Does Wood Expand In Heat?

Temperature change affects water greatly in both directions.  We know that water expands as it gets heated and eventually becomes a vapor. As it cools, it contracts until it reaches 4 degrees C, its molecular density maximum, and then it begins to expand as the temperature falls.

So, it expands when it is heated and also when it drops below that temperature threshold.

Key Points:

  • Wood is affected by both temperature and humidity level changes.  
  • While heat will cause wood to expand, moisture has a greater effect on wood.  The expansion from heat will be minimal, while expansion from moisture will be greater and more dangerous.
  • Hardwoods and softwoods are about the same as regards expansion due to heat.

What about wood, though?  Is wood that dynamic?  Do humidity levels play a part in any expansion or contraction?  It’s very helpful to understand this process and how wood reacts to changes in temperature, and woodworkers should know what to expect.

I used PT wood, ground contact rated, for my large deck out back.  Part of the deck rests directly on the ground, while the other end is supported by 4″x4″x18″  posts that rest on an old concrete pad I didn’t feel like breaking up and removing. 

The planks are 1″ x 6″; and as we leave the cold temperature of winter for the warm temperatures of summer, the change in the seams between the planks will be noticeable.  I know what the rising temperatures and humidity levels will do to those seams, and you probably do, too.

But let’s look a bit more deeply into the title’s question and examine how wood reacts to rising heat.

Does Wood React To Heating and Cooling?

Wood is a natural material that is made up of long, thin fibers. These fibers are held together by a substance called lignin. Lignin is a type of polymer, which means that it is made up of long chains of molecules. When wood is heated, the lignin molecules start to move around more freely. This causes the wood to expand.

This is known as thermal expansion.  When wood is cooled, the lignin molecules slow down, and the wood contracts.

The amount of thermal expansion and contraction that wood experiences depends on the wood species, the moisture content of the wood, and the rate of temperature change. Some woods, such as oak, expand and contract more than others, such as pine.

Woods with a high moisture content expand and contract more than woods with a low moisture content. And woods that are heated or cooled quickly expand and contract more than woods that are heated or cooled slowly.

Wood Grain

The expansion and contraction of wood can cause problems. For example, it can cause wood furniture to warp and crack. It can also cause wood flooring to buckle. To prevent these problems, it is important to use wood that is properly dried and to avoid exposing wood to extreme temperatures.

Here are some tips for preventing wood damage from changes in temperature:

  • Use wood that is properly dried. The moisture content of wood should be between 6 and 12%.
  • Avoid exposing wood to extreme temperatures. The ideal temperature for storing wood is between 60 and 70 degrees Fahrenheit.
  • If you must expose wood to extreme temperatures, do so gradually. For example, if you are bringing wood into a warm house from the cold, let it acclimate for a few hours before bringing it inside.
  • Use a sealant to protect wood from moisture. A sealant will help to prevent wood from absorbing moisture and expanding.
  • Repair any damage to wood as soon as possible. Damaged wood is more likely to expand and contract, which can lead to further damage.

How Does Wood React To Moisture?

Wood is a hygroscopic material that absorbs and releases moisture from its surrounding environment. The amount of moisture in wood is measured as its moisture content, which is expressed as a percentage of the wood’s dry weight.

The moisture content of wood can vary widely, depending on the species of wood, the growing conditions, and the way the wood is stored.

When wood absorbs moisture, it swells. This is because the water molecules fill up the spaces between the cellulose fibers in the wood. When wood loses moisture, it shrinks. This is because the water molecules leave the spaces between the cellulose fibers.

The movement of wood due to changes in moisture content is called wood movement. Wood movement can cause problems in wood products, such as furniture, flooring, and doors. To minimize wood movement, it is important to store wood in a controlled environment with stable moisture content.

Here are some of the effects of moisture on wood:

  • Swelling: When wood absorbs moisture, it swells. This can cause problems in wood products, such as furniture, flooring, and doors. To minimize wood movement, it is important to store wood in a controlled environment with stable moisture content.
  • Shrinking: When wood loses moisture, it shrinks. This can cause gaps and cracks in wood products. To minimize wood movement, it is important to store wood in a controlled environment with stable moisture content.
  • Warping: Warping is a type of wood movement that occurs when wood absorbs or loses moisture unevenly. This can cause wood to bend, twist, or cup. To minimize warping, it is important to store wood in a controlled environment with stable moisture content.
  • Cracking: Cracking is a type of wood damage that can occur when wood absorbs or loses moisture too quickly. This can cause the wood to split or break. To minimize cracking, it is important to store wood in a controlled environment with stable moisture content.

Here are some tips for storing wood to minimize wood movement and damage:

  • Store wood in a cool, dry place.
  • Avoid storing wood in direct sunlight or near heat sources.
  • If possible, store wood in a climate-controlled environment.
  • If you must store wood in an unconditioned environment, check it regularly for signs of moisture damage.
  • If you notice any signs of moisture damage, take steps to dry the wood as soon as possible.

