Bathroom Exhaust Fans

Most people think the bathroom exhaust fans are primarily for removing odors and they are partially correct. The real function is to remove moisture from the bathroom.

Homes without an outside window in their bathroom should ALWAYS have an exhaust fan installed. Moisture and mold are always a problem in these homes. Some local requirements require a fan and the light to be on the same switch. Some homes have the fan installed with a separate switch to turn the fan on and off.

Let us determine how large a bathroom exhaust fan you will need. We are not talking about the physical size of bathroom exhaust fans, just the volume of air the different fans move.

Let’s say your bathroom is 6 ft wide x 12 ft long x 8ft high (6x12x8). That’s 576 cubic feet of volume. The formula we are going to use requires you to divide the amount of cubic feet by 60 then multiply the answer by 8.

This is the recommended number of air changes in an hour. So the answer for the example is 76.8 CFM (cubic feet per minute). The exhaust fan in the picture below would be a good choice for this application.

Here is another example: 10x14x8=1,120 CF. Then divide the 1,120 by 60=18.66 x 8 = 150 CFM. This is twice the volume of the previous example. Okay, now we know the size so let’s go shopping.

Are you looking for an bathroom exhaust fan that does nothing but remove air? Or are you thinking about one that has a built in light or heat lamps? There are many options. One of which is the stand alone model where everything is built into one box. Below is an example of a fan with the light and heat built in.

Perhaps you want one that is really quiet. You can also get a remote fan. This is where the fan is above the ceiling, has multiple intakes and is really quiet.

Speaking of really quiet, fan noise is measured in Sones. (A sone is a unit of perceived loudness).

This is a measurement that a person can determine if the fan suits their needs or not. 4.0 sones are comparable to the average TV. 3.0 sones are comparable to the average office environment.

Some fans sound like aircraft engines ready for takeoff. Look for the lowest number you can afford. 1.2 is a good number. Below is an example of a quiet fan.

The peace and quiet is worth the extra money. Get the correct size fan in CFM for your needs. Now you have a fan with the correct air flow and is quiet.

Installing bathroom exhaust fans is relatively easy. Let’s say you are installing the fan in the ceiling of your bathroom.

  • First, do not install bathroom exhaust fans over the HVAC vents in the floor. You really do not want to pull the freshly heated/cooled air going into the bathroom directly to the outside. You want the moisture laden air to go outside. Also, do not install over the shower.
  • Pick a location for your exhaust fan. I like to push a screwdriver through the ceiling and go into the attic to ensure the space is clear.
  • Then mount two pieces of 2×4 lumber the width of the box between the ceiling joist.
  • Place plastic sheeting over the space below and cut the hole.
  • Mount the box to the frame with screws. Never use nails as they can come loose.
  • The best method is to run the exhaust tubing through the roof. However, a perfectly acceptable method is to run the exhaust tubing to the soffit area. The cut a hole in the soffit and have the exhaust tubing vent to the outside.
  • Don’t forget to put a backdraft damper in the exhaust tubing to keep outside air from blowing back into the bathroom. If you do not do this, you will hear the door in the fan housing rattle every time the wind blows.
  • Install the timer switch and the bathroom exhaust fan project is complete.
Dave Altman

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