Why Does Concrete Sweat?

To practically all of us, concrete seems like a very strong and solid material. So, it may come as a surprise to learn that concrete can actually sweat. But what causes this?

Concrete sweating – also known as Sweating Slab Syndrome (SSS) – happens when warm air in the atmosphere meets cooler concrete, leading to condensation (water droplets) forming on the concrete. It commonly occurs in hot and humid climates such as in Florida or Mississippi.

Now we know how it happens, let’s learn about the effects of concrete sweating and what we can do about it.


As mentioned, condensation is the main cause for concrete sweating. Warm air cools on the cold slabs of concrete, leading to water collecting on the surface.

Another cause can be moisture moving upwards from beneath the concrete, due to a lack of a sufficient barrier beneath the slab.

Even though concrete of course looks really solid, there can be tiny gaps or capillaries within the concrete, allowing water droplets to make their way through.

How to Determine the Cause?

It’s not hard to determine the cause of a sweating floor. You may be wondering if it is due to warm or air, or leakage within the concrete itself.

To find out the cause, you can simply take a piece of plastic and tape it to the floor (a 10 inch piece would do). Then, wait until the floor is showing some signs of wetness – either below the plastic or elsewhere.

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If the area underneath the plastic is dry but the rest of the floor is wet, this is a sign of sweating caused by humidity, since the covered area was protected from condensation caused by the warm air.

However, if the plastic-covered area is wet and the remainder of the floor is dry, this tells you there may be a moisture problem at play.

vapor on concrete floor

How to Fix a Sweating Floor?

The only way to sort a sweating floor is to eliminate or protect against the cause.

You have a number of options:

Sealing the area

  • You can seal off the area (e.g. a garage), to prevent warm air from getting in (however, you cannot seal off vents).

Use a Dehumidifier

  • You can minimise the level of humidity in the area, e.g. by using a dehumidifier.

Improve circulation

  • You can improve the circulation of air in the area. You may wish to use a fan to keep air circulating, so that the warm air can’t settle as easily on the concrete surface, leading to condensation.

Heating the area

Another option is to keep the room or area heated.

This means maintain a certain temperature and the warm air from outside is then close to or at the same temperature as the air indoors, meaning it won’t cool and cause condensation, or at least not as much.

However, if none of these solutions appeal to you or aren’t feasible, it may be best to replace the floor surface with a different material.

This will likely save you money over time as it means you will prevent long-term damp or rot from setting in.

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Plus, replacing the material is neither difficult or expensive.