Determination of Thermal Conductivity of Thermal Insulation Mortar: Standards, Methods and Laborator

Thermal conductivity is the decisive performance indicator of thermal insulation mortar, directly governing the thermal resistance and energy efficiency of building envelope systems. This technical article explains how the thermal conductivity of insulation mortar is determined in accredited third-party laboratories, covering sample preparation, the guarded hot plate and heat flow meter methods, applicable standards such as GB/T 10294, GB/T 10295, ISO 8302, ASTM C177 and ASTM C518, and the key factors that influence measurement accuracy. Materials manufacturers, construction engineers and quality-control professionals will find practical guidance for product certification and compliance with building energy-efficiency codes.

Technical Overview

Thermal insulation mortar (also called insulating plaster or insulating rendering mortar) is a dry-mixed functional mortar composed of a cementitious or gypsum-based binder, lightweight aggregates such as expanded perlite, vitrified microspheres (closed-cell glassy beads) or expanded polystyrene (EPS) granules, and performance-enhancing additives. After mixing with water on site, the mortar is applied as a plaster layer to exterior walls, interior partitions, roofs and floors to reduce heat flow through the building envelope.

Thermal conductivity, denoted by the symbol lambda and expressed in W/(m·K), quantifies the rate of heat transfer through unit thickness of the material under a unit temperature gradient. It is the single most important quality parameter of insulation mortar: the lower the value, the better the insulating performance. Product standards classify insulation mortar into grades with maximum permissible thermal conductivity—for example, GB/T 20473 specifies Grade I materials at no more than 0.070 W/(m·K) and Grade II materials at no more than 0.085 W/(m·K), determined on oven-dried specimens at a mean temperature of 25 °C.

Because thermal conductivity depends strongly on dry density, moisture content, aggregate type and test temperature, its determination must follow standardized steady-state methods under controlled laboratory conditions. The two internationally recognized approaches are the guarded hot plate method and the heat flow meter method, supported by standards including GB/T 10294, GB/T 10295, ISO 8302, ISO 8301, EN 12667, ASTM C177 and ASTM C518.

Testing Samples

Testing is performed on hardened mortar plates prepared under laboratory conditions that represent the factory formulation and site mixing ratio. The principal commercial types submitted for thermal conductivity determination include the following.

Expanded Perlite Insulation Mortar

A traditional inorganic system based on expanded perlite aggregate and cement binder, typically achieving dry densities of 300–500 kg/m³ and thermal conductivity values in the range of approximately 0.055–0.085 W/(m·K). It offers good fire performance and is frequently specified where non-combustible materials are required.

Vitrified Microsphere Insulation Mortar

A high-performance inorganic system using closed-cell vitrified microspheres as the lightweight aggregate, covered by JG/T 283. The smooth, closed pore structure yields lower water absorption than perlite systems, with thermal conductivity commonly in the range of 0.045–0.070 W/(m·K).

EPS Particle Insulation Mortar

A cement-based mortar incorporating expanded polystyrene granules as the insulating aggregate. Due to the very low conductivity of the polymer phase, such mortars can reach 0.045–0.060 W/(m·K), although their fire classification is limited by the organic aggregate.

Aerogel-Modified and Composite Insulation Mortars

Emerging formulations containing silica aerogel powder or hybrid lightweight fillers can achieve thermal conductivity as low as 0.028–0.045 W/(m·K) while remaining Class A fire-rated, and are increasingly tested for premium building retrofit projects.

Specimen Preparation and Conditioning

For thermal conductivity determination, the fresh mortar is cast into flat, plane-parallel plate molds, typically at least 300 mm × 300 mm in face area with a thickness of 30–50 mm representing the installed layer. Specimens are demolded after 24–48 hours, cured for 28 days under standard conditions, and then dried in an oven at 105 ± 5 °C until constant mass is reached. For mortars containing organic aggregates such as EPS granules, a reduced drying temperature of 60–70 °C is used to prevent deformation. Before testing, specimens are cooled to ambient temperature in a desiccator, and their thickness is measured under the actual clamping pressure of the apparatus.

Testing Items

Although thermal conductivity is the core determination, a complete characterization of insulation mortar for product certification typically includes the following related items, because thermal performance cannot be interpreted in isolation from density and mechanical properties.

• Thermal conductivity at a mean temperature of 25 °C (dry condition) — the defining classification parameter
• Dry density — the primary variable correlating with thermal conductivity
• Compressive strength — typically required at 0.20 MPa minimum for Grade I and 0.40 MPa for Grade II systems
• Water absorption and softening coefficient (wet-to-dry strength ratio)
• Linear shrinkage of hardened mortar
• Fresh mortar properties: consistency, setting time and workable density
• Combustion performance classification per GB 8624, relevant to fire safety approval

Testing Methods

Thermal conductivity of insulation mortar is determined by steady-state heat transfer methods, in which a constant temperature difference is maintained across the specimen until equilibrium is reached. Two principal methods are accepted by international and national standards.

