How to Seal Basement Concrete Floor

Concrete is commonly used to make a quick, solid, ‘level’ basement floor that is impenetrable to most insects and critters. If you’re considering painting or carpeting it, or just leaving it bare, you may want to seal it first to keep mold causing moisture out. If you need some suggestions on how to seal basement concrete floors, we’re here to help.

Before sealing a concrete basement floor, do a water test to see if it’s already sealed. If not, clean the floor of dust, dirt, and debris, remove any stains, and use a paintbrush and lint-free roller to apply a surface-level or penetrating sealer. Let dry before replacing furniture or using the floor.

In this guide, we’ll explain how to tell if you should seal your basement floor and if it’s already been sealed, and how best to seal it if the concrete is new or old. We provide a step-by-step guide to sealing a concrete floor, look at the costs involved, and discuss the best sealers. Plus, we look at how to care for and clean a sealed concrete floor. Our goal is to provide you with all the information you need to properly seal a basement concrete floor.

Should I Seal My Basement Floor?

As concrete cures, moisture trapped inside forms microscopic channels or passages to the surface where it evaporates into the air. Unfortunately, those tiny passages remain and can allow moisture and vapor to percolate or ‘wick’ from the ground upward through the concrete and into your home. If the concrete is covered with carpet or wood flooring, the moisture can soak into the flooring and cause mildew, mold, and rot.

The amount of moisture wicking through depends on the ground moisture, humidity, and if the basement has ventilation and an HVAC system. In some locations, between 3 and 5 pints of water vapor can percolate through 1,000 sq ft of unsealed concrete floor in a day. Additionally, if you notice white powdery material on the floor or walls, known as efflorescence, you have a moisture problem. Sealing the concrete helps prevent the movement of moisture and ground vapors into the home.

Concrete doesn’t need to be sealed unless there is a moisture or ground vapor issue permeating through into the home. Sealing the concrete floor minimizes moisture and vapor escaping through the concrete into the living space. It also protects the floor so it will last longer. Plus, it improves your health by decreasing the possibility of mold and mildew growth throughout your home. It should be noted, though, that sealing concrete blocks the passages moisture escapes through, it doesn’t prevent water seeping through cracks or flood water.

To determine if you have moisture penetrating through the floor, tape a 36” by 36” sheet of 6 mil (or thicker) poly, or a plastic bag, to the floor and leave it for 24 hours. Moisture penetrating through the concrete will be trapped under the poly, leaving the surface of the concrete damp and often darker, plus the surface of the poly next to the concrete will be wet.

The amount of moisture penetration will vary, but any penetration indicates the concrete isn’t sealed and that moisture is a concern. If no moisture is noticeable, then the floor may already be sealed, have a moisture barrier under the slab, or ground moisture isn’t an issue.

How to Tell if the Basement Floor Is Sealed?

Prior to painting, acid etching, or covering a concrete floor with epoxy, carpet, or wood, it is recommended that you check to see if the floor has been sealed. In order to adhere to the concrete, paint, epoxies, and sealers need to penetrate into or bond to the concrete’s surface. To protect carpet or wood flooring from moisture, though, the floor typically needs to be sealed.

An easy way to check is the water test. Drip, place, or scatter 1/4 to 1/2 teaspoons of water in different locations around the concrete floor. If the water stays beaded and the concrete’s color doesn’t change, then the concrete has been sealed. However, if the water doesn’t bead but spreads out and is absorbed, and the concrete darkens, then it isn’t sealed.

Another method is to mix a solution of 1-part muriatic acid with 4-parts water and place droplets on different areas of the concrete floor. If it beads and does nothing, then you know the floor is sealed. If the solution fizzes and bubbles, it isn’t sealed. This method is helpful on highly ‘polished’ looking floors, as the smooth finish is usually less porous and may not respond well to the water test. Remember to wash the acid solution drops off the floor when finished too.

It should be noted that many freshly poured concrete slabs are treated with curing or sealing agents to slow the evaporation of moisture during the curing process. The agents, however, will also interfere with the successful application of paint, epoxy, sealers, and acid etching. Surface grinding will remove the sealer or curing agents, and prepare the concrete for other finishes too.

Best Way to Seal a Basement Floor

Concrete sealers are best applied with a brush and roller. Use a brush to get the sealer into the corners and along the edges, and then use a lint-free roller to spread the sealer over the floor. When applying a sealer, the treated floor will have a glistening wet appearance until it dries. A sprayer can be used instead of a roller, but the spray may miss small areas or be uneven in application.

