Does Wood Swell in the Heat?

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When wood comes into contact with heat and humidity, its fibers expand.  Wood is a porous and hygroscopic material. It means that when left on its own it will absorb water readily. The result is that the wood swells. Heat also has a similar effect on wood.

Does wood swell in heat? Yes, it does. Warm temperatures affect wood, just as they also change the humidity levels of the atmosphere. If air is warm, it readily accepts more moisture. If wood comes into contact with such air, the dry cells of the wood take in the moisture from the atmosphere and expand.


Cross section of a felled acacia tree — Photo

Wood expands and shrinks during cold months depending on the humidity levels of the atmosphere. But what is the science behind it?

Your wooden doors, windows, and flooring can change dimensions during the dry winter weather. But you will also see hardwood flooring expand when it comes into contact with the moisture of rain and snow.

During the winter months, there is less moisture in the air, resulting in lowered humidity levels. The moisture content in wood experiences a thing known as equilibrium moisture content (EMC). This is the moisture content that wood tries to achieve to match the moisture level of the atmosphere.

When the moisture content of the atmosphere drops, the wood tries to achieve an EMC similar to the atmosphere. The internal moisture content of the wood drop, resulting in wood flooring and furniture shrinking.

This might seem a bit confusing, so let’s consider a practical example. It is a cold winter day and you get wet in the snow. You come inside and take off your soaking jacket and hang it up to dry.

As you are feeling cold, you stoke up the fire to make the room warm and cozy. But the clothes that you left hanging will start to dry faster due to the increased heat in the room.

If you have several clothes hanging out to dry in the room, you may eventually notice the doors and windows of that room expanding and they become tight in their frames. It occurs due to the combined effect of heat and humidity.

Effects of Temperature on Wood

Brown Wooden cladding in dappled sunlight

The moisture level is the main culprit for making wood expand or contract. Temperature too has a bearing on the condition of wood but it doesn’t affect it as much as atmospheric moisture does.

As we proceed in this article we will delve into the reason why and how temperature affects wood. We will also see how humidity comes to play such a prominent role.

Effect of Humidity on Wood

Moisture affects wood significantly. Being a hygroscopic material, the state of wood constantly changes as weather changes. Wood may release or absorb moisture according to the humidity levels of the environment. Depending on whether it takes in moisture or releases it, wood will either expand or contract.

EMC Explained

 Equilibrium moisture content of wood versus relative humidity of the surrounding air.
Image Credit: Hankwang via Creative Commons

Let us consider the phenomenon of equilibrium moisture content (EMC) of wood. Wood by nature constantly seeks to match its moisture level with the humidity of the atmosphere. In scientific terms, we say that wood wants to achieve EMC levels.

If wood reaches EMC, its moisture level will be the same as the atmospheric moisture. If the atmospheric moisture is higher than that of the wood, it absorbs water and expands.

At lower atmospheric moisture levels, wood tends to release some moisture from its fibers to match the lowered moisture of the atmosphere. This process happens automatically and you can neither make it happen, nor prevent it from happening.

Effect of Cold Temperature on Wood

The commonest effect of cold temperature on wood is a door that gets jammed in its frame as the outside temperature cools down. The expanding and shrinking of wood in such a case is quite simple.

We will now consider a contradictory statement. Wood can contract or expand in cold weather. We tend to associate expansion and contraction with temperature. When we talk of the weather of course, we consider the atmospheric temperature.

But rather than temperature, it is the internal humidity level that plays havoc with the dimensions of wood. Although temperature affects the humidity levels of the atmosphere, it is the moisture content that directly affects the dimensions of wood. As a result, wood can expand or contract in cold weather.

So, when you seek a reason why wood “moves” with weather changes, you should take note of the humidity levels of the wood. It will provide you with an explanation of why the wood changes dimensions and whether it will expand or contract.

Then, why do we consider the temperature at all?

You still need to take note of the temperature because it is what has a bearing on the humidity levels of the atmosphere. Recall the example of hanging wet clothes in a room in cold weather.

The weather in this case is cold, but still, the door expands and gets stuck in the frame. It is because of the raised humidity levels inside the room, not the cold weather outside.

How Much Time Wood Needs to Expand

Due to its hygroscopic nature, wood is always looking for moisture in humid conditions. Similarly, it gives off moisture when the atmospheric air becomes dry.

Denser wood will take more time to expand due to moisture. The pores of such wood are so tightly packed that they block off the moisture to a great extent. Due to this reason, hardwoods take more time to expand or contract due to atmospheric moisture levels. It makes hardwoods more dimensionally stable than softwoods.

For the same reason, you will see that some hardwoods expand or contract more rapidly than others. Different species of hardwood exhibit varying densities which affect the speed and the extent to which they expand or contract with exposure to moisture.

Why Wooden Furniture Cracks

Old brown wooden background with knots and cracks.

Due to the above reason, you will find softwood furniture cracks and warps more readily than hardwood furniture. It makes hardwood furniture more preferred and it also becomes more expensive of the two.

If you want to turn out high-quality furniture, then you need to consider the type of wood that you use. Some softwoods make great furniture, but furniture made from such wood will tend to warp and crack.


Wood is a hardy substance. But to be resilient and durable, the trees from which it comes must withstand significant variation in weather conditions. The level of coping with adverse weather conditions depends on the wood species and its particular characteristics.

The grain direction, the density of wood, the presence of natural oils, and many other such factors decide how hardy the trees of a particular wood species are. The hardier the tree, the more resilient the type of wood that comes from that tree.

We can also enhance the tolerance of wood due to changes in atmospheric temperature and moisture by treating it. But eventually, it is the wood species that decides how vulnerable or resistant to dimensional changes the wood is.

Choose your wood well, and ensure that it is sufficiently dry. That way, you can exercise adequate control on the condition of the wood that you use for your projects.