans and whole-house fans

  • Air purifier, AHAM rule: smoke CADR at least two thirds of the room’s floor area in square feet.
  • Ceiling fan blade span: 29 to 36 in up to 75 sq ft; 36 to 42 in up to 144; 44 to 50 in up to 225; 50 to 54 in up to 400; 60 in or two fans above that.
  • Whole-house fan: about 2 CFM per sq ft of floor area, with 1 sq ft of attic net free vent area per 750 CFM.

Indoor humidity

Season Target Notes
Summer 45 to 50% relative humidity; dew point at or below about 55 °F Above 60% favours dust mites and mould
Winter, mild 35 to 40% Limited by window condensation
Winter, around 0 °F about 30%
Winter, deep cold 25% or less

Dew point comfort bands: below 55 °F comfortable, 55 to 60 noticeably humid, 60 to 65 sticky, 65 to 70 uncomfortable, above 70 oppressive.

Dehumidifiers are rated in pints per day on the 2019 DOE test at 65 °F; a 50-pint unit today is about a 70-pint unit on the old test.

Calculators that use these rules

The guides to the ASHRAE 62.2 formula and to bathroom and kitchen exhaust explain the rules.