Crawl Space Problems: Moisture, Sagging Floors and Vapor Barriers

Last reviewed October 28, 2026 · 6 sources cited

A crawl space is the part of the foundation people see least, and moisture there can soften wood, raise humidity throughout the house and shift piers. Much of what is called foundation repair in a raised house is really moisture control and support repair under the floor.

Signs something is wrong under the floor

• Floors that feel bouncy, sag or are uneven over a part of the house.
• A musty smell, or higher humidity indoors.
• Damp or discolored framing, standing water, or wet insulation.
• Piers that lean, sit unevenly, or leave gaps under beams. InterNACHI notes settlement or rotation of a pier footing causes lowering or tilting, that masonry walls over settled piers will crack, and that frost heaving, often under porches, can raise or tilt a pier.^[1]

Why moisture matters

Oak Ridge National Laboratory's handbook calls keeping a crawl space dry highly desirable. It notes that water can wick into the concrete if the floor is above the footing, that hydrostatic pressure from rainfall or irrigation can enter through a wall, and that perimeter drains are recommended where the crawl space floor is below the surrounding grade. Where the crawl space floor could fall below the water table, it recommends a slab-on-grade foundation instead.^[2]

Vented or sealed? The advice has changed

Older guidance told homeowners to keep crawl space vents open year-round. A University of Missouri Extension publication (undated) says so, and also recommends a 6-mil polyethylene ground vapor barrier, noting that it can stop up to 20 gallons of water vapor per day from rising into a 1,400-square-foot home.^[3]

Later research questioned venting in humid climates. The U.S. Department of Energy's Building America program says vented crawl spaces rely on outside air flushing out moist air, but in humid climates outside air is often more humid than crawl space air, which can cause structural, mold and pest problems. DOE reports its research influenced the 2009 and 2012 editions of the International Residential Code (R408.3), which allow unvented, conditioned crawl spaces, and that Building America research found humidity reductions of over 20 percent and 15 to 18 percent heating and cooling savings.^[4] Which approach suits your house depends on climate, drainage, and local code, so ask a qualified professional rather than copying a neighbor.

Ground vapor retarders

A June 2024 DOE weatherization job aid describes a well-installed ground vapor retarder: at least 6 mil thick with a perm rating of 0.1 or less, covering the ground as completely as possible, extending at least 6 inches up walls, piers and footings, seams overlapped at least 12 inches and taped. It says to ensure the retarder does not interfere with established drainage patterns, and that a retarder helps minimize ground moisture and soil gases such as radon.^[5]

Repairs under the floor

For pier and beam houses, the ASCE Texas Section guidelines list common repair items: floor shimming, framing repairs, added supports, restoring void space, and crawl space moisture control.^[6] See pier and beam foundation repair. If the cause is water, fix the water first; adding piers to a space that keeps flooding does not fix flooding. See water and drainage.