Comparison guide
Thermally modified timber compared with other cladding
There is no single best cladding timber: each option makes different trade-offs between appearance, stability, durability, maintenance and cost. This comparison is a starting point, and our team can help you choose.
The table gives the headline differences. The sections after it explain what sits behind each row: how the timbers are made, how they perform, how they look and weather, and what that means for fixings, finishes and cost. All five are used successfully on UK buildings, so the aim is to match the timber to the job.
Comparison table
| Thermally modified | Siberian larch | Western red cedar | Accoya® | Kebony® | |
|---|---|---|---|---|---|
| Preservation method | Heat and steam | Natural durability | Natural durability | Acetylation | Furfurylation |
| Stability | Very good | Good | Very good | Excellent | Very good |
| Typical look | Rich brown, consistent | Golden-orange, varied | Warm red-brown, light | Pale, light, uniform | Dark brown, rich |
| Weathering | Silver-grey | Silver-grey | Silver-grey | Silver-grey, even | Silver-grey |
| Maintenance | Low | Low | Low | Low | Low |
| Chemical content | None added | None added | None added | Acetyl groups | Furfuryl alcohol |
| Typical position | Mid to premium | Mid | Mid to premium | Premium | Premium |
How each one is made
- Thermally modified timber: softwood or hardwood heated in steam at roughly 190 to 212°C, which changes the wood so it absorbs less water. Nothing is added. The heat also reduces strength and makes boards more brittle.
- Siberian larch: an untreated softwood. Its durability comes from the natural resins in the heartwood, and slow growth in a cold climate gives tight grain. Colour and knots vary from board to board.
- Western red cedar: an untreated, naturally durable softwood that is light and soft, with a clean, straight grain.
- Accoya®: acetylated radiata pine, in which acetic anhydride reacts with the wood so it takes up far less water.
- Kebony®: softwood impregnated with furfuryl alcohol, which is then cured to leave a hard, dense board.
Stability, durability and strength
Stability is the ability to resist swelling, shrinking, cupping and twisting as moisture levels change. Acetylated and furfurylated timber are generally the most stable of the group, followed by thermally modified timber and cedar, with larch moving a little more.
Durability for Accoya and Kebony is usually quoted at the top of the scale in manufacturer data. Class D thermally modified timber is typically durability class 2, which is right for above-ground use. Larch and cedar heartwood have moderate natural durability that varies with the tree and the proportion of heartwood.
Handling differs too. Thermally modified boards are more brittle than untreated timber, cedar is soft and dents easily, and Kebony is noticeably hard. None of them should be used for ground contact.
Appearance and weathering
Thermally modified timber and Kebony start dark and fade towards grey. Larch starts golden-orange, cedar a warm red-brown and Accoya Clear pale, and all of them silver outdoors if left uncoated. Weathering is rarely even at first: sheltered areas under eaves stay darker than exposed elevations. Pigmented oils slow this, and Accoya Color Grey starts grey to avoid the change.
Fixings and detailing
Whichever timber you choose, install it as a ventilated rainscreen with a drained cavity, keep the base clear of the ground and protect cut ends. Stainless steel fixings are the safe default for all five: thermally modified timber, cedar and larch all contain extractives or acids that corrode ordinary steel and stain the boards. Pre-drill near ends, particularly in thermally modified timber and hardwood species.
Cost and value
The position in the table is a rough guide only. A premium grade of one timber can cost more than a standard grade of another, and profile, thickness, length and volume all change the price. Compare like for like: the same profile, grade and finish. Prices in the shop include VAT and UK delivery.
When thermally modified timber is the right choice
- You want stable boards with a consistent dark brown colour.
- You prefer cladding with no added chemicals.
- You are building or lining a sauna.
- You want a wide choice of profiles and species.
When to consider something else
- Siberian larch or British larch: for a natural, golden-toned look and good value.
- Western red cedar: for a lightweight, stable timber with a classic appearance and natural oils.
- Accoya®: for maximum stability and a long service life, including in demanding or wet conditions.
- Kebony®: for a premium, dark finish with high hardness.
- Charred timber: for a dramatic, dark, textured architectural finish.
We stock and supply all of these, so we can give a balanced recommendation rather than pushing one product. Free samples let you compare them side by side.
Shop and specify
Frequently asked questions
Talk to our timber team
Tell us about your project and we will recommend the right species, profile, finish and fire performance. Free samples are available, and we can quote for bespoke milling.