Acetylated Wood: Durability, Fixings and Carbon Data for Designers

Acetylated wood is solid timber chemically modified so that it takes up far less water than ordinary wood. Designers meet it in exterior cladding, decking, windows and doors, and in some structural and water-contact uses.

This guide sets out what Accsys's own documents (2020 editions, with the 2025–2026 editions where they differ), an ICC-ES evaluation report, two 2026 EPDs and one independent review abstract say, and where they stop. Nearly every performance number comes from the manufacturer, so we label it as such. We found no other producer of acetylated solid timber in the documents we read, so there is no cross-manufacturer comparison.

What is acetylated wood and how is it made?

Acetyl groups replace hydroxyl groups

The Accsys guide explains that hydroxyl groups in wood attract water, which makes cellulose swell and shrink and makes the wood more open to attack by organisms. Acetylation reacts wood with acetic anhydride at high temperature to raise the level of acetyl groups, which repel water.^[1] The guide says the wood is modified uniformly through the cross section and the entire cell wall structure is altered.^[1] This is Accsys's description of its own process. A 2017 iForest review places acetylation among the chemical wood-modification processes, alongside furfurylation and resin impregnation, and also discusses how to assess the environmental impact of modification processes. We read its abstract only.^[10]

Process and by-products

In the Kingsport EPD, wood goes into a reactor with acetic anhydride for vacuum-pressure impregnation, about 18 hours at up to 155 °C, and is then dried in a kiln.^[4] The finished product is described as more than 99 % acetylated wood and less than 1 % acetic acid by mass.^[4] The 2015 EPD describes the Arnhem plant as an industrial symbiosis of acetic anhydride and acetic acid production, and allocates the by-product acetic acid economically between Accoya and the acid.^[6] The ICC-ES report says any softwood or hardwood able to reach the minimum bound-acetyl level in its Table 1 can be used, so the process is not tied to pine in that document.^[7]

Residual acetic acid

The guide says Accoya contains a small amount of residual acetic acid, and that batches are released only if the average residual acetyl compounds are 1.0 % or less (mass per oven-dry wood mass) and no individual sample exceeds 1.8 %.^[1] In wet conditions the wood has a pH of 4 to 5.5 in the guide's words, and the guide compares its acid level with oak and western red cedar.^[1] This matters for metals, sealants and adhesives, covered below.

How does acetylated wood perform?

Durability and use classes

Accsys's documents state durability class 1 under EN 350, with a laboratory test to EN 113 and ENV 807 and a field test to EN 252.^[1][2] EN 350 and EN 335 are the European standards for durability classification and use classes; we read only their catalogue entries.^[11][12] The guide says Accoya is suitable for use classes 1 to 4, which run from dry interior to ground or fresh-water contact.^[1] The warranty is 50 years above ground and 25 years in ground or fresh water, and it does not cover attack by marine organisms.^[1][4] The German structural approval (DIBt) described in Accsys's test summary allows outdoor use up to use class 3.2 provided dirt does not accumulate, and says use class 3.2 needs professional design including end-grain sealing.^[3] We did not read the DIBt approval, the FCBA certificate or the warranty. The test summary reports results from several institutes, including a 15- and 20-year Dutch canal-lining test and a 10-year Whakarewarewa field test; the January 2026 edition reports the canal tests at 21 and 26 years, inspected in 2021. We did not open the underlying reports.^[3][15]

Termites and insects

The test summary reports termite trials by LSU (Formosan termites), Timber Products Inspection in Florida, the Australian Forest Research Company and a university in Thailand, with mean mass loss of 0.2 to 0.7 % for Accoya against 82.6 % for untreated radiata sapwood in the Australian above-ground trial, and says it expects good performance south of the Tropic of Capricorn there.^[3] Its FCBA table lists both “not degraded by European subterranean termites” and “susceptible to termite degradation” under its EN 117 and EN 118 tests; the extracted text did not let us match each statement to a test, so we do not draw a conclusion from that table.^[3]

