Refractory Insulation: Ceramic Fiber, Insulating Firebrick, Boards and Microporous

Refractory insulation is the low-conductivity, low-heat-capacity layer of a high-temperature lining, and it comes in 12 product types: insulation wools and their blankets, modules, boards, papers and textiles, insulating firebrick, microporous and calcium silicate backup boards and fiber rigidizers. Which type fits depends on classification temperature, so how are the types told apart?

Insulating refractories, also called insulation refractories, are the high-temperature insulation materials of industrial furnaces, heaters, ladles and ducts, not building or HVAC insulation. Refractory insulation is one of three forms of refractory materials, next to bricks and monolithics.

The 12 types sort into five groups. EN 1094-1:2008 rates wool products by a classification temperature between 850 and 1800 °C (1,562 and 3,272 °F) that sits 100 to 150 °C (180 to 270 °F) above continuous use, ASTM C892-19(2024) gives blanket Types from 732 to 1649 °C (1,350 to 3,000 °F), and refractory ceramic fiber carries Carc. 1B H350i under CLP Annex VI (index 650-017-00-8, applicable 2010-12-01), with an EU limit of 0.3 f/ml (Directive (EU) 2017/2398, transposition by 2020-01-17).

Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.

What Is Refractory Insulation?#

ISO 836:2001 (term 075) defines an insulating refractory as "refractory with a low thermal conductivity and low heat capacity", and EN 1094-1:2008 defines high-temperature insulation wool (HTIW) as man-made mineral wool suitable for heat insulation above 600 °C (1,112 °F).

Shaped insulating products are set apart from dense ones by true porosity (ISO 836:2001, term 116). The US boundary of refractories is 538 °C (1,000 °F) under ASTM C71-12(2024), and a note in ASTM C155-97(2022) sends insulating materials for use below that temperature to ASTM Committee C16 on Thermal Insulation.

Where Insulation Sits Behind the Working Lining#

Refractory insulation sits in the third of three layers: the working lining (hot face), the safety or permanent lining, and the insulating backup layer next to the shell. In cyclic furnaces such as petrochemical heaters and steel reheat furnaces, fiber modules can form the whole lining.

Heat passes through the layers by steady-state series conduction, so each layer sets the temperature of the next. Three checks follow: every interface temperature against the next layer's limit, the anchor tip temperature against its alloy limit, and the shell temperature against personnel and acid dew point limits. Layer order and thickness are part of the design of a refractory lining.

Pores, Fibers and Opacifiers: How Insulation Slows Heat Flow#

Refractory insulation slows heat flow by trapping gas: pores in fired insulating brick, air between fibers in wool products, and pores smaller than the mean free path of air in microporous board. In microporous board, gas conduction is suppressed by the pore size and radiation by an infrared opacifier.

Low heat capacity matters as much as low conductivity in furnaces that cycle, because a lining with low heat capacity stores less heat at each start. Conductivity values on data sheets appear in W/(m·K) or BTU·in/(h·ft²·°F); 1 W/(m·K) equals 6.933 BTU·in/(h·ft²·°F).

Types of Refractory Insulation: 12 Product Types#

There are 12 types of refractory insulation in five groups (insulation wools, fiber product forms, shaped insulating products, backup boards and fiber hardeners), and insulating castables add a cast form that sits with the monolithics. Table 1 compares the 12 types by form, defining chemistry or construction, US and ISO or EN standard, and role.

Rows 1 to 12 are the insulation types of this section; the related row belongs to monolithic refractories. Compositions are in mass %, and the EN 1094-1 compositions are definitions or typical values, not requirements.

High-Temperature Insulation Wool: RCF, AES and Polycrystalline Wool#

High-temperature insulation wool comes in three families: alumino-silicate wool (refractory ceramic fiber, RCF), alkaline earth silicate (AES) and other low bio-persistence wools, and polycrystalline wool (PCW). RCF, also spelled ceramic fibre, is fiberized by spinning or blowing a melt of calcined kaolin or alumina and silica near 2000 °C (3,632 °F), and about half of the melt becomes non-fibrous shot (NIOSH criteria document 2006-123). AES wool is spun or blown from a CaO-MgO-SiO2 melt, while PCW is made by a sol-gel route: a precursor gel is extruded, spun or drawn and fired to mullite or alumina.

