ISO 20589:2025

Overview

ISO 20589:2025 - Glass in building - Determination of the emissivity - specifies a laboratory procedure to determine the emissivity at room temperature of glass and coated glass surfaces. Emissivity values at the standard temperature of 283 K are required to account for radiative heat transfer when calculating the U value and total solar transmittance of glazing according to related glazing standards. The method is based on spectrophotometric specular reflectance measurements at near‑normal incidence in the infrared (5 µm to 50 µm) spectral range.

Key topics and technical requirements

• Scope and applicability
  • Applies to flat, non‑diffusing glass and coated glass that are non‑transparent in the infrared.
  • Not applicable to specimens with rough/structured (diffuse) surfaces, curved surfaces causing off‑angle reflections, or infrared‑transparent glazing components.
• Measurement principle
  • Uses spectral normal (specular) reflectance measurements from 5 µm to 50 µm with an infrared spectrophotometer and a specular reflectance accessory at near‑normal incidence (instrument beam should reach the detector; incidence ≤ 10° is referenced).
  • Supports both double‑beam dispersive spectrometers and FTIR instruments (with drift‑correction procedures from the THERMES project for single‑beam FTIRs).
• Data processing
  • Integrate spectral reflectance to obtain total normal reflectance at 283 K.
  • Convert reflectance to total normal emissivity, then apply a multiplicative correction to yield the corrected emissivity used in glazing thermal calculations.
• Quality and specimen handling
  • Specimens must be clean, undamaged, and large enough for the instrument spot; diffuse reflectance and transmittance checks ensure non‑diffusivity and infrared opacity.
• Reference values and temperature notes
  • For uncoated soda‑lime silicate glass, a default normal emissivity of 0.89 may be used unless coatings affect emissivity. Emissivity is relatively stable between 253 K and 313 K.

Practical applications and users

• Who uses ISO 20589:2025:
  • Glass manufacturers and coating developers (product characterization)
  • Accredited test laboratories performing emissivity measurements
  • Façade designers, building physicists and energy modelers (U‑value and solar transmittance inputs)
  • Certification bodies and compliance assessors for thermal performance
• Practical value:
  • Provides repeatable, standardized emissivity data for accurate thermal modelling of windows and insulating glazing units.
  • Ensures consistency when integrating emissivity into standards‑based calculations for energy performance and building code compliance.

Related standards

• ISO 9050 - Glass in building: light transmittance and solar factor (total solar transmittance)
• ISO 10291, ISO 10292, ISO 10293 - Calculation of steady‑state U values and related glazing thermal performance standards

Keywords: ISO 20589:2025, emissivity, glass in building, spectrophotometric specular reflectance, infrared 5–50 µm, U value, total solar transmittance, coated glass, FTIR, non‑diffusing glazing.