relative humidity scale with dehumidifier setting for basement for mold growth

Basement Dehumidifier Setting: 30%, 40%, 50%, or 60% Humidity?

Having a dehumidifier with a wrong setting can be just as useless as having no dehumidifier. 

Even before you buy a dehumidifier, it makes sense to educate yourself about relative humidity levels. The most basic idea is to keep indoor relative humidity between 30% and 50%

However, when it comes to basement dehumidifier setting, we’re sometimes perplexed – should be set the dehumidifier to 30%, 40%, 50%, or even 60% relative humidity? This is especially important when figuring out the correct dehumidifier setting to prevent mold and mold growth.

Under ‘Mold Course Chapter 2’, in the 3rd Lesson – Humidity, EPA clearly states the following:

Sometimes, humidity or dampness (water vapor) in the air can supply enough moisture for mold growth. Indoor relative humidity (RH) should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible. Low humidity may also discourage pests (such as cockroaches) and dust mites.

The key part that we forget when it comes to basement dehumidifier setting in the temperature. Mold, for example, grows in:
  • Damp environment. Example: Basement with 80% relative humidity.
  • Hot environment. The hotter the indoor is, the more likely it is for the mold to flourish.

That means that we have to aware of the temperature of the basement in addition to the relative humidity. In the end, you’ll find the commentary for 30%, 40%, 50%, and 60% relative humidity setting. 

What Relative Humidity Dehumidifier Setting Is The Best For Mold Prevention?

Answering at exactly what relative humidity percentage mold grows is not easy. Here are a few scientific sources that can help HVAC engineers:

In short, there is no clear answer.

Most HVAC engineers, however, agree that the most appropriate basement dehumidifier setting is 40%.

It might be that 50% would be enough. However, at higher temperatures such as 80°F or 90°F, the likelihood of mold growth is significantly increased. In order to negate this, it’s always safer to set a dehumidifier setting to 40% to prevent mold growth.

Obviously, the basement dehumidifier setting of 30% would be the safest but not the most energy-efficient. If the 40% basement relative air humidity can guarantee the absence of mold growth, all the electricity needed to bring relative humidity from 40% to 30% would be wasted (or would be of no consequence as far as mold growth is concerned).

Here is the full list of basement dehumidifier setting options with commentary:

  • 60%. Too high; mold might grow.
  • 50%. Not safe enough; at high temperatures, there might be a minimal change of mold growth (usually localized to a corner).
  • 40%. Just perfect; very safe with no chance of mold growth.
  • 30%. Unneedlesly low; you will waste energy for reducing humidity from 40% to 30% without increase anti-mold safety.

Leave a comment