Fire-Heated Insulation Foams and Rockwool Lose Strength in Surprising Ways

When a building catches fire, attention usually turns to the flames, the smoke, and the concrete or steel frame. Yet hidden inside many modern walls lies a quieter question: what happens to the insulation core that holds a sandwich panel together? A new experimental study from researchers at the Indian Institute of Technology Patna, published in the open-access journal Results in Engineering, has systematically measured how three of the world’s most common insulation materials—expanded polystyrene (EPS), extruded polystyrene (XPS), and rockwool—lose their mechanical strength after being baked at elevated temperatures. The results challenge a comfortable assumption in the construction industry: that non-combustible rockwool is automatically the safest structural choice. While rockwool famously refuses to burn and holds its shape even at 800 degrees Celsius, the study found that its load-carrying capacity can collapse by more than 90 percent after exposure to temperatures well below those reached in a serious fire.

The research matters because sandwich panels—two thin concrete wythes or steel sheets bonded around a lightweight insulating core—have become a dominant technology in energy-efficient construction. Their popularity stems from fast installation, excellent thermal performance, and reduced lifecycle costs, and building codes increasingly favor them as energy regulations tighten. But the insulation core is not merely filler. It transfers load between the outer layers, maintains composite action, and keeps the wall structurally stable. If heat from a fire passes through the concrete faces and softens the core, the panel’s entire load-transfer mechanism can be quietly undermined long before any visible failure occurs. Despite this, most prior research has focused on fire-resistance ratings and thermal conductivity rather than on how the mechanical properties of the core itself degrade.

The experimental program was unusually comprehensive. The team, led by Abhinav Anand with supervision from Vaibhav Singhal, cut specimens from commercially manufactured blocks of the three materials and subjected them to controlled heating followed by four types of mechanical testing at ambient conditions: compression, three-point flexure, double shear, and tension, all performed on a universal testing machine under displacement-controlled loading according to relevant international standards including EN 826, ASTM C203, EN 12090, and EN 1608. EPS specimens, with a density of 16 kilograms per cubic meter, were heated in a laboratory oven at temperatures from 50 to 80 degrees Celsius. The denser XPS, at 34 kilograms per cubic meter, was tested from 60 to 90 degrees Celsius. Rockwool, at 150 kilograms per cubic meter, was heated in a muffle furnace from 200 all the way to 800 degrees Celsius.

The first striking finding concerned geometry. Beyond 80 degrees Celsius for EPS and 90 degrees Celsius for XPS—temperatures associated with the glass transition of polystyrene, where polymer chains gain mobility and the material loses rigidity—the foams physically warped and bulged. EPS contracted dramatically, shrinking 28 percent in thickness, while XPS deformed anisotropically, expanding on some faces by more than 11 percent while contracting on others. At that point the materials could no longer sustain mechanical loading at all, making further testing impractical. Rockwool, by contrast, showed no melting or distortion even at 800 degrees Celsius, though specimens became noticeably softer, fluffier, and slightly discolored above 200 degrees Celsius—early signs that the organic binder gluing its basaltic fibers together was beginning to break down.

The compression results revealed a subtle hierarchy. EPS lost compressive strength progressively, dropping about 13 percent at 50 degrees Celsius and roughly 32 percent at 80 degrees Celsius, while its initial elastic modulus fell by nearly 48 percent. XPS proved remarkably resilient in compression, losing under 1 percent of compressive strength even at 70 degrees Celsius and only about 9 percent at 90 degrees—yet its elastic modulus still collapsed by 47 percent at the highest temperature, signaling irreversible damage to the closed-cell structure even when peak strength appeared intact. Rockwool told the most dramatic story: minor losses up to 400 degrees Celsius, then a catastrophic 78 percent strength reduction at 600 degrees and over 90 percent at 800 degrees, where the response was governed by loose fiber compaction rather than any true structural resistance, following near-complete burnout of the organic binder.

Flexural and shear tests exposed even greater vulnerabilities. EPS flexural strength fell by 20 percent at 80 degrees Celsius, with stiffness plummeting 55 percent. XPS lost about 22 percent of flexural strength and 15 percent of stiffness at 90 degrees. Rockwool, however, could barely be tested in bending after 400 degrees Celsius: the fibrous matrix delaminated into a loose, fluffy mass that could not support its own weight, with residual flexural strength estimated at a mere 1.57 kilopascals—a 99 percent loss. In shear, the pattern repeated. EPS shear strength dropped 38 percent and its shear modulus 50 percent at 80 degrees; XPS lost 47 percent of shear strength and a striking 73 percent of shear modulus at 90 degrees; and rockwool’s shear capacity fell by more than half at just 200 degrees, reaching an estimated 0.34 kilopascals at 400 degrees, when specimens could no longer even hold the steel loading plates of the test apparatus.

Tensile behavior followed suit, with an important nuance about failure modes. EPS failed brittlely through its core, with cracks propagating along weak inter-bead boundaries, losing 27 percent of tensile strength at 80 degrees Celsius. XPS showed a mixed-mode failure dominated by interfacial debonding near the steel plate boundary, losing about 23 percent of tensile strength at 90 degrees. Rockwool failed progressively through fiber pull-out, rupture, and interlayer delamination—a ductile, distributed damage process—but its tensile strength collapsed by more than 90 percent at 400 degrees, at which point a specimen failed immediately under the self-weight of the test assembly alone. Across all four loading modes, tensile and shear properties consistently deteriorated faster than compressive ones, indicating that heat attacks interfacial bonding and cellular connectivity before it destroys bulk load-bearing capacity.

The overall comparison upends conventional fire-safety intuition. Within their survivable temperature ranges, the polymeric foams kept strength losses below 50 percent, because they were tested below their glass transition points where softening occurs but cellular geometry survives. Rockwool, despite never distorting, suffered strength and stiffness reductions exceeding 90 percent in most loading modes because its mechanical integrity depends almost entirely on an organic binder that burns away between 300 and 400 degrees Celsius. In other words, the material that best resists fire in the visible sense—no melting, no dripping, no combustible gases—is also the one whose structural contribution vanishes most completely. EPS and XPS, meanwhile, released combustible gases and warped at relatively modest temperatures, yet retained a larger fraction of their mechanical function within the ranges they could tolerate.

The authors emphasize that these residual-property datasets fill a critical gap for engineers and modelers. The measurements can support post-fire performance assessment, inform the selection of insulation cores for precast concrete sandwich panels, and provide calibration data for numerical models predicting how panel structures behave under thermal loading. The team also notes an important limitation: only a single commercial density of each material was tested, and since density strongly influences stiffness, strength, and thermal response, broader parametric studies will be needed to develop generalized design equations. For now, the message for the construction industry is clear and slightly uncomfortable—fire resistance and residual structural performance are not the same property, and the non-combustible hero of building insulation may be the first to quietly stop doing its structural job when the heat rises.

Subject of Research: Residual mechanical properties of EPS, XPS, and rockwool insulation materials after elevated temperature exposure

Article Title: Residual mechanical properties of EPS, XPS, and rockwool insulation materials after elevated temperature exposure

Article References: Anand, A., & Singhal, V. (2026). Residual mechanical properties of EPS, XPS, and rockwool insulation materials after elevated temperature exposure. Results in Engineering, 32, Article 113053. https://doi.org/10.1016/j.rineng.2026.113053

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113053

Keywords: EPS, XPS, rockwool, insulation materials, sandwich panels, elevated temperature, fire safety, compressive strength, shear modulus, precast concrete, thermal degradation, structural engineering