Accoya wood
The Wood Guide · The wood register
Modified wood · referenceAccoya
Ordinary softwood that gains the durability of tropical hardwood through a chemical treatment — without heavy biocides. Fifty years above ground, and it barely moves.
- Acetylated wood
- Class 1 · 50+ years above ground
- Exceptionally dimensionally stable
Click a chapter or use the arrows · molecules and cell wall greatly simplified and enlarged.
Acetylation: keeping water out at cell level
Wood rots because it absorbs water. Accoya tackles exactly that — not with a coating around it, but by changing the chemistry of the wood cell itself.
In acetylation the wood is treated with acetic anhydride, a derivative of acetic acid. In the process, the hydroxyl groups in the wood cells — the places where water attaches — are converted into acetyl groups. The result: the wood is much less hygroscopic, so it takes up far less moisture, and becomes both more stable and more durable.
- PreparationThe wood is first dried to a consistent moisture content.
- AcetylationThe dry wood is exposed to acetic anhydride; the hydroxyl groups are converted into acetyl groups.
- ConditioningAfter treatment the wood is rinsed and dried again to the desired moisture content.
The advantages of Accoya
Exceptional durability
Durability class 1 — the highest. More than fifty years above ground, and around twenty-five years in the ground or in water.
Dimensional stability
Greatly reduced swelling and shrinkage. For windows and doors, where a millimetre counts, that is the decisive argument.
Without heavy biocides
The process uses acetic anhydride instead of conventional preservatives, and the wood comes from managed forests.
Long service life
Less maintenance and less replacement over its service life — that is where the economic case lies.
Easy to work
Sawing, planing, drilling and sanding can all be done with ordinary tools.
Accoya costs more than ordinary softwood — the acetylation process takes a lot of energy and time. And the light, blond colour is not to everyone's taste; if you want warm tones, you will need to stain it. Spread the cost over its service life, and the sums usually work out favourably.
What Accoya is used for
The combination of durable and dimensionally stable opens doors that stay closed to ordinary softwood.
Cladding
Weather-resistant and attractive, with an even greying that many architects appreciate.
Windows and doors
The strongest application: here dimensional stability counts most, and that is where Accoya is at its best.
Decking and boardwalks
Durable and slip-resistant. See also wood for your decking.
Garden furniture
A low-maintenance option for outdoor furniture — although you come across it less often than teak or acacia.
Building structures
Beams and columns, thanks to the combination of strength and stability.
Accoya alongside other treatments
Three ways to make ordinary wood more durable, each with its own trade-off.
| Characteristic | Accoya | Thermowood | Pressure-treated wood |
|---|---|---|---|
| Treatment | Acetylation | Heat and steam | Chemical preservatives under pressure |
| Durability | Class 1 — very high | Class 2–3 | Class 3–4 |
| Dimensional stability | Excellent | Good | Unchanged |
| Chemical load | Low | None | Higher |
| Strength | Retained | Slightly more brittle after treatment | Retained |
| Price range | High | Mid | Entry level |
We don't work with Accoya ourselves. Our garden furniture is made of pressure-treated spruce, Douglas fir, teak and other hardwood. Accoya is in our register because it is increasingly the choice for cladding and joinery — and because it shows that "durable wood" does not have to be tropical. More on that in durable wood species for outdoors.
Frequently asked questions about Accoya
What exactly is acetylation?
Treatment with acetic anhydride, a derivative of acetic acid. It converts the hydroxyl groups in the wood cells into acetyl groups, so the wood can take up much less water — the basis of all the other advantages.
How long does Accoya last?
Durability class 1: more than fifty years above ground and around twenty-five years in the ground or in water.
Is Accoya treated with toxic chemicals?
No. The process uses acetic anhydride, not heavy biocides, and the wood comes from managed forests. That is exactly the difference from conventional pressure-treated wood.
Why does Accoya cost so much?
The acetylation process takes a lot of energy and time. Calculated over its service life, the sums often work out favourably, especially compared with wood you have to replace after fifteen years.
Does Accoya need to be treated?
Not for durability. Left untreated, it greys evenly to a silvery grey. If you want colour, Accoya takes stain and paint extremely well — and that finish lasts longer than on ordinary wood, precisely because of the dimensional stability.
Accoya in perspective
Accoya is a reference species in our register — modified wood we don't work with, but do know.
Have a question about your project? Just ask — we're happy to help.
Photo via Wikimedia Commons: Accoya — Sebastian Möricke, CC BY-SA 3.0