Thermally modified timber: standing the test of time

Architects wishing to create a natural, aesthetically pleasing appearance for the exterior of buildings continue to turn to timber for a solution that is both practical and sustainable.

Timber has a life expectancy of decades, and there are undeniable environmental benefits of working with a natural, sustainably sourced material. It’s also very easy to work with, being lightweight and easy to trim and fit when compared to alternatives like PVC.

Unlike PVC, timber cladding is 100 per cent recyclable, so it can be invaluable to building projects that include tough targets relating to carbon neutrality. Projects that require temporary cladding can turn to timber for a solution that can be demounted and reused. What’s more, cladding acts as an insulator, reducing the building’s energy costs and muffling outside noise.

In particular, exotic hardwoods and durable softwoods remain a popular choice: western red cedar is the material that specifiers seem to return to time and again, thanks to its lengthy life-cycle, generally knot-free straight grain and famously rich colour.

However, working with a natural material also has its potential drawbacks as untreated timber has a tendency to warp, swell or shrink due to changes in moisture. Western red cedar’s colour can also vary from light pinkish tones to reddish brown.

While chemical treatments can have harmful environmental effects which counteract timber’s sustainability, thermally modified timber provides a solution to many of the potential drawbacks of untreated wood, increasing its durability without the use of chemicals. Increased interest in these products has led suppliers to develop full thermally modified timber cladding ranges.

This method of treatment involves heating timber to over 200°C with steam used to prevent cracking and burning. Once cooled, the wood is remoisturised to around five per cent.

The principle is simple: heating timber removes most of the moisture, making it less likely to warp or swell. Importantly the treatment also removes resin from the timber, which means that the sugars which fungi could survive on are eradicated thus reducing the potential for fungal attack.

Thermally treated redwood and clear pine products offer the same rich colour as western red cedar, but the non-chemical heat treatment means the timber will weather more evenly, giving a better aesthetic effect with significant cost savings. As both timbers are sourced from Europe and are available with PEFC or FSC certification, there are clear sustainability benefits.

The thermal treatment alters wood in such a way that hardwood timbers such as frake, which otherwise has little commercial value other than in plywood, are suitable for cladding. Not only that, but thermally modified frake and ash cladding both also benefit from a richer appearance and more expensive feel – making them among the most popular high-end cladding options in the past year.

Installers report that thermally modified timber is easier to cut with reduced wear on tools, and as the resin has been removed it’s lighter too, making installation easier. Unlike chemical methods, the treatment penetrates all the way through the wood so thermally modified timber won’t be subject to rot at the core.

Thermally modified timber is unsuitable for structural use as the treatment permanently alters the cell structure of the timber, making it brittle and reducing its tensile strength. When fitting thermally modified timber cladding, installers should follow fitting instructions carefully, and it’s best to use pre-drilled boards and hand-nail boards into place to avoid splitting. Thermally modified timber should be installed using stainless steel fittings only.

As a key component of cladding is weatherproofing, thermally modified timber’s increased weather resistance makes it a sound choice. Offering a host of benefits, easy installation and unbeatable environmental credentials, thermally modified timber will be part of the cladding landscape for a long time to come.

Phil Barman is a hardwood timber specialist at Howarth Timber