Is your bathroom ventilation fan doing a good enough job preventing mold and bacteria growth

What are CFMs? Does your bathroom ventilation fan have enough of them to remove the moist air and prevent mold and moisture damages in your home? Find out here.

What are CFMs?

CFM stands for cubic feet per minute. It is used as a measure of the volume (amount) of air a fan can move in a minute. It is also commonly used as a mode of measurement in air conditioners, air compressors, air scrubbers, negative air machines, etc. The higher the number of CFMs the more air the device will move each minute.

How do you know how many CFMs your bathroom fan has?

Easy, first make sure the fan is turned off, then you will need to remove the cover from your fan. This can be done by pulling the cover gently down you will see it is held up on two sides by metal wires, pinch the wires together on both sides to remove it the rest of the way. Word of the wise, if you haven’t removed the cover before or for a while there is likely quite a bit of dust that has accumulated, so try not to stand directly below your fan when removing the cover or you will get a face full of dust.  Since you have the cover down anyways it is a good idea to vacuum and clean it before replacing it. This should be done periodically to ensure your fan is able to remain efficient. ** Please Note: when you’re vacuuming and wiping around the inside of the fan, make sure you first unplug it, you will see where it is plugged in once the cover is removed.

You will see the same or similar label to the one below.

CFMs label

If you take a closer look at the label you will see how many CFMs your fan has. As you can see this particular fan has 90 CFMs.

fan cfms


Does your fan have enough CFMs to adequately remove the moist air from your bathroom?

We can figure this out using a simple calculation. First you will need to measure the length, width and height of your bathroom.

With this we can calculate how many cubic feet or volume of air in the room. Plug in your dimensions to figure out the cubic feet of your bathroom.

L x W x H = Cubic Feet

We will use the dimensions of the bathroom the fan pictured above was in so you can see the calculation. (11 ft. long, 10 ft. wide, 8 ft. high)

11 x 10 x 8 = 880 cubic feet

Now let us calculate if the fan has enough CFMs for this bathroom.                                         The calculation is as follows:

CFM = cubic feet (L x W x H) x air exchanges in 1 hour

What are the number of air exchanges?

The number of air exchanges is how many times the air will be changed each hour. For an average home bathroom you want your fan to be able to exhaust enough air for there to be 8 air exchanges every hour. The number of air exchanges you will use in your calculation is 8.

Now we will plug in all of the numbers

880 cubic feet

8 air changes

CFM = 880 cu.ft x 8 air exchanges

60 min.

CFM = 117.33


So we can see from the answer we came to that the existing fan in this bathroom will not allow for the 8 air changes in an hour we want. The 90 CFMs is not enough for the size of this bathroom.

In order to correct this, a ventilation fan with a higher number of CFMs will need to be installed or the existing one can remain and a smaller fan can be added above the bath or shower. This will make up the difference that is necessary to obtain the 8 air exchanges we need to remove the appropriate amount of hot moist air and prevent any future damages that moisture will cause. These damages can include mold growth, bacteria growth, water damage, water stains, structural damages and the overall indoor air quality of your home.

You are looking for The CFMs of your existing fan and your answer to be close, they don’t have to match exactly but shouldn’t be too much below. You also don’t want to see a large difference above either, you don’t want to dry the materials out too much this can also create unwanted issues in your home like dry rot, structural damages and balance of airflow throughout your entire household. We want to achieve equilibrium.

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