Sealing Concrete Basement Floor: Step by Step

Sealing concrete basement floors helps prevent moisture and ground gasses like radon gas from leeching up through the concrete and into your home. Moisture can damage flooring and cause mold, mildew, and rot throughout a house, and can be injurious to the health of people and pets too. Use the following step-by-step process to seal your concrete floor.

1. Clean and Prepare Floor

Once you’ve determined the basement floor hasn’t already been sealed, and you’ve made the decision to apply sealer, prepare the floor for sealing. Clear the floor of obstacles such as boxes and furniture. Sweep or vacuum the floor of dust and debris.

Use a paint scraper or scrub brush to remove stubborn dirt, paint, or caulking drips. Apply dish soap to oil stains and scrub to remove any residue that may have penetrated into the concrete surface – remember to rinse it well and let it dry too. You may want to mop the floor to collect any residual dust, just let the floor dry fully before sealing.

2. Fix and Seal Basement Floor Cracks

It’s a flip of a coin whether this step should be first or second since it may create a mess that needs to be cleaned up before applying sealer. Clean out any cracks or pitting of dirt or loose concrete. Use a concrete patch, crack filler, or leveling material to fill in all cracks and pitting.

Use mesh or concrete to fill in large cracks as well. Let the new material dry completely before sealing – you may wish to grind the hardened fill material if it isn’t smooth and level with the adjacent floor.

Cracks at the cove joint where the foundation walls and floor meet should be left open to prevent damage from hydrostatic pressure – groundwater and run-off being forced upward. Additionally, if your floor has cut expansion joints in it, the typical practice is to fill them with compressible foam strips and cap them with silicone caulking to keep them clean prior to sealing the floor.

3. Applying Sealer

Once the floor is prepared, any cracks or joints filled, and the floor is clean and dry, the sealer can be applied. Use a large paintbrush to apply the sealer along the walls and in corners where a roller may have difficulty reaching.

Apply the sealer to the rest of the floor using a lint-free roller. Ensure proper ventilation and follow the manufacturer’s instructions regarding the application, temperature (typically 50°F), drying time, and clean-up.

If using a sprayer, use a brush around the edges and in the corner, and then the sprayer for the main area. Follow the spray instructions on the sealer. It’s common for the nozzle to be 12” to 18” off the floor and a 90-degree nipple to be used.

Apply the spray in a back-and-forth motion so the same area receives a complete coating and no spots are missed. You may wish to tape plastic to the walls to prevent the sealer from getting on them too.

Remember to work toward an exit point so that you don’t need to walk on the freshly sealed floor – don’t ‘paint’ yourself into a corner. The floor should look wet and glisten until it dries. It may also dry slightly darker than unsealed concrete. It typically takes one to two days to dry depending on humidity levels. If applying a second coat, apply it at a right angle to the initial application to ensure full coverage.

4. Types of Concrete Sealer

The most common types of sealer are acrylic, polyurethane, and epoxy. Acrylic may be solvent or water-based and is typically the least expensive. The water-based sealer may change the concrete’s color and the solvent-based sealer may yellow.

Polyurethane sealers can be used both indoors and out, and are almost twice as thick as acrylic sealers, so are more highly protective but take longer to cure. Epoxy sealers are the most durable, impermeable, and resistant to wear and abrasion, and thus the most expensive. Epoxy sealers come in a range of colors as well as clear coats.

There are other types of sealers such as penetrating silicate, silicone, and silane concrete sealers that penetrate into the concrete surface to protect it from the elements. However, while they prevent moisture from entering the concrete and also protect it from stains, some of these sealers still allow water vapor and gasses to escape out of or through the concrete.

Polyaspartic or polyaspartic polyurea sealers are relatively new and cure in 2-hours or less. They can bear extreme temperatures, resist staining and UV rays, and bond with the concrete to form a bubble-free coat up to 18 mils in thickness. Polyaspartic sealers form a clear protective surface that is more durable and resistant to wear, abrasion, and staining than epoxy, and are typically more costly too.

What Is the Best Basement Concrete Floor Sealer?

There are dozens, if not hundreds, of concrete sealers on the market, however, there are two basic types of concrete sealer. One is a surface-level sealer that bonds to the surface concrete and forms a protective coating that prevents liquid or gas movement in either direction.

