Thermal Insulation for High-Temperature Furnaces

Insulation Bricks

Cut furnace heat loss without adding structural deadweight — densities from 0.6 to 1.7 g/cm³, from clay insulating brick to 99% alumina bubble, from our dedicated lightweight brick plant.

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0.19 W/(m·K) Lowest thermal conductivity

0.6 g/cm³ Lightest density grade

≤ 1.2% Fe₂O₃ in premium grades

Dedicated plant Lightweight brick only

4 ISO management systems held

Insulation bricks do two jobs in any high-temperature furnace: they cut heat loss, and they do it without adding weight to a structure that is already carrying its own mass at temperature. A crown or an arch cannot be insulated with dense brick — it would sag under the extra load. The same logic governs kiln cars, boiler walls, reheating furnaces and cement kiln back-up linings.

XZK Group runs a dedicated lightweight brick plant in Xinmi, Henan. It produces four families of insulating refractory — clay insulating brick, high alumina insulating brick, mullite insulating brick and alumina bubble brick — plus lightweight silica brick for crown insulation. Densities run from 0.6 to 1.7 g/cm³ and thermal conductivity from 0.19 to 0.55 W/(m·K).

Because that plant makes insulation and nothing else, the entire insulating layer of a furnace can come from one source with consistent chemistry, density and dimensions — whether the furnace is melting glass, firing ceramics, burning cement clinker, holding aluminium or reheating steel.

What Are Insulation Bricks?

Insulation bricks are refractories engineered for low thermal conductivity rather than maximum refractoriness. They achieve this by introducing controlled porosity into the brick body — either through lightweight aggregates, burnout additives, or in the case of alumina bubble brick, hollow spheres of fused alumina.

The result is a brick that weighs a fraction of its dense equivalent while still standing up to furnace temperatures. That combination — light and heat resistant — is what makes insulation brick useful, because it can be stacked in multiple layers behind a working lining or laid directly over an arch.

Why furnace insulation decides your energy bill

The insulation of all components, and the crown in particular, has a decisive influence on the heat loss of a glass furnace. Modern crown designs target heat losses below 1250 W/m², and insulated concepts at the leading edge achieve under 1050 W/m². Since the tank temperature rises as a result — up to 1600°C or slightly more — only silica bricks with outstanding creep resistance can be used in the working layer, while lightweight silica brick handles the insulation above it.

Types and Grades of Insulation Bricks

Four families cover the insulation requirements of high-temperature furnaces. They differ in base material and density, and therefore in how much temperature each can take before the insulation layer itself must change.

Economical base layer

Clay Insulating Brick

XZK-LC135-1.3 · LC135-1.0 · LC125-0.8 · LC120-0.6

Fireclay-based insulating brick for the outermost layer of a furnace lining. Four densities from 0.6 to 1.3 g/cm³ let you build a graded insulation stack with falling conductivity outward.

Density 0.6–1.3 g/cm³ · Thermal conductivity ≤ 0.25–0.55 W/(m·K) · CCS ≥ 1.5–8.0 MPa

Higher temperature

High Alumina Insulating Brick

XZK-LI140-1.2 · LI140-1.0 · LI140-0.8 · LI135-0.6

Alumina-based insulating brick rated to 1400°C. Used directly behind the working lining where clay insulating brick cannot take the interface temperature, and as tank bottom insulation below the paving.

Al₂O₃ ≥ 48% · Density 0.6–1.2 g/cm³ · Conductivity ≤ 0.25–0.50 W/(m·K)

Lowest conductivity

Mullite Insulating Brick

XZK-JM23 · JM26 · JM28 · JM30

Mullite-based lightweight brick with the lowest thermal conductivity in the range — down to 0.19 W/(m·K). JM23 to JM30 correspond to the familiar JM classification used across the refractory industry.

Al₂O₃ 42–72% · Density 0.6–1.1 g/cm³ · Conductivity ≤ 0.19–0.43 W/(m·K)

Highest purity

Alumina Bubble Brick

XZK-AB90 · AB93 · AB95 · AB99

Made from hollow fused-alumina spheres, giving very low density with high refractoriness and excellent thermal shock resistance. The 99% grade carries almost no iron, making it suitable where contamination matters.

