When designing or retrofitting buildings, understanding how insulation behaves in a fire is as important as its thermal performance. Fire-resistance ratings tell you how long a material or an assembly can withstand fire exposure, while reaction-to-fire tests describe how a material contributes to the start and spread of fire and how much smoke it produces. Both sets of properties influence code compliance, occupant safety, and choices about where and how different insulation types may be used. In short, not all insulation is created equal when it comes to fire — some materials are essentially noncombustible, others are combustible but treated to resist ignition, and some must be shielded by thermal barriers to meet code.
Two common test regimes are often referenced: ASTM E119 (or UL 263) measures fire-resistance ratings of assemblies in time (typically expressed in hours), and ASTM E84 (the “surface burning characteristics” test) reports flame spread and smoke-developed indices for materials. Additional standards such as UL 94 classify small-sample plastics for flammability (V-0, V-1, V-2), and NFPA/ICC code tests (for example NFPA 285) address multi-story wall assemblies with combustible components. These tests, and the resulting classifications, guide how insulation products are specified and installed — for instance, whether they can be left exposed in an attic or must be covered with a gypsum thermal barrier.
Typical common insulation materials show markedly different fire behaviors. Mineral wool (rock wool) is essentially noncombustible, performs well in both surface-burning and assembly fire tests, and is frequently used where fire containment is a priority. Fiberglass is also noncombustible at the glass fiber level, though binders and facings can affect its reaction-to-fire characteristics. Cellulose insulation is made from recycled paper and is treated with fire retardants; it resists ignition better than untreated wood fibers but still differs from mineral wool in tested flame-spread and assembly performance. By contrast, foam plastics (expanded or extruded polystyrene, polyisocyanurate, spray polyurethane foam) are combustible and can contribute fuel to a fire unless they are specially formulated, encapsulated, or part of a tested and approved wall/roof assembly with an appropriate thermal barrier.
Choosing insulation for a project therefore requires balancing thermal goals, moisture control, cost, and fire performance — and following codes that often mandate specific ratings or tested assemblies. Later in this article we will compare the typical ASTM/UL test results and code considerations for each common insulation type, explain how installation details and thickness affect real-world fire resistance, and outline practical strategies (such as using noncombustible materials, encapsulation, or intumescent coatings) to meet safety and regulatory requirements.
Standard test methods and rating metrics (flame spread, smoke-developed, ASTM/UL tests, fire-resistance hours)
Standard fire-performance metrics fall into two broad categories: surface-burning characteristics and time-rated assembly resistance. Surface-burning tests such as ASTM E84 / UL 723 produce a flame-spread index (FSI) and a smoke-developed index (SDI). FSI is reported on a scale where materials are commonly classified as Class A (0–25), Class B (26–75) or Class C (76–200); SDI is reported on a numeric scale (commonly 0–450 in practice) that quantifies relative smoke production. Other bench-scale tests such as the cone calorimeter (ASTM E1354) measure heat-release rate and total heat release, which are strong predictors of fire growth and are often used to supplement surface-burning data. For interior finish performance in realistic room conditions, NFPA 286 (room-corner test) evaluates flame spread and contribution to a developing room fire.
Fire-resistance hours are a different kind of rating and address how long an assembly (wall, floor/ceiling, roof) will maintain structural integrity, insulation, and limit passage of flame and hot gases under standard furnace conditions. Those ratings come from tests such as ASTM E119 / UL 263 and are expressed in hours (commonly 1, 2 or 3 hours for building partitions). These assembly tests account for all components — framing, cladding, insulation, finishes, and fasteners — so the same insulation product can produce very different hour ratings depending on the assembly details. Product manufacturers often list tested assembly designs (UL/ETL listings or manufacturer test reports) that show exactly which combination of insulation and finish achieved a given fire-resistance hour rating.
How these test methods translate to common insulation materials: mineral wool and fiberglass are essentially noncombustible and typically show Class A surface-burning characteristics with low contribution to heat release and smoke; because of that, they commonly help assemblies achieve 1–3 hour ratings in tested wall and floor-ceiling constructions. Cellulose is combustible, but when treated with fire retardants and installed at appropriate density it can show reduced flame spread and lower propensity to sustained flaming; nevertheless cellulose is still a combustible fill and its contribution varies with treatment, density and enclosure, so it generally cannot be treated as noncombustible in assembly designs without specific tested evidence. Foam plastics (EPS, XPS, polyiso, spray polyurethane) are combustible and often have high heat-release rates and smoke production; they typically exhibit higher flame-spread indices and, unless protected by an approved thermal barrier, will reduce or prevent an assembly from attaining multi-hour fire-resistance ratings. For assemblies incorporating foams, tested and listed systems or approved thermal/ignition barriers (or intumescent coatings proven by test) are required to achieve the desired hour ratings—so always rely on manufacturer test data and the specific tested assembly rather than on a standalone “hour” rating for the insulation alone.
