Calcium Silicate Insulation: Thermal Conductivity and Properties

Calcium silicate insulation conducts about 0.05 W/(m·K) at a mean temperature of 38 °C, and the value rises with temperature: one 650 °C pipe insulation goes from 0.050 to 0.093 W/(m·K) between 38 and 371 °C mean. High-temperature grades rated to 927–1,050 °C are denser and run higher, about 0.08–0.16 W/(m·K) at 200–800 °C. ASTM C533 sets maximum values for each type.

Those numbers come from published manufacturer data sheets, listed below. For a plant owner, they are targets: your product will be compared with them line by line, at the same mean temperatures and under the same standard. This article explains how to read calcium silicate insulation properties, what the standards require, and which steps in the plant decide where your product lands.

Calcium Silicate Insulation Thermal Conductivity at a Glance

Three things stand out:

1. Conductivity roughly doubles across the service range. The Type I pipe value at 371 °C is almost twice the value at 38 °C.
2. Pipe and flat values differ. For the same Thermo-1200 product, the pipe test (ASTM C335) gives higher values than the flat test (ASTM C518) from a 93 °C mean temperature upward. Compare like with like.
3. Grade and density move the curve. The 927–1,050 °C products are denser and conduct more at the same mean temperature. Yet the two Promat boards have the same density and different curves, so formulation and crystal structure matter as much as density.

Why Thermal Conductivity Changes with Temperature

Calcium silicate insulation is a highly porous mass of fine crystals. Heat moves through it by conduction through the solid, conduction through the air in the pores, and radiation across the pores. Radiation grows quickly with temperature, which is why every curve in the table rises, and why the Promat curves climb faster at the hot end.

That is also why a single k-value is meaningless without its mean temperature: the average of the hot-face and cold-face temperatures in the test. A pipe at 400 °C with a 50 °C jacket has a mean insulation temperature of about 225 °C, so the 204 °C or 200 °C value is the relevant one, not the value at 38 °C.

What the difference means in practice

As a simple flat-wall estimate (ignoring surface resistance and assuming the face temperatures stay fixed), take 50 mm of insulation between a 350 °C hot face and a 50 °C cold face, a mean temperature of 200 °C:

These are our estimates from q = k × ΔT / thickness, using the data-sheet values above. The point is not which product is “better”: a 1,000 °C grade is not the right choice for a 350 °C steam line. Specifiers match the grade to the operating temperature, so find out which temperature bands your customers actually run before you choose the process route.

ASTM C533 Requirements for Calcium Silicate Insulation

ASTM C533 is the US specification for calcium silicate block and pipe thermal insulation. The current edition is C533-17(2023). Its scope covers use “on surfaces with temperatures between 80 and 1700°F (27 to 927°C)” and it defines three types: Type I, Type IA and Type II (ASTM C533 abstract). In published data sheets, Type I products are rated to 650 °C (1,200 °F) and Type II products to 927 °C (1,700 °F), as in the Johns Manville sheets above.

The abstract lists the test methods behind its physical requirements: apparent thermal conductivity, linear shrinkage after heat soaking, flexural strength, compressive strength, mass loss by tumbling, hot-surface performance, surface burning characteristics, stress corrosion performance and moisture content (ASTM C533 abstract).

We have not quoted the conductivity limits from the standard itself. A 2013 article by the National Insulation Association, written before the current edition, summarized the specified maximum at a 100 °F (38 °C) mean temperature as 0.41 Btu·in/(h·ft²·°F) for Type I and 0.50 for Types IA and II (NIA, 2013). Converted at 1 Btu·in/(h·ft²·°F) = 0.1442 W/(m·K), that is about 0.059 and 0.072 W/(m·K) (our conversion). Check the current edition for the limits that apply now, the limits at higher mean temperatures and the density limit for each type.

A point that buyers in the trade notice: ASTM C533 stops at 927 °C. Products sold at 1,000 °C and above are declared to EN 14306 or to the manufacturer’s own grade, not as “ASTM C533 Type II at 1,050 °C”.

