Basement Humidity Level – What is Ideal?

The basement humidity level should vary between 30% in winter and 50% in summer.

  • 25% is too low and can cause skin conditions and peeling paint.
  • Over 60% is considered too high and can lead to breathing problems, mildew, and fungal growth.

Too much or too little basement humidity will have adverse effects on people–such as dry skin, asthma, and more. It can also cause building problems, from splitting wood to termite infestations.

Basement humidity

This article explains basement humidity levels, what causes humidity, and how to maintain proper humidity levels.

Humidity vs. Relative Humidity

Knowing the difference between humidity and relative humidity (RH) helps us understand why ideal readings differ between summer and winter.

  • Humidity is the amount of water vapor in the air.
  • Relative humidity is the amount of moisture the air contains relative to the maximum amount it could contain at a given temperature.

Heat causes almost everything to expand. Air molecules are no different, meaning that warmer air molecules can hold more water vapor than cold air.

To measure basement humidity, you can purchase mini hygrometers in sets of 3, 4, or 6 for under $20.00. You may want or need RH readings in different rooms–such as the laundry room, family room, or wine cooler.

Basement Humidity Level Goals

Maintaining ideal basement humidity levels can be tricky since all homes are afflicted with a number of variables. The best step to keeping your basement at the proper humidity is to run a humidifier or dehumidifier, depending on the time of the year.

  • Size. Larger areas with more separate rooms make humidity control more difficult.
  • Exterior Temperatures. Exterior temperatures and humidity affect the basement.
  • Basement Uses. Wine cooler rooms need to be cool. Laundry rooms will be warmer.
  • People. A party in the basement adds heat and moisture to the air just from extra people breathing.
  • Etc. Such as windows, leaks, standing water, open sump pits, new construction, etc.

Having multiple hygrometers throughout the basement lets you monitor the relative humidity and make adjustments as necessary to stay within the comfort range.

Note: No need to go crazy if a party causes a temporary bump. It should work itself out within a couple of days.

Summer Basement Humidity Levels

You should keep summer humidity levels as close to 50% as possible, with an upper limit of 60%. Consistent RH above 60% will lead to excess moisture problems.

If your humidity is too high in the summer, here’s what will happen in your basement:

  • Increased Mold and Fungus Growth. Mold can cause breathing problems, black mold symptoms, and peel paint off walls. Mold can grow anywhere and eventually rot or ruin clothes, carpets, framing, etc.
  • Termites. High humidity can create perfect conditions for termite colonies.
  • Dust Mites. Dust mites and other insects thrive in humid conditions.

Winter Basement Humidity Levels

The ideal winter humidity level is around 30%, with an upper limit of 40% and a lower limit of 25%. RH that is consistently lower leads to too little moisture in the air, causing the following problems:

  • Respiratory. Sinus problems like dry cough and an increased possibility of asthma, colds, and flu.
  • Scalp. Excess dandruff and an itchy scalp.
  • Skin. Dry, itchy, and chapped skin, plus an increased risk of Psoriasis.

Causes of Basement Humidity

Basements are humid because they are underground, constructed of concrete, and have the air circulation of a tomb. Many basements leak because of hydrostatic water pressure against porous concrete walls. Standing water eventually evaporates and is absorbed into the air.

Many basements are used for storage, laundry, and a utility area. Quite often, they are closed off by a door, allowing even less air movement. Most of these types of basements are not properly insulated. The R-value of 8” bare concrete walls is 1.35. Condensation will form on them and stay in the basement.

Plus, most people don’t open their basement windows often. All of this makes basements close to airtight, which means that any moisture or water vapor that gets into them cannot get out.

Removing Excess Basement Humidity

Keeping humidity levels low can be a bit of a challenge. Use some or all of the following suggestions to lower your basement humidity.

Stop the Moisture

Seal any gaps and cracks in your foundation walls, floors, and window frames to stop the moisture in your basement. Water accumulating against the exterior of the basement wall creates hydraulic pressure that finds ways into the basement.

Make sure that your landscaping is sloped away from the house. Ensure that your eavestrough is clean and functions properly and that downspouts extend at least 10’ from the house. Install French drains, swales, weeping tile, and waterproofing as required.

Insulate Concrete Basement Walls

The International Residential Code requires basement walls to be insulated in new homes. Builders did not have to meet that requirement years ago. You will solve many of your moisture problems by adding R-10 insulation and a vapor barrier to the insides of your basement walls.

Poor or substandard insulation makes for inconsistent humidity levels. Leaving one part of the basement uninsulated can make for humidity problems in the entire area.

Buy a Dehumidifier

Buy a dehumidifier to remove moisture from the basement air. They are available with humidity meters, timers, auto settings, and much more. Dehumidifiers are rated by the number of pints removed in 24 hours.

Dehumidifiers don’t have large reservoir tanks, so you may be emptying them quite often. Or, with luck, you can set one up close to your floor drain and run a hose for continuous draining.

Mitigate Moisture-Creating Activities

If you shower, cook, or do laundry in the basement, installing fans that vent warm moist air to the house’s exterior will remove many of the problems.

Good quiet fans can move more than 100 cubic feet of air per minute (CFM). A 10’ x 10’ x 8’ high room contains 800 cubic feet of air, meaning a good fan will exchange the air every 8 minutes. Run fans for at least 30 minutes after laundry, showering, or cooking. Ensure your dryer is vented outside and working properly, and tape any holes or joints.