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Relative Humidity - What is it and why does it matter in my crawl space?

Monday, April 13th, 2020 by Erin Dougherty


Outside and Crawl Space Relative Humidity ReadingThis meter reads Relative Humidity (RH) in a space. It allows us to see the difference in conditions outside (left) and in the crawl space (right).

On the Humidity meter, the top, larger number is the Relative Humidity, while the bottom, smaller number is the temperature, in degrees, of the space.

So, what exactly is RH? Relative Humidity is a measure of how much water vapor or moisture is in the air compared to how much water vapor air at that temperature can hold. Approximately, for every 1 degree that air cools, its relative humidity increases 2.2%. When that relative humidity reaches 100% it will condensate or “rain” in the crawl space.

On the day these photos were taken, it was 89 degrees outside and 77 degrees inside the crawl space.  The air cooled approximately 12 degrees as it entered the vented crawl space from the outside. Increasing the relative humidity of the air in the crawl space approximately 26% (from 51% outside to 76% in the crawl space).

At 60% RH, mold begins to grow. Coupled with warm temperatures and organic material, mold now has the perfect environment to thrive in the crawl space. Mold’s food source comes from the paper backing on fiberglass insulation, wood joists of your home, and other organic materials. At 19% wood moisture content, wood rot beings to occur which can compromise the structural integrity of your home if left untreated. 

The difference in relative humidity and temperature between outside and in your crawl space, demonstrates the need to control relative humidity in the crawl space to keep mold growth and wood rot from occurring.

This is done most effectively by encapsulating the crawl space and sealing it off as much as possible from the outside and installing a dehumidifier. This approach works as a system to keep as much air carrying moisture and humidity out of the crawl space as possible. It brings the crawl space into the thermal envelope helping control the temperature, and with the dehumidifier controls the Relative Humidity to keep the space dry and healthy. The benefits reach beyond just controlling RH, it also increases the energy efficiency of your home. Since 40-50% of the air you breathe in your home comes from the crawl space, the system also improves indoor air quality.

Get your crawl space inspected and find out how relative humidity is affecting your home. Call us at 1-833-933-3111 to schedule your free inspection.

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The Drying Co./ThermalTec
236 Industrial Blvd
Suite A
Toano, VA 23168
1-757-645-0270


The Drying Co./ThermalTec
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P.O. Box 637
Toano, VA 23168
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