Dimensional stability

Accsys documents give oven dry to wet dimensional change of 0.7 % radial and 1.5 % tangential, and 0.4 % and 0.8 % from wet to 65 % RH.^[1][2] The guide's movement table uses a different basis (typical characteristic values, green to oven dry, citing an SHR report): 0.8 % radial and 1.6 % tangential for Accoya against 3.4 % and 7.9 % for untreated radiata pine, with a warranty maximum of 2.5 % for Accoya.^[1] We do not mix the two bases. The Teknos coating test (started March 2009) summarised by Accsys reports that after 10 years of natural exposure without maintenance, the coating on Accoya boards showed no significant deterioration, while the coating on western red cedar and Siberian larch had lost its protective function.^[3] Remmers maintenance schedules cited in the same summary go up to 10 years for translucent and 12 years for opaque finishes.^[3] These are Accsys-reported results.

Strength

The DIBt approval qualifies A1-grade Accoya as strength class C22 and A2 as C16 under EN 338, for widths of 100 to 250 mm and thicknesses of 25 to 100 mm, in service classes 1 to 3 and for static or quasi-static loads, with fatigue-relevant stresses excluded.^[1][3] B grade is not strength graded.^[1] Accsys states in its FCBA table that the process does not affect mechanical properties; the test summary also reports Timber Products Inspection's conclusion that Accoya's modulus of rupture and work to maximum load are slightly higher and its average modulus of elasticity slightly lower than untreated radiata pine, with the values shown only in a chart, which we did not read.^[3] The table below shows how the strength figures change basis from document to document.

Where the Accsys documents differ

All four columns come from one manufacturer, so this is not a cross-product comparison. Values measured to different standards or on different bases are kept in their own cells and not reconciled; the 2015 EPD's strength values are labelled EN 310 and the FCBA values are 5-percentile figures, which are not the same as an average or a characteristic value. The table uses the 2020 editions; Accsys's August 2025 datasheet gives an average density of 515 kg/m³ and Class D or E in Europe (see the fact sheet).^[14]

What do acetic acid and low moisture content mean for fixings, glue and coatings?

Metals. The guide says tests showed base metals and galvanised metals in direct or indirect contact with wood containing acids corrode in damp conditions, and strongly recommends stainless steel, corrosion-resistant aluminium or naval brass where moisture or condensation is present.^[1] It recommends EN 10088-1 stainless steel, 1.4301 (A2) or 1.4401 (A4), with A4 for coastal or heavily polluted areas, and says galvanised metals or zinc alloys are not corrosion-proof with Accoya.^[1] The ICC-ES report limits fasteners to Type 304 and 316 stainless steel and requires flashing or another non-corrosive barrier between Accoya and damp cement-based materials.^[7] The guide adds that acetic acid can concentrate in confined spaces such as lock housings.^[1]

Adhesives. According to the data sheet, polyurethane, EPI, epoxy and PRF adhesives give the best results, PVAc results vary greatly and MUF adhesives should be avoided.^[2] The guide says adhesives that need moisture to harden can be affected by the particularly low moisture content, and that acid-catalysed adhesives need advice from the adhesive supplier.^[1]

Sealants and coatings. The guide says rare cases are known where residual acetic acid influenced the curing or long-term performance of sealants, and strongly recommends a compatibility check by the sealant supplier for glazing and double-glazing sealants; a primer can improve adhesion on unfinished wood.^[1] The data sheet states that most commonly used coating systems can be used.^[2] Exposure to products above pH 9 can invalidate the warranty in the guide's words, and some fire retardants may too.^[1]

How does acetylated wood behave in fire?

The Accsys guide says Accoya falls within EN 14915, the harmonised standard for solid wood panelling and cladding, whose excerpt in the guide gives D-s2, d0 for boards of at least 390 kg/m³ above stated thicknesses and D-s2, d2 for the thinnest listed boards, and that the property covers the entire cladding system including fasteners, detailing, sub-frame and cavity.^[1] It says untreated chemically modified wood is classed D like other softwoods, that fire-retardant treatment can reach up to class B, and that the retardant's effect on this wood must be proven by an independent accredited body.^[1] Accsys's January 2026 guide changes this picture: it says that since August 2024 testing with the SBI test (EN 13823) and the single-flame-source test is required to classify any modified, preservative-treated or coated wood cladding, that Accoya boards of average density were rated Class E as 19 mm tongue-and-groove boards over a ventilated 40 mm cavity, about 5 % from the Class D threshold, and that Class D can be reached in some set-ups, such as 22 mm or thicker square-edged boards, a charred finish on 21 mm boards, intumescent or other fire-retardant coatings, or 15 or 19 mm profiles with a fire-retardant impregnation; it adds that the cladding manufacturer is responsible for the system's classification.^[13] The August 2025 datasheet gives Class D or E depending on thickness, profile and construction.^[14] We did not read EN 14915, EN 13501-1 or the classification reports the guide mentions. In the US, both guides report flame spread 95 and smoke developed 155 under ASTM E84, against 70 to 100 for several other woods in their table (southern pines 103 to 195).^[1][13]

