Unlike the brick, drywall, or roofing on your home, the wood flooring installed on your floor is breathing. Yes, it is alive! The very product that creates the warm homey feeling that you walk across every day, which should last the life of your home, is constantly in change. Wood is hygroscopic, comprised of millions of tiny open cells, which are like sponges. Depending on moisture in the air, wood fiber is either accepting moisture and growing in dimension or dissipating moisture and shrinking. This process is similar to a person inhaling and exhaling, except wood flooring is transferring moisture, not oxygen.
In most parts of the country, wood flooring is inhaling (accepting moisture) in the spring and summer, and exhaling (dissipating moisture) in the fall and winter. As this occurs, dimensional change takes place potentially resulting in edge compression and swelling in the humid months and shrinkage resulting in gaps between boards in the dry months. However, this action is not limited to finished floor boards; it also affects the very framed structure the boards are attached to, which compounds the movement.
In many wood framed homes, large gaps between floor boards develop over central framing beams in the winter. This is where two independent floor joist systems meet. Instead of shrinking from the exterior walls, these systems pull apart slightly at an interior junction. Also, where warm air is forced through floor vents, the vent slightly restricts airflow and forces super-heated air down the tongues and grooves of individual boards which can cause gaps around the vent.
During the heating season, some of this may be minimized by use of humidification (adding moisture back into the air), however, when outside temperatures are cold and heat is introduced for comfort, solid wood is going to shrink. The wider the boards, the larger the gap will be. Normal seasonal shrinkage is defined as a gap in the heating season which closes during the humid months. A dime-size gap between 2¼" solid floorboards is considered a seasonal gap if it closes in the summer.