Guarded Hot Plate Method (GB/T 10294, ISO 8302, ASTM C177)

The guarded hot plate is the primary, absolute reference method. Two nominally identical specimen plates are placed on either side of an electrically heated metering section surrounded by a guard ring. The guard ring is maintained at the same temperature as the metering section, suppressing lateral heat flow so that all metered power is forced through the specimens as one-dimensional axial heat flow. Thermal conductivity is calculated directly from the measured heating power, specimen thickness, metering area and temperature difference. The method achieves an accuracy of approximately 1–2 % and is used for type certification, dispute resolution and calibration of secondary apparatus.

Heat Flow Meter Method (GB/T 10295, ISO 8301, ASTM C518)

The heat flow meter is a comparative secondary method and the workhorse of routine laboratory testing. A single specimen is sandwiched between a hot plate and a cold plate, and calibrated heat flux transducers mounted on the plate surfaces measure the heat flow density through the specimen. The apparatus must be calibrated with certified reference materials traceable to national metrology institutes over the relevant conductivity and thickness range. Thermal conductivity is calculated from the measured heat flux, specimen thickness under load and the imposed temperature difference. The method offers repeatability near 1 %, shorter test cycles and lower cost, making it the standard choice for factory production control and commercial testing at accredited third-party laboratories.

Standard Test Conditions

• Mean temperature: 25 °C for standard classification; 10 °C or other mean temperatures may be specified for particular climate design applications
• Temperature difference across the specimen: typically 15–20 K, using natural convection or liquid-cooled cold plates
• Steady-state criterion: consecutive readings of heat flux and temperatures remaining stable within approximately ±0.5 % over two or more observation intervals
• Specimen condition: oven-dried to constant mass, with edges protected to limit lateral heat losses

Calculation and Reporting

For the heat flow meter method, thermal conductivity is computed as the product of measured heat flux density and specimen thickness divided by the temperature difference. The test report should state the measured thermal conductivity, the mean test temperature, the dry density, the specimen thickness under load, the test method and standard used, the apparatus calibration reference, and the direction of heat flow. Results are reported to three significant figures, typically between 0.03 and 0.10 W/(m·K) for commercial insulation mortars.

Factors Affecting Measurement Accuracy

• Moisture content: water has a thermal conductivity of about 0.6 W/(m·K), roughly twenty times that of still air, so even small residual moisture significantly inflates the apparent result
• Density variation and aggregate segregation within the specimen plate
• Contact resistance between apparatus plates and rough mortar surfaces, mitigated by thin compliant interface layers and controlled clamping pressure
• Lateral edge heat losses, especially in thick or low-density specimens
• Thickness error under compressive load, which enters the calculation linearly

Applications

Thermal conductivity determination of insulation mortar serves multiple purposes across the construction materials value chain.

• Product type certification and factory production control for manufacturers supplying insulation mortar to energy-efficiency building projects
• Compliance demonstration with building energy conservation design standards, which prescribe maximum thermal conductivity and minimum thermal resistance for envelope components
• Fire-safety-sensitive applications such as high-rise and public buildings, where Class A non-combustible inorganic insulation mortars are mandated under codes referenced to GB 8624
• External wall insulation and internal insulation systems, roof insulation screeds and floor leveling layers in both new construction and retrofit
• Green building evaluation and energy performance labeling of completed works
• Research and development of formulations, including aggregate gradation optimization, binder content reduction and aerogel or hollow-microsphere modification studies

FAQ

What is the typical thermal conductivity range of insulation mortar?

Commercial products generally fall between 0.045 and 0.085 W/(m·K) in the dry state at 25 °C mean temperature. Perlite systems sit toward the upper end, vitrified microsphere and EPS particle systems occupy the middle range, and aerogel-modified mortars achieve the lowest values, approaching 0.03 W/(m·K).

Which method should be selected: guarded hot plate or heat flow meter?

The guarded hot plate method is preferred for type certification, standard development and dispute resolution because it is an absolute method requiring no calibration. The heat flow meter method is preferred for routine quality control and commercial testing because it is faster and more economical while maintaining accuracy within about 2–3 % when properly calibrated.

Why must specimens be oven-dried before testing?

Water conducts heat roughly twenty times better than the air it displaces in mortar pores. Standard classification values are defined for the dry state, so residual moisture would produce falsely high thermal conductivity and could cause a compliant product to fail.

How long does a complete determination take?

After 28 days of curing and oven drying, the steady-state measurement itself typically takes 4–8 hours depending on specimen thickness and density, with thick, low-density materials requiring longer equilibration. A full test sequence from specimen receipt to report usually requires one to three working days.

What are the acceptance criteria?

Under GB/T 20473, the dry-state thermal conductivity at 25 °C must not exceed 0.070 W/(m·K) for Grade I and 0.085 W/(m·K) for Grade II, with corresponding limits on dry density and compressive strength. Project specifications and green building programs may impose stricter limits.

Does thermal conductivity change in service?

Yes. Mortar exposed to humidity absorbs moisture that raises its effective conductivity. Design codes therefore apply moisture correction factors to laboratory dry values when calculating the thermal resistance of the installed envelope, which is why water absorption is tested alongside thermal conductivity.