Install Local Exhaust Ventilation

The 2012 International Residential Code requires exhaust ventilation to the outdoors for kitchens and bathrooms under the following conditions:

1. When there is no window in the bathroom
2. Above ranges in the kitchen unless:
   
   1. Allowed by the manufacturer AND
   2. Natural or mechanical ventilation is provided.

Despite these code requirements, which have actually been in place for several years in earlier editions of the IRC, we often see recirculating range hoods where they are not technically allowed (and also not a good idea) and bath fan ducts “dead-ended” in the attic. Even though a window may exist, during many times of the year they will simply not be opened due to comfort, allergy, noise, or security issues. Recommended practices to assure effective removal of moisture and energy efficient performance include the following:

• Always provide mechanical exhaust ventilation to the outdoors in bathrooms, kitchens, and other locations prone to moisture, odors, or other air contaminant sources (e.g., a garage shop area).
• Follow these ASHRAE 62.2 guidelines (following) and/or manufacturers’ recommendations when sizing kitchen and bath exhaust. It’s easier to throttle down the installed flow with a damper—but difficult/impossible to boost the flow to an undersized system.
• Measure and adjust actual flow rates to verify they meet design intent.

• Use ENERGY STAR labeled ventilation equipment whenever available. Besides being more energy efficient, these fans have maximum allowable sound levels of 3 sones or less for bath fans and 2 sones or less for range hoods (500 cfm max). This is important. Why? Noisy fans don’t get used.
• Control the bath exhaust fan on a timer or humidistat to assure that 1) the fan runs long enough to adequately remove moisture and 2) the fan is not forgotten and left on too long, consuming more energy than is necessary.

Range hoods present a special case for consideration. With the increasing frequency of high capacity, “commercial grade” range installations, concern arises regarding the very high exhaust rates that manufacturers recommend for the accompanying hood. With the substantial depressurization that is likely to occur as several hundred cfm (or more) is drawn out of the home, there is the potential for backdrafting of combustion equipment that is not sealed—for instance an atmospherically drafted gas water heater.

The 2012 IRC requires a source of makeup air equal in volume for range hoods in excess of 400 cfm. Such outdoor air intakes must be dampered and must be automatically controlled to operate simultaneously with the exhaust hood.

• Vent range hoods directly to the outdoors to remove odors and particulates from the home, rather than simply trapping or recirculating them.
• Follow code or manufacturer’s recommendations for makeup air for range hoods that exceed 200 cfm, whichever is more stringent. This will reduce the possibility of backdrafting combustion contaminants and help assure a safe level of indoor air quality.