How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
Why does my concrete floor slab sweat and get slippery?
Wednesday, August 1, 2018
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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