There are three possible causes of a sweating concrete slab we know of:

1) Missing or damaged vapor barrier under floor slab - The building code requires that a sheet of 6-mil polypropylene (often referred to by the brand name “Visqueen” by builders) or other moisture resistant membrane be installed on the ground under a concrete slab before it is placed.

   Unfortunately, the moisture barrier can be punctured during placing and finishing the slab or just poorly installed with gaps and unsealed laps. This allows ground moisture to rise up through the concrete, especially during periods of heavy rain and corresponding water movement through the ground. The wetness will interfere with the adhesion of flooring materials and make a bare floor slippery.

   Also, a barrier is not required under carports and patios, and was sometimes not installed for garages and screen porches in an earlier era. So, if a homeowner chooses to enclose one of them later on, the moisture barrier under the slab that’s necessary for a conditioned living area may be missing. In our North Florida area, enclosing a carport is a popular way to add living space to an older home, and a sweating floor in the new Family Room is sometimes the result.

    Water vapor can move up through at slab easier than liquid and, because the air conditioning in a newly-enclosed garage or carport will lower the humidity in the room, the higher-humidity air below the slab will be drawn up to seek equilibrium between the two humidity levels.

   There are masonry sealers available that can applied to the floor, but we suggest consulting a construction professional for advice before proceeding, since getting good adhesion of the sealer to the wet floor can be difficult and another retrofit solution may be better for your specific situation.

    For a listing of where a vapor retarder sheet is not required, see our blog post Where is a 6-mil polyethylene (Visqueen) vapor retarder not required under a concrete floor slab on grade?

2) Condensation - In the spring, when the ground is still cool but the air is warm, an especially wet, humid morning will cause condensation to form on a concrete slab in the outdoor air. This can also occur in a garage, which is not exactly outdoors, but also not a conditioned space. The condensation will be more noticeable if the slab has a smooth surface from a layer of porch floor paint. While you cannot stop this from occurring once in a while, applying a coating with slip-resistant granules to the area is one solution to the problem.

Water pipe leak under floor slab - A leaking water pipe will also cause moisture to move up through a floor slab and initially mimic the sweating caused by a missing vapor barrier. But eventually the water will start to pond in an area or bubble up from small cracks. In one home we inspected last year, water squirted up through the joints of the engineered wood flooring underfoot as your walked across it. The culprit was eventually determined to be a long-term water pipe leak under the concrete slab.

   Inspection with a infrared camera can helpful in determining if an under-floor pipe leak is the problem, since it will display the levels of the moisture around the floor, and pipe leaks tend to show up as a plume of moisture in one area. Even with a moisture barrier under the slab, the hydrostatic pressure buildup from the leak will push water through any tiny breaks or areas of deterioration of the plastic.

    Also, see our blog post Why are there score line grooves in the concrete floor of the garage?

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• What are the building code requirements for notching and boring holes in a wall stud? 

• What causes dark or light "ghost" lines on ceilings and walls?

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• What is the minimum ceiling height for rooms in a house? 

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

   Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.