How much carbon does acetylated wood hold?

What the EPDs show

We read two 2026 Accsys EPDs from SCS Global Services covering four products; SCS's directory also lists a third, SCS-EPD-10639 for Accoya Europe, which we did not read.^[16] Both are product-specific, cradle-to-gate with end-of-life and module D, based on 2024–2025 manufacturer data and PCR 2019:14 v2.0.1 (the Kingsport EPD prints the PCR's validity both as 2025 and 2030, the Color EPD as 2025).^[4][5] The Kingsport EPD names Lindita Bushi of the Athena Sustainable Materials Institute as verifier.^[4] The A1–A3 GWP-total is positive, because fossil emissions exceed the biogenic uptake, and the EPD's interpretation says A1 is driven by the upstream production of acetic anhydride and A3 by steam and electricity at Kingsport.^[4]

All values are kg CO₂e per m³ as printed, and the stages are declared separately. The Kingsport EPD says that use of the aggregated A1–A3 stage is discouraged without considering end of life, and we do not add the stages.^[4] We re-read the result tables (text and rendered page) and the printed components do not add up to the printed totals: for Accoya rough from Kingsport, A1–A3 GWP-fossil 1,060 + GWP-biogenic −816 + GWP-luluc 0.774 = 244.8, against a printed GWP-total of 259; for Color rough, 1,280 − 816 + 0.869 = 464.9, against 523. Module C4 shows the same pattern: 5.51 + 850 + 0.003 = 855.5 against a printed 170 for Kingsport, and 2.56 + 840 = 842.6 against 74.0 for Color. We report all figures as printed, do not say which is intended, and have not been able to ask Accsys or SCS.

Read the number with care

The Kingsport end-of-life scenario is 515 kg collected with mixed construction waste, of which 46.4 kg is recycled, 92.7 kg incinerated and 376 kg sent to final deposition, with a biogenic carbon removal of 827 kg CO₂ shown; the Color scenario is 271, 37.9 and 206 kg.^[4][5] Module D is negative (−934 and −474) because the EPDs credit substitution of virgin wood and exported energy; the Kingsport EPD says the primary raw material inputs are 100 % virgin wood.^[4] The Color EPD says Accsys buys renewable energy certificates, which are included in its figures, and offsets some emissions, which are not.^[5] The EPDs warn that EPDs based on different PCRs or calculation models may not be comparable.^[4][5] The EPDs for thermally modified wood on this site come from other programme operators and declare negative A1–A3 totals per m³ for a different product, so we do not set the figures side by side.

The 2015 Norwegian EPD is older (EN 15804+A1, issue and validity fields blank in the copy we read) and declares radiata, Scots pine and beech products.^[6] On 14 October 2026 the EPD-Global (formerly EPD-Norway) list returned no entry for NEPD-376-262 or for Accsys as owner; we could not tell whether that list shows expired declarations, and we do not use its figures. Its text states an expected service life of 60 years for external cladding and decking more than 20 cm above ground.^[6]

Which certificates apply?

