Bathroom ceiling vent fan, heater, light combination (C) Daniel Friedman Bathroom Vent Exhaust Fan Size Requirements & Noise Levels
Suggestions for choosing a bathroom exhaust fan by capacity & sone or noise level

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Bath exhaust vent fan sizing & noise rating choices: here we explain how to determine the necessary capacity in CFM for a bathroom exhaust fan; how to choose a bath vent fan based on its noise level rating in sones.

This article series explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details.

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Bathroom Vent Fan Capacity & Vent Fan Noise Suggestions

Kitchen exhaust fan vent outside terminatinon - through attic, asbestos fabric (C) Daniel FriedmanFan 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.

The huge exhaust fan shown in our attic photo at left was as noisy as hell. When it worked. The blower assembly shook so much that the installer had provided an (asbestos fabric) vibration dampener to avoid shaking the roof sheathing.

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.

Bathroom Exhaust Fan Noise Ratings

Fan noise is 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.

Best choice for a bathroom vent fan:

Look for a high CFM low-Sones bath vent fan. If you don't you may find that people just won't turn on the bathroom exhaust fan - the noise may be too annoying.

Really? Rob Walker, writing for The Workologist in the New York Times pointed out that in some situations such as a bathroom lacking good sound insulation and located next to workplace offices or a conference room, having a noisy bathroom vent fan can improve privacy and avoid unpleasant distractions by masking bathroom noises. [1] When the business is ready to spend on better building soundproofing and sound privacy see these articles where we explain how to track down the sources of building noises and how to improve sound control or privacy in buildings.

Meanwhile, possibly a better solutions to masking bathroom noises on-the-cheap would be a white noise machine that can be left-on during working hours rather than a horrible noisy vent fan that drives everybody crazy and is switched on and off when the bathroom is in use.

Design for Very Quiet Kitchen and Bath Vent Fans

Vent fan manufacturers such as Fantech provide remote fan models. The fan blower/motor assembly is located remote from the air intake in the area to be vented, minimizing fan noise in the building.

For example a bathroom ceiling vent register may be connected by an eight foot or longer duct to a fan located in the building attic where noise transmission to the interior of the building is minimized. Of course the attic vent fan/motor is also connected by further ductwork to blow vented air and moisture outdoors.


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