Blog Post

Water-Based Polyurethanes and Panelization of Athletic Floors

Projects are having trouble with performance of low-VOC finishes on gym floors, but there are ways to avoid problems.

Bona SuperSport water-based sealer is applied to a gym floor. Credit: Jason Thomas Flooring

Wood athletic floors, while unparalleled for all types of play, are typically finished with solvent-based polyurethane coatings that emit high levels of volatile organic compounds (VOC). They need to be finished when installed and then refinished about every ten years. This presents a particular challenge to design teams aiming to earn the LEED credit for low-emitting materials, IEQc4. LEED or not, minimizing VOC emissions in schools is a high priority.

According to reports received by GreenSpec editors, some teams have been experiencing splitting and panelization in the floors using low-emitting water-based polyurethane finishes. Floors that presented these problems were typically being refinished with solvent-based polyurethane, resulting in greater expense and increased VOC emissions. As Brian Feagans, AIA, an architect with Ratcliff Architecture who is engaged in this problem noted in an email dialogue, "as our industry often experiences, when applying a 'new' product or technical approach to a traditional system without tweaking other components in the system, problems can occur."

Splitting and panelization problems

The splitting and panelization was observed in areas that experience wide seasonal swings in relative humidity, including the Northeast and Midwest. These problems mostly occurred where water-based polyurethane had been specified for maple flooring¬--maple is one of the more common and higher-performing athletic floor materials.

Another architect who has researched panelization shared this description of the mechanism behind the problem with GreenSpec: "Water-based finish is not very viscous and it is very hard. As a result, it flows freely between and under the boards. It then can lock the boards together. When the floor contracts during the lower-humidity heating season, large cracks may develop in the floor because the floor is locked in place by the water-based finish--it is unable to shrink normally. Sometimes boards will even split down the middle or along the edges. Panelization is a significant source of complaints within the industry."

Panelization is not a new problem, and can have structural causes beyond finishes, including subfloor material and the type of nailing used. The wood flooring company, All-Wood Flooring describes panelization on its website: "It is caused by flooring boards losing moisture to a drier interior environment and shrinking in groups. The groups of strips, or 'panels' may be any number of pieces and within this 'panel', boards remain tight together. This transfers the aggregate shrinkage, or total shrinkage for all the pieces in the panel, into large gaps between strips on either side of the 'panel'."

Solutions to panelization

Several options exist for addressing this problem. One approach is to maintain constant humidity between 35% and 50%, although this could be energy-intensive and impractical in a large space. Another architect suggested the following: "Spec edge-grain parquet rather than strip maple when LEED IEQc4.3 is a requirement. Edge-grain product works fine with water-based finish because it does not expand and contract on the horizontal plane."


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A third option suggested has to do with changing the application technique. "The typical finish technique is a pretty heavy application of the finish liquid, almost if not literally poured on the surface, with spreading by lambs wool pads. With the low viscosity of water based finishes, this results in a substantial amount of the finish filling the joints between boards leading to edge or side bonding. This in turn means the floor can't move as it is supposed to. The obvious solution to me is 'thinner' applications and more coats. This does mean added costs for the additional labor."

Another potential solution is to use prefinished flooring. This solution is problematic however, as Feagans writes. "We would recommend against any prefinished products for several reasons, including ball bounce [disrupted by grooves] and cleanability [grooves collect dust and dirt]. The bigger issue, however, is that in our K–12 projects, high-use sport floors get sanded down and refinished at least every 10 years, depending on use. Then the [flooring] company will more than likely use the high-VOC oil-based finish because that's what they are comfortable using and can stand behind it on their warranty (and lower price). And the architect likely won't be around to argue the point on the owner's behalf."

Recommendations from manufacturers and installers

The Maple Flooring Manufacturers Association (MFMA) recognizes panelization as a significant problem. In a statement on its website MFMA, cautions installers and end-users that "the use of some water-based finishes has produced a sidebonding effect that can result in localized excessive and irregular separations ('panelization') between maple flooring strips. We strongly recommend that end-users, project architects and specifiers consult with their flooring installer and finish manufacturer to obtain approved procedures for sealing and finishing a raw maple strip floor with water-based products." In other words, anticipate the problem and work with all relevant parties to take steps to minimize the risk of panelization.

