HEALTHY CRAWLSPACE = HEALTHY HOME:
The most important tool in managing a crawl space environment is a properly installed ground vapor barrier, this is simply a heavy weight plastic (3 mil or higher) liner that covers all the open soil. Note in picture 1 how moisture works through the foundation and on top of the barrier that is laid flat on the ground, when installed like this it acts like a "swimming pool liner" and holds moisture under your home. Picture 2 shows that by extending the barrier up the foundation above ground level, any seepage will flow under the barrier and be absorbed back in the earth. The barrier should have all the seams and openings sealed to create one solid barrier across the entire space, this is often called "encapsulation". If a new barrier is being installed over and old one, the old barrier should be removed so moisture is not trapped between the two barriers. A vapor barrier also helps to make your home healthier by keeping damp/musty soil odors and other airborne contaminants that are associated with respiratory related issues from migrating into your home.
CRAWL SPACE VENTING, DO I NEED IT?
At the heart of condensation problems in crawl spaces is often due to foundation vents, why? We know cold air does not hold as much moisture as warm air, thus the reason for dry skin in the winter and sweating in the shade in summer, the exact physics are; for every 1 degree air is cooled, the relative humidity will rise 2.2% to the point it exceeds 100%, then the "air gives up the moisture", also know as the dew point (or condensation). So when hot humid outside air enters the cool crawlspace through the vents the temperature goes down and the humidity goes up causing condensation on all surfaces and saturating the wood promoting mold and rot. Even though many building codes still require ventilated crawlspaces because they have not kept up with changes in building science, the solution is to seal the vents as picture 3 shows and install a proper ground vapor barrier.
CONDENSATION, THE MOST COMMON PROBLEM
Condensation is like high blood pressure, the slow silent problem we don't know about or ignore until its too late. Homes have endless locations where wide hot and cold temperature variations can meet up and "sweat" such as; in attics, crawlspaces, inside walls and electrical panels, on windows, duct work, and pipes. Such areas are at the heart of the "Wet enough long enough" theory and promote ongoing moisture issues that can come and go seasonally as we go from heating to cooling seasons. In crawlspaces condensation is often the cause of musty/damp smells that work into your home by promoting damp soil and wood.
CRAWLSPACES AND MOLD:
Varying levels of mold are often seen in crawlspaces because they offer the ideal environment for mold to grow; dark, cool, long term dampness, and a location that is out of sight and out of mind! Long term correction may involve several steps and possibly the use of proper chemicals (never use bleach), more sever cases may require a crawlspace specialist. The key to a healthy crawlspace is maintaining a consistent environment and avoiding extreme or excessive moisture and temperature variations from occurring under your home.
Mold is naturally in the outdoors and migrates into our homes all the time. However, for mold to be able to establish itself abnormally in our homes the #1 thing to remember is: "Mold is a sign of a moisture problem", for mold to survive it must have the right environment and a food source that is wet enough long enough. More importantly to correct the problem, if you do not address why there is excessive moisture first and only treat the symptom (the mold) the problem will return.