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It is a natural process for water to be drawn to the top of the ground underneath a home in the crawl space. It is also natural for the house to draw that moisture because of the tendency of warm air to rise. As warm air vacates an area, air beneath it rushes to fill the void. This rising moisture is not a favorable condition for the wood framing members of the floor system.
Every crawl space therefore needs a moisture barrier. Polyethylene film at 6 mils thick is used under homes for a moisture barrier. Some of the elements in the soil combine with the rising moisture to cause the poly to decay over a period of years, so it is important to inspect the condition of the moisture barrier each year and be sure it is doing its job.
The crawl space needs to have a balanced condition and if it doesn't, bad things can happen. The four contributing factors are:
* How much moisture is entering the area
* The temperature of the crawl space
* The rate of transfer of any moisture
* Rate of circulation and ventilation
There is no way to stop moisture from rising from the ground, but other items can cause a problem that the moisture barrier might not be able to control. If the roof has a continuing leak or the house does not have positive water flow away from it, moisture can build up.
Downspouts from gutters may be adjusted improperly causing water to flow back to the house, and even an abundance of moisture from the HVAC system can be the villain.
Temperature fluctuations cause humidity issues that can increase the moisture under the home, and all the contributors add to the rate of transfer of moisture into the home. Air circulation within the home can keep the air dryer and decrease the moisture problems and the transfer rate.
In some cases, a dehumidifier for the crawl space is a good additional defense in controlling moisture, but every effort should be made to stop all external sources from adding to the natural moisture from the earth. Sealing the crawl space is recommended to keep out many of the contributors to moisture problems.
Moisture causes wood to rot and mold to form, which will enter the home. Moist
conditions are also ideal for the infestation of subterranean termites, which will destroy the wood framing in short order.