Mineral wool and fiberglass: typical fire-resistance performance
Mineral wool (rock wool or slag wool) and fiberglass are both inorganic fiber insulations with inherently excellent resistance to ignition: the fibers themselves are noncombustible and do not act as fuel in a fire. Mineral wool is particularly stable at very high temperatures and is commonly tested and classified as noncombustible; it resists charring, retains its loft in many fire scenarios, and is widely used where fire containment and thermal stability are required. Fiberglass fibers are also made from glass and likewise do not ignite, but many commercial fiberglass products include organic binders, adhesives, or combustible facings (kraft paper, foil-backed facings) that can char, smolder, or contribute to smoke and flame spread at relatively low temperatures. In short: the bare fiber component of both materials is noncombustible, but product-facing, binders, and installation details affect the real-world fire behavior.
When people ask about fire-resistance “ratings,” it’s important to distinguish between material classifications (noncombustible vs combustible; flame-spread and smoke-developed indices) and fire-resistance ratings in hours, which apply to complete tested assemblies. Mineral wool and fiberglass generally show very low flame-spread indices and modest smoke-developed indices in surface-burn tests (ASTM E84/UL 723), with many mineral wool products rating in the lowest flame-spread bands; exact numbers vary by product and facing. Hourly fire-resistance ratings (1, 2, 3, 4 hours) come from assembly tests such as ASTM E119 or UL-listed designs; mineral wool is commonly specified inside walls, floors, and curtain-wall firestops to help assemblies achieve 1–4 hour ratings, whereas fiberglass can be used in rated assemblies but typically contributes less to structural fire endurance than dense mineral wool or proprietary fireblocking materials. Manufacturers and code listings provide the specific tested assemblies and the contribution of a given insulation product to the overall rating—loose-fill insulation by itself is rarely assigned an hour-based rating apart from its behavior in tested constructions.
For practical decision-making, mineral wool is generally the preferred choice where fire performance is a primary concern because of its higher temperature stability, noncombustibility, and proven use in fire-rated assemblies; fiberglass is acceptable where noncombustibility of the fiber is sufficient and where facings/binders are noncombustible or properly protected. By contrast, cellulose (even treated) and most foam plastics behave differently—cellulose chars and can smolder (treated versions improve performance), and foam plastics typically require thermal or ignition barriers because they have higher flame spread and produce toxic smoke. To determine compliance and expected performance, always use the specific product’s flame-spread/smoke values and the tested assembly (ASTM/UL listing) for the required fire-resistance hours rather than assuming a generic hourly rating for an insulation type.
Cellulose insulation: treated vs untreated fire behavior
Untreated cellulose insulation is made from recycled paper and is inherently combustible; it ignites and contributes fuel to a fire if exposed directly to sustained flame. Because loose-fill cellulose is usually installed at relatively high density, however, it tends to char at the surface and can limit oxygen access to interior material, which slows the rate of flaming spread compared with many foam plastics. Even so, untreated cellulose is prone to rapid ignition, sustained burning, and smoldering under certain conditions, and can produce significant smoke and heat release if it becomes involved in a fire.
Cellulose sold for building insulation is commonly treated with inorganic fire-retardant salts (most often borate compounds) that improve ignition resistance and reduce flame spread and smoldering. Treated cellulose resists brief exposures to small ignition sources better than untreated product, and in standard surface-burning tests it often achieves much lower flame-spread and smoke-developed scores than untreated paper would. Treatment does not render cellulose noncombustible: treated cellulose still chars and will burn if exposed to sustained flame or high temperatures, but the retardants reduce the rate of combustion, peak heat release, and propensity to sustain smoldering, which improves its performance in many real-world installations.
When people ask “What are the fire-resistance ratings of common insulation materials?” it’s important to distinguish between material surface-burning characteristics and the fire-resistance rating of a tested assembly. Surface-burning tests such as ASTM E84/UL 723 produce a flame-spread index and a smoke-developed index and are used to assign Class A (0–25), B (26–75), or C (76–200) designations; mineral wool and most fiberglass products typically fall in Class A (low flame spread, low smoke), treated cellulose often achieves Class A or B depending on product and installation density, while many foam plastics (EPS, XPS, spray polyurethane) have higher flame-spread and smoke values unless protected. Fire-resistance ratings in hours (e.g., 1-hour, 2-hour) come from furnace tests of complete wall, floor, or roof assemblies (ASTM E119 / UL 263) and depend on the combination of insulation, structural elements, and protective layers; a given insulation may be part of assemblies that achieve 1–4 hour ratings, or it may preclude a high rating unless covered by tested thermal barriers. For code compliance and safe design, manufacturers’ test data and tested assembly details should be used to determine the applicable flame-spread classification and the achievable fire-resistance rating for the intended installation.