The other is a penetrating sealer that flows into the concrete’s pores and seals them to prevent moisture movement into the concrete from the surface or up through it. Unfortunately, some penetrating sealers still allow the concrete to ‘breathe’ and moisture and gasses to escape into the living area.

Selecting the best basement concrete floor sealer greatly depends on how the floor will be finished and used, and existing moisture problems. A surface-level sealer will trap moisture and gasses in the concrete, which can lead to bubbling or damage the concrete. A penetrating sealer may stop the movement of vapor and gasses in both directions, or only protect it from surface spills, so read the manufacturer’s info sheet carefully.

A silicate-based deep penetrating concrete sealer that is absorbed by the concrete and penetrates up to 4” to block capillaries, pores, and hairline cracks is best. It solidifies the concrete and prevents the movement of moisture and gasses through the concrete in either direction.

The quick-drying sealer is invisible when it dries and doesn’t change the concrete’s color. Additionally, it doesn’t form a skin or film on the surface, so surface-level sealers like paint and epoxy will still bond to the floor. Plus, carpet and wood flooring will be protected from moisture damage too. However, too much groundwater or run-off – hydrostatic pressure – can damage the concrete as the water seeks an escape route to the surface.

It should be noted, though, that most sealers need to be reapplied every 3 to 8 years. Select a concrete sealer based on moisture control and purpose. If moisture isn’t a concern, most epoxy, polyurethane, or acrylic sealers will do a good job.

Sealing Old Concrete Basement Floor

Similar to new concrete floors, old concrete flooring can be sealed or resealed. It is important to determine if the concrete has been previously sealed unless you know for certain one way or the other. If it has been previously sealed, then you’ll need to remove the old sealer by grinding or acid etching – unless you know it was a solvent-based acrylic sealer, in which case another solvent-based acrylic sealer can be applied over top.

Once the old sealer, any paint, adhesives, oils, dirt, or efflorescence have been removed, sweep and vacuum the surface. Clean up any loose debris, including concrete material from cracks, spalling, or pitting, and ensure the surface is dry. Fill in any cracks or pitting with appropriate material, let dry, and grind smooth if necessary.

The cleaned and repaired concrete floor can be sealed once everything is dry and clean. Use a concrete sealer that will protect the concrete from whatever moisture or wear and abuse it will be expected to withstand. Use a surface-level sealer or deep penetrating one depending on your needs.

How Much Does It Cost to Seal Concrete Floor?

The square footage of the floor, the thickness of the sealer, number of coats required, penetrating or not, all combine to affect the cost. Some products will cover 100sqft per gallon and others between 400 and 600sqft. With a gallon ranging between $55 and $100, the cost will depend on the area being covered.

Expect to pay less for acrylic sealers, and more for polyurethanes and epoxy, and still more for polyaspartic polyurea sealers. Some acrylic sealers may cost $0.50 a square foot, and some polyaspartic sealers up to $4. Doing the work yourself is usually less expensive than having the pros do the work. For the best results, follow the manufacturer’s directions for the product you choose and don’t cut corners.

How to Clean and Care Sealed Concrete Floors?

Cleaning a sealed concrete floor is similar to most other finished hard surface floors. Sweep or vacuum the floor weekly. Use a dust mop to collect material the broom or vacuum missed. Small particles can scratch the finished surface.

Monthly, or as needed, wet mop the surface with warm water and a few drops of dish soap, and then rinse with clean water. Use a clean rag or cloth and a solution with ammonia or vinegar to remove stubborn stains, and rinse with clean water afterward. Clean up spills that could stain as they happen, and let the floor dry before replacing area rugs.

Remember to reapply the sealer as directed by the manufacturer. Alternatively, use a quality floor wax as an additional protective coating to extend the life of the sealer. Most manufacturers also provide cleaning instructions for their products, so check their info sheet for suggestions.

Conclusion

Use a water test to determine if your concrete basement floor has already been sealed. If it hasn’t been sealed, clean dust, dirt, debris, and stains from the surface. Use a large paintbrush and lint-free roller to apply the sealer to the concrete, and let dry before using or replacing furniture or carpets.

Hopefully, we’ve helped you better understand how to seal a basement concrete floor, why sealing is helpful, and what to look for in a sealer for your floor.