- Does it need oiling? No — oiling is optional and purely for colour. Has zero effect on durability either way.
- Does it need treating? Never — no preservative, no fungicide, no biocide ever. The thermal modification is the treatment, done at the factory for life.
- Does it need painting? No — ThermoWood should not be painted. Paints form a surface film that traps moisture and fails earlier than the timber itself.
- Why is it going grey? Normal UV weathering — cosmetic only, not decay. Does not affect durability or service life. Self-maintaining once established.
- How to stop it going grey: Apply Koopmans Houtolie UV oil every 3–5 years. Slows greying significantly but does not stop it permanently.
- Cleaning: Every 1–2 years — soft brush and mild pH-neutral wood cleaner, top to bottom. Never harsh acid or alkali cleaners.
- Service life: 30 years BRE endorsed. The same whether oiled or left to weather naturally.
The most searched maintenance question about ThermoWood cladding in the UK is whether it needs oiling — and the answer that circulates widely online is often wrong. Several guides claim ThermoWood requires oil treatment every 2–3 years for durability, that its service life shortens without maintenance, or that it should be painted. None of this is correct. ThermoWood cladding requires zero maintenance for durability throughout its entire 30-year BRE endorsed service life. Oiling, cleaning, and periodic inspection are options — aesthetic choices and good housekeeping — not structural requirements. This guide sets the record straight on everything: what ThermoWood actually needs, what is optional, what to do about greying, how to clean it correctly, and how its maintenance profile compares to every alternative.
Why ThermoWood Needs No Preservative Treatment — Ever
To understand why ThermoWood cladding requires no treatment, you need to understand what the thermal modification process actually does to the timber. ThermoWood is Scots pine (Pinus sylvestris) that has been heated to 190–212°C in a controlled steam environment — without any chemical additives. At these temperatures, three permanent changes occur that determine everything about its maintenance requirements:
- Hemicellulose breakdown: Hemicellulose is the primary food source for the wood decay fungi (Basidiomycetes) that cause structural timber decay. The high-temperature process permanently destroys the hemicellulose content in the timber. Without hemicellulose, the timber is biologically hostile to decay fungi — they cannot colonise it. This is the fundamental reason no preservative fungicide is ever needed
- Moisture content reduction: The modification reduces the timber's equilibrium moisture content from approximately 12% (standard softwood) to 4–6%. This dramatically reduces the moisture cycling — the expansion and contraction as the timber absorbs and releases moisture — that drives surface coating failure, splitting, and biological vulnerability in untreated softwood
- Durability upgrade: The combined effect of hemicellulose loss and moisture stability upgrades the timber from Durability Class 4–5 (standard pine — not durable) to Durability Class 2 (Durable) under BS EN 350. This is the same durability class as oak. Durability Class 2 means the timber is suitable for above-ground exterior use throughout its service life without preservative treatment
These changes are permanent and irreversible. They are built into the cellular structure of every board. No surface coating can replicate them, remove them, or improve on them. This is why the maintenance question — does ThermoWood need treating? — has a simple and definitive answer: no, because the treatment has already happened, at the molecular level, at the factory.
ThermoWood cladding maintenance is entirely cosmetic — it affects how the cladding looks, not how long it lasts. Oiling maintains the warm honey-brown colour. Cleaning removes surface deposits. Neither activity has any effect on the timber's Durability Class 2 performance or its 30-year BRE endorsed service life. A ThermoWood elevation that has never been oiled or cleaned is performing identically to one that has been meticulously maintained — the only difference is visible on the surface.
ThermoWood Cladding Maintenance Schedule — Complete Guide
Durability Class 2 — no fungicide, biocide, or chemical preservative ever required. The thermal modification is the treatment, permanent and factory-applied.
ThermoWood should not be painted. Paints form a surface film that traps moisture behind it as the boards move. Film failure leads to peeling, moisture ingress, and surface staining. Pigmented penetrating oils are the correct product if colour is wanted.
