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Bathroom Ventilation Improvement Guide InspectAPedia® How to improve bathroom venting, reduce indoor condensation, avoid bathroom mold Bathroom vent fans, required bath vent fan capacity, fan noise and sones Bathroom vent fan ducts, where to route vent air, duct condensation, ceiling leaks Photographs of bad or ineffective bath fan installations

This article explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan installation details. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links show where you are in our document & website.

Why is Bathroom Ventilation Needed?

• Stains, thermal tracking, or mold growth on bathroom walls and ceilings, possibly also on bath vanities and cabinets
• Damaged bathroom window trim
• Bathroom window condensation, and frost forming on bathroom windows during freezing weather
• Frost found in the attic on the roof underside over the bathroom area during freezing weather
• Stains and mold growth found on the roof sheathing in attics or roof cavities over bathrooms
• Damaged or curled roof shingles concentrated over a bathroom
• Leak stains in bathroom ceilings, especially around ceiling penetrations for light fixtures or ceiling vent fans

Guide to the Basics of Installing Bathroom Vent Fans

• Insulate the bath vent ductwork: to minimize the condensation that will otherwise occur as warm moist bathroom exhaust air passes through cool ductwork in the building attic or roof cavity. Our photo (above left) and Carson Dunlop's sketch (above right) shows a long routing of bath vent fan ducts across an attic floor to an exit at the building soffit or eaves and the condensation of moisture in such vent lines. These ducts should be insulated, and a better installation would have shortened that very long duct as much as possible, exiting to a soffit closer to the bathroom. Long vent fan duct runs reduce the ability of the fan to move air.
• Protect the bath fan duct outlet at the building exterior, using approved screening or a louvered fitting so that you do not invite birds or rodents into the building through the ductwork.
• A direct through-wall bath vent fan design may be preferable if the building roof shape, bath location, or other details make it difficult to exhaust a ceiling-mounted bathroom exhaust fan.

Bathroom Vent Fan Capacity & Vent Fan Noise Suggestions

Fan capacity measured in CFM: Bathroom vent fan capacity is measured in CFM (Cubic Feet per Minute) of air that the fan can move. Keep in mind that the CFM capacity of a bathroom vent fan will be reduced by long or irregularly exhaust ducting.

The bath vent fan CFM capacity should be slightly more than the number of square feet of floor of the bathroom area being vented. Multiply the bathroom floor area in square feet by 1.07. For example, a 10' x 10' square bathroom floor area (100 sq.ft. of area) multiplied by 1.07 yields a 107 CFM bath fan requirement. Since bath vent fans are not sold at exactly such odd numbers, buy a vent fan whose capacity is the nearest number just above the calculated CFM required.

Required Bath Vent Fan CFM = (SQFT of bath floor area) x 1.07

Use a still larger CFM bath vent fan if your bathroom includes facilities that generate extra levels of moisture such as whirlpool bath tubs, Jacuzzis™, steam bath, or if occupants (like my daughter Mara) like to take very long hot showers.

Fan noise measured in sones: When purchasing a bathroom vent fan also ask about its sones rating. Sones is a measure of appliance noise. Sones is a linear scale of loudness. Higher sones means a noisier fan. Since bath fan noise often annoys building occupants, a noisy fan will often not be used as much as it should. One "sone" is about as loud as a typical home refrigerator.

Four "sones" is about as loud as normal conversation. Orchestral music has a loudness ranging from 1 to 64 Sones. A higher CFM fan will also have higher sones, but as fan motor and housing quality vary, you may find a higher CFM fan that produces lower sones than some of its competitors. People's subjective experience of sound is a bit complicated. For example, the perceived noisiness of a fan depends also on the frequency of the sound that it is emitting.

Look for a high CFM low-Sones bath vent fan.

Provide Bath Vent Fan Makeup Air

How Proper Bath Vent Fans Save Energy & Lower Heating Bills

Sure, when a bath exhaust fan is running to remove unwanted excess moisture it is also removing warm air from the building. Air to air heat exchangers and exhaust fans are available that can move air into or out of a building while keeping the heat (or cooling) indoors, but we suspect that such equipment is not cost-justified in a private residence. (An air to air heat exchanger ventilator may be very appropriate for a public building, large dormitory bathroom, or gymnasium.)

But using a properly vented bathroom vent fan wastes less heat than is lost by opening a bathroom window. To remove the same amount of bathroom moisture as will be handled by a bath exhaust fan a window would need to be open considerably longer than the bath fan must be run.

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• "About the House - Bathroom Vents", Henri deMarne, New England Builder, November 1985
• "Bathroom Vent Fan Beats Open Window", James Dulley, Poughkeepsie Journal, 11/4/1987 p. 12D.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

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