Foam plastics (EPS, XPS, polyiso, spray polyurethane): flame spread, smoke, and required thermal barriers
Foam plastics (expanded polystyrene EPS, extruded polystyrene XPS, polyisocyanurate/polyiso, and spray polyurethane foam SPF) are combustible materials with distinct fire behavior compared with mineral insulations. In a direct fire exposure they tend to contribute fuel, with EPS and XPS prone to melting, shrinkage and flaming droplets, SPF able to sustain flaming if exposed, and polyiso often forming a char layer that can slow heat release relative to polystyrenes. All foam plastics can produce dense smoke and potentially toxic combustion products; smoke-developed index and smoke toxicity are therefore important performance considerations in addition to flame spread. In standardized surface-burn tests (ASTM E84 / UL 723) foam plastics often score worse than mineral wool and glass-fiber products and may fall into Class B or C unless specially treated, faced, or tested in a particular assembly that improves their classification.
Because foam plastics are combustible, model building codes and standards require them to be separated from interior spaces or exterior exposures by an approved thermal or ignition barrier, or to be used only as part of tested fire-resistance-rated assemblies. A “thermal barrier” is a material that slows heat transfer and delays foam ignition under typical field fire exposures (commonly achieved with gypsum board or other listed materials). For exterior or attic/soffit applications an ignition barrier or other approved covering may be required instead. Designers routinely achieve required fire-resistance ratings (for example 1-hour or 2-hour rated walls and roofs per ASTM E119/UL 263 tested assemblies) by combining foam insulation with rated cladding, cavity barriers, firestops and finishes; the foam itself is rarely relied upon as the sole source of fire resistance. Because performance depends on the whole assembly, manufacturers’ tested assemblies and code-accepted details must be used to demonstrate compliance.
When people ask “What are the fire-resistance ratings of common insulation materials?” it’s important to distinguish material properties (combustibility, flame spread index, smoke-developed index) from fire-resistance ratings, which apply to complete assemblies tested to standards such as ASTM E119/UL 263. Mineral wool and glass-fiber insulations are noncombustible and generally contribute very favorably to fire performance; they commonly help meet Class A surface-burn requirements (ASTM E84 Class A: flame spread 0–25) and are used in assemblies that achieve multi-hour fire-resistance ratings. Cellulose is combustible but often treated with fire retardants that reduce ignitability and flame spread; it still produces char and smoke and therefore usually needs to be installed within tested assemblies for rated separations. Foam plastics are combustible by nature, so their standalone surface-burn classifications and smoke indices are typically less favorable; however, when installed behind approved thermal barriers or within tested rated assemblies they can be part of 1- or 2-hour fire-resistance-rated walls or roofs. Always confirm performance using manufacturer data, tested assembly listings, and the applicable code requirements for the specific installation.
Building-code requirements, installation practices, and mitigation measures (thermal barriers, intumescents, firestopping)
Building codes regulate how insulation may be used by specifying permitted materials, required coverings, and the tested assemblies that demonstrate a given fire-resistance rating. In practice that means combustible insulation (most foam plastics and some spray foams) is generally allowed only when protected by an approved thermal barrier (commonly gypsum board of an approved thickness or an approved intumescent coating) or as part of a tested fire-resistance-rated assembly. Codes also require fireblocking and draft-stopping in concealed cavities, approved sealing of penetrations, and compliance with exterior-wall fire-propagation tests when combustible materials are used in wall assemblies. To preserve the intended fire performance, installation must follow manufacturer instructions and the tested assembly details: insulation must fill cavities without excessive compression or gaps, maintain required clearances to heat-producing equipment and recessed fixtures, and be kept out of unprotected plenums or service spaces unless specifically listed for that use.
Mitigation measures applied in the field include using noncombustible insulation where possible (mineral wool, fiberglass), installing tested thermal barriers over combustible insulation, and applying approved intumescent coatings where code permits equivalency to a gypsum thermal barrier. Firestopping techniques — such as mineral wool backer with listed firestop sealants, firestop collars around plastic pipe penetrations, and cavity barriers at floor/ceiling lines and around wall openings — maintain the continuity of fire-resistance ratings across joints and penetrations. Other practical mitigations include automatic sprinkler systems, limiting the use and thickness of combustible insulation in high-risk locations, and paying careful attention to detail around electrical boxes, chimneys, ducts, and openings so that the insulation does not undermine the tested performance of wall, floor, or roof assemblies.
When people ask about “fire-resistance ratings” for insulation it’s important to distinguish between the material test values and the hourly fire-resistance rating of an assembly. Individual insulation products are characterized by surface-burning tests (flame-spread and smoke-developed indices) and combustibility tests, while fire-resistance hours (1, 2, 3, 4 hours) are assigned to entire assemblies tested to standard furnace tests. Typical behaviors: mineral wool and fiberglass are essentially noncombustible in normal building uses, exhibit very low flame-spread numbers, and are commonly used inside assemblies that achieve 1–4 hour ratings; treated cellulose has improved resistance to ignition and flame spread compared with untreated cellulose but remains combustible and must rely on the surrounding assembly for fire resistance; foam plastics (EPS, XPS, polyiso, spray polyurethane) generally have high flame-spread and smoke potential and therefore require approved thermal barriers, intumescent coatings, or use only within assemblies specifically tested to provide the required fire-resistance rating. Exact ratings depend on the full construction and must be confirmed from the tested assembly details and manufacturer or code-authority documentation.