Fire protection for wood: choosing between M1 impregnation and intumescent varnish
Wood is widely used in interior fit-outs, wall linings, ceilings, decorative partitions and in visible elements of buildings open to the public. When a project calls for protection against fire, the right treatment cannot be improvised: it depends directly on the nature of the substrate, its porosity, its finish and the look you are after.
In practice, two broad solutions are generally considered for the fire protection of wood : flame-retardant impregnation for wood, and intumescent varnish. The two approaches do not work in the same way and are not applied to the same substrates.
Key points
- On bare, absorbent wood, a flame-retardant impregnation can be considered.
- On dense, engineered or already coated wood, intumescent varnish is often the appropriate answer.
- The prescribed spread rate must be respected to obtain the expected performance.
- A poorly prepared substrate, or one altered after treatment, can compromise the final result.
- The choice of treatment must always be tied to the exact type of substrate and to the context of use.
- The technical documents and classification reports must be kept in the project file.
Bare absorbent wood → Flame-retardant impregnation | Dense or coated wood → Intumescent varnish
Why the choice of a wood flame-retardant treatment should not be left to chance
Not all woods respond in the same way to a flame-retardant treatment. Some substrates absorb the product well, while others are too closed, too dense, or already covered with a finish for proper impregnation to take place.
That is why it is not enough to pick an anti-fire product: you have to select a solution compatible with the actual material, with its surface condition and with the result you want. The right treatment is the one that matches the structure of the substrate and the constraints of the job.
To compare the solutions available, see: the whole Alfaflor flame-retardant range .
Technical mistakes that compromise a wood flame-retardant treatment
A few mistakes come up again and again on site and can stop you obtaining the expected result. They concern the substrate, the preparation or the way the product is applied.
- Treating engineered wood like solid timber : OSB, MDF and chipboard do not absorb an impregnation product in the same way.
- Applying a product to an already closed substrate : paint, varnish or primer can prevent penetration or adhesion.
- Not respecting the dose : too small a quantity reduces the performance sought.
- Altering the substrate after treatment : cutting, sanding or over-coating can damage the protected surface.
- Choosing a single product for every substrate without distinguishing the materials actually present.
On site: a wood flame-retardant treatment is not only a product, it is also a method of application. Substrate, spread rate and the steps of the work all have to be consistent with one another.
Flame-retardant impregnation: when to use it
Flame-retardant impregnation is intended for wood substrates able to absorb the product into their structure. It works at the heart of the material, without forming a visible film on the surface, which keeps the appearance of the wood where the substrate allows.
Our flame-retardant impregnation is applied by dipping, by full immersion in a tank: that is the only method validated by LNE classification report No. P258195-DEC/3, on bare softwood of the yellow pine type, 10 mm thick, faces and edges impregnated, at 5.5 kg/m². Pieces that cannot be immersed — a roof structure already erected, an element fixed in place — fall outside that protocol: contact us and we will look at the right solution.
How does impregnation work?
The product penetrates the fibres of the wood. Under high heat it acts on the behaviour of the material and helps limit combustion, according to the system in place. The logic differs from that of a varnish: here the protection does not rest on a layer at the surface but on the treatment being taken up into the substrate itself.
Which substrates are compatible?
Impregnation is above all relevant on bare, porous wood substrates. It can be used where the material really does let the solution penetrate.
- Bare solid timber
- Certain absorbent softwoods
- Roof structures
- Pallets or uncoated wood pieces, depending on the project
To reach this family of products, see: Alfaflor wood flame retardants .
Intumescent varnish: when is it the better fit?
When the wood is not absorbent enough, when it is dense or engineered, or when a surface treatment is more consistent with the substrate, intumescent varnish becomes the appropriate solution.
The principle of an intumescent varnish rests on the formation of a protective film. Under heat that film swells and creates an insulating foam that protects the wood substrate. The reaction forms a thermal barrier that slows the rise in temperature and the spread of flames.
This solution is particularly useful on panels, linings, dense woods and substrates where impregnation is not technically relevant.
See the dedicated range: Alfaflor intumescent varnishes .
Example of application conditions
- Single-coat application, depending on the product chosen
- Spread rate to respect: 200 g/m²
- Target fire classification according to the associated system
- Bare substrate, clean, dry and free of any incompatible earlier coating
| Product | Spread rate | Density | Indicative wet film thickness |
|---|---|---|---|
| Intumescent varnish | 200 g/m² | 1.23 | ≈ 162 µm |
This calculation is useful for checking the wet application when the job calls for close control of the quantities laid down.
What to do with wood that is already painted or varnished?
When a wood substrate is already covered with paint, varnish or another coating, the impregnation generally cannot penetrate properly into the material. In that case it is not the most consistent solution.
An intumescent varnish may be considered, depending on the condition of the substrate, the compatibility of the system and the conditions on site. Before any application it remains preferable to have the choice of treatment validated against the actual substrate and against what the project expects. The fire classification report is established on bare wood, neither painted nor varnished.
Practical tip: before starting work, identify precisely whether the wood is bare, painted, varnished, oiled or covered with a technical coating. This step avoids setting off with an unsuitable treatment.
Comparison table: M1 impregnation or intumescent varnish?
| Criterion | Flame-retardant impregnation | Intumescent varnish |
|---|---|---|
| How it works | Penetrates into the material | Works as a surface film |
| Typical substrate | Bare absorbent wood | Dense wood, panels, less absorbent substrates |
| Look of the wood | Can keep the natural look | Keeps the look, depending on the system used |
| Key condition | A genuinely absorbent substrate | Respecting the spread rate, and preparing the substrate |
| To avoid | A closed or already coated substrate | Application on a dirty or unprepared substrate |
Application advice to secure your project
Whatever system you choose, consistent application and respect for the method remain essential. A good product does not make up for insufficient preparation or a poorly controlled dose.
- Identify the substrate precisely : solid timber, panel, bare substrate or already coated.
- Check the surface condition : cleanliness, moisture, presence of an earlier coating.
- Choose the right product family according to how much the material actually absorbs.
- Calculate the areas to be treated before work starts.
- Respect the spread rate or the coverage associated with the chosen system.
- Keep the references and technical documents after application.
For projects calling for controlled application over large areas, see: the wood treatment service by Alfaflor.
What helps you prepare the compliance of your project?
Compliance does not depend on the chosen product alone. It also rests on consistency between the substrate, the application and the records kept once the work is done.
- Calculate areas accurately
- Respect the coverage associated with the treatment
- Do not alter the protected surface after application
- Keep the technical documents and classification reports supplied
- Check in advance that the substrate and the treatment match
To go further, see also: our page on the fire safety standard and the classifications that go with it.
In summary
Choosing between a flame-retardant impregnation and an intumescent varnish comes down above all to analysing the substrate. On bare, absorbent wood, impregnation may be appropriate. On dense wood, on already coated wood or on certain panels, intumescent varnish is generally more consistent.
The quality of the result then depends on three things: the right product, the right substrate and the right spread rate. To prepare your project, go straight to the Alfaflor wood flame retardants and the intumescent varnish range.