Crawl Space Moisture Readings – What do they mean?

The dew point temperature is the temperature at which condensation forms as the air is cooled. At the dew point temperature, the air is saturated and any further cooling will result in condensation. Both 70F/100% RH crawl space air and 85F/60% outside air have the same dew point temperature: namely 70F.

In the Carolinas we routinely see temperatures and humidity levels far above 85F and 60%. If we vent any crawl space with this air we are allowing higher dew point temperature air to enter the crawl space. We are actually adding moisture to the crawl space, thus making our problems worse. Mechanical fans will exponentially exacerbate the problem.

Vented crawl spaces cause condensation problems. Floors rot, mold or cup because of excess moisture. Ductwork sweats and becomes saturated with water. Duct energy losses go way up because the insulation isn't insulation when it is wet.

Mortgage, pest control and home inspection industries flag crawl space wood moisture contents above 20% as a potential problem. At this wood moisture content, mold can grow. I occasionally see problem-free crawl spaces with wood moisture contents of 16% or less, but not very often. Truly dry crawl spaces simply rarely exist in the Carolinas.

Many homeowners have their crawl spaces tested for moisture yearly but this can be misleading. What time of year are you testing? Most crawl spaces will test low for moisture in dry weather periods. Visual inspections for signs of moisture (mold, fallen insulation, wet earth, wet foundation wall, sweating ducts) are absolute indications of high moisture levels, whether it is winter or summer. If you see any of the signs of moisture, you likely have a moisture issue.