When it comes to basement humidity levels, "normal" is a relative term, and varies with circumstances and according to geographic region. What is "normal" in humid Baltimore can be much different than it is in arid Phoenix. However, most experts, including the Mayo Clinic, say that an idea home humidity level falls between 30 percent and 50 percent.
Outside those boundaries, there are a variety of health problems that can occur:
- Low humidity can make your skin dry and lead to irritation of sinuses and throat. In very dry conditions, your eyes may feel itchy.
- High humidity can make a home feel stuffy and clammy and can cause moisture to condense on windows and walls. Excess moisture can encourage the growth of harmful bacteria, dust mites and molds — all of which can lead to respiratory problems, such as asthma, and allergies.
In some regions, people rarely have problems maintaining a proper home humidity level; those in other areas may habitually struggle with high humidity, while others are chronically dry. In many climates, humidity fluctuates by season, with summers plagued with high humidity and winters fighting dry air conditions.
Basements are subject to unique circumstances. Because these spaces are below grade, they are typically cooler than upstairs spaces, and the closer contact with the ground means that earth moisture can migrate in through the walls and through the basement slab. Maintaining the desired 30 to 50 percent humidity levels can be especially challenging here. Usually the challenge in a basement is not raising humidity, but lowering it.
Determining Humidity Levels
Measure the humidity level in your basement and throughout your house with a hygrometer, a gauge that closely resembles a thermometer and is available at your local hardware store, home improvement center or garden center. There are also visible signals you can look for that indicate you have a problem with excess humidity:
- Spots of mildew or mold on walls or baseboards.
- Condensation on water pipes or plumbing fixtures
- Rust on appliances or furnace fittings.
If your hygrometer shows a relative humidity level of higher than 50 percent, it may be temporary. Check again over a period of a few weeks. If the reading is consistently high, you should take steps to remedy the situation.
Causes of High Humidity
By virtue of the fact that basements are cooler than upstairs spaces and in close contact with the soil, they are naturally prone to condensation and trapping moisture. This natural humidity can be controlled through a combination of sealing the walls and floors against moisture infiltration and increasing air circulation to remove moisture from the air. Plastic or applied vapor barriers placed between foundation walls and floor slabs and finished surfaces may reduce moisture infiltration. Even with the best of structural and mechanical design, though, a basement in the humid days of summer may still soar above 50 percent in measured relative humidity.
- Relative humidity is a measurement of how much moisture air contains compared to the maximum amount it could hold. So a 50 percent relative humidity level contains half of what the air could hold before it begins to "rain" moisture out of the air.
Look also for mechanical problems with plumbing fixtures, such as leaking pipes, or a leaking washing machine or water heater. Water pooling in a basement from these problems can cause overall humidity levels to soar.
Methods of Adjusting Humidity
Whole-home dehumidifier. For very humid climates, a whole-home dehumidifier that fits into the home's furnace/air conditioning system is a possible solution. This is expensive equipment, though, so it should be considered only where chronic high humidity is a problem throughout the house.
In locations where chronic dryness is a problem, in basements as well as the rest of the house, a whole-home humidifier can be installed.
Space dehumidifier. It's more practical to place one or more individual space dehumidifiers in the basement to remove moisture from the air. These can be stored away during the months when humidity is not a problem, and placed exactly where they are needed during the humid months. Space dehumidifiers require that you periodically empty the water reservoir of the moisture they pull out of the air — or you can install them so they drain directly into a floor drain.
Although not normally a problem in a basement, air that is too dry can be adjusted with a space humidifier.
Running vent fans. Moving air will hold more dissolved moisture, so running fans in your basement can reduce the amount of moisture that condenses on surfaces. If your basement has a bathroom with a vent fan, you can also leave this fan running more frequently to keep air circulating.
Running the HVAC fan. If your furnace and air conditioning system has a manual setting for the system fan, move this switch from the AUTO to the ON position to keep air circulating throughout the house during periods of high humidity. This may also have the advantage of making temperatures throughout the house more uniform.