Most problems associated with hardwood flooring are moisture related. Wood has a cell structure that enables it to take on and give off moisture depending on the moisture presented by the environment. Each wood species has a unique “shrinkage factor” which defines how much a certain species will expand and contract as the moisture content increases or decreases. Wood species are considered more or less stable based on whether they expand and contract more or less with changes in their moisture content. When wood absorbs or looses moisture below the Fiber Saturation Point (around 25-30%) wood swells or shrinks. To make matters worse shrinking and swelling is often accompanied by warping.

Wood has always been sensitive to moisture. It absorbs and looses moisture until equilibrium with the surrounding air has been reached. For every hardwood moisture content, there is one specific relative humidity and temperature, when wood does not gain or lose any moisture. This moisture content is called the EMC or equilibrium moisture content. Once the equilibrium moisture content (EMC) is reached and the relative humidity and ambient temperature are not changing, wood is dimensionally stable. Ideal conditions have been reached and no more shrinking or warping will occur.

Hardwood is expected to shrink in dry environments and expand in wetter environments. Wider boards tend to move more than narrower boards. Movement in a 5″ wide plank is more dramatic than in a 2 ¼” strip. Oak flooring is milled at 6-9%. Prior to installation, solid wood flooring should be acclimated in the area where it will be installed, and then tested with a moisture meter to ensure the proper moisture content. A 50% swing in relative humidity produces a moisture content change of 10%. How that affects wood flooring depends on which species is being used. The change in width is 1/16″ inch change in a 2 ¼” wide board. That’s a full inch over 16 boards in a floor. Over the width of a 10 foot wide room, that amounts to more than three inches of total expansion or contraction.

Even if the floor was installed properly and the hardwood flooring was dry at the time of installation, some wood movement can occur when the relative humidity changes with the change of seasons. Hardwood may shrink and small cracks appear during the dry winter months with the heat turned on. The cracks disappear during the wetter summer months.

Today, there better humidity and environmental control systems for interior living conditions than ever before, but not all homes have them. Humidifiers, dehumidifiers, air-conditioners, heating systems and outside ambient moisture conditions can all effect changes in the ambient moisture conditions which may, over time, effect changes in the moisture content of a hardwood floor. Proper acclimation of a wood flooring product prior to install will equalize the moisture content of the floor boards and subfloor to the conditions present at that time, but if those conditions significantly change over time then some corresponding expansion/contraction change in the wood could also be expected. Some seasonal gapping or cupping may result from such changes in the ambient moisture conditions. Controlling the relative humidity within the room will help eliminate this problem. Every hardwood flooring manufacturer has a recommended Relative Humidity Range for their hardwood flooring. If you’re not having this conversation with your customers, you’re only asking for problems. Keep the environment between 35-50% relative humidity. Maintaining a controlled environment is critical to the performance of the hardwood flooring product.

Below are examples showing the difference in the equilibrium moisture content (EMC) between dry winters and moist summers:
70oF and 75% relative humidity, wood moisture will reach 14%.
70oF and 35% relative humidity, wood moisture will reach 7%.
70oF and 20% relative humidity, wood moisture will reach 4.5%.

Thanks to JJ Haines for the info!