Al₂O₃ 90–99% · Density 1.35–1.70 g/cm³ · RUL ≥ 1650–1700°C

Crown insulation

Lightweight Silica Brick

XZK-LS160-0.8 · LS162-1.0 · LS164-1.2

Silica-based lightweight brick for crown insulation, where the working layer is silica and the insulation must match its thermal expansion. Three densities trade insulation performance against compressive strength.

SiO₂ ≥ 91% · Density 0.8–1.2 g/cm³ · RUL ≥ 1600–1640°C

$Insulating refractory castable for glass furnace crowns$

Monolithic alternative

Insulating Castable

XZK insulating castables

Where a brick shape is impractical — over a crown, or around complex geometry — lightweight insulating castables provide the same thermal performance and can be applied by casting or gunning.

Applied in layers behind a dense crown sealing to achieve the target heat flux.

Technical Specifications

Insulation performance is decided by density and conductivity together. Compare grades on both, not on density alone — a slightly denser brick with lower conductivity can deliver a better thermal result.

Mullite insulating bricks

Property	XZK-JM23	XZK-JM26	XZK-JM28	XZK-JM30
Al₂O₃ (%)	42	45	54	72
Fe₂O₃ (%)	≤ 1.2	≤ 1.0	≤ 0.9	≤ 0.55
Bulk density (g/cm³)	0.6	0.8	0.7	1.1
Cold crushing strength (MPa)	≥ 1.5	≥ 2.3	≥ 2.0	≥ 3.5
Thermal conductivity (350°C, W/(m·K))	≤ 0.19	≤ 0.24	≤ 0.21	≤ 0.43

High alumina and clay insulating bricks

Property	XZK-LI140-1.2	XZK-LI140-1.0	XZK-LI140-0.8	XZK-LC135-1.3	XZK-LC120-0.6
Al₂O₃ (%)	≥ 48	≥ 48	≥ 48	—	—
Bulk density (g/cm³)	≤ 1.2	≤ 1.0	≤ 0.8	≤ 1.3	≤ 0.6
Cold crushing strength (MPa)	≥ 8.0	≥ 3.0	≥ 2.5	≥ 8.0	≥ 1.5
Thermal conductivity (350°C, W/(m·K))	≤ 0.50	≤ 0.40	≤ 0.35	≤ 0.55	≤ 0.25

Alumina bubble bricks

Property	XZK-AB90	XZK-AB93	XZK-AB95	XZK-AB99
Al₂O₃ (%)	≥ 90	≥ 93	≥ 95	≥ 99
Fe₂O₃ (%)	≤ 0.5	≤ 0.4	≤ 0.2	≤ 0.1
Bulk density (g/cm³)	≤ 1.35	≤ 1.40	≤ 1.60	≤ 1.70
Cold crushing strength (MPa)	≥ 8	≥ 12	≥ 18	≥ 20
Refractoriness under load (0.1 MPa, °C)	≥ 1650	≥ 1680	≥ 1700	≥ 1700

Lightweight silica bricks

Property	XZK-LS160-0.8	XZK-LS162-1.0	XZK-LS164-1.2
SiO₂ (%)	≥ 91	≥ 91	≥ 91
Al₂O₃ (%)	≤ 1.0	≤ 1.0	≤ 1.0
Bulk density (g/cm³)	≤ 0.8	≤ 1.0	≤ 1.2
Cold crushing strength (MPa)	≥ 2.0	≥ 3.0	≥ 4.0
Refractoriness under load (°C)	≥ 1600	≥ 1620	≥ 1640

Test methods follow GB/T and ISO practice; ASTM equivalents and full certificates of analysis are issued with every shipment.

Where Insulation Bricks Are Used

Insulation goes where heat loss is highest and structural load is most sensitive. The zones below are laid out on a glass tank because it is the hardest case — high temperature, heavy structural load and a corrosive atmosphere at once. The same reasoning governs kiln cars, boilers, reheating furnaces and cement kiln back-up linings.

Lightweight insulation bricks stacked over a furnace crown — Crown insulation in layers — lightweight brick above the silica working layer cuts heat loss without loading the arch.

Tank bottom insulation layer before paving installation — Tank bottom insulation — high alumina insulating brick below the paving maximises energy efficiency in the melting zone.