Micahel Grimaldi, a flooring contractor and owner of All-Wood Flooring in Connecticut emphasized to GreenSpec the complexity involved in selecting the right product and ensuring compatibility, "with every product out there you have to consider the pros and cons of that product in conjunction with the environment and wood species to come up with the best compatible sealer and finish combinations. There are different sealers that change how I apply any type of finish to maple, but this goes even further back into the sanding that is used as well as the grits and how many cuts should be made and to what degree of fineness you stop at before apply sealers and finish. Too fine of a sanding results in sealers or finishes that can lift or peel, and not adhere properly."

Grimaldi told GreenSpec that he has found sealer/finish combinations that address the panelization problem. "What we have done is find sealer products that don't hold [i.e., they don't allow the flooring to lock up and panelize] as well and apply the finish to that to totally alleviate the problem." He went on to say that application technique for water-based finishes is important, "water-based [polyurethane] needs to be applied in more multiple thin coats rather than heavy coats." He said that application technique is important for all finishes, and that lighter more frequent coats with time allowed for proper drying are more effective.

Choosing water-based polyurethanes

Water-based polyurethanes come in one-part and two-part systems. As GreenSpec notes in our product guide to CSI Section 09 93 33 sealants and coatings, "One-part polyurethanes are less expensive but may not have the durability of two-part systems. Two-part polyurethanes contain a resin and a hardener or crosslinker. These polys are considered to be the toughest in the industry, but they are expensive and are usually only sold to trained professionals." Grimaldi concurred that the two-part water based polyurethanes are among the best coatings available, telling GreenSpec, "I love the product and use it on 75% of my jobs."

Successful application of these products requires care to prepare the floor surface properly, completely removing any old finishes, stains, sealants, or waxes that may be incompatible with the water-based product. In some cases a sealer will be needed to prepare the wood for coating. Care should be taken to ensure any sealing products are compatible with the finish that will be applied. Most manufacturers offer low-VOC sealers designed to work with their water-based coatings.

Know your LEED credits and check the addenda

LEED requirements for VOC emissions can be confusing and difficult to pin down. Always check the addenda to make sure the numbers you are working toward are accurate and up to date. One of the designers used as a source for this blog understood the VOC requirement for LEED IEQc4.3 to be 275 g/l or less, meaning that no solvent-based polyurethane could be specified.

However, after reviewing addendum number 100000419 (PDF), and the related discussion on titled "Perplexing Reference Guide language for Table 1, SCAQMD Rule 111," it is clear that for LEED 2009 projects the limit is actually 350 g/l. The following excerpt shared with GreenSpec by David Posada, Sustainability Manager at GBD Architects, both explains the situation and illustrates the complexity that LEED can present. "Even in 2011, projects pursuing LEED 2009 are to follow the VOC limits in Rule 1113 'frozen' at 1/1/2004, and not the lower limits that are scheduled for later effective dates. Thus, from page 483 of the 2009 BD&C Reference Guide, clear wood finishes would still be at the ceiling limit of 350 g/L since it was lowered to 275 on 1/1/05."

Striking a balance between performance and VOC emissions is important. While striving for lower emitting products is essential, this can backfire if those products fail and require further application of products with higher emissions. In the case of water-based polyurethanes, the lowest emitting products are also the highest performing. Make sure that contractors and installers understand and account for the risks of panelization. This may be especially challenging in areas that have experienced multiple instances of panelization, as contractors and clients are more likely to resist using the lower VOC coatings to avoid problems. However, if applied properly, water-based polyurethanes can work in most instances. The payoff for successful application of these projects couldn't be greater--creating durable, high-performing, and chemically safe surfaces for generations of kids to play on.

Published September 8, 2011

(2011, September 8). Water-Based Polyurethanes and Panelization of Athletic Floors. Retrieved from

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