Low-emissivity (Low-E) glass thermal insulation performance testing evaluates the emissivity, thermal transmittance (U-value), and solar heat gain coefficient of coated glazing products used in energy-efficient building envelopes. This guide explains the applicable international and national standards, sample requirements, laboratory test methods, and typical performance indicators that help manufacturers, architects, and accreditation bodies verify the energy-saving performance of Low-E glass.
Technical Overview
Low-emissivity glass, commonly known as Low-E glass, is float glass coated with one or more ultra-thin layers of silver, tin oxide, or other metal-oxide films that selectively reflect infrared radiation while transmitting visible light. Depending on the manufacturing process, products are classified as online (pyrolytic, hard-coat) Low-E glass, with typical corrected emissivity in the range of 0.10 to 0.20, or offline (magnetron-sputtered, soft-coat) Low-E glass, with corrected emissivity often between 0.02 and 0.10. By comparison, ordinary uncoated float glass has a surface emissivity of approximately 0.84, which is why a Low-E coating dramatically reduces radiative heat exchange across glazing.
The thermal insulation performance of Low-E glass operates through two mechanisms: the low-emissivity coating reduces long-wave radiative heat transfer between glazing surfaces (critical in insulating glass units), while the spectrally selective coating controls the transmission of near-infrared solar energy. Because these properties cannot be captured by a single direct measurement, thermal insulation performance testing combines spectrophotometric and infrared reflectometric measurements with standardized calculation procedures, supplemented where necessary by direct calorimetric measurement using guarded hot plate, heat flow meter, or hot box apparatus. Accredited third-party laboratories perform these tests in accordance with ISO, EN, ASTM, and GB standards to support product certification, energy-code compliance, and quality control.
Testing Samples
Sample selection must reflect the final product configuration, including glass thickness, coating type, coating position, and assembly structure. Glazing surfaces are conventionally numbered from the exterior (surface #1), so Low-E coatings in insulating glass units are typically located on surface #2 or surface #3. Specimens should be clean, free of scratches and coating defects, and conditioned to laboratory ambient temperature before testing.
Monolithic Low-E Glass
Single coated panes, commonly 4 mm, 5 mm, or 6 mm thick, are tested to characterize the intrinsic properties of the coating, including emissivity and spectral transmittance/reflectance. Coupons of approximately 100 mm × 100 mm are generally sufficient for spectrophotometric and infrared reflectometric measurements. The coated surface must be clearly identified and marked, and should not be touched or cleaned with aggressive solvents that could damage the thin film.
Low-E Insulating Glass Units (IGUs)
Double or triple glazed units with the Low-E coating on surface #2 or #3, hermetically sealed cavities (typically 9 mm to 16 mm) filled with air or argon, are tested to determine the declared thermal transmittance and solar heat gain coefficient of the assembled product. The declared configuration, including spacer type, gas fill, and glass thicknesses, must match the production configuration, because U-value calculations are highly sensitive to cavity and gas parameters.
Laminated and Heat-Treated Low-E Glass
Laminated Low-E configurations, tempered Low-E glass, and heat-strengthened products are tested in their final processed state. Thermal and chemical processing can slightly shift the optical and emissive properties of soft coatings, so verification after tempering, laminating, or bending is essential. The coating position relative to the interlayer must be documented, since coatings encapsulated in or adjacent to the interlayer behave differently from exposed cavity surfaces.
Testing Items
The core testing items for Low-E glass thermal insulation performance are defined by EN 410, EN 673, EN 12898, ISO 9050, ISO 10292, ASTM E903, and the Chinese national standard GB/T 2680. Typical test items include:
- Corrected emissivity and hemispherical emissivity of the coated surface, determined per EN 12898 or ASTM E408
- Thermal transmittance (U-value) of the glazing, expressed in W/(m²·K), per EN 673, ISO 10292, EN 674, or EN 675
- Solar heat gain coefficient (SHGC), also called g-value or total solar energy transmittance
- Solar direct transmittance, solar reflectance, and solar absorptance integrated over the solar spectral range
- Visible light transmittance and visible light reflectance
- Ultraviolet transmittance for daylighting and fading assessments
- Shading coefficient (SC), calculated as SHGC divided by 0.87
- Light-to-solar gain ratio (LSG), calculated as visible transmittance divided by SHGC, indicating spectral selectivity
Representative performance levels include U-values of approximately 1.0 to 1.8 W/(m²·K) for double-glazed Low-E IGUs, 0.6 to 0.9 W/(m²·K) for triple-glazed argon-filled units with two Low-E coatings, and visible transmittance between 40% and 80% depending on product design. Highly selective products achieve LSG ratios of 1.4 or higher.
