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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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.
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.
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.
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.  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.
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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I am going to install a new bath fan, I am having a new roof put on the house and decided now would be a good time to put the vent on the roof. My question is I got a vent for 6" ducting, I will need a reducer at the fan end to 4" Would this be a good size duct for the fan.? Also I an using metal ducting and it's about six feet from the fan to the roof, Should I angle the duct a little or would it be ok to go straight up.? D.K. 10/19/2013
You've raised several key topics, and your question helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. For example on site I might notice something about your attic and roof structure, ease of routing venting, placement of insulation, and even very basic stuff like - where the heck is your home? Bath ventilation worries may be a bit different in a cold climate than in a warm dry one and different again in a wet humid climate. That said I offer these comments:
For freezing climates we don't want to risk ice accumulation in the vent system - ice can collect from freezing condensate that arrives inside the bath vent duct during hot steamy showers;
For hot humid climates we don't want to have condensate accumulation in air conditioning systems and A/C ductwork, but a bath vent run through a hot attic is less likely to raise that same issue.
I prefer to run a bath vent to outdoors via a horizontal line that goes across an attic and out through a gable-end wall or one that vents down and outside through a roof overhang or soffit. The vent run needs to be designed to drain any condensate outside not back into the bathroom ceiling; in a freezing climate I'd insulate such a vent line as well; If we run a bath vent vertically up through a roof we have two risks I'd prefer to avoid:
The vertical run guarantees that any condensation runs back down into the fan (risking damaging the wiring or fan motor) and back into the bath or bath ceiling.
The vertical run also means another roof penetration. I prefer to minimize the number of roof penetrations on any building since every penetration is a potential leak point, more so if the penetration flashing is not installed correctly.
The vent fan manufacturers installation instructions typically give maximum run lengths and recommended vent diameters for their products; long vent runs and vents that use plastic dryer-type flex-duct (not your case) cut the effectiveness of the fan by adding airflow resistance and thus increase the risk of accumulated moisture too. Metal duct work (your case) is in my opinion always a better installation: smooth interior means better airflow. Metal fan vent ducting also reduces the risk of duct crush or collapse.
I am guessing that for a very short bath vent duct run, going to a larger duct size is fine - it'd make no difference but you're probably not gaining a thing on a short run by using a 6-inch duct to vent a fan that expects to vent through a 4-inch duct.
In my experience inspecting and troubleshooting buildings, I've seen many bath vent fans that seemed ineffective. A fan that nobody uses because it's too noisy means a bathroom that is rarely vented adequately (risking mold, smells, even wet insulation). A fan that is under-powered means even if the fan is used it doesn't do anything.
The fan capacity you need depends on the size of the bathroom being vented - usually calculated in cubic feet. That figure is matched against the fan manufacturer's recommendations for fan capacity measured in cubic feet per minute (CFM). The CFM rating of the fan in turn presumes that the vent routing, diameter, length, and number of obstructing turns and bends is within the company's specifications. In the article above we explain how to calculate the required bathroom vent fan capacity. Also, for bathrooms over 100 sq ft, the HVI recommends a ventilation rate based on the number and type of fixtures as shown in Table 6-12 - data discussed in more detail at BATHROOM VENTILATION DESIGN
Sorry that these notes are a bit long on arm-waving and short on more specific details, but as we've got no information about your particular installation except what's in your original note, I have to stop here.
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References for Bathroom Vent Fan Installation
Fantech in the United States 10048 Industrial Blvd., Lenexa, KS 66215 Phone: 800.747.1762; 913.752.6000 Fax: 800.487.9915; 913.752.6466 www.fantech.net; firstname.lastname@example.org
Fantech in Canada 50 Kanalflakt Way, Bouctouche, NB E4S 3M5 Phone: 800.565.3548; 506.743.9500 Fax: 877.747.8116; 506.743.9600 www.fantech.ca; email@example.com
Nutone Bathroom Exhaust Fan/Light Combination Installation Instructions, Model 8663RP, 8673RP, 8664RP suitable for use
in shower or tub enclosure when used with GFCI protected branch circuit. Suitable for use in insulated ceilings.
Nutone, 4820 Red Bank Road, Cincinnati, Ohio 45227, web search 07/27/2011, original source: http://www.nutone.com/PDF/InstallGuides/8663RPins61784.pdf