Accoya for Sash Windows: Is It Worth the Premium?

Accoya starts as fast-grown radiata pine from New Zealand plantations, a timber so undurable in its natural state that left outside untreated it would be lucky to see five years. It goes into a reactor in Arnhem, comes out modified all the way through its cross-section, and is then sold with a fifty-year above-ground warranty and a Class 1 durability rating, which puts it alongside teak. What happens in that reactor is the whole argument, and it is worth understanding before you accept or refuse the 30-40% your joiner has added to the quote.

The vinegar trick

Wood rots because it holds water, and it holds water because the cell walls are full of hydroxyl groups — free -OH sites that are essentially thirsty. Water bonds to them, the cell walls swell, and once the timber sits above about 20% moisture content the decay fungi have everything they need: food, damp and warmth.

Acetylation swaps those hydroxyl groups for acetyl groups. Kiln-dried boards go into a vessel with acetic anhydride, are heated to around 120°C under pressure, and roughly 80-90% of the accessible hydroxyls are converted. The acetyl group is larger than the hydroxyl it replaces, so it both fills the space and removes the bonding site. The by-product is acetic acid — vinegar — which is why fresh Accoya smells faintly of a chip shop and why the acid content matters later.

The result is not a treatment or a coating. Nothing has been added that can leach out or wear off. The wood's own chemistry has been changed, right through the section, which is why you can machine into it, rebate it, and cut a tenon on it without exposing untreated timber underneath. That is the essential difference between Accoya and a pressure-treated softwood, where you are relying on a preservative that has penetrated only so far.

If the cell walls cannot take up water, they cannot swell, and the fungi cannot get the moisture content up to where they need it. Durability and stability come from the same change.

The numbers

Two of those lines do most of the work. The equilibrium moisture content of 3-5% means the timber simply is not carrying the water that rot needs. And swelling figures of 0.7% radial and 1.5% tangential from bone dry to fully wet are remarkable — the manufacturer's headline is that swelling and shrinkage are reduced by 75% or more against the unmodified wood, and in practice a sash made from it stays the size it was made.

That stability is the reason Accoya's coatings last longer too. Paint fails on timber windows because the substrate moves under it: the film is asked to stretch and shrink through every wet and dry cycle until it cracks at the sharp arrises. Give paint a substrate that barely moves and it stays intact for far longer. The claim is that coatings last three to four times as long, and while that depends heavily on the paint system, the mechanism is sound and it matches what joiners see on site.

Where it earns its keep on a sash window

Not everywhere. The value is concentrated in the members that get wet and the members that are slender.

The cill. This is the strongest case there is. The cill is the most exposed timber on the window, it sits nearly horizontal, and it is the first thing to rot in almost every London house. An Accoya cill removes the failure that our guide to window cills spends most of its length warning about.

The bottom rail of the lower sash. Second most common failure point, for the same reasons: horizontal surface, putty line that cracks, water sitting in the glazing rebate.

The feet of the sash stiles. End grain pointing down into a wet joint. Accoya's water resistance shows up most where end grain is exposed.

Glazing bars. Slender sections that carry putty and take a beating. Stability matters here more than anywhere, because a bar that moves cracks its putty every year and then lets water in behind.

Parting beads and staff beads. Small, cheap, and constantly wetted or rubbed. Accoya beads are widely stocked precisely because timber beads swell, bind against the sash and then get planed down until they no longer seal.

For a splice repair into original timber, the calculation is different. Splicing a modern Accoya foot onto a Victorian redwood stile puts two timbers with very different movement characteristics either side of one glue line, which is exactly the situation that opens joints. Conventional advice is to avoid mixing species across a splice, and for load-bearing splices in original joinery that advice still holds — see splicing new timber into a rotten frame for how those joints are actually cut. Where Accoya makes sense in repair work is a complete new member: a whole cill, a whole bottom rail, a whole bead, rather than a patch.

The awkward parts

None of this is in the brochure, and all of it matters on site.

It is acidic, so ordinary fixings fail. The residual acetic acid attacks common metals. Accoya's own guidance calls for corrosion-resistant fixings to EN 10088-1 — A2 or A4 stainless, equivalent to 304 or 316. Galvanised is explicitly not corrosion-proof in contact with it.

