Concrete may look and feel hard, dry and seemingly water tight but concrete actually resembles a hard, dry sponge more than something solid and impermeable. In fact, concrete’s interior is comprised of thousands of microscopic pores that fill with water as the concrete is being poured and set. Once concrete starts to harden, its hydration reaction to hardening uses some of this water. When moisture near the surface of the concrete slab begins evaporating, these tiny pores dry out.
Once water is depleted in a concrete floor, moisture vapor continues to diffuse through the slab from the soil’s moisture content if no vapor barrier exists between the soil and the concrete. Lack of a moisture vapor barrier means the concrete slab will act as a thirsty sponge, which seriously compromises the integrity of adhesives, flooring materials and coatings by causing bubbling, discolorations and structural deformities.
Moisture Vapor Transmission Causes Flooring Failure
Excess moisture below or inside concrete slabs is responsible for most coating failures. Although moisture in concrete is needed during applications of epoxy materials, this kind of moisture does not cause the coating failures that occur months or years later. Rather, it is the flow of vapor (moisture vapor transmission) that causes the majority of adhesion concrete floor problems. How to significantly reduce vapor transmission in concrete slabs-on-grade is the biggest issue surrounding the moisture vapor transmission phenomenon.
In fact, the negative consequences of unchecked MVT through concrete slabs have increased dramatically over the past few years in the construction industry. Fueled by “fast-track” building practices, restrictions on the use of volatile organic curing compounds and behavioral changes of modern adhesives and floor coverings, the problem of MVT has contributed heavily to discussions about vapor barrier positioning beneath floors, especially in distribution centers and large warehouses where resilient carpet and tile constitutes the majority of adhered finishes.
What is Moisture Vapor Transmission Rate?
MVTR measures the passage of water vapor through any substance, including concrete. Controlling moisture levels is critical in many industries where excess moisture can reduce the shelf life, safety and quality of pharmaceutical and food products.
The American Society for Testing and Materials (ASTM) states that moisture vapor transmission rates of four pounds per 1000 square feet over a 24-hour period is the maximum rate allowable for successfully covering concrete with adhesive, sealer or coating. MVTR can be determined by doing a calcium chloride test involving the placement of unhydrated calcium chloride on a concrete slab and covering it with a domed, plastic lid. The calcium chloride is then weighed (after 72 hours) to determine the amount of moisture it has absorbed from the concrete.
A faster method of testing MVT is called in-situ relative humidity testing, which can be accomplished in minutes instead of hours. The downside of this method is that you must drill a 1/2 inch hole into the concrete to insert a probe for measuring moisture content.
How to test your basement for moisture
Before you put any coating, flooring, or finished wall over concrete, find out how much moisture is coming through it. Testing also tells you where the moisture comes from, which is the first step to fixing a damp basement. Some tests you can do yourself in a weekend. Others are lab-grade methods flooring contractors use before installing finished floors.
- Plastic sheet test. Tape an 18 by 18 inch square of clear plastic tightly to the floor or wall on all four sides and leave it for 16 to 24 hours (this is the ASTM D4263 method). Moisture under the plastic means vapor is coming up or through the concrete. Moisture on top of the plastic means humid air is condensing on cool concrete.
- Relative humidity (RH) meters. A simple hygrometer measures the humidity of the basement air. Readings that stay above about 60 percent mean conditions are right for mold and point to a moisture source that needs more testing.
- Pin meters measure electrical resistance between two pins pressed into a material. Water conducts electricity, so the wetter the material, the lower the resistance.
- Pinless meters send an electromagnetic signal into the material and read moisture from how the signal changes. They do not damage the surface and are good for scanning a large wall to find wet areas.
- Calcium chloride test (ASTM F1869) gives the moisture vapor emission rate in pounds per 1,000 square feet per 24 hours, as described above.
- In-situ relative humidity probes (ASTM F2170) are set into drilled holes to read the humidity inside the slab.
Test during or just after a wet season, when moisture is at its worst, and test in several spots. If readings stay high, find the cause before you finish the space. Common sources include poor grading and gutters outside, cracks, and the joint where the wall meets the floor. Our posts on what causes leaky basements and protecting your home from exterior water damage cover these.
Controlling Moisture Vapor Transmission in Concrete
Installing adequate moisture control vapor barriers and flooring using commercial/industrial grade epoxy or urethane coatings are the best way to control MVT. The lifespan of concrete is prolonged using this type of coating by preventing disintegration of concrete and allowing surface treatments to remain in excellent condition for the duration.
