How to Keep a Concrete Floor Sweating Under Carpet

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How to Keep a Concrete Floor from Sweating Under Carpet

Concrete sweating, also known as sweating slab syndrome (SSS), is a phenomenon in which moisture builds up and condenses on the surface of a concrete slab. It occurs due to the temperature difference between the interior air and the concrete slab. Concrete sweating causes unfavorable indoor environmental conditions or poor air quality. But, how do I keep the concrete floor from sweating?

Put appropriate measures before, during, and after concrete slab construction. Before slab construction, apply retarder to keep off water moisture, and allow your slab to dries out completely. Apply a damp-proof membrane-like epoxy and install a dehumidifier and fan to keep moisture at bay.

Besides, it also puts people and equipment at risk of slipping and falling. Here’s more on causes and how to prevent concrete floor from sweating under the carpet.

Can Moisture Rise Through A Concrete Floor (Into Carpet)?

Moisture can penetrate through a concrete floor making your carpet and other floor coverings damp. It’s a common problem, but you need to worry if the dampness is too high since it can lead to myriad of problem to the flooring and your health and pet for pet lovers. Moisture is known to cause growth of black mold and mildew.

Keeping Concrete Floor from Sweating Under Carpet

What Makes A Concrete Floor Sweat?

Here are some of the things that can make a concrete floor sweat under the carpet:

1. Dew point

Dew point is the most common reason for concrete floor sweat. It normally occurs when warm air collides with colder concrete floors that are below the dew point temperature.

2. Excess floor treatments

The frequent use of floor treatments can also lead to concrete floor sweating. Surface densifiers, paints, and floor sealers reduce the porousness of concrete slabs, making them vulnerable to moisture condensation.

3. Dirty slabs

Dirt, debris, tire marks, and stains might be another reason you are experiencing concrete floor sweating. Dirt not only exposes your concrete floor to moisture but also increases the rate at which moisture soaks and condenses on the concrete floor slabs.

4. Poor drainage

Poor drainage around the concrete slab can exacerbate the dangers of any moisture source. For instance, when there is heavy rain, leaked pipes cause water to collect under the concrete slab.

5. Poorly protected sub-floor

A poorly protected sub-floor might be another reason you are experiencing concrete flow sweating. For instance, lack of vapor retarder between the concrete flooring and sub-floor may cause groundwater to sip into the concrete slabs.

6. Salt deposits

Efflorescence causes salt deposits to settle on the surface of concrete floors. Due to their hygroscopic nature, these salt deposits can draw moisture from the air and concrete flooring.

How to Keep Concrete Floors from Sweating Under Carpet

You can easily prevent concrete floors from sweating under carpet provided you identify the source of the problem. There are several techniques you can use based on the condition under consideration.

How to Keep Concrete Floors from Sweating Under Carpet

Here are suitable intervention methods you can implement before and after the construction of the concrete floor to prevent sweating:

Before slab construction

Application of retarder

This technique can keep moisture from seeping into the floor from beneath the slab. Start by laying a heavy layer of sand on the floor, then a layer of tough polyethylene, and another layer of sand on top of the polyethylene. Once these layers are installed, proceed to install your concrete slab and top flooring.

Allow natural drying

Allow your concrete slap to dry completely before you place your carpet. Don’t use any haste method to dry your concrete slab because you might not end up with a properly dried concrete slab. In general, it takes around 30 days for a finite amount of moisture to dry completely from concrete from a slab.

Keep excavation water-free

To minimize waterholes of moisture rising to the slab, you should be able to keep below-grade excavations dry. Moisture underneath the slab can also weaken the soil’s bearing strength, causing shrinkage and swelling.

After the Construction of the Slab

Apply a dampening chemical

A damp-proof membrane made of liquid epoxy is placed directly on the floor, producing an impenetrable layer. This is utilized for slabs that have moisture difficulties as a result of a lack of moisture prevention methods during the concrete slab’s construction. This damping compound will act as a barrier, preventing moisture from rising through the floor and remaining in the concrete slab. This is the right cure for a damp concrete floor.

