The Behavior of Wood Flooring
As per the National Oak Flooring Manufacturers Association
Any article made of wood will continually expand and contact with changes in relative humidity. Wood, being a hygroscopic material, when exposed to air will dry or pick up moisture until equilibrium is reached between the humidity and the temperature of the air. Moisture absorption causes wood to expand and moisture loss causes wood to shrink.
Generally, wood flooring is expected to expand in wetter climates and shrink in dry climates. How much depends on your particular area and the conditions surrounding the installation. In New Mexico, during the winter months, two things happen. As the temperature drops, the humidity drops. When the temperatures drop homeowners turn on their heating systems and keep doors and windows closed. This heat dries out the home and that, combined with the already low humidity levels, pulls moisture from the wood, causing shrinkage and cracks between boards. This can be somewhat minimized by having a humidifier attached to the heating system and maintaining a relative humidity of 35 to 45 percent whenever possible during the heating season. As the temperature and humidity changes occur in the summer, cracking should remedy itself. As moisture is also absorbed back into the wood, expansion occurs and wood floors return to their original state before the winter season.
It is recommended that you do not fill your wood floor during the winter months When the wood expands in the summer, the wood will either force out the excess fill or boards will warp. Wood must go somewhere as it expands.
Finally, it is important to remember that shrinkage is not a flaw in the wood or an installation related problem. The material is just reacting naturally to changes in moisture levels.