All FAQ
Thermal conductivity (λ, lambda) measures how readily heat passes through a material: the rate of heat flow through unit thickness per unit temperature difference. It is expressed in W/m·K (watts per meter-kelvin). For insulation, lower is better — a lower conductivity means less heat escapes through the insulation layer.
Calcium silicate insulation achieves its low conductivity through the xonotlite crystal structure: the needle-like crystals (6CaO·6SiO&sub2·H&sub2O) create a microporous matrix, 88-92% air by volume, in which heat is blocked by the poor-conducting crystal network and the trapped air. This structure is what distinguishes calcium silicate from dense materials — the same chemistry as solid concrete (which conducts roughly 20 times more heat) but engineered into a high-porosity insulating form.
Conductivity is the single most important property in insulation design because it appears in every calculation: heat loss, surface temperature, required thickness, and payback. It is also the property most often misquoted, because it varies with temperature, density, and moisture — the rest of this guide explains each variation and how to read conductivity data correctly.
For calcium silicate board, thermal conductivity follows a predictable linear relationship with mean temperature. The characteristic equation for the standard grade is:
λ = 0.056 + 0.00011t W/m·K
where t is the mean temperature across the insulation layer in °C
Where the formula comes from: the conductivity of a porous solid has three components — conduction through the solid crystal network, conduction through the trapped air, and radiation across the pores. At ambient temperature, solid conduction dominates; as temperature rises, radiation across the pores grows (radiative transfer scales with T³-T&sup4), which is why the line has a positive slope. The intercept 0.056 represents the ambient-temperature conductivity of the standard grade; the slope 0.00011 is the temperature coefficient measured per grade.
Grade-specific coefficients: LG-Standard (170-250 kg/m³) uses λ = 0.056 + 0.00011t; LG-High Temp (1100°C grade) uses coefficient 0.00012 reflecting its denser matrix; high-density grades (800-900 kg/m³) follow approximately λ = 0.095 + 0.00018t because the denser solid network conducts more and the smaller pores radiate less. The coefficients are production averages — batch certificates carry the measured values.
The conductivity value that matters for design is the one at the mean temperature of the installed layer, not the room-temperature brochure value. The table below gives the reference values for the main grades across the service range.
| Mean Temperature | LG-Standard (170-250 kg/m³) | LG-High Temp (250-270 kg/m³) | MF-HD (800-850 kg/m³) |
|---|---|---|---|
| 20°C (ambient) | 0.056 W/m·K | 0.058 | 0.095 |
| 100°C | ≤0.058 | ≤0.065 | ≤0.095 |
| 200°C | ≤0.065 | ≤0.075 | ≤0.115 |
| 400°C | ≤0.088 | ≤0.095 | ≤0.150 |
| 600°C | ≤0.110 | ≤0.118 | ≤0.185 |
| 800°C | ≤0.135 | ≤0.145 | ≤0.210 |
Representative values from Mingfa QC laboratory per ASTM C518. Batch-specific values issued with every order; the equation λ = 0.056 + 0.00011t reproduces the standard-grade column.
Every insulation material's conductivity rises with temperature, but the rate of rise differs by material structure. Understanding the mechanism explains the grade selection and the design calculations.
Heat travels through the xonotlite crystal network by lattice vibration (phonons). More energy at higher temperature means more active vibration and slightly higher conduction — a modest effect in a highly porous body where the solid is only 8-12% of the volume.
Radiation across the air-filled pores grows steeply with temperature (proportional to T³-T&sup4). This is the dominant driver of the rising slope above roughly 400°C, and it is why the conductivity curves steepen at high temperatures.
Air trapped in the pores conducts a fixed share that grows modestly with temperature (air conductivity rises about 0.5%/10K). In open-pore materials this component is stable; in evacuated or microporous products it is suppressed, which is why those products achieve very low conductivities.
