Many St. Louis homes sit over a crawl space, a shallow, unfinished area between the floor joists and the ground. It is easy to forget about. The soil under it stays damp, the air inside it stays humid, and that moisture rises into the floors, the insulation, and the living space above. If your floors feel cool and damp, or the house smells musty in summer, the crawl space is a good place to start looking.
How moisture gets into a crawl space
Moisture comes from a few places at once. Water moves through the soil, especially after heavy rain, and clay soil common in the St. Louis area holds water and releases it slowly. Warm, humid air from outside enters through open vents and gaps around pipes and wiring. Cold surfaces, such as metal pipes and the underside of the subfloor, collect condensation when the outside air is warm and wet. Each of these sources adds to the others, so a fix usually needs more than one step.
Signs to check on your next visit
You do not need special equipment to start. Bring a flashlight, wear gloves, and look for these conditions:
- Bare soil that looks dark, soft, or wet, or puddles after rain
- Water stains, dark spots, or fuzzy growth on joists, the subfloor, or the underside of the floor
- White, powdery deposits on the foundation walls, which often point to water moving through masonry
- Sagging or torn ground cover, or sheeting that no longer reaches the walls
- Wet or matted insulation hanging between the joists
- Rust on metal fasteners, ductwork, or water lines
- A strong, earthy smell when you open the access door
Note where you see problems and whether they are worse in certain corners. Corners that stay wet often sit near a downspout, a low spot in the grade, or a gap in the foundation.
What a vapor barrier does
A vapor barrier is a sheet of heavy plastic, usually polyethylene, laid over the dirt floor to slow moisture rising from the ground. A good barrier covers the floor fully, overlaps its seams, and extends partly up the foundation walls. Thin or torn sheeting does little. Heavier material with taped or overlapped seams lasts longer and stays in place better.
Check the barrier from the access door. Look for holes, folds that trap water, seams that have pulled apart, and areas where the sheeting is lifted or pushed back from the wall. Lift a corner gently if you can do so safely and check the soil under it. Damp soil directly beneath a torn barrier is a sign that the barrier needs repair or replacement.
Vents: open or closed?
Older St. Louis homes were often built with vents on every side of the crawl space, on the theory that moving air would dry it out. In a humid summer, though, outside air is often wetter than the air in the crawl space, so open vents can add moisture. Many homeowners find that closing the vents during humid months and opening them in dry, cold weather gives better results. The right setting depends on your home, your soil, and whether the space has a vapor barrier. If you are unsure, ask a qualified inspector before you change anything.
Start with the water outside
Much of the moisture under a house comes from water that should never have reached the foundation. Check the gutters and downspouts first. Downspouts should carry water well away from the house, not dump it at the foundation. Look at the grade around the home. Soil should slope away from the walls so rain runs off instead of pooling. Where soil has settled toward the house, add fill and ground cover so the surface drains outward. These steps cost little compared with the damage they can prevent, and they make any work inside the crawl space more effective.
Simple tests a homeowner can do
- Place a hygrometer, which is a simple humidity gauge, in the crawl space and another in a room above it. Compare the readings over several days, including after a rain.
- Check wood joists and the subfloor with a moisture meter if you have one, and compare the readings with dry wood elsewhere in the house.
- Set a piece of dry cardboard or a plastic sheet on the dirt for a day, then look under it for condensation or dampness.
- After a heavy rain, check the crawl space for standing water or fresh damp spots within a day or two.
- Note any changes over a full season so you can see patterns instead of single readings.
When to call a professional
Call a qualified inspector if you see spreading growth on wood, standing water that does not drain, sagging floors above the crawl space, or rot in the joists or beams. A professional can check for water entry points, structural damage, and the condition of the foundation. A written assessment helps you compare options and decide what to do first. Ask whoever inspects the space to explain their findings in plain terms, and to show you the problem areas in person.
A crawl space is not a room you use, but it shapes the comfort and condition of everything above it. Check it at least twice a year, look for changes after storms, and fix water problems at the source before you worry about the air.