How Does Thermally Modified Wood Minimize Swelling and Shrinkage?

Wood naturally expands and contracts depending on environmental conditions. Since this behavior can lead to issues such as cracking, surface deterioration, and loss of form especially in outdoor applications it directly affects material performance. Thermowood technology reduces swelling and shrinkage rates significantly by controlling the hygroscopic behavior of wood. For this reason, Thermowood stands out as a reliable solution for outdoor projects requiring both aesthetics and stability. In this article, we explain this topic in detail.

Why Does Wood Swell and Shrink?

Wood is a living, breathing material that responds to environmental changes. The main reason wood moves is the fluctuation in its moisture content. Therefore, swelling and shrinkage are natural and inevitable processes; however, the right processing methods can reduce this movement to a minimum.

How does wood respond when moisture levels change?

Depending on the humidity of the surrounding air, wood:

Swells when it absorbs moisture,
Shrinks when it loses moisture.

This movement is related to the bound water and free water mechanisms within the cell walls. Sudden humidity changes make this movement more pronounced. Minor movement occurs indoors, while outdoor environments cause much higher levels of swelling and shrinkage.

How does cell wall structure affect wood movement?

The cellular structure of wood can absorb or release water much like a sponge. The following properties of the cell wall determine moisture response:

density,
grain orientation,
lignin and hemicellulose content.

Because hemicellulose can hold large amounts of water, swelling rates are higher in untreated traditional wood.

How Does the Thermal Modification Process Change Wood’s Moisture Response?

Thermal modification is an eco-friendly technology that reorganizes the cellular structure of wood using controlled heat and steam, reducing its reaction to moisture. This process involves no chemicals; it is entirely natural.

What is the effect of high heat on stabilizing the cellular structure?

High temperatures between 190–215°C:

Break down part of the hemicelluloses,
Reduce the water-holding capacity of the cell walls,
Stabilize the lignin structure,
Decrease the swelling tendency of the fibers.

As a result, the “movement” rate of wood decreases significantly, creating a much more stable material.

How does the Thermowood process reduce moisture absorption capacity?

The Thermowood process lowers the equilibrium moisture content of wood to approximately 4–6%.

With reduced equilibrium moisture, Thermowood becomes less reactive to changes in humidity and moisture, which helps improve its performance in outdoor environments. As a result, Thermowood products typically:

• Show improved dimensional stability

• Experience reduced movement due to moisture

• Remain more stable over time

How is hygroscopic behavior minimized?

Thanks to thermal modification:

Cell walls bind significantly less water,
The fibers’ rate of reaction to moisture decreases,
The fiber saturation point is reduced,
Wood becomes a more “stable” material.

This means that hygroscopic behavior the tendency to move with moisture is minimized.

How does performance differ in species like ayous, pine, and iroko?

Each wood species performs differently after thermal modification:

Thermo Ayous: One of the most stable species; highly preferred in façade cladding.
Thermo Pine: Gains durability and offers reliable performance in deck and pergola applications.
Iroko: A naturally oily wood; its dimensional stability improves even further after thermal modification.

Overall, the Thermowood process significantly reduces swelling–shrinkage in all species.

Which species are more advantageous for outdoor use?

Façades requiring high stability: Thermo Ayous
Deck areas exposed to heavy loads: Thermo Pine or Iroko
Pergolas with modern lines: Thermo Ayous or Thermo Pine

When selecting a species, climate, intended use, and surface protection choices must be considered.

Why Is Thermowood the Safest Material for Outdoor Applications?

Thanks to Thermowood technology, wood becomes more durable, more stable, and longer-lasting against the demanding conditions of outdoor environments. This makes it the safest material compared to traditional wood.

Why is stability important in decks, façades, and pergolas?

In these applications, stability is critical for:

Long service life
Installation durability
Surface smoothness
Aesthetic consistency

Unstable wood may crack, separate, or warp over time. Thermowood, with its minimal movement rate, offers ideal stability in outdoor areas.

How does thermal modification improve weather resistance?

Reduced moisture uptake supports long-term durability
Improved dimensional stability in changing climates
Optimized internal structure slows surface aging
UV resistance can be enhanced with protective oils

How does maintenance differ between traditional wood and Thermowood?

Traditional wood:

Requires more frequent maintenance,
Surface workmanship can deteriorate due to swelling and shrinkage,
Moisture-related deformation occurs more quickly.

Thermowood:

Optimized for efficient maintenance planning
Maintains form for longer due to dimensional stability,
Has a longer service life in outdoor conditions.

How Does Novawood’s Thermal Modification Technology Maximize Quality?

Novawood offers high-quality Thermowood products that strengthen the natural structure of wood, minimize swelling–shrinkage rates, and extend service life, using production technologies compliant with Thermowood Association standards.

Novawood’s thermal modification approach ensures exceptional performance in outdoor projects through:

Controlled and homogeneous heating processes,
Species-specific treatment recipes,
High-precision drying techniques,
Standardized moisture content adjustment,
Technologies that ensure homogeneous color and texture.