Crawl Space Science – What’s Going on Beneath Your Feet?!



Do I Have Moisture Problems?

Moisture in a crawl space comes from two sources—the ground and the outdoor air. Exposed earth contributes to a lot of water vapor into the crawl-space air because the earth is damp. In most climates where there are dirt crawl spaces, you can never dry the earth, and the invisible stream of water vapor from the exposed earth goes on forever. Groundwater seepage is another cause for moisture in a crawl space. It enters under the footing, between the footing and the walls, through block walls, and through cracks in pored walls.

Air is another very efficient way to move water. Air, including humid air, moves easily in and out of spaces and brings its moisture content with it. When air is heated or cooled, its relative humidity changes. Warm air holds more moisture than cold air. The relative humidity of air goes up 2.2% for every degree we cool it. When we bring warm, humid air into a cool crawl space, the air is cooled, and the relative humidity goes up. High relative humidity causes condensation and water vapor in the crawl space, causing rot, mold and energy loss.

Do I Have MOLD?

Mold has a purpose. Its purpose is to break down or eat dead organic matter. “Organic” means it was a material that once was living, such as wood. So when something is dead and wet, mold (or other fungi) grows on it and eats it. Since mold spores are everywhere, and our building materials are made from organic materials, this factor cannot be controlled. The only one that we can control in our home is the moisture. Mold needs 60% relative humidity to grow. Mold likes processed organic fibers the best—it will grow on paper and cardboard first. After that it will grow on fiberboard, then plywood, and finally framing lumber. Drywall and fiberglass insulation both have paper in them, so mold will absolutely grow on these items. A crawl-space specialist can test the moisture in your crawl space to see if it is a breeding ground for mold.

Is My Crawl Space Energy Efficient?

When fiberglass floor insulation gets damp from water vapor in a crawl space, it begins to sag. This is called “bearding.” It gets heavy and droops down, making an air space between it and the floor. At that point, it has almost no thermal benefit at all. When fiberglass insulation is a little damp, it loses a whole lot of its insulation value. Insulation should always be installed against the air-boundary surface of the house. In a healthy, properly functioning crawl space, that surface is the crawl space walls and floor. The wood floor system above is full of gaps, joints, and chases for wires, pipes and ducts—so it’s the wrong place to insulate in a crawl space.

Is My Floor System Structurally Sound?

Sagging floors are not only a nuisance, they can leave you wondering how much longer the floor will be able to support the weight of everything on it! Sometimes existing columns are spaced too far apart when the home is built. In this instance, the beam or girder can become overloaded and sag between the columns—and then, so does the floor above it. The floor joists and girders may also weaken if there is mold and wood rot from excessive moisture in the crawl space. This will cause the floor above the crawl space to become bouncy, soft, and it may begin to sag. Finally, weak soil can cause an existing column that is supporting the floor system to settle. If you are experiencing sloping floors, cracks above doors and windows, gaps between floor/ceiling and interior walls, or doors and windows that stick, your floor system may need additional support.

If you think one of these issues may be going on beneath your feet, reach out to a crawl-space specialist for an inspection.


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