Interior concrete flooring always needs a solid barrier underneath it to prevent moisture from seeping up from the ground. A concrete flooring moisture barrier prevents cracking, curling, efflorescence and air quality problems, and it may even be a requirement in building codes and warranties. Here’s what to know about this vital material and how to make the most of it.

Protecting Interior Concrete

The main purpose of a concrete flooring moisture barrier is to prevent water from penetrating the concrete, which is a porous material. It’s necessary for all indoor concrete flooring. You should install any slab on enclosed grade on a moisture barrier over a layer of sub-bases like mud slab, compacted soil, carton forms or stone.

According to the American Society for Testing and Materials (ASTM) standard F2170, a concrete slab should have no more than 75% relative humidity to proceed with flooring installation. It takes around 30 days for each inch of slab to dry. Total drying time depends on air temperature, circulation and ambient humidity.

Turn to the ASTM E1745 for guidance when looking for a vapor barrier. It provides guidelines for testing a product’s performance and classifies water vapor barriers based on their puncture resistance, permeability and tensile strength.

Preventing Problems

Moisture can come from rain, broken plumbing, drainage, humid air, poor ventilation or from below the slab. It can also originate from the water-cement mixture used to make the concrete. The main reason to use a concrete flooring moisture barrier is to protect the floor covering on top of the concrete. Here’s what could happen if you forgo the barrier.


The moisture barrier creates a smooth surface for pouring concrete. The finished floor may be uneven without it, and moisture diffusing through the concrete slab can cause sheet flooring overlays to debond.

Moist concrete can also cause the flooring above to blister, forming small air pockets. This happens when water-based adhesives used to attach the flooring degrade in the presence of water and alkaline salts from the concrete. As water rises from below the slab, it condenses under the flooring and emulsifies or damages the adhesives. It can also lead to floor discoloration.

When moisture corrodes the embedded reinforcing steel, the concrete can suffer from spalling, leading to a dimpled surface. The concrete’s edges may also start to curl over time.


Efflorescence, also called sweating slab syndrome, occurs when water on a concrete slab — often from humidity that originated under the floor — starts pulling salt to the concrete’s surface. It leaves salt deposits as it evaporates, making white marks on the concrete.

Salt is often hygroscopic, so it absorbs water from the surrounding air. Just as water condenses on the outside of a glass, water starts condensing on the concrete. It can lead to a persistently wet floor that poses a slipping hazard.

Alkali-Silica Reaction (ASR)

ASR causes salts in the concrete to react with the silica found in some aggregates, such as Portland cement, to form a gel. This gel will expand and cause severe cracking if the mature concrete slab’s internal relative humidity is 80% or higher. One of the main ways to prevent ASR is to use a concrete flooring moisture barrier to keep the humidity down.

Mold Growth

High temperatures and humidity allow mold to thrive in the porous concrete environment. Carpet creates the perfect recipe for a mold and mildew problem, leading to musty smells and poor air quality. It can even cause allergies and breathing problems for many people. Keeping the concrete dry goes a long way toward preventing mold growth.

Staying up to Code

Most concrete industry experts recommend using a moisture barrier under concrete flooring, and the American Concrete Institute even backs up this recommendation in its ACI 302.2R guidelines.

However, installing a barrier to meet building codes is also important. Although not every building code specifies that you must use a moisture barrier, most have rules requiring you to at least install one inside crawl spaces and below concrete slabs on grade.

Check the ASTM E1643 guidelines to gain a better understanding of building codes. It also outlines how to select a vapor barrier and anticipate the physical demands of the project.

Improving HVAC Efficiency

A building’s humidity levels can rise if you don’t install a moisture barrier. Unless you’re using a dehumidifier, your HVAC system will have to work harder to remove water vapor from the air, leading to higher energy costs and shorter HVAC life spans. Moisture barriers can make a building more energy-efficient, which is better for the environment and your energy bill.

Not Voiding the Floor Covering Warranty

Just as a moisture barrier is important for staying up to code, it’s also a requirement in many floor-covering warranties. You might be able to replace the floor for free within a specified time if you can prove you were using a moisture barrier and still had damage. However, you probably won’t get your money back if you allow preventable water damage to occur. 

Protecting Materials and Equipment

Warehouses with bare concrete floors must take steps to prevent moisture from damaging products sitting on the floor. A wet floor can damage wood, cardboard and paper packaging, and the ambient humidity can also ruin the contents of the packages. The result is high inventory replacement costs and greater waste.

Improving Versatility

Additionally, using a moisture barrier means a building has more potential uses beyond storage or agriculture. You could convert the building for retail or even residential purposes if it’s up to code. A structure with a vapor barrier under the floor will be easier to sell than one without.

A Concrete Flooring Moisture Barrier Is Crucial

Few things make as much of a difference to an interior floor’s integrity as a concrete flooring moisture barrier. It reduces the risk of spalling, blistering, ASR, and other structural and cosmetic issues. In some cases, it’s even required to stay up to code. Luckily, a moisture barrier is usually inexpensive and can prevent a lifetime of costly repairs or replacements.