The thermal conductivity of plastering gypsum is the key parameter that determines how gypsum-based renders and skim coats contribute to the thermal performance of building envelopes. This article explains how the thermal conductivity of plastering gypsum is measured in accredited third-party laboratories, covering the applicable standards (ISO 8302, ISO 8301, EN 12667, ASTM C177, ASTM C518, GB/T 10294, GB/T 10295), specimen preparation, guarded hot plate and heat flow meter procedures, typical result ranges, and practical applications of the measured data.
Technical Overview
Plastering gypsum is a factory-premixed, hemihydrate-based rendering and finishing material widely applied to interior walls and ceilings as a replacement for conventional cement-lime mortars. Because modern building energy codes require accurate calculation of wall thermal transmittance (U-value), the thermal conductivity of every layer in the assembly, including the plaster layer, must be reliably quantified.
Thermal conductivity (lambda, expressed in W/(m·K)) describes the steady-state heat flow through a unit thickness of material under a unit temperature difference. For gypsum plasters, the value depends strongly on dry density, porosity, aggregate type, and moisture content. In the dry state at a mean temperature of 25 °C, ordinary dense plastering gypsum typically exhibits a thermal conductivity of 0.35 to 0.55 W/(m·K), while lightweight formulations containing expanded perlite, expanded polystyrene beads, or vitreous porous aggregates fall in the range of 0.10 to 0.30 W/(m·K), and premium thermal-insulating gypsum plasters can reach values below 0.10 W/(m·K).
The determination of thermal conductivity for plastering gypsum is governed by an internationally harmonized family of steady-state measurement standards, together with the relevant product standards:
- GB/T 10294 and GB/T 10295, the Chinese national standards for steady-state thermal resistance measurement by the guarded hot plate and heat flow meter methods, applied together with the product standard GB/T 28627 for plastering gypsum.
- ISO 8302 (guarded hot plate) and ISO 8301 (heat flow meter), the international reference methods.
- EN 12667, the European standard for determining thermal resistance of building materials by guarded hot plate and heat flow meter methods.
- ASTM C177 (guarded hot plate apparatus) and ASTM C518 (heat flow meter apparatus), the primary North American test methods.
Testing Samples
Ordinary (Dense) Plastering Gypsum
Standard hand-applied or machine-applied gypsum render with mineral aggregate and a dry density typically between 800 and 1300 kg/m³. Samples are prepared by mixing the plaster with water at the manufacturer's recommended ratio and casting flat test panels representative of the factory formulation.
Lightweight and Thermal-Insulating Gypsum Plaster
Formulations incorporating expanded perlite, EPS beads, hollow glass microspheres, or other porous lightweight aggregates, with dry densities commonly between 400 and 800 kg/m³. These products are frequently marketed on the basis of declared thermal conductivity values, making accurate measurement essential for product certification and datasheet claims.
Machine-Applied and Multi-Coat Systems
Multi-layer render systems consisting of a base coat and a finishing coat may be tested layer by layer, or as a combined specimen with the layer thicknesses reproduced as in practice, so that an effective thermal conductivity for the complete system can be reported.
Specimen Preparation and Conditioning
Laboratory-cast slab specimens, typically 300 mm × 300 mm or 600 mm × 600 mm in plan and 20 mm to 50 mm thick, are cured to develop stable hydration products and then conditioned to the dry state. Drying is carried out in a ventilated oven at (40 ± 2) °C, or by desiccation, until constant mass is reached, meaning that successive weighings at least 24 hours apart differ by no more than 0.1 percent. Temperatures above approximately 60 °C are avoided because they can partially dehydrate calcium sulfate dihydrate and alter the pore structure. Specimen faces must be flat, parallel, and smooth, with thickness variation kept within the tolerance stated by the governing method standard, since air gaps at the plate interface introduce systematic error.
Testing Items
A complete thermal characterization of plastering gypsum specimens typically includes the following measured and derived quantities:
- Thermal conductivity (lambda) at a specified mean temperature, commonly 25 °C for GB-based testing and 10 °C for European declarations.
- Thermal resistance (R, in m²·K/W) calculated from specimen thickness and measured conductivity.
- Apparent dry density of each specimen, required for correlating conductivity with mix design.
- Moisture content at the time of test, to verify that the dry-state condition has been achieved.
- Optional conductivity measurements at several mean temperatures (for example 10 °C, 25 °C, and 40 °C) to establish the temperature dependence of the material.
Testing Methods
Guarded Hot Plate Method (GB/T 10294 / ISO 8302 / ASTM C177 / EN 12667)
The guarded hot plate apparatus is the primary, absolute measurement method. Two nearly identical specimens are placed on either side of a central electrically heated metering section surrounded by a guard ring maintained at the same temperature, so that all metered heat flows axially through the specimens toward the externally cooled plates. The heat input, metering area, specimen thickness, and plate temperature difference yield the thermal conductivity directly from first principles, without calibration against other materials. For a symmetric arrangement, lambda equals the product of power and thickness divided by twice the metering area and the temperature difference. The method achieves measurement uncertainty on the order of 1 to 3 percent and is preferred for certification-grade determinations.
