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Relative Humidity: Why You Need to Understand It

Friday, June 28th by Kristina McGovern


Mold and damaged drywall in a basement

When the relative humidity in a basement is high, it can lead to mold and moisture damage.

If you're a homeowner, it's important to understand what relative humidity is, how it can affect your home, and what you can do to control the relative humidity in your basement or crawl space.

When we say the air feels damp, we’re talking about the humidity in the air. But really what we’re referring to is the relative humidity. The relative humidity is the amount of water vapor in the air compared to the maximum amount of water vapor the air is capable of holding at a given temperature and pressure.

What's a High Indoor Relative Humidity?

If the relative humidity is 50%, this means the air is holding half the amount of water vapor it could hold. When there’s no water vapor in the air, the relative humidity is 0%.

According to the Environmental Protection Agency (EPA),
indoor humidity should be kept between 30-60% to decrease mold growth. When the indoor humidity level is higher than 60%, the air is too wet, and the home becomes a breeding ground for mold, pests, and rot.

For more facts about indoor relative humidity, read this article about
relative humidity, temperature, and dew point.

What Causes High Indoor Relative Humidity?

High relative humidity is common in vented crawl spaces, especially in humid climates like in the Southeast. But high relative humidity is also common in basements too.

Water: Water can seep through cracks in the foundation and around basement windows. Water can also come from inside the house from a leaking pipe, water heater, or washer.

Condensation: Moisture from the soil can seep through a concrete foundation. Moisture also evaporates from standing water caused by a leak. When warm air comes in contact with cold surfaces, such as basement walls, pipes, or ducts, this creates condensation.

Outside Air: Damp air from outside can get in through basement windows or crawl space vents.

What Happens When the Indoor Relative Humidity is High?

Most damage in the basement or crawl space itself is obvious, such as mold, rot, wet insulation, wet drywall, and the list goes on. But humidity in the basement or crawl space affects the upstairs too.

Since air flows into the upper levels of your home from the basement or crawl space, it brings the humidity from down there with it.

A brief explanation of how the natural airflow in a house draws humidity (and mold and musty odors) upstairs:

As warm air rises in a home, it leaks out of the upper levels. New air must enter to replace the air that escaped. This creates a suction at the lower levels to draw in replacement air. This process is known as the “stack effect.” It creates airflow in your home from bottom to top. Air from the basement or crawl space is drawn upward into the first floor, and then to the second floor. Therefore, whatever is in your basement or crawl space air is in your house and affecting you and your family, whether you spend time down there or not. If there’s high humidity downstairs, there is higher humidity upstairs than there would be otherwise - and mold spores and damp odors.

The negative effects of a damp basement or crawl space on your home can include:

  • Dust mites (the number one indoor allergen)
  • Sticking (swollen) doors and windows
  • Smelly damp carpets
  • Buckling hardwood floors
  • Condensation/rotting/mold in your attic (as humid air escapes
  • into your attic it can condense against the cold ceiling or roof)
  • Frost or condensation and mold on the inside of windows in cool weather
  • Increased cooling bills (damp air takes more energy to cool)
  • Increased heating bills (damp air takes more energy to heat)
  • Mold upstairs
  • Decreased life of roof sheathing and shingles
  • Decreased life of the paint on the outside of your house
  • Aggravated asthma and allergies

What Can You Do to Control Relative Humidity?

If the relative humidity in your basement or crawl space is high, you need to first find the source of the water or moisture problem before deciding how to fix it. The steps taken to fix a damp basement or crawl space are similar.

If you have a vented crawl space…

Water seepage in the crawl space can usually be fixed by installing a sump pump and drainage system. Water evaporating from the ground or moisture coming through the vents can be solved by encapsulating the crawl space (aka closing the vents and covering the walls and ground with a vapor barrier). A dehumidifier can help control the humidity level and keep your crawl space dry and free of mold.

If you have a basement…

Water in the basement can be solved by fixing plumbing issues and installing a sump pump and drainage system. Cracks in the foundation should be repaired. Installing a dehumidifier can keep the humidity level under control and help prevent mold in the basement.

If you think it’s time you do something about your damp basement or crawl space, contact Basement Systems to find a waterproofer near you and to schedule a free on-site evaluation and estimate.

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