Refractory ceramic fiber was developed in 1942 and became commercially available in 1953. According to ECFIA, the European industry association, AES and PCW have replaced about 50 % of alumino-silicate wool products over three decades, and high-temperature insulation wool is about 2 % of EU synthetic mineral wool production.

Fiber Product Forms: Blanket, Modules, Board, Paper and Textiles#

Wool is converted into five product forms: needled blanket without binder, modules, vacuum-formed board and shapes, wet-laid paper and spun textiles. Modules are folded or stacked blanket, pre-compressed and banded so they can be fixed on anchors or studs.

Each form carries its own bond. Blanket is held together by needling alone, board is wet (vacuum) formed from a fiber slurry with organic or inorganic binders and fillers, and paper is wet-laid with latex as a binder in the ECFIA description. Textiles are spun on an organic viscose carrier and reinforced with glass filament or stainless or nickel-alloy wire.

Shaped Insulating Products and Their Porosity#

Insulating firebrick is the shaped insulating refractory: a fired brick with at least 45 % true porosity (ISO 836:2001). Producers reach that porosity by three routes: a graded organic filler that burns out in the kiln, a foamed slurry fired to an anorthite bond, or hollow alumina spheres in the top classes. The same product is sold as lightweight refractory brick or soft firebrick.

Castables made with lightweight aggregate are the cast form of insulation. They belong to the monolithic family and appear in Table 1 as a related row, because they are mixed, placed and dried like other castables.

Backup Boards: Microporous and Calcium Silicate#

Microporous and calcium silicate boards both sit behind the hot face, never at it. Microporous board is a dry-pressed compact of fumed silica with an infrared opacifier and reinforcing filaments, and liquid water destroys its pore structure, a warning stated in producer literature.

Calcium silicate is made by autoclaving a lime and silica slurry to calcium silicate hydrate, reinforced with cellulose or glass fiber; its low thermal shock resistance keeps it off the hot face. Asbestos reinforcement in North American calcium silicate was replaced by the end of 1972, according to the National Insulation Association. Calcium silicate building boards are construction products outside this page.

Rigidizers, Mastics and Cast Insulation#

Rigidizers are liquid colloidal silica or alumina binders that harden the face of a fiber lining on drying or firing, so the surface resists gas erosion. Mastics carry fiber in a colloidal silica bond. The gas velocity at which a fiber face needs hardening is a producer value, not a standard limit.

How Is Refractory Insulation Classified?#

Refractory insulation is classified by standard temperature classes: EN 1094-1:2008 and ASTM C892-19(2024) for wool products, ASTM C155-97(2022) and ISO 2245:2006 for insulating firebrick, and scope temperatures of C1676/C1676M-25 and C533-17(2023) for backup boards. Each scheme measures something different, so a class number never transfers from one standard to another.

EN 1094-1:2008 also defines the wool families by chemistry. The ASTM blanket scheme is a maximum use temperature, and the firebrick schemes are reheat tests.

Classification Temperature Under EN 1094-1#

EN 1094-1:2008 classifies each wool product by the temperature at which its permanent linear change after 24 h stays within 4 % (blankets, felts, mats, papers) or 2 % (boards, preformed shapes). That temperature is stated to the nearest 50 °C between 850 and 1800 °C (1,562 and 3,272 °F). Table 2 sets out the wool families the standard defines and the status of each composition.

The classification temperature is a shrinkage result, never a maximum service temperature. For amorphous wools, continuous use sits a typical 100 to 150 °C (180 to 270 °F) below it, according to EN 1094-1:2008.

EN 1094-1:2008 was published on 2008-07-01 and replaced EN 1094-1:1997, EN 1094-3:2003 and ENV 1094-7:1993; CEN decided to revise it on 2026-10-21. Its test list covers thickness under 350 or 725 Pa, bulk density after drying at 110 ± 5 °C (230 ± 9 °F), resilience, permanent linear change, tensile strength, and moisture and organic content.

