The 2025 Palisades and Eaton fires destroyed thousands of structures in the Los Angeles area, adding to a pattern that has become familiar across the American West, Australia, and Southern Europe: communities at the edge of wildland burning faster and more completely than anyone expected. Post-fire analyses consistently reveal the same finding. Most homes are not ignited by the fire front passing directly overhead. They are ignited by embers — firebrands carried on wind currents that can land up to a mile ahead of the fire front, accumulate in gutters, roof cavities, and deck joints, and smolder until conditions allow ignition. NIST’s Wildland-Urban Interface Fire Group has documented that ember exposure is the primary cause of structural loss in WUI fires, with embers entering structures through vents, gaps, and openings to ignite materials within.
What those materials are made of matters enormously. When synthetic foam insulation is exposed through a damaged vent or compromised siding, it can melt, release flammable volatiles, and accelerate the fire’s spread through the structure. A material that chars rather than melts — forming a protective carbon layer that insulates the underlying structure from further heat penetration — behaves differently and better. Mycelium composites do exactly this, and they grow on agricultural waste.
Why Mycelium Chars
The fire-resistant behavior of mycelium-based composites is not accidental — it follows from their chemical composition. Mycelium is the vegetative network of fungal threads, or hyphae, composed primarily of chitin — the same structural polymer found in insect exoskeletons and crustacean shells — along with proteins and polysaccharides from the organic substrate on which the fungus grows. Under heat, these components undergo a process called pyrolysis that produces a carbonaceous char layer rather than a liquid melt or a burst of flammable gases.
A foundational 2018 study in Scientific Reports by Jones and colleagues measured the thermal degradation and fire properties of fungal mycelium and mycelium-biomass composite materials using cone calorimetry, finding that the presence of mycelium significantly reduced heat release rates compared to the substrate alone, attributing this improvement to mycelium’s higher charring tendency, which acts as a thermal insulator and limits the supply of combustible gases to the flame front. A 2023 study in Polymer Degradation and Stability by Chulikavit and colleagues demonstrated engineered mycelium as a fireproofing agent for flammable fiber-reinforced polymer composites, confirming that mycelium’s char formation under controlled burn conditions provided measurable protection to underlying materials.
The char layer that forms on a burning mycelium composite does not simply slow ignition — it actively impedes further heat penetration, creating a self-limiting fire behavior that distinguishes mycelium from most synthetic insulation materials. Expanded polystyrene melts, flows, and feeds flames. Mycelium stops.
The Thermal Performance Question
Fire resistance is only part of the requirement for building insulation. A material that resists fire but transmits heat freely into a building is not useful for energy efficiency. Here the mycelium research is encouraging.
A 2025 study in the Journal of Materials Chemistry A documented mycelium-coir composites with thermal conductivity of 0.035 W/m·K — comparable to polymer foam insulation — with the pure mycelium surface film achieving an ultralow 0.015 W/m·K, below the thermal conductivity of still air, attributed to the nanoscale porous architecture of the chitin network. The same study confirmed the composites were more fire-tolerant and hydrophobic than conventional insulation materials. A 2024 review in Frontiers in Sustainable Cities documented mycelium-wood composites as building insulation, noting thermal conductivity values in the range of 0.06 W/m·K across multiple studies — within range of conventional mineral wool and significantly better than many bio-based alternatives. A 2025 study from IOP Publishing specifically tested mycelium composites using UL-94 and limiting oxygen index fire tests, assessing Euroclass classification potential and comparing performance directly against conventional insulation materials.
The WUI Connection
The cross-domain case for mycelium insulation at the wildland-urban interface is more specific than simply “bio-based and fire-resistant.” It rests on the specific failure modes of WUI structure ignition.
The 2026 California Wildland-Urban Interface Code — the most significant restructuring of California’s wildfire construction standards in decades — focuses heavily on materials at the building envelope: vents, soffits, siding, decking, and the insulation behind them. The code requires ignition-resistant materials with flame spread indices not exceeding 25 per ASTM E84. The mechanism of concern is what happens when embers breach the envelope — when they enter through an attic vent, accumulate beneath a deck, or smolder in a gap in siding — and contact insulation material. Synthetic foam insulation is particularly vulnerable at this interface because ember heat can cause it to melt and release flammable volatiles before the main fire has even arrived.
