Excess Moisture in Basements and Crawlspaces

By: Steve Hillenbrand, P.E.

Excess moisture in a crawlspace or basement is a common occurrence, with potentially severe consequences if not addressed in a timely manner. Excess moisture in a crawlspace may originate from several sources, including: surface stormwater (slope run-off, gutter overflow, and downspout discharge), groundwater, mechanical (leak), and condensation. Excess moisture in crawlspaces can be reduced by: a vapor barrier, increased ventilation, and preventing moisture from entering, etc. Long term exposure to excess moisture can lead to rot and fungal growth in wood materials, and ultimately, loss of structural integrity.

Stormwater is the most common source of excess moisture in a crawlspace or basement. Preventing moisture from entering the soil adjacent to the foundation is an efficient means of controlling excess moisture from stormwater. Gutters should not be allowed to stop up and overflow water towards the perimeter of the house. Downspouts should be free from clogs and the water discharge directed away from the house as far as possible. Care should be taken to maintain the downspouts and drainage pipe so they do not become blocked. Buried drainpipes should be checked for clogs periodically by using a garden hose. Compacted soil or other suitable material may be placed along the foundation wall creating a slope away from the house foundation.

If the excess moisture does not appear to correlate with rainfall, if no obvious source around the perimeter of the foundation is observed, or if water appears in the middle of the area or in excavated holes (in crawlspaces), groundwater should be considered a possible source. Installation of a sump pump is a common solution for groundwater intrusion.

The area of excess moisture should be checked for obvious mechanical leaks in both pressurized supply lines and in sanitary lines. A pool of water under a pipe is a good indicator of a leak location. If a pressurized supply line is suspected, the leak indicator at the water meter should be checked for movement. Even a small leak will show up with movement of the leak indicator within 20 to 30 seconds. Make sure all water use in the house or building is stopped while checking the meter. A cycling toilet is a common cause of high water bills and indicator movement.

In a crawlspace a vapor barrier works well if the excess moisture is due to evaporation of ground water or subsurface flow. A heavy-duty plastic sheet covering the floor of the crawlspace usually serves well as a vapor barrier. To receive the maximum benefit, the seams should be overlapped and taped. The plastic can be taped to vertical surfaces such as piers and foundation walls. Some owners are sealing their crawlspaces and providing environmental controls to reduce or eliminate the problems with excess moisture intrusion.

If the excess moisture is widespread with discoloration of the cool surface materials (floor joists under an air-conditioned room, ventilating ducts or duct insulation, etc.) and widespread fungal growth, condensation may be the cause. Condensation is usually not as common in conditioned basements as it is in crawlspaces, but poor ventilation and especially cool surfaces can cause condensation problems even in basements. Having at least 1 square foot of vent opening in a crawlspace for every 150 feet of crawlspace area is usually sufficient ventilation in the summer. The vents may be closed in the winter, especially if the floor above is not insulated or unprotected pipes are present in the crawlspace. A common remedy for condensation problems in a basement is a commercial dehumidifier.