EN 14306, GB/T 10699 and Other Standards

• EN 14306 is the European specification for factory-made calcium silicate (CS) products for building equipment and industrial installations. The current edition, EN 14306:2015, applies to a service temperature range of approximately −170 °C to +1,100 °C (AFNOR listing of NF EN 14306). Like other European insulation product standards, it is built around properties that the manufacturer declares and tests; check the current edition for the characteristics and test methods required.
• GB/T 10699 is China’s national standard for calcium silicate thermal insulation. If your line is built in China, trial results may be reported to it, so ask for results to the standard your own customers use.
• BS 3958-2:1982 is an older British specification for calcium silicate slabs and lags, still listed as current by BSI, with Type I and Type II and thermal conductivity limits at specified mean temperatures (BSI listing).

Calcium Silicate Insulation Density

Density is the property most closely linked to both strength and conductivity, and the published products fall into clear bands:

Lower density usually means lower conductivity at low and moderate temperatures, but also lower strength and more breakage in handling. In the published data, the higher-temperature grades are the denser ones. A small number of manufacturers also make high-density structural grades for around 1,100 °C service.

In the plant, density is set by the formulation, the solids content of the slurry and the forming pressure. On our reference designs, boards are formed in steel moulds on a filter press or hydraulic forming press, with vacuum dewatering at −0.08 to −0.09 MPa and a forming pressure of 2.0–2.5 MPa.

Maximum Service Temperature and Linear Shrinkage

The maximum service temperature of calcium silicate insulation is set by the mineral phase that forms in the autoclave. Tobermorite-based products are rated up to 650 °C; xonotlite-based products up to 1,000 °C, and well-crystallized, high-purity xonotlite up to 1,050 °C. The 650 °C grade can be made by the static process, in which the calcium silicate forms in the autoclave after casting. The 1,000 °C and 1,050 °C grades need the dynamic process, in which a stirred pressure reactor synthesizes xonotlite slurry before forming.

Service temperature is proved by linear shrinkage after a heat soak, not by a label:

Notice that the soak times differ (24 h in the US sheets, 12 h in the Promat sheet). A shrinkage figure without its soak temperature and time cannot be compared.

Strength, Moisture and Corrosion Properties

ASTM C795 covers non-metallic thermal insulation for use in contact with austenitic stainless steel; insulations must pass the C692 preproduction test and the C871 chemical quality-control requirements (ASTM C795 abstract). For a producer, this means controlling leachable ions from raw materials and process water, not only the finished product. How these properties compare with perlite, mineral wool and ceramic fiber is covered in Calcium Silicate vs Perlite, Mineral Wool and Ceramic Fiber Insulation.

What Controls Thermal Conductivity in the Plant

Thermal conductivity is a result of the whole process, so it cannot be bought with one machine.

1. Raw materials. Every formulation combines a silica source, a calcium source and reinforcing fiber: quartz sand or silica flour (or diatomite or silica fume), lime, and glass fiber or pulp. The purity, fineness and reactivity of your local silica and lime decide how the crystals develop in the hydrothermal reaction, and with them density, thermal conductivity and linear shrinkage. Send samples for testing before the line is configured.
2. Process route. Static for 650 °C products; dynamic, with reactors, for 1,000 °C and 1,050 °C products.
3. Autoclaving. On our reference designs, autoclaves of 2 m diameter and 21.5 m length operate at about 1.2 MPa and 190 °C. In our Middle East line, boards are autoclaved at 190–200 °C for 6–20 hours.
4. Drying. In the same line, boards are dried with hot air at 100–180 °C to below 5% moisture. Water left in the board raises its conductivity and its weight.
5. Laboratory. A complete line includes a thermal conductivity tester (heat-flow-meter or guarded hot plate), a high-temperature furnace for linear shrinkage, a compression and flexural tester, and an oven and balance for density and moisture. Density, thermal conductivity, compressive and flexural strength and linear shrinkage are tested batch by batch.

Calcium silicate insulation is not the same product as calcium silicate building board; see Calcium Silicate Board Manufacturing Process for the board.