Do Woods Expand More Due To Heat Or Moisture?

Oak Wood Cracks

We mention wood expansion due to moisture for a reason, and whether it is heat or moisture that influences expansion, more is a fair question.  It’s also a question woodworkers need to know the answer to when storing wood, cutting wood, and assembling projects like wood furniture.

Wood expansion has more due to with moisture than it does due to heat. Wood expansion due to moisture is caused by the absorption of water molecules into the cellulose fibers of the wood. The water molecules fill up the spaces between the cellulose fibers, which causes the wood to swell.

The expansion of wood due to heat is caused by the increase in the kinetic energy of the wood’s molecules. As the temperature of the wood increases, the molecules move faster and take up more space, which causes the wood to expand. However, the effect of heat is much less than the effect due to moisture.

Here is a table that shows the expansion of wood due to moisture and heat:

Moisture Content (%)Expansion (in/ft)
0%0.00001
10%0.0001
20%0.0002
30%0.0003
40%0.0004

As you can see, the expansion of wood due to moisture increases significantly as the moisture content of the wood increases. Moisture plays a much larger role in the expansion of wood than does heat.  

Which Woods Expand More Due To Heat – Hardwoods or Softwoods?

Hardwoods and softwoods expand at about the same rate when heated. However, hardwoods tend to be denser than softwoods, so they will expand slightly more in volume.

The expansion of wood due to heat is caused by the increase in the kinetic energy of the wood’s molecules. As the temperature of the wood increases, the molecules move faster and take up more space, which causes the wood to expand. Moisture plays a larger role in expansion than does heat.

Here is a table that shows the expansion of wood due to heat for some common hardwoods and softwoods:

Wood SpeciesExpansion (in/ft)
Oak0.00002
Maple0.00002
Pine0.00002
Spruce0.00002

As you can see,  expansion due to heat is very small, and it is about the same for both hardwoods and softwoods.

Does Wet Wood Expand More Than Dry Wood When Heated?

Yes, wet wood expands more than dry wood when heated. This is because wet wood contains more water, which causes it to swell more when heated. The expansion of wood due to heat is caused by the increase in the kinetic energy of the wood’s molecules.

As the temperature of the wood increases, the molecules move faster and take up more space, which causes the wood to expand. However, heat has less of an effect on wood than moisture.

Here is a table that shows the expansion of wood due to heat and moisture:

Moisture Content (%) With Rising HeatExpansion (in/ft)
0%0.00001
10%0.0001
20%0.0002
30%0.0003
40%0.0004

As you can see, the expansion of wood due to moisture increases significantly as the moisture content of the wood increases. Again, it’s moisture that has the greater effect vs heat.

For example, a piece of wet wood with a moisture content of 20% will expand about twice as much as a piece of dry wood with a moisture content of 5% when heated to the same temperature.

The expansion of wood due to heat can cause problems in wood products, such as furniture, flooring, and doors. To minimize wood movement, it is important to store wood in a controlled environment with a stable moisture content.

I’m glad the cold weather is leaving us for the warmth of summer.  The cold weather of winter had its effect on the wood in my deck, but the effect of the warmer summer weather and its higher humidity level will be more dramatic.

Those seams between the decking will expand, but now we know it is due more to the humidity level than the higher temperatures.  That higher moisture level in both the air and the wood will lead to enough expansion to close the gaps between the decking.

What Is Equilibrium Moisture Content?

Equilibrium moisture content (EMC) is the moisture content of wood at which the rate of water absorption or loss is equal to the rate of water evaporation. EMC is affected by the relative humidity (RH) and temperature of the surrounding air. The higher the RH, the higher the EMC will be.

The lower the temperature, the lower the EMC will be.

The EMC of wood varies depending on the species of wood. Hardwoods typically have a lower EMC than softwoods. The EMC of wood also varies depending on the growing conditions. Wood that grows in a moist environment will have a higher EMC than wood that grows in a dry environment.

The EMC of wood is important to consider when using wood in construction or furniture making. If wood is used in an environment with a higher RH than its EMC, it will absorb moisture and swell. If wood is used in an environment with a lower RH than its EMC, it will lose moisture and shrink. This can cause problems such as gaps, cracks, and warping.

To minimize problems caused by changes in moisture content, it is important to store wood in a controlled environment with a stable RH. It is also important to use wood that has been dried to the correct EMC for the intended use.

Yes, we need to be aware of temperature in woodworking, but paying attention to moisture level and the internal humidity level in the wood we are working with is even more important.  Heat will do its part, and we know that the warmer the air, the greater moisture it can hold – we feel it on hot, humid days.

So does wood.  Warm temperatures bring more moisture, so expect the wood expansion dynamic at play.

1 thought on “Does Wood Expand In Heat?”

  1. It was interesting to me when you explained that it is important to be aware of the temperature in woodworking. Are there stains and other treatment forms that can be used to increase the heat resistance of wood? It seems like you would want to use some kind of thermally treated wood when you are building a deck.

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