Determination of Thermal Conductivity of Foam Glass: Standards, Methods and Laboratory Practice

Thermal conductivity is the single most important performance parameter of foam glass (cellular glass) insulation, with declared values typically falling between 0.038 and 0.060 W/(m·K) at a mean temperature of 10 °C. This article explains how the thermal conductivity of foam glass is determined in accredited third-party laboratories using the guarded hot plate and heat flow meter methods in accordance with ISO 8302, ISO 8301, ASTM C177, ASTM C518, EN 12667, EN 13167 and GB/T standards, covering specimen preparation, test conditions, calculation and reporting.

Technical Overview

Foam glass, also known as cellular glass, is a rigid, lightweight thermal insulation material manufactured by firing crushed (typically recycled) glass together with a carbon-based foaming agent at approximately 850–1000 °C. The resulting structure consists of hermetically closed glass cells filled with gas, which gives the material an exceptional combination of properties: it is non-combustible (Euroclass A1), impermeable to water and water vapor, dimensionally stable, chemically resistant, and serviceable over an extremely wide temperature range of roughly −260 °C to +430 °C.

• Density: typically 100–165 kg/m³ for boards and blocks
• Thermal conductivity: typically 0.038–0.060 W/(m·K) at a mean temperature of 10 °C
• Cell gas: air or combustion gases, so no blowing-agent aging drift as with polyurethane
• Completely closed-cell structure, resulting in negligible moisture pick-up during testing

Accurate determination of the thermal conductivity (lambda value, λ) is essential for product certification under harmonized product standards such as EN 13167, for factory quality control, for energy-performance declarations of buildings, and for the thermal design of cryogenic and high-temperature industrial installations. The measured λ value depends on density and cell structure, mean temperature, specimen thickness and, to a much lesser extent than with open-cell materials, moisture content. Because foam glass is dimensionally stable and non-hygroscopic, its measured thermal conductivity is highly reproducible, but the measurement still demands strict adherence to standardized steady-state methods.

Testing Samples

Board and Block Specimens

Flat board or block specimens are the standard form for thermal conductivity testing. Depending on the apparatus, typical specimen sizes are 300 mm × 300 mm or 600 mm × 600 mm, with thicknesses of 20–60 mm as delivered or as cut. For the guarded hot plate method, a matched pair of specimens with near-identical thickness and density is required, one placed on each side of the central heater plate. The heat flow meter method requires only a single specimen. For reliable heat flow meter results, the specimen must provide a thermal resistance R of at least 0.5 m²·K/W as required by EN 12667; a 40 mm thick foam glass specimen at λ ≈ 0.05 W/(m·K) yields R ≈ 0.8 m²·K/W, which comfortably meets this requirement.

Pipe Sections and Shaped Products

For cellular glass pipe sections (shells) and other preformed shapes, thermal conductivity is determined on flat specimens cut from representative production material. This is standard practice because both the guarded hot plate and heat flow meter apparatus require planar, parallel-faced specimens. The flat specimens are taken from the same production batch, density and cell structure as the shaped product so that the measured λ is representative.

Specimen Preparation and Conditioning

Specimens must be cut so that the two faces are flat, parallel and perpendicular to the intended heat flow direction, with thickness uniformity typically within ±1%. Specimens are conditioned to constant mass at (23 ± 2) °C and (50 ± 5) % relative humidity, or dried where appropriate, and weighed before and after testing to verify that no moisture change occurred. Thickness is measured at multiple points and, where required by the standard, determined under the actual clamping load of the apparatus.

Testing Items

The core scope of a foam glass thermal conductivity determination typically includes:

• Thermal conductivity λ in W/(m·K) at specified mean temperatures (most commonly 10 °C for building product declarations, and 23 °C or other temperatures on request)
• Thermal resistance R in m²·K/W calculated as R = d / λ
• Thermal conductivity as a function of mean temperature, λ(T), for cryogenic (for example LNG service down to −165 °C) and elevated-temperature applications
• Declared thermal conductivity λD and conversion to design values for conformity with EN 13167
• Supplementary determinations: specimen density, dimensions, and moisture content verification to support the validity of the λ result