Crown insulation — the largest single heat loss

The crown radiates heat across its whole area, so it dominates furnace heat loss. Insulation must reduce that loss without adding deadweight to the arch, because a crown that sags is a structural failure. Lightweight silica brick matches the thermal expansion of the silica working layer above it, while low-density grades keep the added load minimal.

Typical grades: XZK-LS160-0.8, XZK-LS162-1.0, XZK-LS164-1.2, silica insulating castables

Tank bottom insulation layer

Below the bottom paving and sub-layer, insulation reduces heat loss from the melting zone and raises the temperature the working layer sees — which itself improves melt quality and energy efficiency. The key requirement is withstanding the interface temperature without creeping under the weight of the paving above.

Typical grades: XZK-LI140-1.0, XZK-LI140-1.2 (high strength), XZK-LC135-1.0

Regenerator casing and outer wall

The regenerator casing has the best cost-performance ratio when the insulation stack is graded: denser material inside, lightweight outside. Insulation brick reduces the shell temperature and protects the steel structure from thermal distortion.

Typical grades: XZK-LC135-1.0, XZK-LC125-0.8, XZK-JM26

Suspended walls and doghouse

Suspended structures carry their own weight and cannot tolerate heavy insulation. Lightweight high alumina brick provides thermal protection at a fraction of the mass. In the doghouse area, it also backs the working lining against batch dust erosion.

Typical grades: XZK-LI140-0.8, XZK-LI140-1.0

Forehearth and feeder insulation

In the conditioning section, temperature uniformity matters more than raw insulation. Low conductivity mullite grades hold a stable thermal profile along the channel, which translates directly into consistent gob weight and fewer forming defects.

Typical grades: XZK-JM23, XZK-JM26, XZK-JM28

High-purity and high-temperature zones

Where the insulation layer itself meets high temperature or contamination risk, alumina bubble brick provides low density with high refractoriness. The 99% grade carries under 0.1% Fe₂O₃, making it suitable where iron contamination cannot be tolerated.

Typical grades: XZK-AB93, XZK-AB95, XZK-AB99

On Site — Back-Up Insulation in Place

Insulation is never the hot face, but it is always visible in these photographs — behind the crown corner, inside the suspended wall panels and around the forehearth channel.

Suspended wall panels fully installed, before commissioning.

Fitting the bottom course of a suspended wall.

Crown corner (springing) with the insulating back-up courses already laid. This transition decides how the crown and the side wall move against each other.

Forehearth channel insulation lining during installation.

Why Insulation Layers Fail — and How to Prevent It

Insulation is often treated as a secondary detail, and that is exactly why it fails. Four failure modes account for most insulation problems in high-temperature furnaces.

Lightweight insulation bricks after firing at XZK Group — Lightweight insulating brick after firing — density and pore structure are controlled together to hit the specified conductivity.

Quality inspection of insulation brick dimensions and density — Dimensional and density inspection — consistent brick size is what keeps an insulation stack tight and free of thermal bridges.

Structural overload — using dense brick for insulation

The problemThe instinct to specify a stronger brick for insulation backfires: added mass makes a crown or suspended wall sag, closing expansion joints and distorting the arch.

XZK solutionSpecify by conductivity and density together. A 0.8 g/cm³ high alumina insulating brick insulates better than a 1.2 g/cm³ grade while adding a third less load to the structure.

Interface temperature exceeded

The problemInsulation is chosen for its cold-face performance and then exposed to a hot face above its rating. Clay insulating brick used directly behind a working lining at 1400°C will shrink and lose its insulating value.

XZK solutionGrade the stack by temperature. High alumina or mullite insulating brick immediately behind the working lining, clay insulating brick only in the outermost layer.

Thermal expansion mismatch

The problemWhere the working layer and insulation layer expand at different rates, joints open at temperature and hot gas bypasses the insulation entirely, creating local hot spots on the shell.

XZK solutionMatch expansion, not just conductivity. In a silica crown, lightweight silica brick is the correct insulation because it shares the working layer’s expansion behaviour.

Moisture and thermal shock during dry-out

The problemInsulating brick absorbs moisture during installation or storage. Fast heat-up turns that moisture to steam inside a low-permeability body and spalls the brick.

