How is your crawlspace growing mold while three seasons out of four, your home feels comfortable and dry?  Sure, it may feel rather humid on the occaisional summer day, but most the time the house feels fine…  So how does your crawlspace trap humidity?  Even with your vapor barrier, moisture will condense and breed mold, termites, dust mites and more.

Here’s the answer.  Air in your crawlspace is always more humid than the air in the house.  And this is because cool ground temperature keeps the crawlspace colder than the air in your house.  Because of this temperature difference, your home’s living areas can feel dry, while below, crawlspace humidity runs rampid.

Here are a couple terms to know:

1. Dew Point = Temperature at which water vapor condenses
2. Relative Humidity (R.H.) = % of moisture present in relation to of moisture the air can hold.

In other words, relative humidity is the percent of water in the air.  When R.H. hits 100%, water will condense, rain or snow.  Dew Point is the temperature when this happens. Here are a few scenarios to show what happens in your home and crawlspace during typical weather conditions.

NOTE: Crawlspace R.H. (relative humidity) should try be 40% or less in order to keep Dew Point around 10-15 degrees below the crawlspace temperature.  EPA recommends 40% – 50% R.H.  This will control dust mite and mold populations.  A crawlspace dehumidifier should always be used during humid or rainy weather.  Humidity in the living area above, can be as high as 50% or 60%, based on comfort.

As the winter air enters, the humidity drops slightly within the crawlspace.  Notice relative humidity is 62%, beyond what the EPA considers acceptable.  As the air migrates upward, it warms greatly and relative humidity drops drastically.  The house feels dry, but the crawlspace still requires dehumidification.  To control dust mite populations, it is recommended that the crawlspace be kept at 40% R.H. all year long.  Unfortunately, most dehumidifiers will not work at a 45° temperature.  Though, our Dri-CrawlSpace^® dehumidifiers will run just fine all the way down to 33°.

Still winter time, but here we have a cold rainy day.  It will still feel dry indoors, but notice as the outside air enters the crawlspace, relative humidity only drops to 70%.  During this type of weather, your crawlspace dehumidifier should be running.  Be sure to choose a Dri-CrawlSpace^® Dehumidifier because it is one of the few that can operate in temperatures this low.

This is the type of weather where mold really thrives, especially if it were to rain.  It’s too cold to run the air conditioner, so high humidity goes unchecked.  If it were raining, relative humidity outside would be 100%.  This would raise the dew point to 55°.  With the crawlspace at 58°, humidity would rise to a whopping 89%.  All your cold water pipes and exterior walls would be dripping wet.

Here’s a nice warm day with comfortable 50% humidity. But because it is so warm, dew point is also going to be warm.  Any surfaces below 60° will form condensation.  Look how high the relative humidity gets as the air cools to 65° in the crawlspace.  At 83% relative humidity, your crawlspace is sure to grow a good amount of mold, as well as make a fine home for spiders, termites, cockroaches and other pests.

The mild summer night is the worse case scenario for allergies. As temperatures near the dew point, dust mites and mold will multiply and run rampid throughout your home.  Notice how quickly food goes bad in this weather.  Also, if you notice more bugs in your home,  these are indications of high humidity.  A good crawlspace dehumidifer like the Dri-CrawlSpace^® can keep up with these humid conditions and make your home comfortable while taking some strain off your air conditioner.

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