Water, water vapor, humidity and moisture all have something in common: they all damage wood floors. The opposite is true when you have extreme cold and dry weather. This can also damage wood floors. A lot of people do not know why their floors are suddenly swelling up or shrinking. People often do not know why their floors suddenly have gaps in the floorboards or are buckling up. It is most often a problem related to moisture.
Humidity swings will always affect wood. Humidity swings are fluctuations or quick changes in the humidity. This is often related to weather. You could have several days of hot weather with lots of humidity or several days of extreme cold and bone dryness. Both of these extremes will affect your wood floors.
Wood naturally sucks up moisture from the atmosphere. Scientifically speaking, wood is hygroscopic. Hygroscopic simply means that not only does wood take in moisture but it also retains it. Water usually will not escape the wood on its own. Wood will expand when it comes in contact with water as the water seeps into the wood. Even after the wood has dried, it will still be affected by the water.
To understand it from the manufacturing side, when wood leaves the sawmill, it leaves "green". It typically has a moisture content of around 30%. This moisture level is too high for wood floors as it will dry out and shrink. Floor manufacturers dry out wood to a level of between 5 to 8%. This is according to NOFMA (National Wood Flooring Association) standards. When the wood is dried out from 30 to 9%, the wood loses approximately 7% of its size.
Wood floors are going to be at their the best in temperatures between 68 and 72 degrees and a relative indoor humidity of 40% and 60%. This is ideal all year-round. Fortunately, that is comfortable for most people as well. To save money in the heat, people open their doors and windows to let in the cool air. Little do they know, they are bringing in the humidity. This will cause your floors to buckle and pop. Also, if you live in a climate like Florida and decide to conserve energy when you go away for a few weeks by turning off your air conditioner, you may come back to hardwood floors that have swelled up because of excess humidity. You need to keep your home at a relatively consistent temperature.
I am constantly asked about warped hardwood floors. Warped floorboards occur when one face (side) of the board has significantly more moisture than the other. Almost every time there is cupping and crowning, it is because of moisture in the wood. When you have different amounts of moisture in a floorboard, the wood will expand and contract. If you live in a climate where you experience humidity swings often, you may see it in your hardwood floors. It is the warm, humid weather that makes wood expand. Dry weather makes hardwood floors contract. This will happen to wood floors, no matter how new or old they are.
Now that you understand the effects of humidity on wood floors, it is a good idea to detect moisture in floorboards. When floorboards warp because of humidity, it is known as cupping and crowning. Cupping is when the floorboards warp upward. Cupping is concave. The floorboards are raised up because of the higher than normal moisture content. The floorboards expand and puff up because the moisture in them. The edges of the floorboards are raised. You can see this in any style of wood floor.
Imagine crowning to be like the opposite of cupping. Crowning is concave. Crowning is when floorboards warp downward. A floor crowns because it loses excess moisture. A cupping floorboard has a shrunken underside. The cupping floorboards flattens in the middle. It makes the edges of the floorboards lower.
Not all floors warp the same. An older floor will warp less as it already has gaps in it. A new wood floor that was laid with small spaces will warp less because there is more room for the wood to expand. A wood floor laid very tightly will warp and buckle more as it has no place to move when the wood swells with moisture.
Now that you understand how moisture affects wood floors, I recommend reading on to learn about what you can do to prevent moisture problems or how to deal with them in my article "Humidity Swings and Wood Floors".