Thermal Conductivity Testing of Double-Layer Insulated Glass Substrates

Thermal conductivity testing of double-layer insulated glass substrates quantifies how effectively the glass panes and the sealed air or gas cavity resist heat flow, yielding the thermal conductivity, thermal resistance, and U-value data required for energy-efficient glazing design. This article outlines the sample types, measurable parameters, standard laboratory methods governed by ISO, ASTM, EN, GB, and JIS standards, and the typical applications of the test results. The data obtained supports product certification, building energy code compliance, and manufacturing quality control for insulated glazing units worldwide.

Technical Overview

A double-layer insulated glass unit (IGU), also referred to as double glazing or hollow glass, consists of two parallel glass substrates bonded to a hollow spacer bar with primary and secondary sealants, forming a hermetically sealed cavity filled with dry air or an inert gas such as argon or krypton. Heat crosses this assembly through several parallel paths: conduction through each glass substrate, combined conduction, convection, and radiation across the cavity, and edge transfer through the spacer and sealant system. Because soda-lime silicate glass itself exhibits a relatively high thermal conductivity of approximately 0.9 to 1.1 W/(m·K), the insulating performance of the unit is derived mainly from the low-conductivity gas layer and from low-emissivity coatings that suppress radiative exchange across the cavity.

Laboratory testing therefore serves two distinct purposes: measuring the intrinsic thermal conductivity of the glass substrate material itself, and determining the effective thermal transmittance (U-value) of the assembled unit under standardized boundary conditions, typically a 20 °C interior and 0 °C exterior environment with standardized surface heat transfer resistances. Testing is conducted by accredited third-party laboratories in accordance with internationally recognized standards published by ISO, ASTM, EN, GB, and JIS organizations, ensuring that declared thermal performance values are reproducible and comparable across markets.

Testing Samples

Base Glass Substrate Coupons

Flat coupons are cut from the raw sheet glass used for each lite of the unit. Common substrate types include annealed float glass, tempered safety glass, heat-strengthened glass, chemically strengthened glass, borosilicate and aluminosilicate glass for display and photovoltaic applications, and low-emissivity (Low-E) coated glass. Coupons typically measure 100 mm to 300 mm square, with thicknesses ranging from 2 mm to 12 mm, and are suitable for transient and laser flash measurement techniques that require smaller, precisely prepared specimens.

Assembled Double-Layer Insulated Glass Units

Complete glazing specimens are prepared with cavity widths of 6 mm to 20 mm and standard configurations such as 4 mm glass, 12 mm cavity, and 4 mm glass. Specimens of 500 mm × 500 mm are commonly required for guarded hot plate testing under EN 674, while heat flow meter apparatuses generally accept panels of 300 mm × 300 mm or larger. Spacer options include conventional aluminum, warm-edge stainless steel, and thermally broken composite profiles. The specimen edges are masked with low-conductivity material to suppress lateral heat losses during steady-state measurement.

Component and Material Samples

Individual components that influence edge thermal performance are also sampled, including spacer bar sections, primary polyisobutylene and secondary polysulfide, silicone, or polyurethane sealants, and molecular sieve desiccant. Gas samples extracted from the sealed cavity are analyzed to verify fill concentration, because the thermal conductivity of the fill gas directly governs the center-of-glazing U-value.

Testing Items

The core measurable parameters for double-layer insulated glass substrates and assemblies are listed below:

• Thermal conductivity (λ) of the glass substrate in W/(m·K), typically reported at a mean temperature between 10 °C and 30 °C
• Thermal resistance (R-value) of the glazing assembly in m²·K/W
• Thermal transmittance (U-value) of the double-glazing unit in W/(m²·K), evaluated in the vertical position under standard boundary conditions
• Thermal diffusivity and specific heat capacity of the glass substrate, from which thermal conductivity is derived in transient methods
• Corrected surface emissivity of low-emissivity coated lites, a key input for calculated U-values
• Cavity gas fill concentration, including argon and krypton content and gas purity
• Temperature dependence of thermal properties across the service range of the glazing product

Testing Methods

Guarded Hot Plate Method

The guarded hot plate method is the primary absolute steady-state technique, performed in accordance with ISO 8302, ASTM C177, EN 674, GB/T 10294, and JIS A 1412-1. The specimen is clamped between a central metering hot plate surrounded by a guard ring and a cold plate; the guard ring is maintained at the same temperature as the metering section so that all electrical power dissipated in the metering area flows unidirectionally through the specimen. The method achieves an accuracy of approximately ±1% to ±2% and is regarded as the reference (referee) procedure for determining the U-value of glazing units.

