Choosing timber for sash windows is about more than appearance. It affects how much the window moves with changes in moisture, how well its painted finish lasts and how it stands up to decades of exposure.
Accoya is increasingly specified for traditional timber sash windows because it combines the appearance and workability of real wood with enhanced durability and stability. If you are deciding whether the upgrade is worthwhile for your property, call our team on 02080 162058 to talk through the options.
What Is Accoya Wood?
Accoya is made from radiata pine that undergoes a process called acetylation, which permanently modifies the timber at a molecular level. The result remains real wood: its enhanced durability comes from the acetylation process rather than conventional preservative treatment.
Wood naturally contains chemical groups that readily interact with moisture. Acetylation changes how the timber responds to water, significantly reducing the swelling and shrinking that normally occurs as humidity changes. The result is still real wood: it has not been coated or pressure-treated with a preservative to create its core performance.
Accoya is classified as Durability Class 1 (very durable) against wood-destroying fungi and carries a 50-year manufacturer warranty for above-ground use, subject to the warranty conditions. It is also highly resistant to wood-destroying insects.
Why Does Dimensional Stability Matter for Sash Windows?
A sash window relies on accurately made moving parts. Timber that repeatedly absorbs moisture and moves can contribute to sashes sticking, joints being placed under stress and gaps changing through the seasons.
Accoya undergoes substantially less swelling and shrinkage than conventional untreated timber. That stability helps the components retain their intended shape and tolerances.
It also benefits the painted finish. Paint on a timber window has to move whenever the wood beneath it expands and contracts. Reducing that movement places less stress on the coating, and testing by Accoya has found improved coating life compared with non-modified woods.
That does not make an Accoya window maintenance-free. Exterior joinery still needs appropriate finishing, inspection and maintenance. The advantage is that a more stable substrate can help the coating system perform for longer before significant maintenance is required.
Is Accoya Better Than Oak or Sapele for Sash Windows?
There is no single timber that is automatically right for every property. Oak and Sapele are established hardwoods used in exterior joinery, while Accoya achieves its performance by modifying a faster-growing softwood.
For sash windows, Accoya’s strongest advantage is its dimensional stability. Independent testing and manufacturer data also give it a clearly defined durability classification and warranty, making its expected performance easier to specify.
Oak may be chosen where its natural grain and appearance are part of the design, particularly for clear or stained finishes. Sapele is another durable joinery timber and has traditionally been used where hardwood performance is wanted.
Cost is less straightforward. Accoya is a premium material and will normally cost more than standard softwood, but there is no useful universal rule that it will always cost more or less than oak or Sapele. Timber prices, window dimensions, profiles, glazing, finishes and manufacturing requirements all affect the quotation.
When comparing options, look beyond the raw timber price. The stability of the material, expected coating maintenance and suitability for the design can matter more over the life of the window.
Can Accoya Be Used to Repair Existing Sash Windows?
Yes, where replacement timber is needed and Accoya is appropriate for the repair. A sash window with localised decay does not necessarily need to be removed in its entirety.
During sash window repairs, deteriorated sections can sometimes be repaired or replaced while sound original joinery is retained. The extent of any decay, condition of the joints and overall structure of the window need to be assessed first.
The London Sash Window Company offers Accoya among its timber options for repair and restoration work. This can be useful where durable replacement sections are required but retaining as much existing fabric as practical remains the priority.
Where a window has deteriorated beyond sensible repair, Accoya can instead be specified for bespoke replacement sash windows.
Does Accoya Suit Period Homes and Conservation Areas?
Accoya can work particularly well for period sash windows because it can be machined into traditional profiles, glazing bars, sash horns and other details rather than forcing a property into a modern window style.
However, choosing Accoya does not itself make a replacement acceptable in a conservation area or listed building. What usually matters is the overall design: proportions, materials, glazing pattern, profiles, opening method and how closely the new joinery preserves the building’s character.
Listed buildings require particular care, and conservation areas may also be affected by local planning controls or Article 4 directions. Check the property’s status and any required approvals before ordering replacement windows.
From an environmental perspective, Accoya is produced from timber sourced from certified, well-managed forests. Its acetylation process is non-toxic and does not rely on adding conventional wood preservatives containing biocides. You can read more about the material itself in London Sash’s guide to Accoya wood.
Choose the Timber as Carefully as the Window Design
Accoya’s durability and dimensional stability make it a strong option for traditional timber sash windows where reduced movement and longer coating performance are priorities.
It is not automatically necessary for every repair or replacement, so the condition of the existing windows, design requirements and budget should guide the choice. Contact The London Sash Window Company to arrange a free survey and discuss which timber is appropriate for your sash windows.
Frequently Asked Questions
How long does Accoya last in sash windows?
Accoya carries a 50-year manufacturer warranty against fungal decay for above-ground applications, subject to its warranty terms and correct use. That should not be confused with a promise that every component, coating or item of window furniture will last 50 years without maintenance.
Does Accoya rot?
Accoya is classified as Durability Class 1 and is highly resistant to fungal decay. No timber window should simply be neglected, however. Correct detailing, installation, drainage and maintenance of coatings remain important for protecting the complete window assembly.
Do Accoya sash windows still need repainting?
Yes. Painted Accoya windows still require maintenance, but the timber’s reduced swelling and shrinkage can help coatings last longer than they would on less dimensionally stable wood. Repainting intervals depend on the coating system, exposure, colour, installation and ongoing condition.
Is Accoya suitable for listed buildings?
It can be, but suitability depends on the building and proposed work. Alterations to a listed building may require listed building consent, and the conservation officer may consider the material, profiles, detailing and glazing. Confirm the approval requirements before committing to replacement work.