Why Does Wood Shrink and Expand with Temperature Changes?

Wood is a natural material with unique properties that make it both beautiful and functional for a wide range of uses, from furniture to flooring to construction.

However, if you’ve ever noticed that your wooden floors creak or your wooden furniture warps in response to temperature changes, you may have wondered:

Why does wood shrink and expand with temperature changes?

Understanding the science behind wood’s behavior in varying temperatures can help homeowners, woodworkers, and designers better manage their projects and preserve the integrity of their wooden items.

In this article, we’ll explore the reasons why wood shrinks and expands, how it responds to environmental changes, and what you can do to minimize these effects.

The Science Behind Wood’s Movement

Wood is a hygroscopic material, meaning it naturally absorbs and releases moisture from the air.

This moisture absorption or release affects the wood’s internal structure, causing it to expand or contract.

Temperature changes often coincide with shifts in humidity, making wood’s expansion and contraction a response to both factors: temperature and moisture levels in the air.

To understand why wood shrinks or expands with temperature fluctuations, let’s break it down:

1. The Structure of Wood

Wood consists of cellulose fibers, hemicellulose, and lignin.

The cellulose fibers are hydrophilic (water-attracting), and the spaces between them, known as the cell lumen, can absorb or release water.

When humidity increases, the wood absorbs moisture, causing the fibers to swell. Conversely, when the air is dry, the moisture within the wood evaporates, leading to shrinkage.

In terms of temperature, warmer air can hold more moisture than cooler air.

Thus, when the temperature rises, it can cause the moisture in the wood to evaporate more quickly, leading to shrinkage.

On the other hand, colder temperatures often correlate with drier air, which can also lead to the wood contracting if the moisture content decreases.

2. The Role of Humidity

The primary factor influencing the expansion and contraction of wood is relative humidity, which is directly impacted by temperature.

As the temperature increases, the air’s capacity to hold moisture increases, leading to a drop in relative humidity.

When the relative humidity decreases, the moisture in the wood evaporates, causing it to shrink.

Conversely, when the temperature drops, the air’s ability to hold moisture decreases, which can raise the relative humidity and cause the wood to absorb moisture, leading to expansion.

For example, in the summer months, when temperatures rise and humidity levels are higher, wood will absorb moisture and expand.

In the winter, when temperatures drop and humidity levels fall, the wood will release moisture and contract.

3. Wood’s Grain Orientation

Wood doesn’t shrink or expand evenly in all directions. The way wood behaves depends on its grain orientation:

• Radial and Tangential Shrinkage: Wood’s shrinkage is most noticeable along the tangential and radial directions (across the grain). The longitudinal direction (along the grain) is much less affected by moisture changes, which is why wooden boards often expand and contract across their width and thickness rather than length.
• Tangential Movement: The tangential direction refers to the direction parallel to the rings of growth in the tree. This is where the greatest movement occurs. A piece of wood will expand or shrink most significantly across its width as it absorbs or loses moisture.
• Radial Movement: Radial movement refers to the direction going out from the center of the tree. This movement is less than tangential but still significant.

Because of this, when wood is cut into boards or planks, the width and thickness of the pieces are more likely to change than the length. This behavior is a key consideration in woodworking, especially when crafting wooden furniture or installing flooring, where maintaining the dimensional stability of wood is essential.

What Causes These Changes in Moisture?

The main factor that causes wood to lose or gain moisture is the surrounding environmental conditions, specifically temperature and humidity:

• High Temperature: When temperatures increase, wood tends to lose moisture, causing it to shrink. This is because warmer air holds less moisture, which encourages evaporation from the wood.
• Low Temperature: Conversely, in colder temperatures, air holds less moisture, and wood can absorb moisture from the air if humidity levels are higher, causing it to swell.
• Humidity Changes: The fluctuation between dry and humid conditions plays a significant role. In a dry environment, such as during winter, the wood will release moisture and shrink. In a humid environment, such as during the summer or in tropical climates, wood will absorb moisture and expand.

The amount of expansion or contraction depends on the wood species and its inherent moisture content. Some woods, like oak and walnut, are more stable and less prone to dramatic changes in size. Others, like pine or spruce, may show more noticeable movement.

How to Minimize the Effects of Wood Expansion and Contraction

Although you can’t eliminate the natural movement of wood, there are steps you can take to minimize the impact and preserve the appearance and functionality of your wooden items:

1. Climate Control

Maintaining a stable indoor environment is one of the most effective ways to reduce wood movement. Keep indoor temperature and humidity levels consistent, especially in areas where wooden furniture, flooring, or structural components are present. Using a humidifier in dry winters or a dehumidifier in humid summers can help stabilize the air.

2. Acclimate Wood Before Use

If you’re working with raw wood, it’s crucial to let it acclimate to its environment before cutting or installing it. Allow your wood to sit in the room where it will be used for several days (or even weeks) to adjust to the local temperature and humidity. This ensures that the wood won’t shrink or expand after installation.

3. Use Wood Finishes

Wood finishes, such as varnish or polyurethane, can help seal the wood and reduce moisture absorption. This protective layer creates a barrier that slows the wood’s response to environmental changes, keeping it more stable over time.

4. Proper Wood Selection

If you’re planning a woodworking project, choose wood species that are known for being more stable and less affected by humidity changes. Woods like teak, cherry, or mahogany are more resistant to movement, while woods like pine and birch are more prone to warping and shrinking.

5. Leave Room for Expansion

When building or installing wooden structures, make sure to leave small gaps for expansion and contraction. This is especially important in flooring or cabinetry, where wood is often installed in tight fits. These gaps will allow the wood to move without causing damage or creating unsightly cracks.

Conclusion

Wood’s natural tendency to shrink and expand with temperature and humidity changes is due to its hygroscopic nature and cellular structure.

When wood absorbs or loses moisture, it changes its dimensions, with the most noticeable movement occurring across the grain.

While this is a natural and unavoidable property of wood, understanding the science behind it can help you take steps to minimize its effects.

By controlling the climate, acclimating wood before use, applying finishes, and selecting more stable wood species, you can preserve the beauty and integrity of your wood products for years to come.