Engineered Wood Flooring Performance
January 19, 2012
Typically, engineered wood flooring is made from layers or veneers of solid wood glued together to make a suitable flooring plank. The grain directions of the different veneers are typically rotated 90 degrees to the previous layer to reduce the overall width expansion/shrinkage of the engineered flooring. The wear surface or top veneer is most often thicker than the remaining or core veneers. As a performance issue, the thicker the top veneer the more engineered flooring performs similar to solid wood flooring. The purpose of all the layers is to restrain the dimensional change across the grain with an adjacent longitudinally oriented veneer. With a significant moisture change and a thick top layer, the unequal dimensional changes of the core and face can result in dramatic distortion such as cupping, crowning, and or bowing.
The expansion/shrinkage rate of engineered flooring is near 0.5% from green to oven dry for both width and length. For a significant change in moisture, say 4%, a 4” x 48” long board can shrink/expand about .032” (1/32”) in length and have a .003” change in width. This compared to a solid oak board would be .010” (<1/64”) in length and .059” (1/16”) in width. This 4% change in moisture can result in noticeable end gapping from shrinkage or end lift from expansion of engineered flooring.
In the HPVA/ANSI EF 2009 standard for engineered flooring two paragraphs, “Construction” and “Moisture Content”, address conditions related to assembly of the flooring piece and moisture content of shipment and the purposes of the requirements. Basically the product should be a consistent size and flat enough to be properly installed per instructions. Also, there is a moisture content requirement of 5% to 9% at time of shipment in order for the flooring to perform in a typical indoor environment. There are exceptions for flooring that is specifically manufactured for arid climates or areas with generally high humidity.
So what does this have to do with installation and performance of the flooring? First, engineered flooring is marketed as being more suitable for environments with more adverse moisture conditions. Applications such as below grade slabs and basements fall into this category. With these applications the potential is in place for greater moisture change than solid flooring. The construction of the product makes it more stable than solid wood so the flooring should not react to the extent of solid wood where “extra” moisture is present. This is true for width of the product but may not be true for length. Extra moisture can result in end lift or extra drying can result in end gaps. Many instructions call for no acclimation because of the tight fit of the configuration so some moisture change is inevitable. Again, always make a moisture test on the installed product if you expect a significant gain in moisture; some slight spacing at ends may be necessary during installation. There is little you can do for a significant expected loss of moisture and related end gaps.
Another performance concern with engineered flooring is cracks in the top veneer. During the manufacture of the veneers cracks are inevitable. For rotary peeled veneers the actual cutting and flattening of the veneer will always result in latent veneer cracks or lathe checks. Even with sliced veneers, checks occur as the veneer is bent as the cutting knife slices the log. Fewer checks generally occur as the bending is not as severe as a peeled veneer. After drying, assembly, and finishing these checks are filled and are no longer an issue. They can be noticed later if environmental change occurs, particularly when related to shrinkage. Many cracks can also manifest themselves if the finish is not flexible enough to accommodate the movement during seasonal change or acclimation. In addition, if the construction is unbalanced, particularly as related to moisture, cracks can occur. For instance, if the core stock is dryer than the face, as the face shrinks it is restrained by the core and fractures occur at the latent checks.
Another cause of face checking is wet maintenance. When the flooring is wetted the water can soak into the latent cracks causing cyclical expansion and shrinkage as repeated exposure occurs. The finish breaks and the condition becomes worse. This almost always shows at the ends of the boards more than along edges, as the end grain open cells can potentially absorb more moisture. The ends will be rough.
As always, using a moisture meter on the flooring before installation can determine average moisture content and the expected changes during seasonal cycling. Too much change and the flooring will develop performance issues such as cupping, crowning, gapping, and cracking. In particular, cracking and cupping may occur in arid environmental situations as the flooring acclimates after installation, especially if the flooring is not specifically manufactured for the drier environment. Areas with a heavy heating season such as the upper Midwest also fall into this category. Many manufacturers recommend that the relative humidity be maintained between 35% and 55% for standard product to properly perform. This equates to 6.9% to 10.1% EMC, an average of 8.5%. In many areas of the country this environment cannot be maintained without installing extraordinary humidification systems during winter heating. Extraordinary humidification may also result in problems such as condensation with air infiltration.
Engineered flooring with an exotic species as the face veneer can also have similar issues to the temperate North American hardwood species. Many are considered quite stable with a low percentage of shrinkage/expansion from green to oven dry. However, since they have a lower fiber saturation percentage, the expansion/shrinkage is more concentrated. For instance Brazilian tigerwood shrinks approximately 8% tangentially from green to oven-dry, considered a low shrinkage factor. However, its reported fiber saturation is about 20%. This translates into more concentrated movement for each percentage change in moisture content compared to temperate hardwoods. In addition, exotic species, particularly South American woods, may have chemical inclusions within the wood fiber that can create discoloration or finish interruptions as the flooring cycles seasonally. White spots that develop in Brazilian cherry after light exposure are an example of this.
Finally, the integrity of the adhesive bond within the flooring as well as the adhesive bond to the substrate affects performance. First, the glue bonds between the flooring veneers within the flooring should not fail because of any normally lived-in humidity condition. The HPVA/ANSI standard bond line tests are very aggressive in that the test pieces are soaked and then forcibly dried at an elevated temperature of 120 degrees F. Even relative humidity near 10% if found in a residence doesn’t approach the stresses generated by the soaking and drying cycle of the ANSI test. Veneer bond failure or delamination only occurs when the adhesive fails as a result of a manufacturing error or a catastrophic condition such as a flood occurs within the home.
The adhesive bond performance between the flooring and the substrate is dependent on the proper choice of adhesive, the proper application technique, and the proper installation of the flooring. The flooring contractor is generally responsible for these procedures. Clean, dry and flat is the mantra for the substrate. Proper spread rates and working times as the directions instruct are the guidelines for the adhesive. Any condition that is not properly followed can result in an improper bond and related noises and movement.
The manufacturer of the flooring is responsible to provide a product that will perform under normal environmental living conditions. If their product is specifically designed for conditions other than those that will be encountered at the installation in question, then selecting another product suitable to the conditions should be considered. The installation responsibility rests with the flooring contractor. Taking moisture readings of the flooring to determine suitability for the area’s geographic environmental conditions and advising the consumer if performance is in question can help with the final installation decision. Consider with caution if the flooring is not at the final projected acclimated condition and how it will perform during the acclimation adjustment. If a significant change is expected, not proceeding with the installation, changing the product, or changing the expected environment to a more suitable condition are options for the consumer to consider. As stated in many instruction sheets, once the flooring is installed you have accepted all aspects of the product and site conditions as being correct. Any later issues can become your sole responsibility.