Design implication: because conductivity rises with temperature, the thickness calculation must use the conductivity at the mean temperature of the proposed layer — the average of the hot and cold face temperatures — rather than the ambient value. Using the ambient value undersizes the insulation by up to 40% at high service temperatures. The mean-temperature method is the one implemented in the thickness calculator and in every quotation from the engineering team.
Density is the primary dial that calcium silicate manufacturers turn to balance thermal performance against mechanical strength. The relationship is intuitive: more solid material per cubic meter means more conduction paths, so conductivity rises with density.
| Density (kg/m³) | Typical Use | λ @ 100°C | Compressive Strength |
|---|---|---|---|
| 170-250 | Pipe insulation, kiln backing, vessels | ≤0.058 W/m·K | ≥2.0 MPa |
| 250-270 | 1100°C zones | ≤0.065 | ≥2.5 MPa |
| 350-450 | Fire board, structural protection | ≤0.072 | ≥4 MPa |
| 800-900 | Ladle backup, load-bearing | ≤0.095 | ≥8-13 MPa |
Selection rule: specify the lowest density that meets the mechanical requirement. Where the insulation carries load (ladle backup, furnace casing support), the dense grade is mandatory despite its higher conductivity — the load requirement decides. Where no load exists, the low-density grade delivers more thermal performance per thickness and per dollar. The trade-off table above is the one engineers use at specification stage, and the engineering team confirms the density recommendation per application.
Conductivity comparisons between materials are only valid at the same mean temperature and only meaningful where both materials can survive the service condition. The table below presents the comparison at 100°C mean, where most materials are still within their rating.
| Material | λ @ 100°C (W/m·K) | Max Service Temp | Notes |
|---|---|---|---|
| Calcium silicate (170-250 kg/m³) | 0.058 | 1000-1100°C | Rigid, load-bearing, A1 non-combustible |
| Mineral wool | 0.040 | ~650°C | Lower conductivity at low temp; drops out above 650°C |
| Ceramic fiber | 0.060 | 1260-1430°C | Lower density, fragile, no compressive strength |
| Perlite block | 0.060 | ~650°C | Comparable at low temp; budget choice |
| Microporous / aerogel | 0.020 | 900-1000°C | Best conductivity; cost and fragility limit use |
| Fiberglass | 0.040 | ~230-260°C | Cheapest below its temperature limit |
The honest comparison: at 100°C, mineral wool and fiberglass beat calcium silicate on conductivity — a fact the industry acknowledges. What calcium silicate sells is not the lowest number at low temperature, but the combination that exists nowhere else: conductivity below 0.10 W/m·K while carrying 2-13 MPa of compressive load at 1000°C+ for 20+ years. When the service temperature, load, or service life rules out the softer materials, calcium silicate is the engineering answer — and its conductivity at those temperatures is fully competitive with ceramic fiber.
Conductivity is measured on laboratory apparatus, and the measurement standard determines the comparability of the result. The two methods used for calcium silicate insulation are described below.
The reference method: a flat specimen sits between a hot plate and a cold plate, with a guard ring eliminating edge losses. The heat input through the known area at steady state gives conductivity directly. Slow but definitive; used to validate other apparatus.
The production method: a calibrated heat flux transducer measures the heat flow through the specimen between plates at set temperatures. Fast enough for batch testing; the standard method cited on Mingfa batch certificates.
Both methods set hot and cold plate temperatures so the average equals the target mean (100/200/400°C etc.). The certificate states the mean temperature of each reported value — read it before comparing numbers across suppliers.
European certificates may cite EN 12667 or ISO 8302 instead of the ASTM methods; the values are comparable when the mean-temperature basis matches, but the certificate should always be checked for the method and mean temperature before the number is used in a calculation.
Conductivity is the input to every thickness calculation. The method below is the one used across industrial insulation design and implemented in the online calculator.
Step 1 — heat flux target: define the allowable heat loss q (W/m²) or the maximum cold-face temperature. For personnel protection, the surface must stay below 55-60°C where accessible; for energy design, q is set by the payback economics.
