Why does a crawl space stay damp for days after the rain stops?
The ground under a house holds water, and the air in the space stays humid until the moisture is removed. Soil releases moisture slowly, and a shut space with little air movement keeps the joists damp for a long time.
A crawl space is often cooler than the air above the house and has limited airflow. Water that has soaked the soil evaporates into the space slowly, and it condenses on the cool joists and ducts. That condensation keeps the wood wet even when the standing water is gone.
This is why a crawl space can look dry from the access hatch while the framing above the soil still reads wet. The technician measures the joists and the subfloor, not just the ground.
How does the moisture reach the floor above?
Moisture moves up through the joists, the subfloor and the bottom plates, and it reaches the flooring from below. The floor can cup, lift or soften even when the room above looks normal, which is why the crawl space is checked as part of the floor job.
Wood takes up moisture from the air and from the ground in contact with it. The subfloor sits directly on the joists, so the moisture in them moves up into the subfloor and then into the flooring.
Flooring in a room above a damp crawl space may show the damage first at the edges, under cabinets and by the outside walls. The crew looks at those places from underneath and marks them on the drying plan.
What moves the cost of a crawl space moisture job?
The depth of the standing water, how much of the framing is wet, how much insulation must come out and how hard the space is to reach all decide the job. The price is explained and approved before work starts.
A crawl space with a little water and a dry set of joists is a smaller job. A space with water across the ground, wet insulation hanging from the joists and framing that has been wet for weeks takes more removal and more drying time.
Low clearance, tight access and the need to cut an opening in the floor for the equipment add time. The crew explains the access before it begins, and it gives you the scope in writing before any change to the job.
Should the crawl space vents be closed to keep the moisture out?
Vents are not closed by default. Whether they should be open, closed or changed depends on how the space is drying and the outside conditions. The crew tells you what it sees, and the choice is made with the drying plan in mind.
A closed space in humid weather can hold moisture for longer, so a crew may ask for vents to stay open while it works. A drowned or blocked vent should be cleared so that water does not pool at it.
Changes to the vents, the grading or the vapour barrier are done as separate work. The crew notes the findings, and you or a contractor you choose can decide what to change.
What should a homeowner check after the crawl space has been dried?
Look for a musty smell, new stains on the subfloor or floor above, and any change in the floor's feel. Check the space after the next two or three heavy rains, and call if you see water or moisture returning.
Use a flashlight to look at the joists and the ground from the access hatch. Note any new damp patch and any change in the wood from the record the crew left.
Keep the access hatch clear and the vents unblocked, and check the gutters and downspouts after storms, since water from the roof that lands near the foundation feeds the crawl space.
