Understanding Equilibrium Moisture Content

- After a tree is cut down and sewn into lumber the wood reacts to its environment especially the moisture in the air.

In the wood industry, moisture is an issue that needs to be dealt with before a product can be produced.

That's why lumber is killing dried before it is used.

Many people do not realize how moisture in the air affects wood even after it's dried.

Understanding equilibrium moisture content or EMC is key to avoiding moisture problems in wood after it's dried.

EMC is the moisture content the wood would eventually reach over a period of time.

Here is a cracked door panel from a Chestnut wash stand that most likely was affected by moisture fluctuations throughout the years.

Wood is hygroscopic which means it will gain or lose moisture until it is in equilibrium with the moisture in the air around it.

EMC is related to temperature and relative humidity.

If the temperature remains constant and the humidity increases, the EMC will increase.

So for example, at 70 degrees with a relative humidity of 40%, the EMC is 7.7%.

If the temperature remains at 70 degrees and the humidity increases to 77%, the EMC is 14.9%.

How does this affect the wood?

Because wood is a hygroscopic material, improper storage of dry lumber will cause the moisture content to increase or decrease.

For example, if wood is dried to a 7% moisture content but stored in an area with a higher EMC it will begin to gain moisture until it reaches the EMC for the conditions in which it is stored.

The opposite is true if it is stored in an area with a lower equilibrium moisture content.

Problems can arise with wood products that are not stored or used in an area with the proper EMC.

The picture frame in this photo was made with wood that had a moisture content that was too high for the conditions where the frame was being stored.

You can see that the gap in the joint increases over time.

This was caused by the lumber shrinking after the frame was made.

If it had gained moisture the gap would have been on the outside corner rather than the inside corner.

Wood begins to shrink as its moisture content starts to get below about 30%.

Most hardwood lumber is dried to around seven to 8% moisture content when most of the shrinkage has already taken place.

Wood used or stored for the EMC is higher than the moisture content of the wood will swell.

If the EMC is too high the swelling could cause joints to fail or warping the product that's made from the wood.

The same thing can happen in reverse.

If wood is brought into a much drier environment and loses enough moisture, the shrinkage could calls work or joint failure in the product.

In Pennsylvania, the outdoor EMC ranges from about 11 to 14%, depending upon a time of the year and the weather conditions.

The same would be true in a building such as a shed that has no heat or air conditioning.

So storing lumber that was killed dried to 7% moisture content in a shed with no climate control would cause the lumber to increase in moisture content over time.

To illustrate how EMC can affect wood, let's take a look at a real world example.

The lumber that was used to build a table in this picture was dried to 6% moisture content.

It was stored in an area that had an EMC of around 13% for about a year before the table was made.

In the winter when the heat comes on in the home which has forced air gas heat, the EMC in the house drops to around 4%.

As a result, the table begins to lose moisture and the top begins to work, which you can see here when a straight edge is laid across the tabletop.

In fact, there was so much pressure created that drawer can not be opened.

Notice how tight the drawer is to the tabletop just above the drawer knob and the obvious gap on the right side of the drawer.

In the summertime, when the humidity increases the tabletop begins to gain moisture and swell relieving the pressure on the drawer, and it can be opened.

Understanding how moisture even in the vapor form affects lumber is important not only for diagnosing wood problems but also in preventing those problems from occurring in the first place.

Properly drying and storing lumber before it is manufactured into a product can save you trouble in the long run.