ThermoWood does not develop the rough fibrous surface breakdown of treated softwood. No sanding back, stripping, or surface preparation is needed at any point in the service life.
Only if maintaining the warm honey-brown colour. Slows natural greying. Has zero effect on durability if skipped. South-facing elevations: every 3 years. North-facing: every 4–5 years.
Brush and wash to remove algae, surface deposits, and organic matter from shadow gaps and board faces. Purely cosmetic — no effect on service life if skipped, but keeps the elevation looking well-maintained.
Check A4 stainless fixings are tight and no boards have shifted. ThermoWood's reduced moisture movement means fixings stay stable — but a periodic check is good practice on any exterior timber elevation.
ThermoWood Cladding Going Grey — What Is Happening and Is It a Problem?
The greying of ThermoWood cladding is the maintenance question we receive most frequently — and it is almost always framed as a problem requiring a solution. It is not. The greying of ThermoWood is a normal, expected, and structurally irrelevant process that affects only the cosmetic appearance of the surface. Understanding exactly what is happening prevents unnecessary intervention and unnecessary expenditure.
The science of greying
When ThermoWood is installed, its surface contains lignin — the structural polymer that gives wood its brown colour. Ultraviolet radiation from sunlight drives a photochemical reaction that degrades the surface lignin over time, bleaching the colour and eventually producing the characteristic silver-grey patina. The process is identical to the greying of Siberian larch, Douglas Fir, and all naturally durable or modified timber species in UV exposure. It affects only the outermost surface layer — typically less than 0.5mm deep on ThermoWood.
The silver-grey beneath this surface layer is unchanged, structurally sound, Durability Class 2 timber. Scrape away the grey surface and the warm brown colour of the modified wood is immediately visible underneath. The grey is a colour change, not a material change. It does not spread deeper over time. It does not indicate moisture ingress, biological activity, or deterioration of any kind. A ThermoWood elevation that has turned silver-grey is performing identically to a freshly installed one.
Is the greying even? What if it looks patchy?
ThermoWood greying is influenced by UV exposure, which varies across an elevation. South and west-facing sections grey faster than north-facing sections. Areas under roof overhangs or in deep shadow grey more slowly than exposed sections. On a large or complex elevation this can produce some variation in the degree of grey at any given time — one section may appear silver-grey while an adjacent section still shows warm brown tones.
This is normal and self-resolving. Within 18–24 months of installation, all sections of an elevation will have progressed to a consistent silver-grey regardless of initial exposure variation. If colour consistency before this natural equilibrium is a project requirement, the solution is to oil the full elevation at installation — this slows and evens the greying process across all aspects simultaneously.
Several third-party guides claim that ThermoWood's service life shortens dramatically without oiling and that it requires treatment every 2–3 years. This is incorrect and contradicts Lunawood's own technical documentation and the BRE assessment. The greying of ThermoWood cladding is cosmetic. The 30-year BRE endorsed service life applies whether the cladding is oiled or not. Do not apply unnecessary treatment products on the basis of inaccurate advice found elsewhere online.
How to Oil ThermoWood Cladding — If You Want to Maintain the Colour
If you want to maintain the warm honey-brown colour of ThermoWood cladding, oiling is the correct approach. The right product, applied at the right time, in the right conditions, will significantly slow the greying process and keep the elevation looking warm and characterful for years longer than untreated boards.
The right product
Koopmans Houtolie is our recommended oil for ThermoWood cladding — a penetrating linseed-based wood oil with UV stabilisers. It penetrates into the modified wood fibres rather than forming a surface film, and the UV protection works from within the timber. It does not crack, peel, or blister. It is available in clear (natural wood tone) and several pigmented shades. Do not use paint or varnish — these form surface films that trap moisture behind them as ThermoWood's surface moves through seasonal moisture cycles, leading to film failure and surface staining.
