Humidity In The Home

Relative Humidity in the Home

Relative Humidity (RH) refers to the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. For example, at 75ºF and 50% RH, the 75º air is 50% full of water. Even though the water content of the air throughout a building may be the same, the relative humidity can vary dramatically from one area to another based on temperature. Since warmer air holds more water than colder air, RH increases as temperature goes down and vice-versa. For every one degree change in temperature, the RH changes by about 2%. For example, when the 75º air that is 50% full of water is cooled to 74º, the RH rises to 52%.

Humidity and Hardwood Floors

There are many ways that water can cause damage to hardwood floors. Leaky pipes, weather related water coming into a home from outside, and even a spilled drink that doesn't get claned up can cause serious problems for hardwood flooring. But there is a less obvious culprit for issues that many homeowners have with their hardwood flooring: HUMIDITY. Wood will retain or lose moisture in accordance with the amount of moisture in the air. As relative humidity in the air changes, your hardwood flooring will change as well - slowly expanding and contracting based on the conditions around it. Some expansion and contraction is normal, but too much of either can cause serious problems. 

Optimum Conditions

When relative humidity is within the proper range for your hard wood flooring, it will stay flat and smooth with tight joints.

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Hardwood flooring during optimum relative humidity conditions.

Floor Cracking and Separation

In winter, when a building is heated and the air is dry, wood flooring loses some of its moisture. This causes contraction or shrinking of the wood which can lead to cracks and separation between boards. If the separation between boards becomes excessive or homeowners simply prefer to minimize cracks, they should add moisture to the environment.

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Flooring during dry conditions. Some separation between boards is normal and acceptable.


Cupping is when the edges of a board are higher than the center of the board. High humidity is often the cause as excessive moisture causes the wood to expand. As the boards expand and are compressed together, the pressure is relieved by the board deforming at its edges which results in the cupping.

floor illustration cupped1

Cupping occurs when the bottom of the wood flooring remains wetter than the top surface, which dries faster than the bottom.


Crowning is the opposite of cupping: The center of a board is higher than the edges. Moisture imbalance is sometimes the cause of crowning if excessive moisture is introduced on the top of the floor. However, a common cause is that the floor was previously cupped, but was sanded at the wrong time-before the moisture content returned to normal and the board flattened on its own.

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This illustration shows a cupped floor that was sanded on top while the bottom is still cupped. 

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Crowning occurs when the bottoms of the boards eventually dry and flatten out, leaving the tops of the boards with a convex profile.

Prevent Problems by Monitoring and Controlling Humidity

1) Be aware of the recommended relative humidity percentage for your hardwood flooring.

2) Monitor the relative humidity level in rooms where the flooring is installed with your TH1901 Thermo-Hygrometer. Remember, humidity can vary from room to room especially when there is a temperature difference, so a small, portable thermo-hygrometer is the ideal solution.

3) If humidity is too low, raise the moisture level with a portable humidifier, leave the dishwasher open after it runs, turn off the bathroom exhaust fan after a shower, or hang wet clothes to dry near the furnace. The ideal solution would be to install a humidifier in the furnace. 

4) If humidity is too high, a dehumidifier is often the best solution. If cupping has already occured, do not sand the floors flat unless you are sure that the boards have completely dried back to normal.

5) Clean and care for your flooring according to manufacturer recommendations. Avoid using mops or cloths that leave excessive water on the floor. Never let a spill of water dry on the floor.