US Blanket Types and European Wool Classes Side by Side#

ASTM C892-19(2024), a Committee C16 specification, sorts fiber blanket into Types I to V by maximum use temperature, from 732 °C (1,350 °F) to 1649 °C (3,000 °F). Grades 3, 4, 6, 8, 10 and 12 sort it by density, conductivity and tensile strength. The full Type and Grade tables are covered under the ASTM C892 specification for fiber blanket.

Inch-pound values govern in C892, and the SI values are for information only. A C892 Type is a use limit set by the specification, while an EN 1094-1 class is a measured shrinkage result, so the two scales are never equivalent even where the numbers sit close.

Shaped Insulation Groups: ASTM Versus ISO 2245#

ASTM C155-97(2022) classifies insulating firebrick by bulk density and by reheat change on the 9 in (229 mm) dimension at the group temperature, and the group number times 100 approximates the test temperature in °F. ISO 2245:2006 sets groups 80 to 180, where the group equals the test temperature divided by 10 at which the permanent change in dimensions stays at or below 2 %, adds Class L by bulk density, and excludes ceramic fiber products.

Neither group is a service temperature: both report a reheat test, and the C155 density limits are not meant for engineering calculations. The group tables of both standards are set out under ASTM C155 and ISO 2245 classification of insulating firebrick.

Which Refractory Insulation Suits Which Temperature and Service?#

The right refractory insulation is the type whose classification temperature clears the hot-face or interface temperature with margin and whose form survives the zone's gas velocity, chemistry and moisture. Wool or modules suit low heat storage, insulating firebrick suits load-bearing backup and hot faces, and microporous or calcium silicate suits zones where backup space is short.

A specification should name the standard and class, not a brand, because the class is what a buyer can test on delivery.

Insulation Types by Temperature Band#

Table 3 compares the temperature bands of the insulation types; each band is a class range from ECFIA or a standard scope or class, never one product's rating.

Backup boards cover the low end of the scale, wools and blanket the middle, and polycrystalline wool the top. ASTM C533-17(2023) also caps dry density at 240 kg/m³ (15 pcf) for Type I and 352 kg/m³ (22 pcf) for Types IA and II, with moisture at no more than 2 % by weight.

Insulation by Industry and Lining Zone#

Table 4 compares five industries by the zones that take refractory insulation and the types used there.

Petrochemical heaters and steel reheat furnaces take full fiber linings, because they cycle and gain from low heat storage. Aluminum, glass and ladle zones use fiber, brick or board as backup behind a dense working lining that meets metal, slag or glass.

Gas Velocity, Chemical Attack and Moisture#

Refractory insulation fails in four ways at the hot face: shrinkage above the classification temperature, devitrification and embrittlement, fluxing by alkalis, and erosion by gas velocity. Alkali fluxing follows the same mechanism as other forms of slag and chemical attack.

Backup boards fail by water rather than heat. Microporous board loses its porosity when wetted by water, oil or castable slurry, and calcium silicate takes up water from wet castable during installation, so both stay dry and covered while castable is placed against them.

Binders and Additives in Refractory Insulation#

Refractory insulation is bonded in three ways: needled wool blanket carries no binder, boards, papers and textiles use organic binders or carriers that burn out, and microporous, calcium silicate and rigidized products use inorganic bonds. Table 5 lists the bond and the additions of each type.

Each binder decides how the lining behaves on its first heat-up. The same binder families, such as colloidal silica, calcium aluminate cement and organic temporary binders, run through dense products too, and they are covered under refractory additives and binders.

Melt Additions and Binder-Free Fiber#

Melt additions change wool chemistry before fiberizing: EN 1094-1:2008 gives alumino-zirconium-silicate wool a typical composition of ZrO2 below 20 %, Al2O3 below 37 % and SiO2 above 48 %.

