Concrete floors can develop moisture when the cool concrete temperatures clash with the warmer inside air temperatures. Concrete moisture buildup is called sweating.
Concrete floor sweating under carpet is sometimes called Sweating Slab Syndrome, or SSS. Concrete sweating happens when moisture builds up and then condenses on the surface of the concrete underneath the carpet.
SSS becomes a problem when it makes the carpet wet. Wet carpet is more than simply an uncomfortable nuisance or the awful feeling of your socks becoming wet when you walk across the soft flooring; it can cause serious issues like mold and carpet damage.
Reasons For Concrete Floor Sweating Under Carpet
The bottom line, scientific explanation as to why concrete floors sweat boils down to it being because of the difference between the interior air and the temperature of the concrete slab. The concept that hot air rises apply here.
The concrete slab tends to be cooler than the air above it. When a rug or carpeting is placed on top of the already cool concrete, it will trap the moisture and create water issues.
A few reasons why concrete floors sweat includes:
- Dew point – Dew point is the Number One reason why a concrete floor begins to sweat. Normally, it happens when warm air hits the cold concrete flooring that is below the dew point temperature.
- Rushed Construction – A lot of home construction is rushed, so the concrete is not allowed enough time to completely dry adequately. It may look dry long before it has dried entirely.
- Water Mixture Didn’t Dry – Sometimes, the proportion of water mixed with cement is too wet and not given enough time to dry out thoroughly. Also, rain, snow, or sprinkler systems near the construction site can wreak havoc with moisture levels.
- Wrong vapor retarder – Often, concrete sweats because the wrong vapor retarder is used or is torn.
- Sloping – Sometimes, the slope of the land may be causing moisture to collect and cause problems either inside, outside the home, or both.
- Leaky Pipe – Even if the concrete slab was constructed properly after it has been in place, water can gather because of a leaky pipe or worn-out and broken pipes.
- Floor Treatments – Overusing floor treatments can cause concrete floors to sweat. Paint, floor sealers, or surface densifiers all make concrete more susceptible to moisture condensation.
- Dirt – Dirt buildup, debris, or stains can also cause concrete to sweat.
- Poor drainage – If moisture is not allowed to drain off of the concrete slab, it will sit stagnant waiting to cause trouble.
- Poor Sub-Floor Protection – If the sub-flooring is poorly protected, with a lack of vapor retarders between the concrete and the subfloor, it could cause groundwater to seep in.
Moisture on and in concrete can take as much as 48 hours to dry. Even when the surface of the concrete looks dry, it could be retaining moisture for as many as 30 days.
Dangers of Concrete Floor Sweating
Besides the unpleasant feeling of moisture that is a breeding ground for bacteria and mold, the moisture can penetrate through a concrete floor making the carpeting damp as well. It can make the carpeted floor slippery. If a floor is wet and slippery, slips and falls are bound to happen.
This moisture buildup can also cause health issues for you or your pet if mold begins to develop.
How To Tell If The Concrete Is Sweating
Water issues can be the result of concrete sweating or some other problem. If the concrete is gathering moisture, you will probably notice the carpet is wet. You may even notice an odor.
If the problem is not from concrete sweating but rather from a leaky pipe, you will potentially even notice a decrease in water pressure or that your water bill has increased which would indicate there is a leak somewhere that needs to be tended to.
If the water damage has been left unnoticed for a while, you may notice moving or cracking in the foundation.
If you are able to lift a rug or carpeting, check the concrete for white or gray powdery stains that indicate minerals from water sitting on it have created efflorescence. Efflorescence is the result of mineral deposits from water causing salt deposits to sit on the surface of concrete and draw moisture from the air onto the concrete.
You may also notice mold growing on either the concrete or the carpet.
If you notice the carpet is wet, take these steps:
- Check the foundation for any cracks.
- Check for mold.
- Make sure all downspouts on the outside of your home are draining 10 feet, 3.048 meters, away from the home. If there are no downspouts, have them installed. Alternatively, you can install a French drain or an interior drain.
- Use an RH Relative Humidity test that will test for moisture below the surface of the slab. If you are able to lift a piece of carpet and get to the concrete slab, you can also test for beaded moisture. If the moisture is beaded, it is likely a vapor barrier problem. If there is no beaded moisture that forms, it’s likely due to humidity. Improving ventilation and controlling temperatures will help alleviate any humidity problems. Another easy way to test where the moisture is coming from is to tape a 16-inch, 0.40 meter, square piece of plastic to the floor. Wait for the floor to show signs of wetness. Remove the taped plastic. If the concrete patch that was sealed with the plastic is completely dry, and the rest of the floor is wet, it is a good indication that the floor is sweating because of humid air. If the patch is wet and the rest of the floor is dry, it indicates that there is another moisture problem.
- Ventilate the room by opening windows and turning on any ventilation fans
- Use a dehumidifier or fan
- To clean up stains left on the carpet from moisture, use baking soda and vinegar or an antifungal spray, and vacuum with a wet/dry vac that has a HEPA filter.
How To Prevent Concrete Floor Sweating Under Carpet
From the beginning of construction, the best way to prevent concrete floor sweating under the carpet is to do so from the start. Take steps to prevent the moisture from developing before the carpet is laid on top of the concrete.
The best time to prevent the concrete from sweating is to do so before construction by taking these steps:
- Lay down a heavy layer of sand on top of polyethylene before laying down the concrete slab.
- Apply retarder to keep moisture from seeping in from underneath the slab.
- Allow the slab to dry completely, and then apply a damp-proof membrane-like epoxy. Also, install a dehumidifier and fan to keep the moisture at a minimum.
- Allow the concrete to dry completely before placing any carpet on top of it. Typically, it will take at least 30 days for all moisture to dry entirely from the concrete.
- Keep moisture from forming under the slab during construction which can cause shrinkage or swelling.
Unfortunately, construction is not always properly done to minimize concrete slab sweating. If this is the case, there are some steps to take after the slab construction that include:
- Form a damp-proof membrane by applying liquid epoxy to the concrete.
- Create air movement by using a fan. The fan will also minimize the temperature difference between the floor and the ceiling and increase the surface evaporation rate.
- Use a dehumidifier to remove any excess moisture from the air and to cut down on the chance of condensation from forming.
- As a final step, use a penetrating sealant to seal the concrete and keep the moisture out. The sealant will retain all moisture on the floor and create a barrier between the concrete and the carpeting flooring on top.
You can also use a levelling compound to fill in any dips, a sealant to prevent moisture from accumulating, an underlayment that is as thick as possible to prevent moisture from rising up through the flooring and help to retain heat from the carpet, or use track strips around the walls of the room to secure the carpet.
When To Ask For Professional Help
If you see mold from the wet carpet that covers a large area, call in a professional to remediate the mold issue. You may also need to have the carpet replaced.
Concrete sweating is usually the result of different temperatures, but it can cause carpeting on top of it to get wet and lead to mold issues. The best way to prevent concrete sweating is during construction, but there are steps you can take if construction is completed, and concrete floor sweating under your carpet is happening now.