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
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Ventilation in bathrooms is important to prevent moisture damage to wall and ceiling surfaces, decay of wood trim, saturation of building insulation, and mold contamination. Especially in bathrooms where a shower is used, large amounts of moisture are added to room air and are concentrated in this area.
Our photo (left) shows a horrible bathroom ceiling vent fan ductwork job: multiple ducts sprawl around in the attic, all joining to terminate at an attempted through-roof vent that has fallen back into the attic. Notice how wet the roof sheathing is? These conditions are inviting an attic mold problem too.
Some signs of excessive, uncontrolled bathroom moisture include:
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
Bathroom vent fan duct routing suggestions
Our photo at left shows a bath vent fan that spills directly into a building's attic. Better design would have routed this exhaust outside.
Vent to outside: Make sure that the bath vent fan carries moist air all the way outside of the building.
Do not spill bath vent air into the building attic or roof cavity where it will condense on and damage building insulation, roof sheathing, possibly framing, and where it will certainly encourage mold growth.
Metal vent ducts: good bath vent duct design uses solid metal ducting, not flexible "dryer vent" material. Solid ductwork has a smoother interior surface that improves airflow, though it is indeed more trouble and a bit more cost to install.
Terminate the bath vent duct: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant, terminate the bath vent down through a building soffit at the roof eaves.
If you were not convinced that failure to properly spill bath vent air and moisture outside can lead to attic mold, notice our photo (left).
Brown mold was found growing on the attic side of roof sheathing in this new home only where the bath vent moisture was spilling into the soffit (but not outside).
Slope the bath fan vent duct downwards towards its building exit - this will avoid condensation accumulating inside the ductwork and dripping back into the building ceilings or insulation.
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.
Avoid through-roof bath vent exits: Our photo (left) shows a typical attempt at venting a bath into a ridge vent - this direction of vent exit may seem convenient but we don't like it much.
In the photo (left) the droopy flex-duct will certainly invite bath moisture to condense and run back to the home's ceiling rather than exiting at the ridge.
Our sketch (below) shows a bath vent fan exiting up through the roof.
The through-roof vent approach gives us another roof penetration, a possible leak spot, and it almost assures that condensing moisture will drip down the vent duct and into the bathroom ceiling.
Do not vent bath fans into a crawl space: you're only putting more moisture into an area where it is already going to be a problem, inviting mold growth on wood surfaces and hidden mold growth in building insulation. See Mold in Fiberglass Insulation.
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
For the bath fan to be effective, and considering that usually the fan is running while the bathroom door is closed, provide an opening to allow makeup air to enter the bathroom from an adjacent building area. Makeup air might be supplied by any of the following:
A warm heating air supply register in the bathroom (if your building is heated by warm air)
A supply grille covered opening cut near the bottom of an interior wall between bathroom and adjoining room, or in the bottom of a bathroom door.
A supply grille covered opening cut near the top of an interior wall between bathroom and adjoining room, or in the top of a bathroom door.
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.
Carson Dunlop, Associates, 120 Carlton Street Suite 407, Toronto ON M5A 4K2Toronto. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
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