Testing Methods

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

The guarded hot plate apparatus is the primary (absolute) steady-state method. Two nominally identical specimens sandwich a central electrically heated metering section that is surrounded by a guard ring maintained at the same temperature, suppressing lateral heat losses. The outer faces of the specimens are held against cooled or heated plates at a lower (or higher) temperature. Once steady state is reached, the thermal conductivity follows directly from one-dimensional Fourier conduction: λ = Φ × d / (A × ΔT), where Φ is the heat flow rate through the metering area A, d is the specimen thickness and ΔT is the temperature difference across the specimen. Because the method is absolute and requires no calibration against reference materials, it offers the lowest uncertainty, typically 1–3 %, and is preferred for precision measurements, reference work and certification testing.

Heat Flow Meter Method (ISO 8301, ASTM C518, EN 12667, GB/T 10295, JIS A 1412)

The heat flow meter apparatus measures the heat flux through a single specimen with calibrated heat flux transducers mounted in the hot and/or cold plate. The apparatus is calibrated using certified reference materials traceable to national metrology institutes, after which the thermal conductivity of the test specimen is obtained by comparison: λ = Φ × d / ΔT, with Φ taken from the transducer output. Modern instruments commonly use 600 mm × 600 mm plates, allowing testing of full product thicknesses and representative material. The method is faster than the guarded hot plate and is the workhorse for routine quality control and commercial certification of foam glass boards, with typical laboratory uncertainties of 2–5 %.

Test Conditions, Calculation and Reporting

Common test conditions are a mean temperature of 10 °C (for example plates at 25 °C and −5 °C) for building applications per EN 13167, and 23 °C or application-specific mean temperatures otherwise, with a temperature difference of typically 20–30 K between the plates. For cryogenic service, λ is determined at several mean temperatures to construct a λ(T) curve; the thermal conductivity of foam glass decreases markedly at low temperatures because both gas conduction and radiative transfer are reduced. A complete test report identifies the material and product standard, the method and apparatus used, specimen density, thickness, moisture condition, mean temperature and temperature difference, the measured λ (and R) values, the measurement uncertainty, and the date of test.

Applications

Reliable thermal conductivity data for foam glass underpin its use across the full range of its service temperatures:

• Building insulation: flat and pitched roofs, load-bearing floor slabs, plinth and basement insulation, façade systems, and green roofs, where the declared λ at 10 °C feeds directly into energy-performance calculations
• Cryogenic engineering: insulation of LNG, ethylene and liquid-air storage tanks, air separation units and cold boxes, designed using λ(T) curves determined at sub-ambient mean temperatures
• Petrochemical and process industry: pipe and equipment insulation, underground steam and district-heating lines, and skid-mounted process modules operating up to approximately 430 °C
• Infrastructure and marine applications: tunnel, bridge-deck and marine insulation where vapor-tightness and dimensional stability are decisive, all qualified through standardized thermal testing

FAQ

What specimen size and how many specimens are required?

Typical specimens are 300 mm × 300 mm or 600 mm × 600 mm with a thickness of 20–60 mm. The guarded hot plate method requires a matched pair of specimens; the heat flow meter method requires one specimen, with a minimum thermal resistance of 0.5 m²·K/W. Accredited third-party laboratories issue precise sampling instructions before testing begins.

Which method is more accurate for foam glass?

The guarded hot plate method (ISO 8302, ASTM C177) is the absolute reference method with the lowest uncertainty. The heat flow meter method (ISO 8301, ASTM C518, EN 12667) is fully accepted for certification and quality control and is faster and more economical for routine testing.

At which mean temperature is the declared value determined?

For building products under EN 13167, the declared thermal conductivity is referenced to a mean temperature of 10 °C. Other mean temperatures, including cryogenic or elevated-temperature points, are selected to match the intended application.

Does foam glass require aging correction like polyurethane foams?

No. The cells of foam glass are filled with air or combustion gases rather than blowing agents, so the thermal conductivity does not drift with time and no long-term aging allowance is applied.

How long does a determination take?

Once specimens reach steady state, a measurement itself typically takes several hours. Including conditioning, density and dimension checks, and reporting, accredited third-party laboratories generally complete a determination within three to seven working days.