March 16, 2022

Hardwood Flooring and Moisture Control

Hardwood flooring adds character and value to any home, but proper care and maintenance must be taken to ensure your hardwood flooring will last for decades to come. It's important to remember that hardwood floors are a natural product, and are capable of storing large amounts of water in the structure of the wood. 

 

Moisture in Hardwood Flooring

Being a product of living trees, hardwood can store moisture in two ways: the first is free water that is found in the cell of the wood, similar to how water is trapped in a sponge; the second is bound water, which is chemically bound to the wood structure.An image of wood cellular structure with locations of free water and bound water indicated.

Wood flooring will perform best when the interior environment is controlled to stay within a stable environment and the wood is installed at a moisture content corresponding to those interior conditions. Most wood flooring manufacturers dry their flooring to 6-9% MC, which directly coincides with a relative humidity range of 30-50% and a temperature range 60 to 80 degrees Fahrenheit. This 6-9% range is likely to be the average of all types of wood products used in a normal household environment, assuming common heating and cooling equipment is used to ensure human comfort.

According to the National Wood Flooring Association (NWFA), Canada has a relatively consistent "moisture map", where the moisture content of the wood should be kept. In the provinces of British Columbia, Alberta, Saskatchewan, and Manitoba, the moisture content of hardwood flooring should remain between 6-9%, while Ontario and Quebec should fall between 6-11%, and Nova Scotia 8-12%. The exceptions being the North Coast region of British Columbia (9-12%) and Northern Quebec (5-9%). 

Testing Moisture in Wood

Moisture in hardwood floors can be tested using devices called moisture meters. These come in different forms, but the most common are pinless (also referred to as non-destructive or non-invasive) and pin-type. Pinless moisture meters are placed directly on top of the hardwood floors and can usually measure to a depth of 1 inch, which is enough for most hardwood floors. Pin-type meters are destructive and are usually required to place the pins in the direction of the grain of wood. 

With both types of moisture meters, the moisture content of the hardwood floors should be recorded in the first 2-3 seconds of reading, to avoid instrument drift.

Installation Considerations

There are a few things to consider before installing hardwood flooring, with regard to moisture content and humidity levels. The first is where you will be installing the flooring, i.e. on top of wooden subfloor or over concrete (such as in a basement). 

Wooden Subfloors:

It is important to check the moisture content of wooden subfloor before installation. Due to the variance in material and glue used in subfloors, readings of moisture content can also be taken from surrounding wood features, such as wood beams, 2x4's, etc.. Subfloors should be conditioned to the expected end use moisture content levels to ensure no damage is done to the hardwood floors to be installed. 

Image of wooden subfloor that has been damaged by water resting on the surface.

In order to reduce the potential for vapor or moisture related issues, a vapor retarding barrier can and should be used. However, it is essential to not use an impermeable membrane, since this can trap moisture with the hardwood, and lead to subfloor rot, decay, and mold growth.

Concrete Sublfoors:

Concrete subfloors should be checked for moisture content regularly 30 days after it is poured. 

One common method of testing concrete subfloor moisture content is through the use of the Calcium Chloride Test (ASTM F1869), as well as Relative Humidity Probe Test (ASTM F2170). Other methods also exist, such as electronic probes (ASTM F2659) and Plastic Sheet Test (ASTM D4263). 

The #1 thing to remember: Always follow the manufacturers instructions for moisture control and vapor retarding systems before installing hardwood flooring over concrete subfloor.

Sources of Moisture

Sources of moisture in a house can vary greatly, and include but are not limited to:

  • Ground water 
  • Crawl spaces
  • Surface water
  • Generated within the house (mopping floors, baths/showers, cooking, etc.)
  • HVAC systems

As we talked about before, ground water effects can be mitigated through the use of a proper vapor barrier below the concrete slab before it is poured. Probably one of the easiest sources of moisture to mitigate is the moisture generated by activities such as mopping. Wet mopping in general should not be done frequently, and water should not be left standing on hardwood floors to avoid absorption into the hardwood.

A very common issue seen is also through improper HVAC system use. The NWFA suggests the following important points with regards to HVAC systems:

During humid seasons and in humid climates (when the average RH remains above 50%), dehumidification systems may be necessary if air conditioning alone does not control the RH levels within the home.

  • During dry seasons and in the arid climates (where the average RH remains below 30%), supplemental humidification may be necessary in the home to sustain adequate RH levels.
  • Rooms or basements where HVAC vents are closed-off and are not conditioned to the same temperature and humidity levels as the rest of the interior space can result in sub-climates within the home, which can have adverse effects on any adjoining floors within the home.
  • When air conditioning and heating systems are not used or are completely shut down for an extended period of time, the air exchange necessary for the performance of the home and the wood floor is sacrificed. Floors will shrink or swell due to this limited air movement and inconsistent humidity levels. Sunlight through windows can generate heat, creating abnormal humidity levels, which may fluctuate from day to night. Controlling the atmosphere during and after the installation is critical to avoid issues. This is often referred to as the greenhouse effect.

The relative humidity of a home can vary greatly depending on seasonal conditions and sun exposure.

 

Reference:

National Wood Flooring Association Technical Publications. Photos contained within this post are courtesy and copyright of the NWFA and not owned by Magna Hardwood Floors International Inc.

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