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Fire Performance of Biobased Materials

What Euroclasses mean, what Part B asks for, and how to specify timber, hemp, and natural insulation safely

5 min readUpdated November 2026

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The short answer

Biobased materials are not automatically unsafe, and they are not automatically safe either. Many are combustible, which limits where they can go. Whether a design is acceptable depends on the classification of the actual product, the build-up it sits in, the height and use of the building, and its distance from the boundary.

"It's natural" and "it's fire treated" are not fire data. A specification needs a classification report or test evidence that matches the product and how it will be used.

Watch out. This guide is an introduction, not a fire strategy. It summarises guidance for England as of November 2026. Wales, Scotland, and Northern Ireland have their own guidance. Confirm the design with your building control body or a fire engineer.

Reaction to fire and fire resistance are different

These two terms are often confused, and a material can do well on one and poorly on the other. Heavy timber is a good example: it is combustible, but it can keep carrying load for a predictable time in a fire.

  • Reaction to fire

    Question it answers
    How readily does it ignite and spread flame?
    Applies to
    A product, tested to BS EN 13501-1
    Example
    B-s3, d2

  • Fire resistance

    Question it answers
    How long does it keep doing its job in a fire?
    Applies to
    An element or assembly, such as a wall or floor
    Example
    REI 30

How to read a Euroclass

A Euroclass has three parts. The letter describes combustibility, from A1 (non-combustible) to F (no performance determined). The s number describes smoke, from s1 (least) to s3 (most). The d number describes burning droplets, from d0 (none) to d2 (most). All three matter: B-s1, d0 and B-s3, d2 are not the same result.

The older national classes, such as Class 0, have been removed from Approved Document B for reaction to fire. If a product sheet only quotes them, ask for a current BS EN 13501-1 classification. Removal of national classes for fire resistance takes effect on 2 September 2029, subject to transitional arrangements.

How common biobased materials tend to perform

These are starting points to help you ask the right questions. Results vary between products, so use the supplier's classification rather than this table.

  • Untreated timber cladding

    Typical position
    Around Euroclass D
    What to check
    Thickness, density, fixing, and whether treatment is needed for the location

  • Fire-retardant treated timber

    Typical position
    Can reach Euroclass B
    What to check
    The full class including s and d, and durability of the treatment outdoors

  • CLT and heavy timber

    Typical position
    Combustible, but design often assumes a charring rate of about 0.65 mm/min
    What to check
    Fire resistance design, exposed areas, and connections

  • Wood fibre insulation

    Typical position
    Often Euroclass E
    What to check
    The product's class and the tested build-up

  • Sheep's wool insulation

    Typical position
    Often Euroclass E
    What to check
    The classification standard, as older sheets may use other tests

  • Cellulose insulation

    Typical position
    Ranges from B-s2, d0 to E by product
    What to check
    The specific product and installation method

  • Hempcrete and other plastered biobased walls

    Typical position
    Varies with the mix, finish, and thickness
    What to check
    Test evidence for the same mix and build-up

  • Straw bale

    Typical position
    Plastered walls differ greatly from exposed bales
    What to check
    Tested wall build-up, and protecting bales during construction

Typical behaviour. Always check the product's own test evidence.

What Approved Document B says about external walls

Height is the main dividing line for dwellings. The table below summarises the main provisions in Approved Document B Volume 1 (Section 10). Other routes and exemptions exist, so treat it as a guide to the conversation with building control.

  • 11m or less

    B-s3, d2 or better
    External surface 1m or more from boundary
    No provisions
    Insulation and filler in the wall
    No specific limit in paragraph 10.7

  • More than 11m

    A2-s1, d0 or better
    External surface 1m or more from boundary
    A2-s1, d0 or better
    Insulation and filler in the wall
    A2-s1, d0 or better for a storey at 11m or more, with an exception for compliant masonry cavity walls

  • Relevant building with a storey at 18m or more (regulation 7)

    A2-s1, d0 or A1
    External surface 1m or more from boundary
    A2-s1, d0 or A1
    Insulation and filler in the wall
    All materials must meet A2-s1, d0 or A1, other than those exempted by regulation 7(3)

Approved Document B Volume 1, England. Heights refer to the storey or building measure defined in the document.

Note. Rules are under review. A Building Safety Regulator consultation on Approved Document B, which closed on 1 July 2026, covered external walls and the scope of the combustible materials ban. No outcome had been published when we last checked, so check the latest position on GOV.UK.

Inside the building and on the roof

Exposed wall and ceiling linings have their own limits. In dwellings, most rooms and circulation spaces need C-s3, d2, small rooms up to 4m2 can use D-s3, d2, and other circulation spaces such as common areas in flats need B-s3, d2. This affects exposed CLT and timber boarding.

Thatch and wood shingles with no established test performance are treated as EROOF(t4). That normally means keeping them at least 6m from the relevant boundary, with further limits on larger buildings and terraces. Approved Document B also notes that closer distances can be considered where precautions such as those in the Dorset Model are used.

Why the build-up matters

A product's rating does not automatically transfer to the finished wall. Cladding that reached a class in one test arrangement may behave differently over a different substrate or air gap. Fire barriers and cavity barriers matter too, particularly behind ventilated cladding.

  • Does the test report match the product, thickness, density, substrate, and fixing proposed?
  • Is the classification from a current report, and who issued it?
  • If a treatment is claimed, was the treated product tested, and is it suitable for external use?
  • Where do cavity barriers go, and do they suit the cladding and insulation?
  • Have any substitutions been checked against the fire strategy?

Tip. Raise fire early. Changing a cladding or insulation late in a project can affect the wall build-up, moisture strategy, and budget at the same time.

Using Matera

Before you shortlist a product, ask the supplier for its BS EN 13501-1 classification report and details of the tested build-up. Keep these alongside the carbon data so that fire and sustainability are compared together.

Sources

Where the facts in this guide come from. Standards and guidance change, and some sources are suppliers or trade sites, so check the original before relying on a detail.

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