Paint needs thinking about. Two specific problems. Opaque coatings filled with calcium carbonate — chalk — can blister on Accoya because the acid reacts with the filler, so the paint system needs to be one the manufacturer has confirmed for use on it. And end grain should be sealed before coating, which on a sash window means the ends of every rail and every bar, not just the visible faces. The general order of work in painting sash windows still applies, but the product choice is narrower.

Glazing rebates must be sealed before the glass goes in. This applies to any timber, but on Accoya it is stated as a condition rather than a recommendation, and unsealed rebates are the most common cause of sealed glazing units failing early.

It blunts tools and smells. Cutters and blades dull faster than on redwood, so carbide tooling is the norm and joiners tend to machine Accoya at the end of a run rather than the start. Machining releases the vinegar smell, which needs extraction in the workshop. Neither affects you as a customer except in the price.

It looks like what it is. Radiata pine is pale, with a fairly bland, even grain and occasional large knots. Under paint that is irrelevant, and virtually all sash joinery is painted. If you want a clear-finished or stained window, look at it in person before committing.

It is not for everything. Accoya is not warranted against marine borers and permanent immersion in salt water is not recommended, which is irrelevant to a Fulham bay window but tells you the modification has limits.

What it costs, and how to judge it

On a full replacement box sash window, Accoya typically adds 30-40% over softwood. Take a £2,400 softwood window and you are at £3,100 to £3,350 before glass and decoration.

Judge it by where the money goes. Specifying the whole window in Accoya buys you a lot of durability in places that were never going to rot anyway — the head, the inner linings, the top sash, everything sheltered by the reveal. Specifying the cill, the bottom rails and the beads in Accoya and the rest in good slow-grown redwood buys you most of the benefit for a fraction of the premium, and it is what a lot of experienced London joiners quietly do. Ask for that as an option; a firm that refuses to price it is selling a product rather than a solution.

The other honest comparison is against maintenance. A well-made redwood sash window that is redecorated every six to eight years lasts a very long time — most of the original windows in London are proof of it. Accoya's advantage is real, but it is largely an advantage in neglected conditions. If you repaint on schedule, the difference in service life narrows considerably. If you know you will not, it widens.

Will a conservation officer accept it?

Usually, for repairs and new joinery, but do not assume it.

The two objections you may meet are species substitution and appearance. Accoya is radiata pine, not the Baltic redwood or Douglas fir the window was originally made from, and some conservation officers take the view that repairs to historic joinery should be like-for-like in material as well as profile. On a Grade I or Grade II* building, expect that argument. On an unlisted house in a conservation area, painted joinery of the correct profile is generally judged on how it looks, and Accoya under paint looks like painted timber.

Where it tends to be welcomed is on cills and other wholly renewed components on a building whose windows have already been repaired repeatedly. A conservation officer would usually rather see one durable new cill than the same cill replaced three times in thirty years.

Either way, on a listed building the material specification belongs in the application, not in a conversation on site afterwards. Our guide to getting listed building consent for window work explains what to submit.

Where it sits against the alternatives

Against redwood, Accoya buys durability and stability at a 30-40% premium, and it needs stainless fixings and a compatible paint system.

Against hardwood such as sapele or European oak, Accoya is lighter, more stable and easier to work, and it does not have oak's tannin problem with ferrous fixings — though it has its own acid problem instead. Oak in a sash window is heavy, which matters because every kilo has to be counterbalanced in the pockets.

Against uPVC, it is not really a comparison. A timber sash window is repairable member by member; a uPVC one is not repairable at all, and when a component fails the window is replaced. The full case is in uPVC sash windows against original timber.

What to do next

If your existing windows are original and sound, none of this applies to you yet — the timber in a Victorian sash is usually better than anything on sale today, and the right move is repair and redecoration rather than replacement. Work out which category you are in first, using the probing test in what can be saved and what genuinely needs replacing.

If you are commissioning new joinery, ask for the quote three ways: all softwood, all Accoya, and Accoya on the cill, bottom rails and beads only. The third figure is usually the interesting one. Then check that the specification names stainless fixings and a paint system the timber manufacturer approves, because those two lines are where the fifty-year claim quietly stands or falls.

If you want someone to tell you honestly whether new timber is needed at all, our sash window restoration work starts by probing every member before anything is quoted, and the wider sash window repair service covers repairs, overhauls and new joinery across London.