XZK solutionControlled dry-out schedules and correct storage. Our engineers supply recommended heat-up curves with the lining design, and grades are selected with permeability in mind for thick stacks.

How to Select the Right Insulation Brick

Work through interface temperature first, then conductivity, then load. Selecting on price per brick rather than on delivered thermal performance is how insulation budgets get wasted.

Thermal conductivity testing of insulation brick samples — Thermal conductivity measurement at 350°C — the single most important figure when comparing insulation grades.

Insulation brick density and dimensional check before shipment — Density and dimension verification — both drive the delivered insulation performance of the finished stack.

If the application is…	Interface temp.	Priority	Grade
Crown insulation over silica working layer	to 1500°C	Match silica expansion	XZK-LS160-0.8 / LS162-1.0
Directly behind a working lining	to 1400°C	Temperature resistance	XZK-LI140-1.0 / LI140-1.2
Tank bottom insulation below paving	to 1200°C	Load bearing + insulation	XZK-LI140-1.2
Regenerator casing, inner layer	to 1200°C	Temperature + strength	XZK-LC135-1.3
Regenerator casing, outer layer	to 1000°C	Lowest cost insulation	XZK-LC120-0.6 / LC125-0.8
Minimum conductivity required	to 1260°C	Lowest conductivity	XZK-JM23 / JM26
High purity, contamination sensitive	to 1700°C	Purity + refractoriness	XZK-AB95 / AB99
Kiln car top or tunnel kiln lining	to 1300°C	Low mass + low conductivity	XZK-JM23 / LC125-0.8
Boiler, reformer or process heater wall	to 1200°C	Lowest conductivity	XZK-JM23 / JM26
Cement or lime kiln back-up lining	to 1200°C	Temperature + strength	XZK-LI140-1.2 / LC135-1.3
Complex geometry, no brick possible	to 1300°C	Formability	XZK insulating castables

Three practical rules:

Grade the stack, don’t use one brick throughout. A three-layer insulation stack with falling density outward outperforms a single-grade stack of the same total thickness, at lower cost.
Check the interface, not just the cold face. The temperature where insulation meets the working lining decides which family you can use. Everything else follows from that number.
Account for the weight you are adding. On a crown or suspended wall, insulation mass is structural load. Ask for the density figure alongside the conductivity figure, every time.

Manufacturing and Quality Control

Insulating refractories are defined by their pore structure, which makes process control more demanding than for dense brick — an inconsistent pore structure shows up directly as inconsistent thermal performance.

100,000 MTAnnual refractory capacity

20CNC presses, 630–2000 t

4 × 108 mFully automatic tunnel kilns

93Core patents held

Four automatic batching lines feed 20 CNC presses ranging from 630 to 2000 tonnes, so brick density is controlled by press force rather than operator judgement. Firing runs through four 108-metre fully automatic tunnel kilns and five ultra-high-temperature shuttle kilns, giving the uniform soak these materials need to complete densification.

Quality control covers the full chain: incoming raw material assay, in-process density and dimensional checks, and finished-brick testing for bulk density, apparent porosity, cold crushing strength and refractoriness under load. Every shipment leaves with a certificate of analysis.

Clay, high alumina and mullite insulating bricks are produced by combining lightweight aggregates or burnout additives with the base material, then firing to a controlled density. Alumina bubble brick uses hollow fused-alumina spheres as the aggregate, which is what allows both low density and high refractoriness in the same body.

XZK Group holds ISO 9001, ISO 14001, ISO 45001 and ISO 50001 certification, is recognised as a National High-Tech Enterprise, and operates the Henan Provincial Engineering Technology Research Centre for Sintered Refractories for Glass Furnaces. We have contributed to standards including JC/T 2574-2020 (Sillimanite bricks for glass furnaces), GB/T 4984-2023 (chemical analysis methods for zirconium-containing refractories) and T/CSTM 00853-2024 (chrome-zirconia-corundum bricks for glass furnace regenerators).

Industries We Serve

Glass

Crown and tank bottom insulation, regenerator casing and forehearth channels in float, container and fibre glass furnaces. Crown insulation is the most demanding case — see the zone notes above.

Cement and lime

Rotary kiln and preheater back-up insulation, cooler and calciner linings, where every degree of shell temperature is paid for in fuel.

