Mastering Moisture in Your Subfloors

Photos courtesy of Sika Corporation

Many years ago, a customer of mine called me to look at her mother’s hardwood flooring. She told me there was some serious buckling and warping, but nothing could have prepared me for what I was about to see. The solid hardwood was installed by a general contractor who did not account for moisture. As a result, this wood floor looked like the waves of Lake Michigan. In some areas, this floor buckled 10” off the floor. The force of the hardwood buckling actually broke the wall base in some places. I performed some moisture testing (applicable ASTM test methods), and unfortunately, discovered that moisture was the culprit. For some professionals, managing substrate moisture can be a mystery. Let’s unravel some of it here.

So, you’ve performed your testing according to ASTM test methods and discovered moisture (or the potential for moisture) in your substrate, but what are your options to deal with it? The answer is not always simple because there are a lot of variables, including subfloor type, installation method, and more. Of course, the most important thing you can recommend is to stop the moisture at its source whenever possible. Assuming that has been attempted, and the situation hasn’t improved, the industry has several different ways to mitigate or slow the passage of moisture, so let’s start with the simplest remedy of high moisture adhesive.

Modern adhesives can be an answer to moisture present in a substrate, especially concrete. Sometimes special trowel notches are used, or spacers are embedded in the glue to help to achieve a certain film or layer thickness during the installation process. However, certain care must be taken to understand if this application will achieve the results you’re hoping for. Some high moisture adhesives claim to control moisture, while others simply tolerate it and don’t offer protection to your hardwood flooring itself. It also is important to read the instructions and the warranty literature to make sure the concrete slab is prepared properly, does not require profiling, and the application techniques are correct before you commit to this method. High-moisture adhesives generally are more expensive than standard adhesives, but could be part of your solution.

When it comes to concrete substrates, true moisture mitigation often is considered the best line of defense against moisture vapor emissions coming from the slab itself. This is especially true of the older slabs where an under-slab vapor barrier may not be present or has degraded due to age, or even a new concrete slab where the added moisture content, drainage, or grading, as well as environmental conditions, could contribute to the problem. Some products advertise easy application without the need for grinding or shot blasting to achieve a profile, but this can be misleading. Once again, make sure you read all the installation details, including the cleaning instructions because there are many contaminants and impurities on the surface of a concrete slab that will prevent a moisture mitigation product from doing its job properly. The process of moisture mitigation application can range from simpler to more involved, but generally, they’re all teachable if you’re willing to learn. Your local manufacturer’s rep can provide training to get the best results. These are some of the more-common types of moisture mitigation systems.

It is important to have your moisture testing results so you can compare them to the solution and be sure the product will meet or exceed your test results.

  • Acrylic and latex-type products often are less expensive and may offer some protection, but only up to a certain level.
  • Urethane solutions can offer even higher protection and sometimes apply in one to two coats.
  • Epoxy two-component systems offer the best protection up to 100 percent relative humidity in most cases. Certain precautions must be taken during installation for proper disposal and safety. Epoxy application is something that can be learned and is an additional skill set that can help you stand out amongst your competition.
  • Plastic or poly sheeting still is utilized in properly prepared crawlspaces where hardwood will be installed over plywood or OSB substrates. This is a simple and cost-effective method, but ensure any flashing, overlapping, or taping of the seams is done thoroughly with no places for the moisture to escape. Also, pay attention if any cross ventilation is present as recommended by NWFA guidelines or manufacturer recommendations.

When it comes to traditional plywood subfloors, there is a newer method available referred to as liquid-applied vapor retarder. Traditionally, hardwood floors have had either tar paper or a similar semi-permeable vapor retarder installed onto the plywood subfloor using staples or fasteners. Aside from the additional time installing the paper, if a glue assist was required or recommended by the manufacturer, in the past, you had only two options.

The first option is to remove this vapor retarder completely because a bead of adhesive on top of a slip sheet of paper does not do any good. I admit I was never fond of this strategy because I have now eliminated the protection I had from moisture in the substrate. The other option is to cut slits into the vapor retarder to allow the adhesive from your glue assist to contact the plywood substrate underneath. The problem here is obvious: we now have slits or holes in our vapor retarder anywhere we have applied the adhesive.

These updated methods for semi-permeable vapor retarders allow for a single coat applied to the plywood or OSB, often dry within an hour, and then allow for the use of penetrating hardwood fasteners, including staples or cleats with the addition of glue assist. While these products cost more than a paper roll, a great deal of time can be saved by eliminating the installation of the paper itself and having a more solid method for your glue assist hardwood installations. Solving the common problems of substrate moisture does not have to be a mystery when you have access to a variety of solutions.

Jeremy Waldorf is regional business manager for Schönox HPS North America, LLC in Michigan and Toledo, Ohio. He can be reached at

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One thought

  1. Excellent article. The information was presented in an accurate, concise, and simple manner. The mitigation methods discussed in this article can be applied by any competent installer.

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