Step 2 — mean temperature: estimate the mean temperature across the layer. For a first pass, assume the cold face at a reasonable value (e.g., 50-150°C depending on duty) and average with the hot face. Read λ at that mean from the certificate or the equation.
Step 3 — thickness: t = λ × ΔT / q. Check the resulting cold-face temperature; if it differs materially from the assumption, iterate with the new mean. Two iterations converge in practice.
Worked example: hot face 650°C, target cold face 150°C (ΔT = 500 K), target q = 1,000 W/m². Mean = 400°C, λ = 0.056 + 0.00011×400 = 0.100. t = 0.100 × 500 / 1000 = 0.050m = 50mm. Verify: cold face at 50mm with mean 400°C gives exactly the target. If q = 500 W/m² were required, t doubles to 100mm.
Standard-density calcium silicate board (170-250 kg/m³) measures approximately 0.056 W/m·K at ambient, rising to about 0.058 at 100°C mean, 0.088 at 400°C, and 0.135 at 800°C mean. High-density grades (800-900 kg/m³) run higher: about 0.095 W/m·K at 100°C. Always read conductivity with its mean temperature.
Water conducts heat about 23 times better than air (0.6 vs 0.026 W/m·K). When water displaces air in the pores, conductivity rises dramatically — wet calcium silicate can lose more than half its thermal resistance. The protection is the weather jacket and vapor barrier; treated water-repellent grades resist absorption where wet service is unavoidable, and wetted board dries with full property recovery.
Three reasons: different mean temperatures (0.058 at 100°C vs 0.088 at 400°C is the same material), different density grades, and different test methods or specimen conditioning. Compare only after normalizing to the same density, mean temperature, and test standard — the batch certificate is the only document that states all three for the actual delivered material.
For calcium silicate, no meaningful aging occurs: the xonotlite crystal structure is thermally stable to its rating, there are no organic binders to degrade, and no blowing agent to diffuse out. Measured conductivity after years at rated temperature remains within a few percent of the certificate value — provided the system protects the material (weather jacket intact, no moisture ingress, design temperature not exceeded).
Conductivity and R-value are two sides of the same property. Conductivity (λ) is the material property; R-value is the resistance of a specific thickness: R = t / λ. Every R-value statement implies a conductivity and a thickness — and a mean temperature.
| Thickness | R @ 100°C (λ=0.058) | R @ 400°C (λ=0.100) |
|---|---|---|
| 25mm | 0.43 m²·K/W | 0.25 |
| 50mm | 0.86 | 0.50 |
| 75mm | 1.29 | 0.75 |
| 100mm | 1.72 | 1.00 |
The trap: the same 50mm board has R = 0.86 at 100°C mean and R = 0.50 at 400°C mean — a 42% difference from temperature alone. Specifications that quote R-value without the mean temperature, or that carry ambient-temperature R-values into high-temperature designs, undersize the insulation by the same margin. The fix is one line in the specification: "R-value at the mean service temperature, conductivity per ASTM C518."
The conductivity family for Mingfa products spans the density range from lightweight pipe insulation to load-bearing refractory board. The reference table below consolidates the values for specification work.
| Product | Density | Max Temp | λ @ 100°C | λ @ 400°C | λ @ 800°C |
|---|---|---|---|---|---|
| LG-Standard board | 170-250 kg/m³ | 1000°C | 0.058 | 0.088 | 0.135 |
| LG-High Temp board | 250-270 kg/m³ | 1100°C | 0.065 | 0.095 | 0.145 |
| GF-1100 fire board | 350-450 kg/m³ | 1100°C | 0.072 | 0.105 | 0.155 |
| MF-HD high density | 800-850 kg/m³ | 1000°C | 0.095 | 0.150 | 0.210 |
| MFDJ composite brick | 300-400 kg/m³ | 1000°C | 0.075 | 0.110 | 0.160 |
| MFBL glass furnace brick | 600 kg/m³ | 1000°C | 0.090 | 0.120 | 0.170 |
Values in W/m·K, representative per ASTM C518. The technical data sheet for each product carries the full temperature-conductivity curve; batch certificates carry the measured values for the delivered batch.
