Ideal humidity level for finished basement?7 min read

Ideal humidity level for finished basement?7 min read

Want to know the ideal humidity level for a comfortable living?

dew drops in window due to indoor humidity
Early Morning Dew Drop due to Indoor Humidity

Humidity, regardless of whether it is in the living area finished or unfinished basement, there is no difference in the level that must be maintained.  There is one thing common though.  Ideal humidity level must be maintained to feel comfortable.

In the rest of the article, I will let you know what you need to know and where to maintain your humidity level in the finished basement and other living areas to be absolutely comfortable.

Ideal humidity level for a finished basement is 30 to 50% relative humidity. Anything less than 60% should be the goal.

Well, if I tell just this, I will be like one of the other thousands of blogs rattling the same thing. Like a broken record.

Since most of the world is using RH as a gauge, I am going to talk about it.  But, I will give you a better way to measure to make sure your home and basement living conditions are comfortable.

Let me get that straight.  Relative humidity (RH) is not the right way to measure.

What is relative humidity (RH)?

Let me try to define it, the scientific way.  I will take you back to your high school physics class.

The partial pressure of water vapor 

——————————————————-  x 100

Saturation vapor pressure

Sorry, I didn’t mean spoil your mood, however, that is the correct definition. Let’s pick this formula apart and try to understand correctly.

The partial pressure of water vapor:  It is directly related to the number of water molecules present in the air.  So, more water molecules in the air, more the pressure there is.

The partial pressure of water vapor:  It is then the maximum number of water molecules that can exist in the air.

It’s not humidity in the abstract term that we are referring to.  It is important to note it is a relative term.

So, that in English would translate to the below.

Under the given condition,

Amount of available water molecules (0.5 in Hg)

——————————————————————————– x 100   = 62.5 %

Total water molecules that can be present (0.8 in Hg)

Side note: Hg is the symbol for mercury in the scientific table. That’s why you see Mercury in the conventional thermometers.

The above formula boils down to this…

Amount of water vapor in the air divided by the maximum possible amount.

Okay! Well, if you are still confused, don’t sweat, I will further explain and show what you need to pay attention to.

Relative humidity and Dew point

Warmer air can hold more moisture.  That’s the truth.Calculate Dew Point using temp and relative humidity

So, in summer months since the outside temperature is in the 80 ° Fahrenheit and above range, the outside air is going to hold on to more volume moisture or vapor.

Colder weather conditions, in fall and spring, are going to hold much lesser.

And in the coldest winter months, water vapor is going to be the least.

As we saw earlier, RH is going to tell how much water molecules in proportion to what is the maximum that the air, given the condition and temperature, can hold.

Dewpoint is the temperature at which water vapor turns in to visible liquid, i.e. Water.

The Dewpoint indicates the absolute level of moisture in the air.  An absolute level is much easier to wrap your head around.  What do you say?

So, in essence, a 100% RH at 50° F is going to hold less moisture than 50% RH at 95° F.

How do I know?

There is a non-linear scale that shows the dewpoint and relative humidity in relation to the temperature.  You can use this DewPoint calculator and play around.

Look at the Dewpoint.  Higher the due point, more moisture it is in the air and hence it’s going to be uncomfortable.

Dew point at 100% RH at 50° F temperature is 55-degree Fahrenheit – Comfortable

Dew point at 50% RH at 95° F temperature is 74-degree Fahrenheit – Yukies!

A Good rule of thumb:  It’s comfortable when the dew point is less than 60 degrees.

Well, now you know relative humidity is a poor measure of how humid it is.  Dew Point is a much more objective, and in my opinion, the correct measure of humidity.

So, when someone says 50% Relative humidity is good, don’t believe.  Unless they also say at what level of temperature.

Reasons for high humidity

  • Moisture seeping from under the ground
  • Moisture seeping from the side of the walls
  • Air leaks and outside humid air gets in

Problems with low and high relative humidity

When the humidity levels are too high below are some of the risks:

  • Mold growth and other microorganism infestation (e.g. bacteria)
  • Discomfort in living conditions
  • Stuffy nose etc.

When the humidity levels are too low below are some of the risks:

  • Uncomfortably living condition – Dry air sucks moisture out of the body
  • Itchy skin and rashes due to dryness
  • Catching a cold or infection
  • Wood damage due to dryness
Basement Indoor Humidity Levels, Health Risks and Optimal Zone

Tips for having a healthy living condition below-grade and Solutions to keep humidity under control

  • Extend the downspouts away from the house. At least by 10 feet or more.
  • Make sure exterior water is not puddling near or close to the foundation. Any water that gathers should run away from the houses’ foundation.
  • Fix any moisture leak issues, including sealing basement floors, fixing cracks in the wall and leaky window sills.
  • Some old dehumidifiers might not have humidity level monitor. If so, place a hygrometer in a central location away from the dehumidifier to get a really good reading on the basement humidity level
  • If the Landry room is in the basement, make sure that the dryer is vented outside.
  • Place a Vent fan, if your basement has a shower in the bathroom. Run it while you take a steam shower.
  • Place a dehumidifier in your basement. Keep the RH at around 50%. If you can tap into the drain line at the main feeder, run the drain line from the dehumidifier.  At a minimum run it into the sink drain.

Don’t sweat if you don’t have a dehumidifier lying around. Here are some hacks to make your very own homemade DIY dehumidifier.  It costs less than $5.

Ideal humidity for the basement in Winter

Winter is generally a better living condition with respect to humidity.


Didn’t you skim the article and just landed here for the answer?

Ok, if so I am going to briefly touch here.

When the temperature is colder, air can only hold a lesser number of water particles.  Which is to your advantage.  Even if the relative humidity is high, it is only in relation to maximum possible water particles that the air can hold.

So, a high relative humidity is not such an issue.  Because the absolute amount of water content is still going to be less than ideal for mold growth.

If you keep your heat on in the winter months, your basement will stay dry and warm.   This will obviously make the relative humidity below 40%.

Ideal humidity for the basement in Summer

In summer, the temperature is going to be high and hence the moisture content will be high.

It really depends on whether you are in a humid region like Florida or dry region like Arizona.

In any case, keeping the relative humidity below 50% in summer is going to keep you out of trouble.

Dehumidifier running all the time:

Check your factory settings. Some of them come in set at 42%.  You don’t need it that low.

Increase it to a more comfortable level like 55% and you will fix that issue.

Place it in a nice location where the dehumidifier can pull the air from all the space around

Pro tip: Run the plumbing from dehumidifier to a drain line (or sump pump), so it’s self-emptying and maintenance free. 🙂 

In Conclusion:

While relative humidity is the metric used widely, it only tells half the story. Always combine it with the temperature or at least the season to know the correct relative humidity level to maintain.

Dew point is a better measure. Always keep it under 60 and your family will be comfortable.

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