FAQ

What is the thermal conductivity of calcium silicate insulation?

Standard 650 °C calcium silicate pipe and block insulation measures about 0.050–0.051 W/(m·K) at a 38 °C mean temperature in published data, rising to about 0.08–0.09 W/(m·K) at 371 °C. Higher-temperature grades conduct more, about 0.08–0.16 W/(m·K) between 200 and 800 °C. Always compare values at the same mean temperature and test method.

What is the density of calcium silicate insulation?

Published standard grades rated to 650 °C are under 240 kg/m³. A 927 °C ASTM C533 Type II block is about 288 kg/m³, and 1,000–1,050 °C boards about 300 kg/m³. Higher density adds strength and high-temperature stability but usually raises conductivity. Density is set by formulation, slurry solids and forming pressure.

What is the maximum temperature for calcium silicate insulation?

It depends on the mineral phase. Tobermorite-based grades are rated to 650 °C, xonotlite-based grades to 1,000 °C, and high-purity xonotlite to 1,050 °C. ASTM C533 covers surfaces from 27 to 927 °C, while EN 14306 covers approximately −170 to +1,100 °C. Linear shrinkage after a heat soak is the test that supports the rating.

Is ASTM C533 Type II rated to 1,050 °C?

No. ASTM C533 covers surfaces up to 927 °C (1,700 °F), and Type II products are rated to that limit in published data sheets. Calcium silicate products sold at 1,000 °C or 1,050 °C are declared to EN 14306 or to the manufacturer’s own grade. Ask suppliers which standard and soak test stand behind a temperature claim.

Does moisture affect calcium silicate insulation?

Yes. Calcium silicate absorbs water unless treated, and wet insulation conducts more heat and adds weight. Even a water-resistant grade is described by its manufacturer as not hydrophobic, so jacketing should follow installation quickly. In production, boards are dried to a low moisture content before packing, below 5% in our Middle East line.

Planning a Calcium Silicate Insulation Plant?

See how the 650 °C, 1,000 °C and 1,050 °C grades are produced, section by section, on our calcium silicate insulation production line page. The calcium silicate plant setup guide sets out the planning sequence, from raw material tests to commissioning.

Sources

• ASTM International, ASTM C533-17(2023) Standard Specification for Calcium Silicate Block and Pipe Thermal Insulation: https://store.astm.org/c0533-17r23.html
• National Insulation Association, “Exploring Insulation Materials: Granular Insulations”, Insulation Outlook, March 2013: https://insulation.org/io/articles/exploring-insulation-materials-granular-insulations/
• Johns Manville, Thermo-1200 Calcium Silicate Pipe & Block Insulation, data sheet IND-303 (04/08/25): https://JM.com/content/dam/jm/global/en/industrial-insulation/calcium-silicate/thermo-1200/JM_IND_Thermo-1200_DS.pdf
• Johns Manville, Super Caltemp Gold 1700 Block Insulation, data sheet IND-305 (03/12/24): https://www.jm.com/content/dam/jm/global/en/industrial-insulation/calcium-silicate/super-caltemp-gold-1700/IND-305-Super-Caltemp-Gold-1700.pdf
• Promat, PROMASIL-1000L and PROMASIL-1100 SUPER technical data sheet (2017-09): https://media.promat.com/pd36061/original/1067776668/promasil-1000l–1100-super_tds_en.pdf
• AFNOR, NF EN 14306 (December 2015) Thermal insulation products for building equipment and industrial installations – Factory made calcium silicate (CS) products – Specification: https://www.boutique.afnor.org/en-gb/standard/nf-en-14306/thermal-insulation-products-for-building-equipment-and-industrial-installat/fa059236/46716
• BSI, BS 3958-2:1982 Thermal insulating materials – Calcium silicate preformed insulation (slabs and lags): https://knowledge.bsigroup.com/products/thermal-insulating-materials-calcium-silicate-preformed-insulation
• ASTM International, ASTM C795-08(2023) Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless Steel: https://store.astm.org/c0795-08r23.html