Atlanta Home Inspection-certified member ICC,

Many crawl spaces are dirt and are located in the humid southeastern United States and do not have a crawl space dehumidifier or vapor barrier.It is not uncommon in the southeast to see crawl space humidity levels at 50% to 90%.Dirt crawl spaces were vented based on the faulty assumption that these vents would allow moisture to escape from the crawl space.  In fact, many outdated state codes still require this.  We now know that these vents actually increase the relative humidity level.  The natural upward air movement draws air from the crawl space vents up into the living space of the home.  Did you know that up to 40% of the air you breathe in your home was once in the crawlspace.Vented dirt crawl are potential problems.Soil contains high levels of humidity that natural airflow patterns move upwards into your home. The these problems need to be addressed eliminating the source(s) of moisture.  Immediately fix any groundwater, plumbing or other obvious leaks.  Make sure gutters and downspouts are clear and drain away from the foundation of the house.  Remember, the longer you ignore moisture problems in your crawl space, the more expensive the repairs will be.
Wood with a moisture content of 20% or more is highly susceptible to wood rot and damage is possible at lower percentages depending on other factors.
Excessive Moisture Warning Signs:

Wood surfaces that look dull and discolored
Splits across the wood grain
Heavy condensation on windows
Blistering or peeling paint
Water stains on framing and sheathing inside walls
Mold spots on furniture
An evident musty odor
Standing water in your basement or crawlspace
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:

Dew Point = Temperature at which water vapor condenses
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.
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.
The lower the outside temperature, the lower indoor humidity needs to be to prevent condensation. Single-pane windows will have a much bigger problem with condensation than double-pane windows will. Condensation from windows or walls can cause damage and promote mold, don't let your humidity get too high.Moisture needs to be removed from the air of a home in the Summertime. This is normally done by an air conditioner or de-humidifier. In either case the moistue is removed by drawing air across a cold evaporator coil, the air is cooled to it's dew point, water condenses on the coil and runs down a drain. An air conditioner can remove several gallons of water a day, de-humidifiers are rated in pints per day. Recommended humidity levels for Summer are between 50% and 60%. Air can hold only a limited amount of water vapor, and that amount depends on the air temperature. When air at a certain temperature contains all the vapor it can hold, it’s said to have a relative humidity of 100%. Thus, when it holds only half the water vapor it can hold, the relative humidity is 50%. Cooler air is capable of holding less vapor than warmer air. So air at 30º F and 100% relative humidity actually contains less water than air at 70º F and 100% relative humidity.
       Excess humidity contributes to the deterioration of a home. Excessive humidity can pass through walls and freeze in the insulation. In spring it melts, damaging your ceiling and walls. Or, humidity can force its way out through siding to form blisters under your exterior paint.

You mean moisture can actually go through walls?You bet. It’s because of a force called "vapor pressure". Moisture in wet air tries to flow toward drier air to equalize itself. This flow acts independently of air currents. In winter, inside air is much more humid than colder outside air. So, the vapor pressure, or equalization process, actually forces the inside moisture through cement, wood, plaster, and brick, toward the outside.