House Building
Choosing The Perfect Bathroom Exhaust Fan
By Mike Finley
Tue, 9 Nov 2004, 07:26 PST

Because 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 x width x 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 = 10 x 6 x 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. 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.

Bathroom fans are measured by SONES. 4.0 Sones is the sound of normal television (Ridiculously loud for a fan) 3.0 Sones is office noise (Still very loud for a fan) 1.0 Sone is the sound of a refrigerator and 0.5 Sone is the sound of rustling leaves. A very quiet bathroom fan that will just make a gentle whoosh is a fan at 1.0 Sone or below.

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.