Moisture and Hardwood Flooring: Achieving Equilibrium
By: Jeremy Tate Jun 30, 2016 Category: Moisture

Equilibrium is defined as “a state in which opposing forces or influences are balanced.” This balance is crucial to ensuring a floor will be stable in its environment. Moisture management can be one of the biggest issues facing a flooring contractor. Failure to account for issues such as moisture content, relative humidity, and seasonal variation can cause a myriad of problems. The ability to identify potential issues and select the proper remedy can be the difference between making money and losing money on a job. The following is an outline of how to best deal with moisture in differing situations.

Jobsite Inspection

The National Wood Flooring Association (NWFA) recommends first ensuring that the building is enclosed, and heating and air conditioning is operational. Optimally, the temperature should be between 60 and 80 degrees Fahrenheit for at least five days before the start of installation. Relative humidity should be between 30% and 50% during this time. In this “sweet spot,” flooring is most stable and less susceptible to dimensional fluctuations. If permanent heating and air is not available, portable heaters, dehumidifiers, and fans can be used to assist in maintaining target temperatures until permanent heating and A/C is operational.

Next, the moisture content of the subfloor and flooring material should be checked. For strip flooring (3.25” wide and smaller), moisture content of flooring material should be within four percentage points of the subfloor. For plank flooring, moisture content should be within two percentage points of the subfloor. Ideal moisture contents vary throughout the country depending on climate but are typically between 6%-9%. These parameters assume the use of an industry-approved moisture barrier between the flooring material and subfloor (more on this later). If the moisture contents are outside of these guidelines, there should be an acclimation period for the flooring material. This acclimation should occur on the jobsite and should be in “normal living conditions.” Industry standards suggest flooring should be separated with sticks or stacked in a way that promotes air circulation about the bundles of wood. According to the NWFA:

The worst-case scenario is one in which wood flooring is stored at the jobsite in an uncontrolled environment — especially one that is subject to excessive moisture and humidity conditions. It does no good at all — in fact it is likely harmful — to store wood flooring at the jobsite under conditions that don’t reflect those normal environmental conditions. Garages, basements and exterior patios, for example, are not acceptable areas to store wood flooring.

There is no set length of time that an acclimation period should last. Once the relative moisture contents of the flooring material and the subfloor are within the suggested parameters, the flooring is ready to install. Attention should always be paid to the manufacturing guidelines for special considerations of some materials. Ignoring these guidelines could void the manufacturer’s warranty.


Before installation is not the only time a contractor must be concerned with moisture. Moisture can cause problems after installation as well. If the above guidelines are ignored or if excess moisture is introduced to the jobsite, floors will swell which will result in cupping. Cupping is when the edges of the flooring are higher than the center of the boards; the surface has a concave shape (Wood Floor Cupping). Cupping is not always necessarily a problem. Slight cupping is to be expected with wood flooring in the summer as moisture contents naturally rise resulting in the wood swelling a bit. Floors are expected to then shrink in the winter resulting in visible cracks. (The amount a floor will swell/shrink depends on species and width.) Cupping is only a problem when the problem persists year round. If a cupped floor is sanded when excess moisture is also present, the floors will result in a crown when the excess moisture drains and the jobsite approaches an equilibrium moisture state.

Ideally a cupped floor should be allowed at least one full seasonal cycle to both observe the effects of the changes of moisture in the floor as well as to allow excess moisture to exit the jobsite. For some floors, this time will be necessary to fix the problem. If the cupping persists through the seasonal cycles then a resand may be necessary. It is best for this resand to occur outside of extreme outside temperatures.

Moisture Barrier

An NWFA-approved moisture barrier should be used over wood subfloors. Underlayment papers such as Aquabar “B”, HWD-15, and Silicon Vapor Shield are commonly used in the wood flooring industry. These papers all have permeation rates of between 0.7 and 10, which is the industry standard. Use of underlayment paper helps to reduce moisture-related problems but does not completely eliminate moisture or moisture-related problems (NWFA).

Higher numbers for perm rates indicate higher rates of moisture passing through the moisture paper. Use of a underlayment paper with a perm rate below 0.7 can trap water in the subfloor or between the paper and the flooring which has the potential to cause cupping. Use of paper above 10 perms can cause too much moisture to pass through the paper making it ineffective. According to their respective data sheets, Silicon Vapor Shield has a perm rate of 0.71 (with nail holes). Aquabar “B” has a perm rate of 0.87 perms. HWD-15 has a perm rate of 5 perms.

Underlayment paper should be rolled out on top of the subfloor before racking out hardwood. The NWFA recommends overlapping the paper about 4 inches. An added benefit of using underlayment paper is it allows wood to slide more easily across the subfloor. This allows for wood to be more easily positioned which increases workability.

Another difference between the papers is the ability to use with radiant heat. Silicon Vapor Shield is able to be used with radiant heat. Aquabar “B” is also able to be used with radiant heat but has a maximum heat of 85°F. HWD-15 is not recommended for use with radiant heat. For more information regarding underlayment paper, please read the NWFA Installation guide as well as the respect data sheets for the products: NWFA Installation Guide, Aquabar “B”, HWD-15, Silicon Vapor Shield .

Crawl Space

Crawl space conditions can be a major source of moisture-related issues. This is an oft-overlooked factor in hardwood flooring installation. Wood flooring exposed to excess moisture from below will result in cupped floors as the board will swell in response to a higher moisture content. Moisture barriers can help alleviate the effect of changing moisture conditions in the crawl space but has its limitations. Excess moisture in a crawl space can be caused by a variety of reasons: Failure to properly install ground cover, inadequate drainage resulting in standing water, inadequate crawl space ventilation, improper pipe insulation, among other factors can cause relative humidity and moisture content to be above ideal conditions causing cupping and in extreme cases complete failure of flooring.

Myth of “Bad Wood”

When wood flooring fails, the wood itself is sometimes blamed as being “bad wood.” Wood flooring is a hygroscopic material that is influenced by its environmental conditions. If industry and manufacturer-recommended guidelines for installing and maintaining floors are disregarded, wood floors will often fail. This is not the result of “bad wood” but a natural reaction of wood flooring to high moisture contents. Wood swells when it is exposed to high humidity, and shrinks when it is exposed to low humidity.

Just because a contractor “has been doing this for 30 years and has never had this problem” does not mean the flooring can be blamed. The wood flooring itself should not be blamed when job site conditions are problematic. Flooring contractors need to evaluate each job and communicate which factors could be problematic and what the best way to deal with the factors are. Attention should be paid to the order in which flooring is installed relative to other parts of the house. Flooring should NOT go in right after the drywall is hung if the conditions are not right, and they rarely are. Flooring is most likely to succeed when it is installed later on in the project after ground cover has been put down and moisture contents are in the target ranges and optimally when the air conditioner is running. Paying attention to these details will help eliminate many of the problems faced by a flooring contractor.

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Moisture and Hardwood Flooring: Achieving Equilibrium
By Jeremy Tate on 30 Jun 2016