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Why Does CFM Matter?

What is CFM?

CFM stands for Cubic Feet per Minute, and is used to measure how much air a fan or piece of equipment can move.  For dehumidifiers, CFM is important because moisture can only be extracted from air that can be moved through the dehumidifier.  Simply put, if your dehumidifier cannot move any air (CFM = 0), then it cannot extract any moisture.

How much CFM do I need?

The CFM needed is based the square footage and level of humidity in the space you want to dehumidify.  Technically we're interested in the cubic footage (square footage x celing height), which will determine how many cubic feet of air that we need to cycle through the dehumidifier.  Determining the level of humidity will indicate how many times the air should be cycled through the dehumidifier every hour (know as ACH, or Air Changes per Hour).  The higher the relative humidity the more often that the air needs to be cycled through the dehumidifier.

If you have a hygrometer (an inexpensive device that can measure the relative humidity), you should be able to test the humidity levels in your basement or crawlspace.  If you do not have a hygrometer or any other way to get a fairly accurate measurement of the relative humidity, you can typically come up with a good estimate from the table below.

Relative Humidity Description Recommended ACH
Damp
60-70%
Space feels clammy or smells musty in humid weather. This is typical for normal basements and crawlspaces. 3
Very Damp
70-80%
Space smells of mildew and has frequent moisture spots on walls or floor. 4
Wet
80-90%
Mold and mildew present, moisture visible on walls or floor. 5
Very Wet
90-100%
Space frequently has standing water present. 6

To calculate the minimum CFM that your dehumidifier should have, take the cubic feet of your basement or crawlspace, multiplied by the number of times per hour that air should be changed (ACH), and then divided by 60 minutes.  The resultant number should give you the CFM that your dehumidifier should have.  Below is an example to calculate CFM:

  1. 65% relative humidity = 3 air cycles per hour
  2. 1,000 ft² * 7 ft ceilings = 7,000 ft³
  3. 7,000 ft³ * 3 ACH / 60 minutes = 350 CFM

The CFM calculation should serve as a guide for sizing your dehumidifier.  If you find that your suggested CFM is much higher than any dehumidifier you can find, you can always install more than one unit to increase the dehumidification.  If the dehumidifier you choose has a higher CFM rating that what you need, then the dehumidifier will run less frequently to achieve the desired relative humidity levels (in other words, it's OK to get a larger dehumidifier than you need).