Concrete Moisture Barriers

Choosing the right concrete vapor barrier coating and installing it correctly is a good way to help minimize moisture vapor.  A cold basement slab and a damp floor is certainly not ideal for your customers. Initially, old floors remained damp and cold because they did not have a vapor barrier beneath them. This left a path through which vapor from the soil migrated into the slabs. In fact, this is what made the damp feeling become permanent. When water vapor moved through the concrete floor, it delaminated the sealers, emitted efflorescence, discolored the floor and damaged aggregates. This forced the slab to curl or even condense beneath the floor.

Nowadays, by using concrete vapor barrier sealant, the moisture beneath floors can be controlled or even be eliminated. This article provides some essential information that can help you understand how water vapor or moisture moves through a slab, and how concrete vapor barriers can be used to solve this the problem.

Problems Caused By Moisture in Concrete

Natural systems tend to remain at equilibrium. For example, when hot coffee is poured into a cup, it transfers the heat to the surrounding air until the temperature around the room and that in the cup is the same. This is what is usually referred to as entropy; the law of thermodynamics.

Basically, when there is a higher concentration of chemicals in an area, the compounds will always move towards regions that have got lower concentration. (Le Chatelliers principle).  This is applicable for areas that have a lower or higher relative humidity.  The movement of vapor is referred to as diffusion.

Therefore, this means that if there is a difference in the humidity in the concrete slab and that of air that’s immediately above it, moisture will try to move into the air. However, when the surface dries, it draws moisture from the bottom.

This can be a big problem since water vapor from this floor will always make the room humid. This predicament mostly affects homes that are tightly constructed. If the construction started with a wet concrete and the concrete beneath the slab is wet without any damp proofing initiated, it leads to a damp floor. This makes the moisture to condense beneath the object placed on the slab.  Once the floor becomes wet, it tends to promote mold growth.

In a case of decorative concrete, there are 3 major problems associated with moisture coming out of the concrete slabs. Firstly, if a slab with a lot of moisture movement is stained with copper salts, it becomes discolored or black. Secondly, the water vapor moving through the slab condenses into liquid water and forms leachable calcium hydroxide that finally forms efflorescence on the surface of the slab. This results in high pH levels on the surface of the slab.

The main problem for decorative concrete especially when a lot of moisture moves through the slab is that it exerts pressure on the surface. In a case where too much moisture moves through the slab and there is an impermeable layer that is placed over it, the moisture migration delaminates the overlay or the sealer. Sealers that allow vapor passage could be used. Topical membrane sealer requires a moisture vapor emission rate of 5 or 3 pounds /1000 sq. ft. per 24 hours.

In commercial applications, moisture can cause a great havoc when the wood, the carpet, or the vinyl flooring is installed on a damp concrete. The reason behind this is that a high alkalinity and dampness in the concrete leads to warped flooring and mold emulsified adhesives. The problem has made many researchers to carry out a lot of studies on this topic. Most of them have discovered that using very low water levels to cement ratio of about (0.5 or less) or concrete moisture barrier products like Spray-Lock SCP 327 can help solve the problem.

Concrete Moisture Barriers

The barriers ensure that all the problems associated with the movement of moisture in a slab concrete go away fast as the slab dries. For this to happen, all the sources of additional water should be eliminated. The main source of water is moisture, which is abundantly available in the soil beneath the slab. Therefore, once the ground is removed from the equation, the problem gets solved. Using a concrete vapor barrier has been found to be the best way of accomplishing this. Vapor retarders have been in use since the 1950s. In recent research, it has been demonstrated that the traditional layer polyethylene plastic (6 –mil Visqueen) under the slab is rarely effective because of two reasons:

1. The grade of its material allows water vapor to pass through it.

2.6 –Mil plastic normally gets damaged when concrete and reinforcement is put in place. This results into holes that increase the amount of water vapor in the slab.

The thin plastic also called vapor retarder works by slowing down vapor, but it does not effectively stop it. Using a true vapor barrier with features that conform to the ASTM E-1745 standards can effectively solve the problem. The specification has 3 classes of vapor retarders Class A, B and C. The term barriers and retarders are used interchangeably.

For the three classes of retarder, the permeance that measures the rate at which vapor passes through should be not more than 0.3 perms. Currently, there are materials with less than 0.03 and 0.01. These are materials with low permeability, which eliminates moisture migration from the ground. It allows the slab to dry out quickly and remain dry.

According to ACI 302.2R-06; a guide to concrete floors that receive moisture flooring materials, it is estimated that a concrete with a w/c 0.5 can dry to  a MVER of 3/pounds/1000sq.ft/24 hours in less than 82 days if a vapor barrier is used. This can be compared to 144 days when it is exposed to underneath vapor.

The other important feature of a vapor barrier is that it is resistant to tear and puncture. According to ACI 302.1 guide, the minimum thickness of vapor barrier should be about 10 mils. This result were verified in studies carried out by Concrete Construction Magazine. When thinner plastic is used, it may not stand up construction abuses. The minimum values of puncture resistance and tensile strength increase from class A to C and have been specified by ASTME-1745.

A vapor barrier of 10- mil has been found to be sufficient for residential construction because it increases puncture resistance although it cannot completely isolate the slab from the ground moisture. There are new permeability barriers including those from W.R Meadows Stego, Polyguard, Reef, Raven, Interwrap, fortifiber, Grace Construction products and Layfield. This material is 15 mills, 15/1000 of an inch or more. When a thicker material is used, it is less susceptible to puncture and tear and has a lower permeance.

Installation of a Vapor Barrier

The vapor barrier should be installed in such a way that it prevents water vapor from infiltrating the slab. ASTM E1643 provides more details. Here are some of the tips you can use:

• The exterior slab may not need a vapor barrier especially if it will be sealed. It is important that you find a penetrating concrete sealer, which can transmit water vapor.
• Generally, the concrete slab should be placed directly on top of the barrier whereas the sub base should be placed below it. A thin layer of sand should be placed on the gravel subbase close to the vapor barrier.
• Vapor barriers may be placed under the sub case. In a case of a closed in building, dry sub base material may be used.
• Do not punch holes in your vapor barrier since they allow bleed water to escape.
• Don’t pound form stakes through vapor barrier.  Some of the supports come with a number of bearing pads. You may also use new Vapor Stake to seal the vapor barrier, cut it off and leave it in the slab.
• The vapor barrier should be sealed with a tape provided by barrier manufacturer. Ensure that there is a 6-inch overlap of the seam.
• Ensure that you seal all block outs and pipe penetrations.  Details on this can be read from Howard Kanare’s book, which can be acquired from Portland Cement Association. It provides details on this aspect of Maryland Decking.
• A vapor barrier should be run up onto the barriers footing, but make sure you seal it to the foundation.
• Ensure that the vapor barrier is protected during the construction. If there is any damaged spot, let it get repaired using a manufacturer’s tape.

The positioning of vapor barrier has been a subject of discussion for many years. In the 1980s and 1990s, it was recommended by the ACI that a blotter layer is used in addition to the vapor barrier.  These recommendations changed in 2001 and required that the concrete be directly placed on top of vapor barrier so as to stop water from being filtered through the bottom. This is what results to longer bleeding time and results in slab curling.  This article has shown that the subbase can result in a continued flow of vapor into the slab. According to experts, it is recommended that a slab be placed directly on the barrier only if the sub base is protected. This will make it remain dry throughout. Curling could be controlled if a well-graded aggregate is used while the slabs lower part is reinforced with a concrete moisture barrier.