Iron and steel

Ladle and tundish backing insulation, hot blast stove casings, annealing and reheating furnace linings — insulation that cuts the fuel bill per tonne.

Ceramics and glass fibre

Kiln car tops, tunnel and shuttle kiln linings and fibre drawing bushings, where lightweight brick also cuts the mass that has to be pushed through the kiln.

Non-ferrous metallurgy

Aluminium holding and melting furnace roofs, copper anode and shaft furnace back-up linings, where low-iron grades prevent contamination of the melt.

Petrochemical and power

Boiler, reformer, cracking furnace and process heater insulation, where low conductivity lowers shell temperature, protects the steel structure and cuts fuel consumption.

Frequently Asked Questions

What is the difference between insulating firebrick and dense firebrick?

Density and purpose. Dense firebrick is the working lining — it resists the process temperature, chemical attack and mechanical wear. Insulating firebrick sits behind or outside it and resists heat flow. Insulating brick has low strength by design; using it as a working lining will destroy it.

What does the JM classification mean?

The JM series (JM23, JM26, JM28, JM30) is a widely used industry classification for mullite-based insulating firebrick, where the number relates to the temperature rating. Our XZK-JM grades follow that convention, so they can be specified as direct equivalents in most lining designs.

Can insulating brick be used in a glass furnace crown?

Yes, and it should be. But the correct material depends on the working layer: where the crown is silica, lightweight silica brick is used because it shares the same thermal expansion. Using a different base material would open joints at temperature.

How much can insulation reduce furnace heat loss?

It depends on the starting point and the stack design. Modern insulated crown concepts target heat losses below 1250 W/m², with optimised designs achieving under 1050 W/m². Because furnace temperature rises as losses fall, the working layer must be able to handle the higher temperature — which is why creep resistance in the crown working brick matters more after insulation is improved, not less.

What is alumina bubble brick used for?

Where low density and high refractoriness are both required. The brick is made from hollow fused-alumina spheres, so it weighs a fraction of dense alumina while still rated above 1650°C. It is used in high-temperature insulation layers, kiln furniture and anywhere contamination from iron must be avoided.

What is your minimum order quantity and lead time?

Standard insulating brick shapes carry no practical minimum beyond a full pallet. Custom dimensions depend on moulds. Typical lead time is 25–40 days, and we hold stock of common JM and LI grades for short-notice repairs.

Do you provide heat-up schedules for insulation layers?

Yes. Because insulating brick is sensitive to moisture spalling, we supply recommended dry-out and heat-up curves with the lining design, matched to the grade and stack thickness you specify.

Do you make insulating bricks for furnaces other than glass tanks?

Yes — that is most of what the lightweight plant does. The same grades go into cement and lime kiln back-up linings, ceramic kiln cars and tunnel kilns, hot blast stoves and reheating furnaces, aluminium holding furnaces, boilers and reformers. Selection is driven by interface temperature, stack thickness and structural load, not by the industry label.

Why Buy Insulation Bricks from XZK Group

Lightweight brick is not a sideline for us. XZK Group operates a dedicated insulating brick plant, separate from the dense refractory lines, with its own mixers, presses, kilns and density control. That is how we hold tolerances on density and thermal conductivity that plants treating insulation as a filler product cannot.

We supply the full stack — clay, high alumina, mullite, alumina bubble and lightweight silica — from one source, so the layers are designed to work together rather than assembled from whatever grades happen to be available at the time.

Our application engineers work from the furnace, not the catalogue: interface temperature, stack thickness and structural load determine the specification. Where a brick shape is impractical, we supply insulating castables for the same duty.

Get your insulation stack reviewed — free

Send us your furnace data or current lining drawing. Our engineers will propose a graded insulation stack with the conductivity and load figures for each layer, and return a quotation within 24 hours.

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Related refractory products

Insulation bricks are specified together with the working lining they back. Depending on the zone they are used with clay insulating bricks, high alumina insulating bricks, mullite insulating bricks, alumina bubble bricks, lightweight silica bricks, insulating castables, low porosity fire clay bricks and mullite bricks. Browse all refractory products or see the insulating bricks range for zoning guidance. For duties that cannot be met with a brick shape, see monolithic refractories; for the full furnace picture see glass furnace refractories.