What is Thermowood?

Fresh Toast

Thermowood ®, also known as thermally modified wood, is modified by a process of being heated (baked) to 250 degrees in a special oven with the doors closed and the air pumped out. During this heat treatment, chemical and structural changes occur within the timber which alters and improves some of the basic characteristics. This results in a product which is a more durable and stable timber. Think of it this way: when you toast bread in a toaster it becomes stronger and more durable, just like the wood.

Swallow Thermowood Greenhouses

Swallow, who are one of the leading independent garden building construction companies, specializing in high-quality timber greenhouses, introduced Thermowood in 2014.


View the range of Swallow Greenhouses

Thermowood benefits

What are the benefits of Thermowood?

  • Has high durability
  • Has excellent finish
  • Is insect and mould repellent (no chemical preservatives required)
  • Is environmentally friendly (no need to treat the product)
  • Has no resin bleed
  • Shrinking and swelling is reduced up to 90%
  • Warping is reduced to a minimum
  • Does not expand or contract
  • Makes painting easier (similar to painting metal)
  • Thermal qualities are improved


Thermwood is rated as class 2, which is a highly durable material, hence the standard 10-year crafter's guarantee against rot. However, due to the exceptional materials and workmanship, thermowood greenhouses are known to last for many years past than the initial guarantee.


Once dried to a moisture content of around 6% the density is between 350kg m3 to 480kg m3, natural variation will occur between individual boards. 

Decay Resistant

During the high temperature experienced treatment process the rot-causing hemicelluloses are destroyed, allowing it to have high resistance to the majority of decay fungi.

Dimensional Stability

The thermal treatment process reduces the amount of moisture present in the timber, giving it around 40% more stability than untreated softwood. Aiding in reducing atmospheric changes.

Thermal Conductivity

It has around 20% less thermal conductivity than regular kiln-dried softwoods.

Is it better for the environment?

Due to the way Thermowood ® is treated using only heat and water vapour, rather than chemicals used normally, it allows the process to be much more environmentally friendly. Which is why the majority of people choose this process, along with its strength and durability.