Conductivity clauses in specifications fail in predictable ways. The specification language below prevents the three most common failures.
"Thermal conductivity ≤0.10 W/m·K" without a temperature invites a supplier to quote at the most favorable mean. Correct language: "≤0.065 W/m·K at 100°C mean and ≤0.095 W/m·K at 400°C mean, per ASTM C518."
Conductivity is grade-specific. "Calcium silicate board, density 170-250 kg/m³" in the same clause ties the number to the duty. Without density, the supplier quotes the grade that meets the number, not the grade that meets the load.
Brochure "typical values" are not a compliance basis. Correct language: "Batch certificates with measured conductivity per ASTM C518, matching delivered pallet labels, shall accompany each delivery." That single clause converts the specification from aspirational to auditable.
Mingfa's technical team reviews conductivity clauses in tender documents free of charge and confirms whether the grade, density, and certificate requirements are internally consistent before submission.
Every 0.01 W/m·K of conductivity difference translates into measurable energy cost over a plant's operating life. The business case below shows why conductivity is worth specifying precisely.
The arithmetic: heat loss through insulation is proportional to conductivity at a fixed thickness and temperature difference. A material with λ = 0.058 at the service mean loses 42% less heat than one with λ = 0.100 — the difference between the standard grade and the high-density grade at the same thickness. Over a 4-inch steam main at 400°C running 8,000 hours per year, each 0.01 W/m·K of conductivity costs a measurable annual fuel sum per 100m of line; specifying the right grade and verifying the batch certificate is a direct operating-cost decision, not a paperwork exercise.
The lifecycle view: the energy saved by correct insulation specification compounds over the 20-25 year service life. Industry lifecycle data puts the energy saving ratio of calcium silicate insulation above 11,000:1 over its service life relative to the manufacturing energy — and the ratio improves when the specified conductivity is actually delivered. Batch verification closes the gap between the design basis and the installed reality.
For energy audits: plants conducting insulation energy audits should request batch certificates for installed material and compare the certificate conductivity at the service mean against the design basis. Where the audit finds a mismatch, the correction (re-specification at next maintenance, or targeted replacement) is a documented, quantifiable energy measure — the kind that features in ISO 50001 energy management submissions.
Above 800°C, the radiative component of conductivity grows faster than the linear model predicts, and material selection must account for the divergence. This section covers the high-temperature behavior that matters for 900-1100°C duty.
The linear model's limit: the equation λ = 0.056 + 0.00011t is accurate through approximately 800°C mean for the standard grade. Above that, radiative transfer across the pores accelerates, and the measured conductivity runs above the linear projection. The LG-High Temp grade, with its higher density (250-270 kg/m³) and finer pore structure, holds the radiative growth in check better than the low-density grade — which is one of the reasons the 1100°C rating pairs with a denser matrix.
Specifying for 1000°C duty: request the batch certificate conductivity at the 800°C mean as the design input for high-temperature layers, and apply a margin where the mean exceeds 800°C. The technical data sheet for the LG-High Temp grade carries the full conductivity curve to 1000°C; the engineering team provides the curve on request for design calculations at extreme duty.
Comparative note: in the 800-1100°C range, calcium silicate's conductivity (0.13-0.18 W/m·K depending on grade) is competitive with ceramic fiber at the same mean temperatures, while providing the compressive strength ceramic fiber lacks. Where the design carries load at extreme temperature, calcium silicate is the material that combines both properties.
Buyers and quality auditors can verify a supplier's conductivity claims with the checklist below — no laboratory required.