Boron, titanium and zirconium oxides act as stabilizers in RCF melts (NIOSH criteria document 2006-123, citing the RCF industry coalition, 1996). Chromia-doped RCF grades also exist; their oxide level is a product value. Once fiberized, wool and needled blanket stay binder-free.

Organic Binders, Burnout and First Heat-Up#

Organic binders give board, paper and textiles their green strength and burn out on the first heat-up, with smoke and a loss of strength. Board loses strength after binder burnout, and paper and textiles lose their latex binder or viscose carrier the same way.

Needled blanket and modules give no binder smoke on first firing, as two producers' blanket literature states. Board made with an inorganic binder avoids off-gassing and keeps its strength when hot.

Colloidal Silica, Opacifiers and Hydrothermal Bonds#

Colloidal silica (CAS 7631-86-9, EC 231-545-4) bonds rigidizers and mastics, and as fumed silica it forms the body of microporous board. Its cell hazard line reads "No Annex VI entry; ECHA C&L notified: not classified (85.7 %); minority H373 (8.6 %)", and the same sol bonds castables as colloidal silica binders.

Opacifiers in microporous board, such as silicon carbide, titanium dioxide or zircon, block radiant heat at high temperature. Calcium silicate takes a third route: a hydrothermal bond formed when the lime and silica slurry is autoclaved.

Standards and Test Methods for Insulation Data Sheets#

Insulation standards come in two layers: classifications and specifications that define the classes (EN 1094-1:2008, ASTM C892-19(2024), ASTM C155-97(2022), ISO 2245:2006, ASTM C1676/C1676M-25, ASTM C533-17(2023)) and test methods that produce the data-sheet values.

API Std 976 (2nd edition, October 2024) adds a third layer for fiber linings: it sets installation quality control, inspection and testing of AES and RCF fiber linings. Designations and editions across all refractory families are listed under refractory standards.

Classification and Product Standards by Insulation Type#

Table 6 compares the US and ISO or EN standards for each insulation type; side by side means the same product family, not equivalent requirements.

Three standards in Table 6 belong to ASTM Committee C16 on Thermal Insulation (C892, C1676 and C533), while C155 and C71 sit with Committee C08 on Refractories.

Thermal Conductivity and Shrinkage Test Methods#

Table 7 compares the test methods behind insulation data sheets, ASTM and ISO or EN side by side for the same property, never declared equivalent.

Fiber conductivity is measured by the ASTM C201-24 calorimeter, while insulating firebrick, calcium silicate and microporous board are measured by other methods. A conductivity comparison across methods is indicative only, so a data sheet must name its method.

Shrinkage follows the same split: wool products report permanent linear change after 24 h, and insulating firebrick reports permanent change in dimensions at the group temperature. Methods and values by product are compared under thermal conductivity of refractories and insulation.

Sourcing, Installing and Handling Refractory Insulation#

Sourcing refractory insulation starts with a specification that names type, standard class and density, continues with a lining fixed on anchors or laid in brickwork, and ends with fiber and dust rules for installation and tear-out.

What an Insulation Specification and RFQ Contain#

An insulation specification and RFQ contain the five items listed below.

• Type and form: wool family, blanket, module, board, paper, textile, IFB or backup board.
• Standard and class: EN 1094-1 classification temperature, C892 Type and Grade, C155 or ISO 2245 group, C1676 or C533 Type.
• Density and thickness, with dimensions for shapes and modules.
• Quantity in m², pieces or metric tonnes, with the hot-face temperature and zone.
• Documents: safety data sheet and CLP statement for every wool.

Price depends on quote factors, not on a list: class and grade, raw-material origin, shape complexity, quantity, packaging, freight and installation. No dated price range for insulation products is printed here, so each request is quoted on its own specification.

Installing Fiber Linings, Shaped Insulation and Backup Boards#

Fiber blanket and modules are fixed on studs or anchors welded to the shell, insulating firebrick is laid with thin joints of a water-retaining IFB mortar, and backup boards are cut and fitted dry. Anchor alloys and spacing for fiber linings are set out under refractory anchors.

