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Choosing The Perfect Bathroom Exhaust Fan

by Michael Finley

 J-CAN HOUSING W/RADIATION DMPRBecause moisture is a bathrooms biggest enemy, you must vent the hot, moist air out of the bathroom. Without doing this paint will peel, doors will warp and you run the risk of mold.

The first thing you should consider is a timer instead of a switch. The success of your fan relies on a two-fold approach. The Home Ventilating Institute recommends that your fan be capable of achieving 8 air changes per hour, with the exhaust of air continuing for 20 minutes after use of the bathroom. Using a timer will help you achieve the extra run time without having to remember to come back and turn it off. Plus with the new quiet fans available it isn't uncommon to forget to turn a bathroom fan off now because we don't have that load roar to remind us it is on. To achieve the air exchanges we need to correctly size the fan.

  1. First find the volume of the bathroom. Volume = length × width × height
  2. Find the CFM (cubic feet per minute). Volume ÷ 7.5
  3. This CFM is the minimum airflow required to achieve 8 air exchanges per hour.

Example: Volume of the room = 10 × 6 × 8 = 480

CFM = 480 ÷ 7.5 = 64
In this example you would only purchase a fan that has a CFM listed on the box greater than 64. 

Next comes noise... balance noise against air volume!!

Choose your fan based on how quiet you want it. Personally I want the quietest fan I can afford. Ideally you don't even want to know it is on. However, the price goes up as the noise levels go down, so you have to keep your budget in consideration.  Also, the noise level goes up as the speed of the fan increases.  A real balancing act!

Bathroom fans are measured by sones, a subjective measurement of how a sound is sensed.  A sound that is "grating" or annoying will rate higher in sones than it would in decibels, a more scientific measurement.

Here are some examples of sone measurements: 

4.0 sones is the sound of normal television (which would be ridiculously loud for a normal bathroom fan, though I've heard some this loud!)
3.0 sones is office noise (still very loud for a fan)
1.0 sone is the sound of a quiet refrigerator (about as good as it gets!!)
0.5 sone is the sound of rustling leaves.

 Non-Duct Fan-LightA very quiet bathroom fan that will just make a gentle whoosh is a fan at 1.0 sone or below. 

Keep your new quiet fan quiet by installing it correctly!

So after you spend the extra money on a fan that is quiet, don't make common mistakes in the installation to negate that low sone fan. Always use screws in the installation, not nails. Nails will vibrate lose eventually and create noise. Use 4 inch venting not 3 inch, the bigger the venting the quieter it will be. Make turns in your venting gradual; avoid 90-degree bends if possible to reduce air noise in the venting.

The only other considerations are features and looks. You can get a fan with a light in it; you can even get a fan with a heating element in it. Looks run from just a white grill to just about anything you can imagine. The limits are just your budget and your tastes. But start with CFM and Sones ratings first, then looks. Doing this will ensure you have a bathroom fan that not only looks good but is doing its job, which is protecting your bathroom from the perils of moisture.

This article provided by Michael Finley of
Rocky Mountain Custom Home Improvements, Inc

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