Testing Methods
Spectrophotometric Determination of Optical and Solar Properties
A UV-Vis-NIR spectrophotometer equipped with an integrating sphere is used to measure spectral transmittance and reflectance from approximately 300 nm to 2500 nm, in accordance with EN 410, ISO 9050, ASTM E903, and GB/T 2680. The spectral data are weighted with the standard solar spectral irradiance distribution and the photopic response of the human eye to calculate solar direct transmittance, solar reflectance, and visible light characteristics. The solar heat gain coefficient is then derived as the sum of the solar direct transmittance and the secondary heat transfer factor toward the interior, the latter being calculated from solar absorptance and the thermal properties of the glazing.
Emissivity Determination by Infrared Reflectometry
Coating emissivity is determined per EN 12898 or ASTM E408 using a calibrated infrared reflectometer or a Fourier-transform infrared (FTIR) spectrometer operating in the mid-infrared region, which dominates room-temperature thermal radiation. Near-normal infrared reflectance is converted to emissivity using Kirchhoff's law for opaque bodies, where emissivity equals one minus reflectance, and corrected to hemispherical emissivity using standard conversion factors. Emissivity is the single most influential coating parameter in U-value calculations for insulating glass units.
Calculated U-Value Determination
EN 673 and ISO 10292 define the standardized calculation method for the thermal transmittance of multiple glazing. The calculation uses measured surface emissivities, glass thicknesses, cavity widths, gas fill properties (air, argon, or krypton), and standard boundary conditions with an external surface heat transfer coefficient of 23 W/(m²·K) and an internal coefficient of 8 W/(m²·K). This method produces the declared U-value used in product datasheets and energy performance documentation, and its inputs must originate from validated laboratory measurements.
Guarded Hot Plate, Heat Flow Meter, and Hot Box Methods
Direct thermal measurement methods verify calculated values or evaluate complete products. The guarded hot plate method (ISO 10291, EN 674) establishes steady one-dimensional heat flow through a glazing specimen between temperature-controlled plates. The heat flow meter method (ISO 10293, EN 675, ASTM C518) determines thermal transmission using calibrated heat flux transducers. The guarded and calibrated hot box methods (ISO 8990, ASTM C1363) accommodate full-size insulating glass units and facade panels by maintaining a controlled air temperature difference across the specimen until steady-state heat flow is achieved. Specimen sizes for calorimetric methods typically range from about 500 mm × 500 mm up to full product dimensions, depending on the apparatus.
Applications
Test results for Low-E glass thermal insulation performance are used across the building industry and glass supply chain, including:
- Energy-efficient curtain walls and structural glazing facades in commercial and public buildings
- Residential windows and doors in both cold climates, where Low-E coatings reduce winter heat loss, and hot climates, where they reject solar heat gain and lower cooling loads
- Skylights, conservatories, and overhead glazing with demanding solar control requirements
- Green building rating documentation and building energy code compliance reports
- Manufacturer research and development, new coating qualification, and routine quality control of coating production lines
- Import and export compliance, product datasheets, and technical documentation for architectural tendering
FAQ
What emissivity value qualifies glass as low-emissivity glass?
Standards generally treat coated glass as low-emissivity when the corrected emissivity is 0.20 or lower. Commercial soft-coat products typically achieve corrected emissivity between 0.02 and 0.10, while hard-coat pyrolytic products range from approximately 0.10 to 0.20.
What is the difference between U-value and solar heat gain coefficient?
The U-value measures conductive and radiative heat transfer through the glazing driven by an indoor-outdoor temperature difference, so it governs winter heat loss and summer conduction gains. The solar heat gain coefficient measures the fraction of incident solar radiation that enters the building, including directly transmitted sunlight and inward-re-radiated absorbed heat. Both parameters are needed to characterize overall thermal insulation performance.
Can a single coated pane be tested, or is an assembled insulating glass unit required?
Coating characteristics such as emissivity and spectral transmittance are measured on single coated panes. However, the declared U-value and solar heat gain coefficient of a product must be determined for the complete insulating glass unit configuration, because cavity width, gas fill, and number of panes strongly influence the results.
Which standards are most commonly applied?
European projects typically reference EN 410, EN 673, EN 674, EN 675, and EN 12898. International methods include ISO 9050, ISO 10291, ISO 10292, ISO 10293, ISO 8990, and ISO 10217-series practices. American methods include ASTM E903, ASTM E408, ASTM C518, and ASTM C1363. In China, GB/T 2680 governs the determination of optical and solar characteristics, together with related national calculation standards for glazing U-values.
What sample sizes and test durations are typical?
Spectrophotometric and emissivity measurements usually require coupons of about 100 mm × 100 mm and can be completed within one to three working days. Calorimetric methods such as guarded hot plate or hot box testing require larger specimens, often 500 mm to 2000 mm on a side, and typically require several days to reach and document steady-state conditions.
Does tempering or laminating change the test results?
Yes. Heat treatment can slightly increase the emissivity and shift the color and spectral response of soft coatings, and lamination changes the optical stack. For conformity assessment, testing should always be performed on specimens representing the final delivered product state, whether annealed, tempered, heat-strengthened, or laminated.