Once the grade limitations are determined, it's time to look at other potential sources of moisture. During the building of a new home, moisture can enter the house in a number of ways. Because builders usually work on tight, demanding schedules, homes are sometimes built trapping moisture that needs time to evaporate. The amount of moisture introduced by framing lumber and the wet trades can be significant. Any of these may cause water to penetrate the foundation walls or rise through the ground surface into the foundation or crawl space.
Rain water falling on the ground or from the roof can pass through or under the foundation walls.
In some areas, heavy soils may retain surface drainage and cause water pressure against the foundation walls or slab. Surface drainage problems may also occur because the crawl space is below the finish grade outside the house. The minimum depth of the crawl space should be 24 inches under the floor joists or 18 inches under the girder. An 18-inch clearance beneath any ductwork is also desirable. Moisture problems in crawl spaces may be due to the construction of a crawl space in an area of high water table, improper grading of the lot for drainage or the omission of moisture control devices such as vapor barriers, ground cover and ventilation openings.