During the past several years, installers have gradually been switching to urethane adhesives to install hardwood floors, a move designed to comply with the Clean Air Act-banning of the manufacture of 1-1, 1 Trichloroethane. In that time, urethane adhesives have become some of the top selling hardwood adhesives on the market.

In 1998, there were more than a dozen urethane adhesives available, ranging from around $80 to more than $200 per 5-gallon container. Unfortunately, many contractors don’t realize that they have a choice when deciding on a urethane adhesive, and often end up paying for a product that may be over-engineered for their particular installation.

When chlorinated solvent adhesives ruled the market, there was not much of a difference between any two particular products. The physical properties of the various adhesives were very similar; therefore, it didn’t matter which product was selected. Urethane adhesives, on the other hand, are made of polymers that can be selectively modified, allowing the properties to be changed to fit a variety of installations.

There are four basic physical properties - elongation, bond strength, water vapor permeability rate, and set time – that should be considered before purchasing a urethane adhesive. Paying careful attention to these variables will help the contractor to complete a successful installation.


Elongation is a measure of how much a product will stretch before breaking. Wood expands and contracts due to swings in temperature and humidity, which ultimately affects the dimensional stability of the wood. If the adhesive is too brittle (has a low elongation), it will not be able to stretch with the wood, and the adhesive bond will fail. If there will be changes in temperature or humidity levels throughout the year, choose a urethane with a higher elongation percentage. If these changes are not a factor, a urethane with a lower elongation percentage may be selected.

It is also important to consider the wood being installed. Solid wood expands and contracts much more than engineered wood, so a high elongation percentage (no less than 80%) would be important for the project, whereas engineered wood performs well with adhesives containing a lower elongation percentage.

Bond Strength

Bond strength determines how difficult it will be to remove a piece of wood once it is cured. Typically, urethane adhesives are known for their tenacious bonds. However, many manufacturers are cutting back on “extreme” bond strength in order to make the product easier to use if a repair needs to be made.

A contractor can choose from an adhesive with a bond strength of as much as 1000 psi to as low as 140 psi. If the floor will never be removed, will never require repairs, or is a commercial installation that will encounter heavy traffic, use an adhesive with high bond strength. If damage from flooding or weather is anticipated, or if the wood being installed often requires repairs, select a urethane adhesive with lower bond strength.

Water Vapor Permeability Rate

This determines the amount of water that is transferred through the adhesive from the slab into the wood. Hardwood adhesives have vapor retarding characteristics, but are not considered vapor retarders.

Water vapor permeability is usually measured in grams/meter-24 hours-mmHg. Although these measurements may seem intimidating, when comparing adhesives, simply look at the first number. If one adhesive is 4.5 x 10(-5) grams/meter-24 hours-mmHg, and another is 9.0 x 10(-5) grams/meter-24 hours-mmHg, the adhesive that is 4.5 has double the moisture protection, because 4.5 is two times lower than 9.0. If the contractor has moisture-related concerns, he or she should choose an adhesive with a lower value (Note: Never install wood flooring if the moisture level exceeds the hardwood manufacturer’s recommendations).

Set Time

Set time determines how quickly the product adheres to the slab. By manipulating the polymer in urethane adhesives, a product can be produced that sets quicker or slower depending on environmental conditions.

Generally, if the conditions include high temperatures and high humidity, urethanes will cure fast. In low temperatures and low humidity, they will cure more slowly. In the later instance, the contractor will want to purchase a product specifically formulated for a low temperature/low humidity environment.

By closely examining the physical properties of a specific installation, contractors will see a significant savings, both in installation times and costs.