Vapor Permeability of Wood Floor Finishes

“Wood finishes can slow changes in the moisture content of wood, but cannot entirely prevent moisture from affecting an installed wood floor.”

A few years ago, some friends of mine were refinishing an old gym floor in a small-town high school. They called me to come take a look at an odd anomaly happening with the floor during the resand, and give them my thoughts.

The floor was about 50 years old, and hadn’t been resanded in more than 30 years. As with most high school gyms, the floor received a couple fresh coats of finish every summer. Doing the math, this would equate to somewhere between 60-65 coats of finish on this old floor. The gloss was nice, but due to the changing of the school logos, and some new funding, the time had come to resand, repaint, and update to a more durable water based finish.

The reason I was called to look at the floor was because, despite this being a vintage 50-year-old wood floor, during the resand process, the floor started gapping and slightly cupping after all of the old finish had been sanded off. Keep in mind, this floor was being resanded during the summer months, when there was no air conditioning, and high humidity levels.

The guys were ready to begin applying their first seal coat, but afraid of what the floor was doing, why it was doing it, and how it could affect their finish and paint application processes. (If you’re in the business of refinishing gym wood floors, you may have seen this before.) The reason this happens is pretty simple. The floor had been suffocated and protected from the surrounding elements for nearly 30 years. Once the old finish had been sanded off, the floor was re-introduced to the surrounding conditions in the gym and able to breathe again, similar to coming out of the water for a breath of fresh air. Having 60-65 coats of poly on the floor had virtually created a nearly impermeable plastic membrane over the wood floor.

Let’s take a look at how this happens.

Moisture vapor moves through materials naturally. The more porous a material is, the easier it is for moisture vapor to flow through it. Vapor permeance is a property that describes the ease with which vapor molecules diffuse through a material. The ability of moisture vapor to freely travel through a floor system is based on what’s in its way.

With all of this finish build-up on the surface of the floor, it is protected from all of the surrounding conditions, right? Well, we know that moisture content changes in wood flooring can be slowed, but not entirely prevented, by protective coatings. However, we really have never driven into what this actually means.

For starters, it all depends on which direction the moisture vapor travels. In a residential floor setting, the floor assembly is the primary component inside of the home that directly affects the performance of the wood floor. The least vapor-permeable component of an entire floor assembly (subflooring, insulation, vapor retarders, and flooring) determines how easily moisture vapor will be able to travel through it. This entire assembly will only allow moisture vapor to flow through at a rate based on the least permeable product within the system. Where the permeability is very low (i.e., < .1 perms), moisture can accumulate at this point, potentially leading to decay, mold growth, or fastener corrosion at that point within the assembly.

Some flooring manufacturers have gone as far as to seal all six sides of the flooring planks to help minimize the rate the flooring takes on moisture. Likewise, many OSB manufacturers have developed “high-performance” panels that have been sealed on all six sides to minimize the rate they take on moisture.

The U.S. Department of Agriculture, Forest Products Laboratory has published the ratings for what is known as Finish Moisture-Excluding Effectiveness (MEE) for some of the more common finishes used on wood. MEE of a finish is a measure of its resistance to diffusion of water vapor (that is, a measure of the permeability of a coating to water vapor) as a way of evaluating and comparing different types of coatings. A coating with a high MEE has a low vapor permeance, and a coating with a low MEE would be more permeable. This MEE value depends on a number of variables including: the coating film thickness, defects and voids in the film of finish and of the wood it was applied to, length of exposure to moisture, and others. Obviously, the MEE of any given finish is that only of the film of finish itself, and does not take into account gaps between boards, or holes in the surface of the boards.

This data is useful but has not been directly applicable to our industry, as the focus is primarily with exterior applications. So what does the MEE, or permeability of finish have to do with the long-term performance of a wood floor?

In the gym floor case I reviewed with my friends, we knew the moisture control system below was completely intact and doing its part to block moisture from infiltrating from below. The ambient conditions within the space above the floor are what caused the erratic movement within this floor once the finishes were removed. What we knew was that this floor was completely encapsulated until the removal of finish occurred.

Where moisture control below the floor is not intact, and moisture is able to find its way into the wood flooring, the floor will begin to cup. Where moisture control below is intact, a floor will only be affected by the conditions above the floor. From the consumer’s perspective, it is often perceived that more finish equates to better protection.

The point of this article is not to suggest applying more and more finish to your customers’ floors as a method of moisture protection.

The point of this article is to shine a light on the importance of understanding the flow of moisture through the flooring system, how moisture from above or below can affect a wood floor. Finishes can slow the effects of seasonal moisture changes in wood flooring, but will not prevent the potential damage caused by an improperly maintained facility.

7 thoughts

  1. I wonder if any moisture readings were taken through the planks. Gaps and cupping in the planks seems to indicate a moisture loss and low humidity cupping, rather than a moisture gain from the air above. Is it possible that the problems were caused by a loss of moisture from the planks after the finish was removed?

    1. I would agree with this idea. It might not have showed up on a moisture meter, but a slight loss of MC from the surface of the board after sanding would cause an imbalance between the top and bottom of the boards such that they would deform in flatness. They would also shrink some in width, and gaps would appear.

  2. Hi John and Tom,
    Absolutely a possibility! The flooring went through a moisture shock when exposed to the elements. What we know is that the cupping was caused by a moisture imbalance through the thickness of the wood, that had previously been protected by a nearly impermeable membrane of finish.
    Thanks for your comments!

  3. I would like to understand how we can study this information and provide a universal set of guidelines? Without an accurate protocol and training of mil thickness development in the sand and finish process, a contractor can possibly contribute to moisture imbalances in a wood floor. I am glad that this is in the discussions and is good fruit for thought. I hope that if a study is done, it is done so with the purpose of preventing problems by education. And, that it is not sidetracked by the politics of releasing any data that would effect sales of a product.

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