Relative Humidity: Why You Need To Understand It

Wednesday, August 26th, 2020 by Kal Aldrich


As a homeowner, it is crucial to understand what relative humidity is, how it can affect your home, and what you can do to control its levels in your crawl space or basement. When someone says the air feels damp or moist, they are talking about the humidity in the air. But really what they are referring to is the relative humidity. Relative humidity is the amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at a given temperature and pressure.

What determines when relative humidity is too high?

If the relative humidity in a home is 50%, the air is holding half the amount of water vapor it can hold. Therefore, when there is no water vapor in the air, the relative humidity is 0%. According to the Environmental Protection Agency, indoor humidity should be kept between 30-60% to reduce 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. Humidity measurement tools can be found at most home goods stores.

To reduce indoor relative humidity, we recommend a professional-grade dehumidifier. Click HERE to read more.

What causes high indoor relative humidity?

The high relative humidity is common in vented crawl spaces and basements, with one of the causes being water. Water can leak through foundation cracks and around basement windows, and it can also come from a leaking pipe, washer, or water heater. Another cause is condensation, which occurs when soil moisture seeps through the concrete foundation and meets with pipes, ducts, or basement walls. The last cause is outside air. Damp outdoor air can get in through the basement windows or crawl spaces, causing increased humidity levels in your home.

What happens when the indoor relative humidity is high?

Most damage in your basement or crawl space is obvious, such as mold, rot, wet insulation, wet drywall, and so on. But humidity in the basement or crawl space can affect other levels of the home. Since air flows into the upper levels of your home from the basement or crawl space, it brings humidity and poor air quality with it.

As warm air rises in a home, it leaks out of the upper levels. New air must enter to replace the air that escaped, which is referred to as the “stack effect,” from bottom to top. With that said, the humidity that is in your basement or crawl space is affecting you, your family, and your home.

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

-          Smelly, damp carpets

-          Rotting floors

-          Condensation, rotting, and mold in the attic

-          Poor air quality

-          Frost, condensation, or mold on the inside of windows in cool weather

-          Increased cooling and heating bills

-          Decreased value in roof sheathing and shingles

-          Reduced like of the paint on the outside of the home

-          Aggravated asthma and allergies

-          Dust mites

If you have a vented crawl space, water seepage in the crawl space can usually be fixed by getting one of our crawl space waterproofing systems installed. Water evaporating from the ground or moisture coming through the vents can be solved by encapsulating the crawl space with a vapor barrier, drainage matting, and crawl space drain. Click HERE to learn more. 

If you have a basement, water in the basement can be solved by getting one of our many wall protectants, a sump pump, and an internal drainage system installed. Our Aprilaire Dehumidifier can also keep the humidity level under control and prevent mold in the basement. To learn more about our Aprilaire Dehumidifier and what it can do for you, click HERE. Do not wait until it is too late to do something about your damp crawl space or wet basement and contact American Waterworks to schedule your no-obligation, FREE estimate today!


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