Humidity

No matter how strong or expensive your hardwood floors are, they can still incur damage as a result of exposure to extreme changes in humidity. This is also true of bamboo, laminate and cork floors, as they are all manufactured with woody materials (e.g. cork is made of bark; bamboo is made of grass shoots). Spills should be cleaned up immediately.

The more moisture there is in the air, the more moisture hardwood floors can absorb. When hardwood planks soak up moisture, they expand and swell up. When the humidity level drops below 35%, any moisture within the hardwood planks dries up, consequently causing the planks to shrink. You can safeguard your floors from this predicament by maintaining a steady indoor humidity level. One of the ways to accomplish this is by investing in a dehumidifier (for high humidity) and/or a humidifier (for low humidity). For example, indoor humidity tends to be higher on rainy days. On these days you can use your dehumidifier to help reduce indoor humidity to a more agreeable level. Ideal indoor humidity level for hardwood floors is somewhere between 35% and 60%. 

Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature. Humidity may also be expressed as specific humidity. Relative humidity is an important metric used in forecasting weather. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.

The Relationship of Wood and Water
First and foremost, it's important you understand the relationship between wood and water. Trees contain vertically-aligned fibers, also known as vessels, that work in similar function to a soda straw. These vessels draw water from the ground and transport it up to the tree's branches. Although these fibers dry up once the wood is cut, they can still absorb water and moisture from the air. This characteristic is the reason floors made with wood materials expand and contract in low humidity and swell in high humidity.

Ideal humidity levels vary by geographic region, but should generally be kept in the 30 to 50 percent range. Although slow, seasonal fluctuation is normal and expected, rapid and extreme humidity changes can damage a floor.