When to apply
- First application: Before installation or within 3 months of installation, before significant greying begins. This is the most important intervention — fresh ThermoWood absorbs oil deeply and the UV protection is most effective when applied to an unweathered surface
- Subsequent applications: Every 3 years on south and west-facing elevations with high UV exposure. Every 4–5 years on north-facing and sheltered elevations. Apply in dry conditions — minimum 48 hours dry weather before and after. Spring and early autumn are ideal — avoid midsummer heat which can cause oil to dry on the surface before penetrating
- If ThermoWood has already greyed: Oil applied to greyed ThermoWood will temporarily darken the surface (wet look) but will not restore the original brown colour. The surface lignin has already been degraded by UV and cannot be reversed by oiling. If colour restoration is genuinely required on a greyed elevation, light abrasion followed by a pigmented oil in a warm brown tone is the most effective approach
How to apply
Clean thoroughly with a mild pH-neutral exterior wood cleaner to remove all dust, algae, and surface deposits before oiling. Oil applied over contaminated surfaces penetrates unevenly and can trap organic matter against the timber surface. Allow 48 hours minimum drying time after cleaning before applying oil.
Oil in dry, overcast conditions — not in direct strong sunlight (oil dries on the surface before penetrating) and not when rain is forecast within 24 hours. Optimal temperature range is 10–25°C. Cold conditions below 5°C significantly slow penetration and can prevent proper absorption.
Apply Koopmans Houtolie by brush along the board grain direction, working methodically from top to bottom. A flat brush gives the most even coverage. Work oil into shadow gaps and board edges — end grain absorbs oil faster than face grain and benefits from additional application. Wipe away any excess that pools in corners or recesses.
Allow 24–48 hours before the cladding is exposed to rain. The oil needs time to penetrate and cure in the timber. A second coat on the first application — applied within 4–6 hours of the first while the surface is still slightly tacky — significantly improves penetration depth and UV protection longevity.
How to Clean ThermoWood Cladding
Cleaning ThermoWood cladding is straightforward but requires more care than cleaning standard softwood — ThermoWood's modified surface is more sensitive to harsh acid or alkali-based cleaners. The standard brush-and-wash approach used for spruce can damage ThermoWood's surface if the wrong product is used.
Routine cleaning (every 1–2 years)
Use a dilute mild pH-neutral exterior wood cleaner — diluted more generously than the standard rate for softwood. Apply with a soft brush, working from the top of the elevation downward. Scrub gently along the grain direction. Rinse thoroughly with clean water from top to bottom. Allow to dry fully before any oil application.
For algae or biological staining (north-facing and shaded elevations)
Apply a diluted biocidal exterior wood cleaner — use a milder dilution than for spruce and test on a small inconspicuous area first. Allow to dwell for 10–15 minutes, then scrub gently and rinse thoroughly. On heavily shaded elevations where algae returns quickly, improving air circulation by trimming back overhanging vegetation is more effective than more frequent chemical treatment.
Pressure washing
Acceptable for ThermoWood — fan-tip nozzle only, low-medium pressure, minimum 30cm distance from the boards. Work top to bottom. Never use a concentrated jet nozzle which can damage the modified surface layer and force water behind boards into the cavity. After pressure washing, allow 48 hours drying time before oiling.
