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Is hempcrete fire resistant?

5 min read

The fire performance of hempcrete — what tests show, including our own fire test, and how it compares to other materials.

Fire resistance is a critical consideration for any building material, and it's a question that comes up consistently when architects, builders, and self-builders are evaluating hempcrete. At first glance, the presence of hemp shiv, a plant-derived, organic material, might raise concerns. In practice, the picture is far more reassuring than the raw ingredients suggest.

Why Hempcrete Behaves Differently to Its Components

Understanding hempcrete's fire performance starts with its composite structure. Hemp shiv in its loose form is flammable. That much is true. But in a finished hempcrete mix, the shiv is fully encased within a mineralised lime binder, and this encapsulation fundamentally changes how the material responds to heat and flame.

The density and nature of hempcrete, with the plant-derived component locked inside its binder matrix, makes it very difficult to set fire to. The lime binder acts as a protective shell around the organic particles, limiting their exposure to oxygen and slowing combustion significantly.

There's a further structural advantage in cast-in-situ hempcrete construction. Because hempcrete is typically cast around a timber frame, the hempcrete wall itself forms a physical barrier between any external fire and the load-bearing timber structure, protecting the building's structural integrity even if the outer surface of the wall is exposed to flame.

Fire resistance also isn't static: it improves over time. As the lime binder continues to carbonate after construction, fire-resistance performance increases. French manufacturer Isochanvre has long characterised hempcrete as effectively non-flammable on this basis, though that specific claim comes from the manufacturer rather than independent test data.

What the Tests Show

Formal fire-resistance testing of hempcrete has been limited in scope, and most of the available data comes from tests carried out on proprietary products, both precast and cast in situ, conducted to demonstrate compliance with national regulatory frameworks. Despite this, the results are consistently positive.

BRE Group Test (UK, 2009)

One of the most significant tests was conducted by the Building Research Establishment (BRE) in 2009, on a 3m × 3m wall of Tradical® Hemcrete® (Test Report 250990). The wall was:

  • Non-rendered and non-plastered
  • Cast from layers of hemp-lime mix poured into a mould
  • Reinforced with eight vertical timber studs
  • Subject to a vertically imposed load of 135kN
  • Tested in accordance with BS EN 1365-1:1999

The internal face of the wall was exposed to fire. The wall resisted for 73 minutes in respect of integrity, insulation, and load-bearing capacity, comfortably exceeding the common 60-minute fire-resistance threshold required in many building regulations.

Hempcrete Block Wall Test (France)

Testing carried out in France examined a 250mm-thick wall constructed from hempcrete blocks laid in lime mortar. This wall remained intact for 1 hour 40 minutes before the mortar joints failed. No re-ignition occurred after the test, and no toxic gases were emitted. We haven't been able to independently trace the original test report for this one, so treat the specific timing as reported rather than independently verified.

The authors of this test observed that a solid cast wall of hempcrete would be expected to offer superior fire resistance to a block-and-mortar construction, since there are no mortar joints to represent a weak point.

ASTM Testing (USA)

This is the section most worth being precise about, because two different things get conflated online: a formal standardised test, and an informal demonstration.

The real, nameable test is ASTM E84-19B, conducted for Idaho-based Hempitecture in February 2020. This test measures flame spread and smoke development on a 0 to 450 scale, where 0 is the best possible result. Hempcrete scored 0 on both indices. The testing agency reportedly noted that the only material they'd previously seen perform this well was mineral wool.

More recently, in July 2024, Pennsylvania-based Americhanvre ran a full-scale ASTM E119 one-hour fire resistance test at an Intertek facility: a 12-inch spray-applied hempcrete wall assembly, finished with lime plaster on both faces, was exposed to 1,700°F for sixty minutes followed by a hose-stream test. The exterior face reportedly never rose above ambient temperature during the test, and the wall passed.

Separately, and this is the demonstration that often gets confused with the above, there are informal tests involving a direct flame (blowtorch or open fire) applied to a hempcrete surface, showing minimal surface damage after several minutes. These are useful as illustrations of the encapsulation effect described above, but they aren't standardised tests and shouldn't be cited as if they were.

One of these informal demonstrations is our own. We put a hempcrete block onto a roaring fire with a piece of timber inside it, left it for 75 minutes, then broke the block apart. The timber was untouched and could be picked up by hand straight away. You can watch it here: our 75-minute hempcrete fire test.

The Cladding Burn Test

One illustrative real-world fire scenario involved a hempcrete wall with an external timber cladding system. The cladding burned away, as did the cladding battens, and the supporting frame sank flush into the face of the wall. But the hempcrete itself did not combust, and neither did the structural timber frame on the internal face of the wall, a striking demonstration of hempcrete's capacity to contain a fire and protect the structure behind it.

Gaps in the Current Evidence Base

Hempcrete's fire performance isn't fully characterised yet. Testing to date has been relatively limited, and several variables haven't been systematically studied:

  • The effect of different binders (natural hydraulic lime, pozzolanic mixes, cement-lime blends) on fire resistance
  • The influence of varying mix ratios, specifically the hemp-to-binder ratio
  • How render and plaster finishes affect fire performance, both positively and potentially negatively
  • How results from different studies can be meaningfully compared, given that testing methodologies and product specifications vary

The lack of a standardised testing procedure that accounts for hempcrete's unique material properties, and accommodates the range of shiv types, binder materials, mix ratios, and application techniques in use, makes direct comparison between studies difficult. Developing such a framework would be a valuable step forward for the industry.

Hemp shiv also behaves differently in different forms. Hemp paper, for instance, appears somewhat less flammable than comparable plant-based materials in isolation, though this doesn't change the need for robust, material-specific testing standards for hempcrete as a finished construction product.

Practical Takeaways for Builders and Specifiers

Based on the evidence currently available, here's what can be said with reasonable confidence:

  • Hempcrete is difficult to ignite in its cast form, due to the encapsulation of hemp shiv within the mineralised lime binder.
  • Test data supports a 60-minute fire-resistance rating for cast-in-situ hempcrete, with individual tests exceeding this (73 minutes in the 2009 BRE test; a full hour at 1,700°F in the 2024 Americhanvre ASTM E119 test).
  • 90- or 120-minute fire resistance may be achievable through adjustments in specification, though this requires further testing to confirm.
  • Hempcrete improves in fire resistance over time as the lime binder carbonates, a property not shared by many conventional materials.
  • No toxic gases were recorded in the French block wall test, a meaningful consideration for occupant safety during a fire event.
  • Renders and plasters may further improve performance, but their specific contribution hasn't yet been formally quantified.

For projects where fire resistance is a regulatory requirement, the existing test data provides a reasonable basis for specifying cast-in-situ hempcrete at 60 minutes. Specifiers seeking ratings beyond this should work closely with their certifying body and, where necessary, commission product-specific testing.

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