Heat Flow Meter Method

The heat flow meter method, conducted per ISO 8301, ISO 8990, ASTM C518, EN 675, GB/T 10295, and JIS A 1412-2, places the specimen between two temperature-controlled plates fitted with calibrated heat flux transducers. The apparatus is calibrated using certified reference transfer standards traceable to national metrology institutes. The technique offers repeatability of roughly 1% to 2%, shorter cycle times, and lower specimen preparation effort, making it well suited to routine quality control and production verification.

Transient Plane Source and Laser Flash Analysis

For substrate-level characterization, the transient plane source (hot disk) technique under ISO 22007-2 measures thermal conductivity directly on small coupons, covering a wide conductivity range with fast turnaround. Laser flash analysis per ASTM E1461 measures the thermal diffusivity of the glass substrate; when combined with independently determined density and specific heat capacity, the thermal conductivity is calculated from the relationship λ = α · ρ · cp. These transient methods are particularly valuable for evaluating specialty substrates such as borosilicate and aluminosilicate glass used in electronic displays and thin-film photovoltaics.

Calculated U-Value Confirmation

In parallel with physical measurement, the U-value of the double-glazing unit is calculated in accordance with EN 673 and ISO 10292, using measured gas properties, cavity geometry, and the corrected emissivity of coated surfaces determined per EN 12898 together with spectral data per ISO 9050. Agreement between measured and calculated values validates the declared thermal performance. For whole-window performance, center-of-glazing results are subsequently combined with frame and edge effects using numerical methods per ISO 10077-2 or verified by hot box testing per ISO 12567-1 and ASTM C1363.

Applications

Thermal conductivity and U-value test results for double-layer insulated glass substrates are applied across the construction, energy, and electronics industries:

• Energy code compliance for windows and curtain walls, where maximum glazing U-values are mandated by national building regulations
• Documentation for green building certification programs and environmental product declarations
• Quality control of Low-E coating performance, gas filling processes, and warm-edge spacer systems during IGU production
• Research and development of advanced glazing, including krypton-filled cavities, triple-cavity designs, and vacuum-assisted configurations
• Substrate qualification for display panels, OLED lighting, and building-integrated photovoltaic modules where through-thickness heat dissipation is critical
• Performance verification of refrigerated display cabinets, cold-chain enclosure vision panels, and rail and marine glazing

FAQ

What is the typical thermal conductivity of the glass substrate itself?

Soda-lime silicate float glass, the most common substrate, exhibits a thermal conductivity of approximately 0.9 to 1.1 W/(m·K) at room temperature. Borosilicate glass typically falls between 1.1 and 1.4 W/(m·K), while aluminosilicate glass used for display substrates can reach higher values depending on composition. These figures explain why the sealed gas cavity, rather than the glass, provides the primary thermal resistance of the assembly.

What U-value can a standard double-layer unit achieve?

A conventional air-filled configuration of 4 mm glass, 12 mm air cavity, and 4 mm glass yields a center-of-glazing U-value of approximately 2.8 W/(m²·K). Introducing a Low-E coating and filling the cavity with approximately 90% argon reduces the U-value to roughly 1.1 to 1.6 W/(m²·K), while krypton fill and optimized cavity widths can achieve values near 1.0 W/(m²·K).

What specimen size and conditioning are required?

Guarded hot plate testing under EN 674 generally requires specimens of at least 500 mm × 500 mm, whereas heat flow meter apparatuses accept panels of 300 mm × 300 mm or larger. Specimens must be free of defects, have clean surfaces, and be stored in a stable laboratory environment until thermal equilibrium is reached before testing. The desiccant within assembled units must be fully activated so that the cavity moisture content reflects the intended service condition.

How long does a steady-state test take?

Reaching thermal steady state typically requires 4 to 8 hours after the plates stabilize, depending on specimen thickness and cavity gas. A complete test campaign, including mounting, equilibrium verification, and duplicate runs at multiple mean temperatures, usually spans one to three working days.

Can the measured center-of-glazing U-value be used directly as the window rating?

No. The laboratory result represents the center-of-glazing performance of the double-glazing unit only. Whole-window ratings require the glazing value to be combined with frame conduction and spacer edge effects, either through numerical simulation per ISO 10077-2 or through calibrated hot box measurement of the complete window per ISO 12567-1 or ASTM C1363.