Over the past twenty years, thermally modified wood (TMH) has been positioned in Europe as a sustainable alternative to tropical hardwood.Manufacturers consistently refer to the European standard EN 350, which classifies wood species into durability classes based on laboratory tests.However, what many designers and construction professionals are unaware of is that the test methods on which these classifications are based were, until recently, incomplete.Only since 2020 has the new standard EN 113-2:2020 mandated a modified testing approach for modified wood, including an additional, crucial fungal species and a recommended accelerated pre-weathering.Time to lay the facts on the table.
What does the 'natural durability' of wood mean?
Natural durability refers to a wood species' resistance to biological attack (fungi, insects, organisms).
In Europe, this is tested in the lab, where the mass loss after fungal attack determines the durability class the wood falls into:
Class Description Mass Loss
1 Very durable < 5%
2 Durable 5–10%
3 Moderately durable 10–15%
4 Low durability 15–30%
5 Not durable > 30%
So much for the basics.
However, the way this durability is tested has recently undergone significant changes, and this has major consequences for thermally modified wood.
The situation before 2020: outdated standards, flawed insights
Before 2020, manufacturers often relied on:
EN 113:1996
CEN/TS 15083-1:2005
EN 350-1
These standards have been officially withdrawn due to outdated methods and insufficient relevant data.
Nevertheless, many manufacturers continue to rely on these standards, leading to unrealistic sustainability claims and systematic misleading practices in the market.
The valid standard since 2020 is: EN 113-2:2020.
Why the old test failed for modified wood
The traditional laboratory test exposed wood to two fungi for 16 weeks:
Coniophora puteana (brown rot)
Trametes versicolor (white rot)
This method was sufficient for natural wood, but gave a distorted picture for modified wood species, such as TMH.
Why?
1. Distorted initial moisture content
Modified wood comes out of the kiln with a low moisture content (~4%), while reference wood has around 12%. Because moisture contributes to mass, modified wood appears less affected, while the difference is mainly due to the starting conditions.
2. Delayed fungal activity
Fungi only become active from around 20% moisture content. Because modified wood absorbs less moisture and starts dry, this threshold is often reached much later than with reference wood.
As a result:
- Biological damage starts too late
- The actual damage is underestimated
➡️ Solution: accelerated pre-aging, so that the moisture behavior more realistically matches outdoor applications.
3. Incomplete fungal selection
The traditional test used only two fungi.
But some fungi are better adapted to modified cell structures, such as:
Rhodonia placenta: an aggressive brown rot fungus that is capable of attacking TMH (and modified bamboo fibers).
This fungus was completely absent from the old test methods.
What changes with EN 113-2:2020?
The new standard is designed to provide a more realistic assessment of wood species, and modified materials in particular.
The three most important changes:
Mandatory inclusion of 'Rhodonia placenta'
This fungus is now used as standard in testing modified wood.
Recommended accelerated pre-weathering
This makes moisture behavior more realistic, and fungal attack is accurately estimated.
New testing under similar conditions
Avoid an advantage for kiln-dried wood compared to natural materials.
What does this mean for thermally modified wood?
The impact is significant:
Sustainability claims based on the old standards are no longer reliable.
New tests according to EN 113-2:2020 show that TMH often achieves a lower durability class than previously claimed:
Durability classes 1–2 will likely drop to classes 3–4, sometimes even 5.
The standard distinguishes between natural and artificial wood species, essential in a market that demands transparency.
Yet, there are hardly any (or no?) complete test results available that comply with EN 113-2:2020.
This indicates that the sector is reluctant to make actual performance publicly available.
What do we do about this?
In our search for Lesser Known Timber Species (LKTS), we notice how limited and inconsistent data is today.
Sometimes the durability class of an untreated wood species varies between classes 2 and 3, depending on the source.
This difference is important for outdoor applications.
That's why we are launching our own testing campaign according to EN 113-2:2020, focusing on untreated wood species.
Modified wood species can optionally be included in the testing, but that responsibility lies primarily with the producers themselves.
The goal: a clear, fair, and comparable picture of the true, biological durability of lesser-known wood species.
Conclusion
The introduction of EN 113-2:2020 marks an important step towards fair and transparent durability testing.
This revised standard is particularly crucial for modified materials such as thermally modified wood (TMH) or other modified fibers (bamboo, etc.): it requires the inclusion of fungi that specifically target the weak points of these materials and, with accelerated aging, provides a more realistic test framework.
Thermal modification certainly offers advantages:
The wood becomes more dimensionally stable, with significantly reduced shrinkage and swelling.
➤ This makes certain wood types more suitable for outdoor applications, where they would have been difficult to use without modification.
The reduced moisture absorption slows down biological degradation, which in practice results in a limited improvement in durability.
At the same time, there are important caveats:
Thermal modification makes the cell wall more brittle, making the wood more mechanically vulnerable to prolonged loading, wear, or aging.
The fire reaction class typically deteriorates significantly after thermal modification, with an increased risk of rapid ignition and flame spread compared to untreated wood.
Those who want to build sustainably with wood must therefore look beyond traditional claims and make conscious choices based on current, realistic, and transparent test data.