API Std 976 (2nd edition, October 2024) covers the installation quality control, inspection and testing of fiber linings. Modules go on walls in soldier course with batten strips and on roofs in parquet, a layout given in producer installation manuals.

Fiber Classification, Exposure Limits and After-Service Cristobalite#

Refractory ceramic fiber is classified Carc. 1B H350i under CLP Annex VI index 650-017-00-8, with notes A and R (ATP01, Regulation (EC) No 790/2009, applicable 2010-12-01). The EU binding limit is 0.3 f/ml as an 8-hour TWA (Directive (EU) 2017/2398, transposition by 2020-01-17). Aluminosilicate RCF entered the REACH Candidate List on 2010-01-13 and zirconia aluminosilicate RCF on 2011-12-19, and RCF is not on Annex XIV (Commission Regulation (EU) 2017/999, recital 23; ECHA authorization list read 2026-10-27). IARC places RCF in Group 2B (Vol. 43, 1988; Vol. 81, 2002). In the US, NIOSH recommends 0.5 f/cm³ as a TWA up to 10 h (criteria document 2006-123), ACGIH sets a TLV of 0.2 f/cc, A2 (1999), Cal/OSHA sets a PEL of 0.2 f/cc, and federal OSHA has no RCF-specific PEL (OSHA chemical data 951, read 2026-10-27).

AES wool falls under Annex VI index 650-016-00-2 (mineral wool with Na2O+K2O+CaO+MgO+BaO above 18 %, Carc. 2 H351, notes A, Q and R; ATP01, applicable 2010-12-01) and is not classified where the Note Q criteria are met, by producer declaration. The EU RCF limit is not applied to AES or PCW.

Alumino-silicate and AES wool may devitrify in service to crystalline phases including cristobalite, while PCW and other low bio-persistence types do not, according to ECFIA. NIOSH defines after-service RCF as fiber exposed above about 1000 °C (1,832 °F) and partly converted to cristobalite. Respirable crystalline silica, IARC Group 1 (Monograph 100C, 2012), is limited to 50 µg/m³ with an action level of 25 µg/m³ (29 CFR 1910.1053, effective 2016-06-23) and to 0.1 mg/m³ in the EU (Directive (EU) 2017/2398, transposition by 2020-01-17). Each hazard has its own page:

• ceramic fiber health rules: full RCF, AES and PCW rules, tear-out and waste
• crystalline silica in refractories: after-service cristobalite and silica limits
• AES and low bio-persistence wool vs RCF: the substitution choice
• asbestos in old refractories: legacy calcium silicate and other pre-1973 insulation

How Does Refractory Insulation Compare With Dense Refractories?#

Refractory insulation trades strength and slag resistance for low conductivity and low heat storage, so dense brick or castable carries the hot face where slag, metal or load act, and insulation sits behind it or lines clean, cyclic furnaces. Fiber stores less heat than brick, while insulating firebrick carries load as backup; the choice between them is weighed under ceramic fiber vs insulating firebrick.

Refractory bricks and monolithic refractories form the dense working lining, and the porosity split of ISO 836:2001 draws the line between a dense shaped product and a shaped insulating one. Insulating castables cross that line in cast form.

Which Insulation Withstands the Highest Temperature?#

Polycrystalline wool reaches the highest band of the insulation types, with a class range of 600 to 1700 °C (1,112 to 3,092 °F), against 600 to 1400 °C (1,112 to 2,552 °F) for alumino-silicate and low bio-persistence wools (ECFIA). ASTM C892-19(2024) Type V blanket reaches 1649 °C (3,000 °F), and the EN 1094-1:2008 and ISO 2245:2006 scales end at 1800 °C (3,272 °F), as classes, not service ratings.

Does Refractory Insulation Need a Dry-Out Before Service?#

Binder-free wool blanket and modules need no cure, while the organic binders and carriers of board, paper and textiles burn out with smoke on the first heat-up. Insulating firebrick holds no hydraulic water, so its mortar and the adjacent castables set the heat-up, and microporous board must be kept away from castable water and steam. Schedules for those castables are covered under refractory dry-out.