In modern construction, concrete perimeter foundations are generally poured in such a way that the top of the foundation wall (where the “mudsill” is placed), is at least 8 to 10 inches above “grade.” This allows for an adequate amount of space from both any surrounding soil, and from any ground water. But in older construction, it was common to pour foundation walls that were level with the surrounding soil (on grade), and sometimes even lower than the surrounding soil.
This condition leads to both potential termite problems and water damage. When the mudsill (which is the horizontal framing atop the foundation that the ground floor studs are fastened to) comes into contact with the surrounding soil, it provides a clear path for termites to move from the soil into the framing. And any surrounding ground water can also be easily “wicked” into the mudsill, where it can then be transferred to the other framing.
Both of these conditions are potentially very problematic. The “quick fix” for this problem is to simply pull the soil away from the structure, crating as much distance from grade to the mudsill as possible. When this can’t be done, it is sometimes necessary to extend the foundation up to provide the necessary space between the framing and grade.
In the attached photo, the foundation vent is seen almost directly “on grade.” Foundation vents are commonly placed directly above the mud-sill, so it is fairly easy to determine the spacing between grade and framing by simply looking at how close these vents are to the surrounding soil. When these vents are placed as depicted in the photo, they are generally indicative of potential termite and water-intrusion problems.