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Basement & Crawl Space Glossary

Relative Humidity

Relative Humidity

Relative Humidity
95 47%
90 58%
85 69%
80 80%
75 91%
70 102%
65 113%
60 124%
55 135%
50 146%
45 157%
Almost all air holds moisture.  However, only a certain amount of moisture can be held become the air becomes overloaded and cannot hold any more.  Relative Humidity refers to how saturated the air is with this moisture.

Warm air can hold much more moisture than cold air.  As a consequence, warm air will much more readily pull up and store water there.  This is why water evaporates much more quickly off surfaces in the summer.  As moisture enters warm air, the Relative Humidity will rise slowly.  In the winter, when the air cannot hold much water, it pulls less moisture up, but the relative humidity will still rise and fall at normal rates because less moisture will make more of a difference.

If you take warm, humid air and lower its temperature, its relative humidity will rise at a rate of about 2.2% for each degree it's lowered.  For example, the air outside is about 80 degrees with an 80% relative humidity.  If you lower this air to 75 degrees, the relative humidity should rise to approximately 91%.  Lowering the same air another five degrees to 70 degrees should raise the relative humidity another 11%, but that would bring the humidity to over 100%!  Once this happens, the air becomes overloaded with water and drops it on any cool surfaces nearby in the form of dew, or condensation.

The natural temperature of a crawl space is about 50 degrees.  When warm summer air is pulled into a crawl space, the temperature drops and the air has more moisture in it than it can handle.  This moisture is dropped on pipes, ducts, and wood.  In time, the wood in a crawl space will become permanently moist, making the perfect environment for mold growth, dust mites, rot, rodents, termites, carpenter ants, and other pests.

The mold spores and dust mite waste formed in crawl spaces and basements will rise up into the home.  The reason for this is the stack effeck.  Warm air rises in the home and leaves through the upper levels, and air is sucked out of the crawl space.  The humid air will make the home harder to heat and cool, and the allergens in the basement and crawl space air will cause various health problems in all areas of the home.

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