Top quality adhesives keep moisture problems from becoming a big problem later.

Many installers feel that when they install a wood floor over concrete they are rolling the dice with moisture issues and hoping they don’t receive the dreaded customer call that there is a problem with the floor. When that call does come it usually results in a merry-go-round of finger pointing and inspections with everyone trying to avoid being caught with the replacement costs of a floor. Understanding the characteristics of the products being used and the environment in which they are installed can go a long way in preventing unnecessary callbacks or even worse floor replacements.

Wood is a natural product composed of cells that expand and contract based on the availability of moisture. Solid wood products are more susceptible to moisture in the form of cupping and the wider the product the more it is likely to occur. Engineered products however are more stable due to the laminated layers. The more plys, the more stable the product. As installers excessive moisture is the enemy of a perfectly installed wood floor and it can come from several sources.

The Concrete Slab: Surprisingly it takes only a small amount of water to cause concrete to cure. The balance of the water is used to make the handling and finishing easier. This water is often trapped in the slab by the use of curing agents, typically applied by the contractor to increase slab strength by slowing the rate at which the water can leave it.

It leaves in the form of vapor. Testing a slab for this is vital to understand how much moisture the wood will be exposed too. A calcium chloride test will tell you how much moisture in the form of weight is being passed from the slab in a 1,000-ft area over a 24-hr period. A gallon of water weighs approx 8 lbs. If you tested and calculated a reading of 8 lbs, this would be the equivalent of spreading 1 gallon of water over a 50-by-20ft area every day. It is easy to understand why wood floors react when exposed to these amounts of moisture.

Note: If you have tested, and know that you have a moisture problem, make sure that you use a recommended moisture barrier approved by the adhesive manufacturer you are using.

Humidity: This is the moisture content in the air and depending on where you live in the country this condition will affect you different ways. The ideal humidity level is around the 50 percent mark. Lack of humidity can result in wood shrinkage and customer complaints of gapping and cracking. Too much humidity will result in cupping. The edges of the wood will become higher than the center, as the wood takes on moisture and tries to return to its natural form – a round tree. Acclimating wood for a minimum of 48 hrs or longer will allow the wood to stabilize to its new environment (either expanding or shrinking) before it is bonded in place permanently. This assumes that the new space is a functional controlled environment. Installing wood in a newly constructed home in high humidity areas without functional climate control can lead to a greenhouse environment inside the house. Moisture drawn into the air from new paint, drywall, wood framing and trim, can easily be absorbed by a kiln dried flooring product with damaging consequences.

Adhesives: There are a wide variety of adhesives available for installing wood floors. Recognizing their attributes will help you either save money or potential headaches. As mentioned earlier water and wood don’t mix well so it is important to understand how to use them correctly.

Using a reliable adhesive gives the advantage of working on a wood floor without any slippage.

Latex Based Adhesives utilize water to make it easier to mix all the components together and to make it easier to trowel it on the floor. When applying this type of adhesive more is not always better.

First, use the recommended trowel notch size because it has been tested by the manufacturer to have the strongest hold with the least moisture impact to the floor.

Secondly, allow the glue “flash” time recommended on the bucket. Doing this will provide a stronger initial grab and allow time for the now unneeded moisture to evaporate rather than be absorbed into the flooring. Time will vary based on humidity levels and floor porosity. Although we are talking very small amounts of moisture, it may, if not applied correctly, be enough to affect sensitive products such as bamboo, exotic species, and thin gauge or extra wide solid products. Engineered products, which are constructed of a series of plys, are more dimensionally stable and therefore are more resistant to changes in moisture conditions. These products are typically the most common choice for water-based adhesives.

Resin Based Adhesives work well across all wood products as long as the same procedures as mentioned above are followed. They have excellent initial grab strength and also have great residual tack days after an install is complete which works well to eliminate hollow spots.

Moisture Cure Urethanes solve issues relating to moisture absorption in sensitive products as they contain no moisture but actually require moisture to cure. These adhesives have a much slower grab time than that of their water based counterparts, requiring a “working off the product” installation method, and typically have a strong odor related to the use of isocyanates that can cause issues in occupied environments. While they also may cure or set waterproof they are often represented incorrectly as moisture barriers. Using any adhesive as a moisture barrier only is not an advisable option. Most manufacturers have a barrier coat system that requires special instructions to be followed to achieve acceptable moisture conditions for wood flooring.

Modified Silanes: These are a new addition to the United States wood flooring market. Modified Silane or MS is a polymer developed in Japan but used extensively in Europe. Similar to urethanes this moisture cure adhesive is 100 percent solids, moisture proof when dry, and has the quick strength and flexibility of a silicone without isocyanates or a noticeable odor. It trowels well and requires no flash time. Cleanup is remarkably easy.

As the installer you cannot always control the conditions or the purchase of the products described above. However as the installer you become accountable for the installation once complete. The following are a must:
  • Always test concrete slabs for moisture before installing any wood. If the floor is above the manufacturers’ recommendation have it sealed with a sealer approved by the adhesive manufacturer.
  • Acclimate the material and maintain climate control before and after installation where possible to limit dimensional changes.
  • Use the best adhesive for the product type and read and follow the instructions regarding trowel notch size, flash time if required, and rolling.

Making the builder/retailer/customer aware of potential conditions that may affect the success of the floor performance and then providing installation and performance based product solutions to combat those issues will help in enhancing your professional reputation while limiting your liability. A proactive approach will improve your chances for success and is far better than rolling dice.