A supplier that cannot produce a certificate meeting all six points is quoting from a brochure. The distinction matters because the delivered material's conductivity is what the plant's energy performance — and payback — depends on.
The conductivity requirement in practice varies by application because the service mean temperature and the allowable heat loss differ. The application notes below show the typical specification language per duty.
| Application | Typical Spec Clause | Why This Basis |
|---|---|---|
| Steam pipe, 250-400°C | λ ≤0.075 W/m·K at 200°C mean, ASTM C518 | Mean across 50-75mm layer runs 150-250°C |
| Furnace casing, 500-800°C | λ ≤0.11 W/m·K at 400°C mean | Backup layer mean near 400°C |
| Kiln backing, 900-1100°C | λ ≤0.15 W/m·K at 800°C mean, LG-High Temp | High mean requires the high-temp grade curve |
| Fire protection assembly | A1 classification + λ data to 800°C for thermal modeling | Assembly rating computed from the full curve |
| Ladle backup (dense grade) | λ ≤0.095 W/m·K at 100°C mean, density 800-850 | Density fixed by load; conductivity follows |
The pattern in every row: the conductivity clause names a mean temperature tied to the actual service, a density grade tied to the load, and a test standard tied to the certificate. That triple is what makes the clause auditable and the quotes comparable.
The terms conductivity, resistance, and transmittance are used interchangeably in casual specification language — and each error propagates into the thickness calculation. The definitions below fix the terminology.
Material property: heat flow through unit thickness per unit area per unit temperature difference. Unit W/m·K. Independent of thickness. This is what batch certificates measure and what grade selection compares.
Layer property: R = t/λ, unit m²·K/W. Scales with thickness. What the design calculation solves for. R-values add across layers in a composite build.
Assembly property: U = 1/R_total including surface films, unit W/(m²·K). What the heat-loss calculation uses for the complete wall or pipe system. U = q/ΔT.
A specification that says "conductivity 0.5 m²·K/W" mixes units of conductivity and resistance; a spec that says "R-value 0.058 W/m·K" mixes the property and the number. The engineering team reviews specification terminology on request and returns corrected clauses with the property, value, unit, and test method aligned.
Conductivity values circulate through four document types, with very different authority. Knowing which document carries the value you need prevents specification errors.
| Document | Authority | Use |
|---|---|---|
| Batch certificate | Highest — measured per delivered batch | Compliance verification, project quality file, acceptance |
| Technical data sheet | High — production-range values per grade | Design calculations, specification development |
| Brochure / website | Medium — marketing values, often best-case | Initial screening only; never a compliance basis |
| Industry reference texts | Medium — general values, may predate current production | Background understanding, rough estimates |
The practical rule: design from the technical data sheet, verify against the batch certificate, and treat anything else as indicative. Mingfa issues technical data sheets per product line on request and batch certificates with every order, so the design basis and the compliance basis come from the same source.
The conductivity guide closes with the five-step workflow that converts the information above into a correct specification.
The engineering team supports each step: grade and density confirmation, conductivity data at the service mean, specification clause review, and acceptance support. Free A4 samples and the thickness calculator are available for the design phase, and every quotation includes the calculation basis.
Mingfa Insulation manufactures calcium silicate board, pipe sections, and refractory grades in Laizhou, Shandong since 1991, exporting to 70+ countries. Batch test certificates per ASTM C518 are issued with every order, free A4 samples are available for evaluation, and the engineering team responds to conductivity and specification inquiries within one business day.
The conductivity values in this guide are representative production figures verified through ASTM C518 testing at our QC laboratory; for design work, request the technical data sheet for the specific grade and the batch certificate values for the delivered material. Where the design mean temperature exceeds the tabled range, the engineering team provides the measured conductivity curve on request.
Contact the team with your operating conditions — temperature, geometry, and mechanical duty — for a written conductivity basis and thickness recommendation, or use the online thickness calculator for pipe insulation sizing.
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