A mycelium insulation panel at this interface behaves differently. It does not melt. It chars, forming a protective layer. And because mycelium composites are grown rather than petrochemically synthesized, they produce minimal toxic volatiles under heat — an important consideration both for occupant safety and for the firefighters working in smoke from burning structures, who face documented elevated cancer risks from WUI fire exposures according to recent occupational health research.
What Remains Speculative
No mycelium insulation product has achieved commercial-scale deployment in WUI construction or received building code certification specifically for fire hazard severity zones. The thermal performance and fire resistance data available are primarily from laboratory samples, not field-installed building envelopes exposed to weathering, moisture cycling, and the mechanical stresses of real construction. Long-term durability in humid conditions is a concern: mycelium composites are biodegradable, and their performance in permanently damp wall assemblies has not been established. Scaling production to construction volumes while maintaining consistent density and fire performance across batches requires process engineering that has not yet been demonstrated commercially. Regulatory pathways through ASTM certification and building code adoption are lengthy. Cost competitiveness with mineral wool or foam board at scale remains to be established.
Why It Matters
More than 70,000 communities in the United States are classified as being at elevated risk from WUI fires. California alone updated its WUI building code in January 2026 precisely because the scale of destruction has made the inadequacy of existing materials standards undeniable. The rebuilding that follows major WUI fires — in Los Angeles, in Maui, in communities across the Western states — represents a direct opportunity to deploy better materials. A bio-based insulation panel that grows on agricultural waste, provides competitive thermal performance, produces a protective char rather than toxic volatiles under fire exposure, and sequesters carbon rather than adding embodied carbon to construction would address multiple dimensions of the WUI building problem simultaneously. That combination does not currently exist in any certified commercial product. Mycelium composites are the most plausible path to it.
Closing Human Dimension
Every wildfire season, communities are rebuilt in the same locations using largely the same materials, often facing the same results the next time fire returns. The mycelium growing in the dark on agricultural waste does not know it is a candidate for a better kind of building material. But the biology it embodies — the charring chemistry, the insulating nanoporous network, the capacity to grow into any shape from locally available organic matter — is already doing the work that fire safety engineers are trying to replicate synthetically. Learning to use it is less an innovation than a recognition.
Sources
1. Chulikavit, N. et al. (2023). “Fireproofing flammable composites using mycelium.” Polymer Degradation and Stability. https://www.sciencedirect.com/science/article/abs/pii/S0141391023001714
2. Jones, M. et al. (2018). “Thermal Degradation and Fire Properties of Fungal Mycelium and Mycelium-Based Composites.” Scientific Reports. https://www.nature.com/articles/s41598-018-36032-9
3. “Mycelium–coir-based composites for sustainable building insulation.” Journal of Materials Chemistry A (2025). https://pubs.rsc.org/en/Content/ArticleLanding/2025/TA/D4TA07869A
4. Candido, A. et al. (2024). “Mycelium-wood composites as a circular material for building insulation.” Frontiers in Sustainable Cities. https://www.frontiersin.org/journals/sustainable-cities/articles/10.3389/frsc.2024.1412247/full
5. “Influence of additives and fabrication route on the thermal and fireproofing of mycelium-based insulation composites.” IOP Publishing (2025). https://iopscience.iop.org/article/10.1088/2634-4505/ae7153
6. NIST Wildland-Urban Interface Fire Group. “Firebrands and ember exposure in WUI fires.” https://www.nist.gov/el/fire-research-division-73300/wildland-urban-interface-fire-73305
7. WFCA. “What is WUI (Wildland Urban Interface) and How It Works.” https://wfca.com/wildfire-articles/what-is-wui-wildland-urban-interface/
Idea generated by Grok. Article expanded with Grok, substantially rewritten with Claude Sonnet 4.6. Published at artificialideas.org.
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