What not to use
- Acid-based cleaners (brick acid, stone acid): React with ThermoWood's modified surface and cause irreversible discolouration
- High-alkali cleaners: Cause surface discolouration and can raise the grain
- Undiluted bleach: Damages timber fibres and causes uneven discolouration — particularly visible on ThermoWood's modified surface
- Abrasive scouring pads or wire brushes: Scratch and damage the modified surface layer
ThermoWood Maintenance vs Other Species — Comparison
| Species | Preservative needed? | Oiling | Cleaning | 30-yr maintenance cost/m² | Scaffolding for maintenance? |
|---|---|---|---|---|---|
| ThermoWood | Never | Optional 3–5 yrs | 1–2 yrs | £0 | Never |
| Siberian Larch | Never | Optional 2–3 yrs | 1–2 yrs | £0–£10 | Never |
| Charred Timber | Never | Never | 2–3 yrs (gentle) | £0 | Never |
| Nordic Spruce | Every 2–3 yrs | Included in treatment | Before each treatment | £45–£75 + scaffolding | Yes — every 2–3 years on house elevations |
The maintenance comparison illustrates the practical significance of specifying ThermoWood or larch over treated softwood on a house elevation. For a full species-by-species cost and lifespan comparison see our timber cladding lifespan guide and our ThermoWood vs Siberian larch guide.
Maintenance by Elevation Aspect — South, North, East, West
ThermoWood's maintenance requirements do not vary by elevation aspect for durability — but they do vary for colour retention, because UV exposure drives the greying rate and UV exposure is aspect-dependent.
| Aspect | UV exposure | Greying rate | Oiling frequency (if wanted) | Cleaning frequency |
|---|---|---|---|---|
| South-facing | Highest | Fast — 12 months to grey | Every 3 years | Annually |
| West-facing | High (afternoon sun) | Medium-fast — 12–18 months | Every 3–4 years | Every 1–2 years |
| East-facing | Medium (morning sun) | Medium — 18–24 months | Every 4 years | Every 1–2 years |
| North-facing | Low | Slow — 24–36 months | Every 4–5 years | Every 1–2 years (algae risk higher) |
Note that north-facing elevations, while they grey more slowly, are more prone to surface algae accumulation due to slower drying and less UV. This does not require more intensive maintenance — just a slightly more attentive cleaning approach when algae is visible. See our timber cladding cleaning guide for the detailed approach.
Does ThermoWood cladding need oiling?
No — oiling is optional and purely for colour maintenance. It has zero effect on ThermoWood's durability or service life either way. If you are happy with the natural silver-grey weathered appearance, no oiling is ever required throughout the 30-year service life.
Does ThermoWood cladding need treating?
Never — the thermal modification is the treatment, permanent and factory-applied. No fungicide, biocide, or preservative stain is required at any point. ThermoWood is Durability Class 2 (Durable) without any surface treatment throughout its 30-year BRE endorsed service life.
Why is my ThermoWood cladding going grey?
Normal UV photochemical weathering — the same process that greys all exterior timber. It affects only the surface layer (less than 0.5mm) and has no effect on durability or service life. Self-maintaining once established. To prevent greying, apply Koopmans Houtolie UV oil every 3–5 years. To embrace the grey, do nothing.
Can ThermoWood cladding be painted?
Technically yes, but it should not be. Paints form surface films that trap moisture as ThermoWood's surface moves seasonally, leading to film failure, peeling, and surface staining. If a coloured finish is required, use a pigmented penetrating oil — not paint. Koopmans Houtolie is available in a range of pigmented finishes that penetrate the timber rather than sitting on the surface.
How often should ThermoWood cladding be cleaned?
Every 1–2 years for most UK elevations. Use a dilute mild pH-neutral exterior wood cleaner — not harsh acid or alkali products which damage ThermoWood's modified surface. Clean top to bottom, rinse thoroughly. Allow 48 hours drying before oiling.
How long does ThermoWood cladding last without maintenance?
30 years — the full BRE endorsed service life applies with zero maintenance. The durability comes from the thermal modification, not from surface coatings. ThermoWood that has never been oiled, treated, or cleaned will last the same 30 years as one that has been meticulously maintained — the only difference is cosmetic.
ThermoWood Cladding — UK Stock, Zero Maintenance, 30-Year Service Life
We supply Lunawood ThermoWood cladding in all profiles from UK stock. FSC and PEFC certified. We also stock Koopmans Houtolie for those who want to maintain the warm honey-brown colour. Nationwide delivery in 7–14 days.