Heat Flow Meter Method (GB/T 10295 / ISO 8301 / ASTM C518 / EN 12667)
The heat flow meter method is a comparative technique in which a single specimen is sandwiched between isothermal hot and cold plates, and the heat flux density through the specimen is measured by calibrated heat flux transducers. The apparatus must be calibrated using certified reference materials traceable to national metrology institutes, covering the thermal resistance range of the specimens being tested. Thermal conductivity is calculated as the measured heat flux density multiplied by thickness and divided by the temperature difference. The method is faster, requires less operator effort, and achieves uncertainties of approximately 2 to 5 percent, which makes it the standard choice for routine factory quality control and pre-screening, while the guarded hot plate remains the arbitrating method.
Test Conditions and Measurement Procedure
A typical test employs a mean temperature of 25 °C with a temperature difference of 15 K to 20 K between the plates, for example a hot plate at 35 °C and a cold plate at 15 °C. After assembly, the specimen is allowed to reach steady state, defined as heat flux and plate temperatures remaining stable, typically within 0.5 percent, for at least 30 minutes. Specimen thickness is measured under the actual test load at several positions, and edge guarding or peripheral insulation is applied to suppress lateral heat losses. Equilibrium times of several hours per specimen are normal, and replicate specimens are recommended to demonstrate reproducibility.
Calculation and Reporting
Thermal conductivity is expressed in W/(m·K) at the stated mean temperature, and thermal resistance is reported as thickness divided by conductivity. A compliant test report should include:
- Complete product identification, batch, and mix proportions, including water-to-plaster ratio.
- Specimen dimensions, dry density, and moisture content at test.
- The measurement method, apparatus, calibration status, mean temperature, and temperature difference.
- Thermal conductivity values for individual specimens and the reported mean, with measurement uncertainty.
- Any deviations from the standard method and the date of testing.
Quality Assurance
Accredited third-party laboratories maintain the validity of thermal conductivity results through annual calibration verification against certified reference materials, participation in interlaboratory comparison programs, documented uncertainty budgets, and strictly controlled specimen conditioning procedures. Because moisture is the dominant interfering variable for gypsum-based materials, dry-mass verification immediately before testing is a mandatory quality checkpoint.
Applications
Measured thermal conductivity data for plastering gypsum serve a wide range of engineering, regulatory, and commercial purposes:
- Compliance demonstration for building energy codes, where the plaster layer contribution is included in whole-wall U-value and thermal bridge calculations.
- Input data for building energy simulation and dynamic thermal modeling software.
- Product certification and datasheet declaration of lambda values for lightweight and thermal-insulating gypsum plasters.
- Environmental product declarations, which require verified thermal performance data for the use-stage modules.
- Research and development of new formulations, such as plasters modified with expanded perlite, aerogel, or phase-change materials, where conductivity is a primary optimization target.
- Factory production control and batch-to-batch consistency verification for manufacturers.
- Thermal retrofit assessment, where the added insulation benefit of replacing dense cement render with lightweight gypsum plaster must be quantified.
FAQ
What is a typical thermal conductivity value for plastering gypsum?
In the dry state at a mean temperature of 25 °C, ordinary plastering gypsum usually measures between 0.35 and 0.55 W/(m·K), lightweight gypsum plaster between 0.10 and 0.30 W/(m·K), and specialized thermal-insulating gypsum plaster below 0.10 W/(m·K). The value rises noticeably with moisture content and density.
Which standard should be selected for testing?
The choice depends on the target market and specification. Projects in China typically require GB/T 10294 or GB/T 10295 together with the product standard GB/T 28627, European declarations rely on EN 12667, and North American specifications cite ASTM C177 or ASTM C518. The underlying physical principles of all these methods are equivalent, so results are broadly comparable when specimen conditioning and mean temperature are matched.
Why must specimens be dried to constant mass before testing?
Water has a thermal conductivity of about 0.6 W/(m·K), far higher than dry gypsum, so even a few percent of residual moisture can inflate the apparent conductivity and mask the true dry-state performance. Drying at approximately 40 °C removes moisture without dehydrating the gypsum crystals, and constant mass confirmed by successive weighings ensures the reported value is reproducible.
Guarded hot plate or heat flow meter: which method is better?
The guarded hot plate is an absolute method with the lowest uncertainty and is preferred for certification and dispute resolution. The heat flow meter is faster and more economical, making it ideal for routine quality control, provided the apparatus is properly calibrated with certified reference materials.
How long does a thermal conductivity test take?
Specimen casting, curing, and drying to constant mass generally require several days to a week depending on thickness. The steady-state measurement itself, including apparatus stabilization, typically takes from four to twenty-four hours per condition, so a complete test with two mean temperatures is usually finished within one to two weeks of laboratory time.
Can samples be taken from plaster already applied on site?
Yes. Panels or cores can be cut from cured field plaster, provided the extracted pieces are large enough, representative, and reconditioned to the required dry state, and their faces are made flat and parallel before testing. Laboratory-cast specimens remain the reference approach for product declarations because mix water content and compaction are fully controlled.
How does density relate to thermal conductivity in gypsum plasters?
Thermal conductivity generally increases with dry density, because solid gypsum conducts heat far better than air-filled pores. Lightweight aggregates and foaming increase porosity and lower conductivity, but they also reduce strength and abrasion resistance, so mix design always involves a trade-off that must be verified by actual measurement rather than estimated from density correlations alone.
