Thermally Modified Wood vs Accoya for Coastal Builds
By Special Contributor
Thermally Modified Wood vs Accoya: Which Modified Timber Suits a Coastal Build?
Compare thermally modified wood and Accoya on durability, movement, saltwater limits and fixings before specifying either on a coastal elevation.
Salt spray, fog and ultraviolet punish exterior timber far harder within a mile of the water than anywhere inland. Modified wood is the usual answer, and it splits into two products that reach stability by opposite routes.
Accoya is acetylated radiata pine, chemically altered so the cell walls have far less left to bond with water. Thermally modified timber is cooked, with nothing added to it at all. Choosing between them comes down to how wet the elevation gets and how much you are prepared to spend.
What actually separates them
The Thermo-D process, defined in the ThermoWood Handbook published by the International ThermoWood Association, takes Nordic pine and spruce through high-temperature drying from 100 to 212°C, holds them at 212°C plus or minus three degrees, then reconditions them back to a moisture content of 4 to 7%. The heat drives off the extractives that decay organisms feed on.
Accoya’s figures come from the Accoya Wood Information Guide, version 4.0, January 2026. Durability is Class 1 tested to EN 113 and to AS5604. Density is 515 plus or minus 80 kg/m3, and equilibrium moisture content at 65% relative humidity is 3 to 5%.
Dimensional stability is the standout gap. Accoya’s shrinkage measured to ISO 4469 is 0.7% radial and 1.5% tangential from wet to oven dry. On Accsys’s own comparison of a 145 mm board taken from soaked to oven dry, Accoya moves 2.2 mm tangentially against 13.1 mm for Siberian larch, 7.3 mm for western red cedar and 11.0 mm for Douglas fir. If you have watched cedar siding cup on a beachfront wall, that spread is the explanation, and it is the reason Accoya cladding gets specified on exposed elevations in the first place.
Where the hard limits fall
Both materials resist decay for the same underlying reason. Saprotrophic fungi become active once timber moisture content exceeds 20%, and thermally modified timber stays significantly below 20% irrespective of the humidity around it.
They part company over contact with water and ground. The International ThermoWood Association states plainly that ThermoWood is not recommended for structures in direct contact with soil or water, and its suitability table places Thermo-D at use class 3.2, above ground in prolonged wetting conditions, which covers cladding and decking and stops there.
Accoya goes further out. Accsys rates it for EN 335 use classes 1 to 4 and AWPA use categories UC1 to UC4A, while its use class table extends to class 5, contact with salt or saltwater splash zones. The warranty covers splash-zone use such as marina decking, and Accoya is suitable in full saltwater immersion, though the warranty excludes attack by marine organisms. Accsys reports more than a decade of testing in Northern European waters without attack, a 10-year service life position for that region, and US Pacific coast testing still ongoing with no attack after four years.
What goes wrong, and with what
Neither material is flawless. Thermally modified boards are not available as strength-graded timber and must not be used for load-bearing structures, and the caramel tone weathers to silver-grey without a UV-stable finish. Accoya carries a real price premium, gives off a faint vinegar smell when freshly cut, and its residual acetic acid attacks ordinary steel.
The fixings point is the coastal-critical one. Accsys instructs stainless-steel fasteners conforming to EN 10088-1, in A2 or A4 quality, and adds that in coastal regions or near highly polluted industrial zones you should always use A4. Its environment table ticks A4 for coastal outdoor use and leaves A2 blank in that column, while its application table accepts A2 for cladding generally. Worth checking what your supplier actually ships: UK charred timber producer TimberSol sells both materials but stocks A2 stainless cladding screws, which are fine inland and short of the coastal specification.
Which material for which job
For cladding above ground on a ventilated cavity, both perform well. Thermowood is the cheaper route to a dark facade, and black thermowood cladding takes pigment and charring readily, which is why it has become the common compromise on projects that want a modern dark exterior without the Accoya budget.
Decking near the waterline is a different question entirely. Accoya wins outright there, because Accsys warrants splash-zone use while the ThermoWood association rules its own product out of water contact altogether.
Whichever you specify, the detailing decides the outcome. A ventilated cavity is required to maintain the Accoya warranty, at a depth of a quarter to three quarters of an inch depending on climate zone. Seal end grain on every cut, use a UV-stable finish or a charred surface, fix with A4 near the sea, and plan the re-oiling before the scaffolding comes down. Salt air punishes shortcuts.
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