When a homeowner or architect asks us about new timber windows, particularly for a period property or listed building, one of the first questions we ask is: what timber are you specifying? The answer shapes everything: the performance, the maintenance, whether planning will accept it, and how long the finished windows last.
Over 40 years, we’ve worked with oak, Sapele, ash, pine and many others. Each has its place. But for exterior timber windows, especially in the UK’s wet climate, we increasingly specify Accoya as our material of choice. Here’s why.
| 01What is Accoya? | 02Why it outperforms | 03Accoya vs oak & Sapele |
| 04Listed & heritage | 05When to specify it | 06More guides |
1 | What exactly is Accoya? |
Accoya isn’t a species of tree. It’s a process. Radiata pine, a fast-growing and sustainable softwood, goes through a modification process called acetylation, which permanently changes the wood’s cell structure.
1 Radiata pine A fast-growing, sustainable softwood from FSC®-certified forests. |
2 Acetylation The free hydroxyls in the cell walls that attract water are replaced with acetyl groups, which don’t absorb moisture. |
3 Accoya A permanently changed timber that resists swelling, shrinking, warping and rot. |
The result is a timber that behaves in a fundamentally different way to untreated wood. It doesn’t swell, shrink, warp or rot the way conventional timber does, because the cause of those problems has been removed at a molecular level.
50+ Years durability Above ground, independently tested to Durability Class 1 | 75% Less swelling Than untreated pine, so far fewer paint failures and joint problems | Class 1 Durability The highest biological durability rating, the same class as teak | FSC® Certified source A fully certified, sustainable timber supply chain |
2 | Why Accoya outperforms the alternatives |
Compared with the timbers we’ve traditionally used for exterior joinery, the differences are significant, and they directly affect how a window performs over its lifetime.
Exceptional durability Accoya achieves Durability Class 1, the highest rating under EN 350. It resists rot, fungal decay and insect attack without toxic chemical preservatives, and has been independently tested for 50+ years above ground. |
Dimensional stability Timber windows most often fail because the wood moves: it absorbs moisture, swells and distorts, breaking paint films and misaligning joints. Accoya moves up to 75% less than untreated timber, so you get fewer paint failures, tighter seals and longer between maintenance. |
Better paint adhesion Because Accoya doesn’t swell and shrink with the weather, paint and stain finishes last significantly longer than on conventional timber. That means less maintenance for homeowners, and a consistently high-quality finish on heritage projects. |
Genuine sustainability Accoya is made from fast-growing, FSC-certified radiata pine, not tropical hardwood. The acetic acid produced by the process is recovered and reused. It’s the most credible sustainable choice in premium exterior joinery. |
Conservation & planning acceptance Conservation officers and planning authorities across the UK are increasingly accepting Accoya for heritage and listed building work. Its stability and performance make it a credible alternative to tropical hardwoods, which are becoming harder to source. |
Thermal performance Accoya has a lower thermal conductivity than many hardwoods. With modern double or triple glazed units, Accoya windows can comfortably meet current Building Regulations, even when retrofitted to older properties. |
3 | Accoya vs oak & Sapele |
This comparison covers the timbers most commonly specified for exterior joinery in the UK. It isn’t theoretical. It’s drawn from 40 years of our own manufacturing experience.
| Accoya | European oak | Sapele | |
| Durability class (EN 350) | Class 1 | Class 2 | Class 3 |
| Dimensional stability | ●●●●● | ●●●○○ | ●●○○○ |
| Paint retention | Excellent | Good | Moderate |
| Conservation acceptance | Growing | Established | Varies |
| Suits heritage profiles | ✔ Yes | ✔ Yes | ✔ Yes |
| Cost compared with softwood | High (worth it) | High | Medium |
4 | Accoya for listed buildings & heritage projects |
It’s one of the questions we’re asked most often by architects and conservation officers. The honest answer is that it varies by local authority, but the trend is firmly in Accoya’s favour.
Local authorities have moved towards accepting modified timber where it can match or beat the performance of the materials originally used. With its Class 1 durability and dimensional stability, Accoya increasingly fits that brief, particularly where the traditional specification would have been high-quality hardwood.
Crucially, Accoya takes profiles and mouldings in exactly the same way as conventional hardwood. We can machine it to match original Victorian or Georgian sightlines, reproduce heritage mouldings and make sliding sash windows to period specification, without the movement problems that affect less stable timbers.
5 | When should you specify Accoya? |
Accoya isn’t always the right answer, but it’s the right answer more often than many people realise. Here’s our honest guide.
Period & character properties For a Victorian, Edwardian or Georgian home where you want new windows that look original but perform better than the originals ever did. Stable, durable and beautifully paintable. |
Listed & conservation area properties Where planning needs a timber window that performs reliably for decades without repeated maintenance, Accoya is increasingly the preferred choice. We can support your application with technical data if needed. |
High-end new build For developers and self-builders who want to specify once and not revisit the decision for 30 years, Accoya’s long maintenance intervals and durability make it excellent value over the life of the building. |
Exposed or coastal locations Seaside and exposed hillside properties are hard on exterior joinery. Accoya’s resistance to moisture movement, salt air and wind-driven rain makes it far more resilient than conventional options. |
We’re one of a small number of Accoya Approved Manufacturers in the UK. Based in Chester, we manufacture bespoke timber windows, doors and joinery for residential and commercial clients across the North West and beyond.
Call us: 01244 371571Send us an enquiryor email enquiries@parryjoinery.co.uk