Use of a fan

Another method that can help keep concrete floors from sweating under carpet includes the use of a fan to help ensure air movement in the room. Fans minimize the temperature difference between the ceiling and the floor while simultaneously increasing the surface evaporation rate.

Install a dehumidifier

A dehumidifier may also be used to remove moisture from the air and minimize condensation. slabs sweat because air can’t easily escape the surface. The last approach is to seal the concrete and keep moisture out by using a penetrating sealant.

How to Seal A Concrete Floor to Stop Moisture

Sealing your damp concrete can help eliminate moisture. This method is also applicable even though you have other options for reducing concrete sweating. When you apply a seal to your floor, you’re sealing the sub-floor to create an unbreakable barrier between it and your flooring. The floor sealant retains all moisture on the floor, regardless of its source. This method will go a long way toward sealing your floor.

The process of sealing your floor has been outlined in the steps below:

1. Check for dampness

To do so, lay a transparent plastic sheet on the floor and leave it for 24 hours. Then look to see if there is any moisture on it. It’s still moist if there’s dampness.

2. Clean and dry the floor.

Sweep your floor first and then rinse it with a mop. Clean the grime with soap and water. After that, rinse and dry vacuum to eliminate any remaining water. It will take days to completely dry the floor. You can use a dehumidifier to remove the moisture and ensure that the area is dry before moving on to the next step.

Read more: How to clean sealed concrete floor.

3. Seal the Floor

Before sealing its good to check and fix cracks on concrete floor. To prevent bubbles as the floor dries, apply a thin coat of sealant. To accomplish this, seal one section at a time. It is recommended that you use a concrete wetness protection sealant. It’s available as an epoxy concrete sealant.

All of the floor’s edges should be targeted. Leave the first coating to dry for 24 hours before you apply the second coat. You can keep layering your sealant until it’s to your liking.

What do you put under the Carpet on Concrete?

In most cases, many homeowners install damp-proof membranes on concrete slabs to prevent concrete sweating. Unfortunately, these damp-proof membranes are not effective. That said, these are some of the things you can put under the carpet to prevent moisture from rising.

1. Leveling compound

Apply self-leveling compound on the concrete slab to fill all the dips and shallow troughs before you apply a flooring sealant. Pour self-leveling compound between ⅛ inch and 1 inch and level off the compound with a leveling trowel. Use a bubble level to make sure your concrete floor surface is horizontal.

2. Sealant

Another thing you can put on the concrete slab before installing the carpet is a concrete sealant. Applying concrete sealant over your concrete slab will prevent moisture from accumulating on the surface. Your carpet and the flooring underlayment are likely to be damaged when there is no concrete sealant.

3. Underlayment

You also need to install a concrete underlayment before installing the carpet. The thicker the underlayment the better the results. A thicker underlayment not only prevents the moisture from rising through your floor but also retains heat under the carpet. Use an adhesive to fix the underlayment to the concrete floor. Leave a 1-inch space between your walls and underlayment.

4. Tack strips

Use a strong purpose-made adhesive to attach the strips around the perimeter of your room. Once the strips are ready, cut your carpet and lay it into the concrete surface while securing it using tack strips you installed earlier.

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Joe McGuinty
I’ve been working with floors for over 12 years. I started as a flooring contractor, primarily in materials selection. Then, I switched careers into accounting, so my wife and I began buying, renovating, and re-selling homes on the side. You’d be surprised how much value you can add to a home simply by adding new floors.

1 thought on “How to Keep a Concrete Floor Sweating Under Carpet”

  1. Hi, I hope you don’t mind me asking but I have mould rising up through the carpets onto my furniture and clothes. The mould is actually growing inside my sons board games and it seems to be mainly in the bedrooms (ground floor flat) It is ruining all our stuff. What should be done as I think the mould is affecting me and my autistic sons health? I am council but they don’t seem to really care. I have already thrown away a lot of our stuff which has been ruined. I have lived here for 1 year 5 months and still suffering. I actually swapped my 3 bed house for this 2 bed flat and nobody told me it was mouldy and damp. I also have black mould growing in various places around the property. Should I be moved out while the council fix the concrete flooring? Its an old property so it probably hasn’t been correctly damproofed when built. Linda


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