Accsys's Cradle to Cradle Certified certificates are Full Scope Gold, standard version 3.1, for Accoya Wood and for Accoya Color, both assessed for professional installers only; the Accoya Wood certificate expires on 1 November 2026, less than three weeks after this page's date, and the Accoya Color one on 7 November 2026. The C2C registry entries, checked on 14 October 2026, show the same dates and also list Material Health Platinum for both.^[8][16] The FSC chain-of-custody certificate lists dimensional timber and solid wood among its product groups and says the certificate alone is not evidence that a given product is FSC-certified; FSC Search showed it as valid on 14 October 2026.^[9][16] We read no PEFC certificate; the 2015 EPD lists FSC and PEFC chain-of-custody certificates among the production facility's certificates.^[6] The ICC-ES report evaluates the wood for above-ground and ground-contact use as preservative-treated wood under US codes, with fasteners limited to Types 304 and 316 stainless steel.^[7] For structural use the DIBt approval and the FCBA certificate described by Accsys are summarised above; we did not open either.^[3]

When to specify it, and when not

Where the documents support it

• Cladding, decking and joinery where movement and coating life matter. The data sheet gives 0.7 % radial and 1.5 % tangential oven dry to wet change and says no finish is needed for longevity and stability, and Accsys reports a 10-year coating test and Remmers maintenance schedules favourable to Accoya.^[2][3] For comparison, our thermally modified wood profile reports that the ThermoWood handbook recommends a surface treatment for weather-exposed use.
• Ground or fresh-water contact. Accsys warrants 25 years in ground or fresh water, and its summaries cite a Dutch canal test inspected at 20 years (2020 edition) and 26 years (2026 edition); ICC-ES evaluates it for ground contact under US codes.^{[1][3][15][7]} The thermally modified wood profile reports that the ThermoWood handbook does not recommend contact with soil or water.
• Structural members within the approved sizes and static loading. DIBt-approved strength classes C22 and C16 exist for A1 and A2 grades, in the stated section sizes, in service classes 1 to 3, for static or quasi-static loads.^[1][3] For large spans or members outside those sizes, the documents we read give no basis; see glulam and cross-laminated timber. We found no sourced comparison of Accoya's structural performance with them.

Where it may not suit, or needs extra care

• Hardware and fixings. Stainless steel (304 or 316; A2 or A4 in the guide) or equally resistant metals are needed, and ordinary or galvanised steel is not recommended; the guide advises isolating less resistant hardware and warns of confined spaces such as lock housings and non-ventilated humid rooms.^[1][7]
• Glazing, sealants and glued assemblies. Ask the sealant and adhesive suppliers for compatibility with acetylated wood; the documents we read call for a check, not a universal pass.^[1][2]
• Fire. The classification belongs to the whole cladding system in Accsys's guides, and retardants may affect coatings, adhesives and the warranty.^[1] The 2026 guide reports Class E in one common build-up and Class D only in some set-ups.^[13] Where Class D or higher is required, specify a tested system.
• Marine and tropical conditions. The warranty does not cover attack by marine organisms, and the ICC-ES report requires the ground-contact treatment level for above-ground uses in tropical climates.^[1][7]
• Appearance. The guide says uncoated boards bleach and then grey, may look blotchy in between, grow surface mould in damp or shaded places, and can show rare reddish-brown extractive stains or green stains from copper preservatives in a sub-frame; boards have a discoloured zone up to about 6 mm deep from the process.^[1]
• Carbon. The A1–A3 figure is positive in Accsys's EPDs, driven by acetic anhydride, steam and electricity; the net depends on end of life.^[4] Treat any carbon claim as an EPD-by-EPD question, not a material property.

What we could not support

The documents we read give no guidance on climate zones beyond the termite note and the marine caveat, and no independent field data on service life beyond Accsys's test summary. We found no price, installation-cost or maintenance-cost figures and do not give any.

Where is acetylated wood used, and who makes it?

The 2026 EPDs name decking, cladding, siding, windows and doors as applications.^[4][5] We found no project documents we could verify for this profile, so it has no projects section. Accsys's resources page also lists a Tricoya flyer, an acetylated wood-fibre product; we did not read it and it is outside this profile.

Suppliers whose documents we read

Frequently asked questions about acetylated wood

The bottom line on acetylated wood

Acetylated wood, as Accsys documents it, is a stable, class 1 durable timber for exterior cladding, decking, joinery and some structural uses. Its trade-offs are practical: stainless fixings, sealant and glue checks, fire classification that needs a tested system, and a positive production-stage carbon figure in the EPDs we read. Ask for the batch's durability documentation, the current EPD and certificates, the fire test for your build-up and the fixing specification.

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