A vapor barrier’s effectiveness in controlling moisture movement is measured by its perm rating. A permeance of one perm is one grain of water vapor that transmits at one square foot per hour per one inch of mercury vapor pressure difference. Consequently, good vapor barriers have lower numbers. Materials with permeance ratings higher than 1.0 are typically not considered to be genuine vapor barriers.
Anti-moisture vapor transmission flooring prevents erosive forces from permeating concrete and causing serious degradation. When concrete slabs intended to receive an impermeable surfacing or coating are being placed, an effective moisture vapor barrier must be established. Also be aware that adhesion issues caused by MVT are not limited to epoxy bonding to concrete. Any other non-breathing films will react in this manner as well.
Once the barrier is in place, using high quality concrete and professional placement techniques are essential to completely eliminate MVT. Structural integrity and configuration of the concrete slab should be considered, along with precision design of expansion and control joints. A properly cured, well-placed concrete slab will provide a dense, hard concrete surface exhibiting the desired low permeability.
Common uses for vapor barriers in a home
A conventional vapor barrier is a sheet of plastic or rubber placed in a wall, floor, or foundation assembly to slow the movement of water vapor. You will find them in several places:
- Under concrete slabs. Polyethylene sheeting laid on the gravel before the pour keeps ground moisture from wicking up into the slab. Older homes and many garages and basements were poured without one.
- On the outside of basement walls. Many new homes get a membrane or coating on the outside of the foundation before backfill to keep groundwater out of the walls. Depending on where you live, building codes may require dampproofing or waterproofing here.
- In crawl spaces. Plastic sheeting over the soil cuts down the moisture that rises into the crawl space and the floor framing above.
- In showers and bathrooms. A barrier behind the backer board keeps shower water and steam out of the wall cavity. See how to waterproof showers and bathtubs.
- Behind exterior siding and in walls and attics, where the right type and position depend on your climate and local code.
Sheet barriers work when they are installed perfectly and stay intact. The problem is that they get punctured during construction, seams open up, and they stretch, crack, and tear over time. Under a slab or on the outside of a basement wall, there is no way to reach a damaged barrier without tearing out the concrete or digging. When a slab was poured without one, or the original barrier has failed, the fix has to be made from the top of the concrete.
The SANI-TRED® Solution
Epoxy or polyaspartic systems simply do not work for extended periods of time on exterior applications and often fail because they can’t hold back the vapor pressure exerted on them in an outdoor setting. The same thing goes for many basement floors or other areas where moisture vapor pressure is an issue, resulting in concrete floor problems. SANI-TRED on the other hand is a different animal.
Its flexible rubber “tentacle-like fingers” penetrate deep (sometimes up to 1.5 inches deep) within prepared concrete. When moisture vapor/pressure presses against them they expand and swell tightly against the concrete capillaries making it impossible to blow off the coating. Learn more about using SANI-TRED as a concrete vapor barrier sealer on our site.
SANI-TRED as a moisture vapor control coating
After testing, get the concrete as dry as you can, then apply PermaFlex. It soaks into the pores, bonds to the concrete, and cures into a seamless rubber membrane that stops moisture vapor from reaching the surface. Cracks and joints are filled with a rubber patching compound made by mixing LRB (Liquid Rubber Base) with TAV. Because the membrane is bonded into the concrete instead of lying on top as a separate sheet, there is nothing to rip or tear, and you can put paint, stain, flooring, or a decorative finish over it.
Contractors use it this way on residential and commercial floors. Josh Owens of Owens Concrete Staining used it on a floor where the existing sealer was failing: “This floor had a high moisture reading and was causing the current sealed floor to fail. We decided to use PermaFlex as the vapor barrier paint for concrete before resurfacing the floor and designing, staining, and sealing with a solvent urethane.” Colt Weller of Supremecrete installed SANI-TRED as an industrial floor coating at the University of Northwestern Ohio to resist cracking and moisture vapor transmission.
SANI-TRED products are solvent free, non-flammable, and low VOC. The basement system carries a lifetime warranty and blocks 99 percent of radon gas.
- See the SANI-TRED basement waterproofing system
- Read about the basic product line
- Garage and concrete floor coatings
- Calculate the cost of your project
- Call 1-866-784-3308 to talk with a SANI-TRED tech about your moisture readings