Many older homes have a white powder visible on foundation walls and concrete slabs. This powder is actually salt, which makes its way to the surface of the concrete when it becomes saturated with water. This process, known as “efflorescence,” (which means to “flower out” in French) is strong evidence for the homeowners that they have water problems that they probably need to address.
In the below photo, the efflorescence appears along the lower 12″ of the perimeter foundation. This part of the foundation is completely “below grade,” meaning that the level of the soil on the outside of the foundation is higher than section of the foundation wall where the efflorescence is prominent.
In older homes, there was rarely any waterproofing applied to these foundation walls prior to the “back-filling” of the soil against the foundation wall. Additionally, there were rarely any drain lines (French drains) or gravel placed against the foundation prior to the back-filling. This lack of drainage, combined with a lack of waterproofing, often leads to the water intrusion issues that result in efflorescence.
This efflorescence itself is rarely a problem, but it signals to the homeowner that moisture is likely getting into the crawl space of the home. And this moisture often creates mold problems both within the crawl space, and in the living space directly above.
To deal with this moisture, homeowners are encouraged to speak with both drainage specialists in order to try to prevent the naturally occurring ground water from making its way into the crawl space, and also, vapor barrier specialists, who can install their barriers directly over the exposed soil (or concrete rat-proofing) of a crawl space. These barriers are very effective at reducing both the humidity and water vapor that are common in crawl spaces. And these measures go a long toward dealing with mold growth issues within the home.