Overview
ISO 10292:1994 - "Glass in building - Calculation of steady-state U values (thermal transmittance) of multiple glazing" provides a standardized method to calculate the steady‑state U value (thermal transmittance) for the central area of sealed multiple glazing units. The standard applies to clear and coated glass, and to materials opaque in the far‑infrared. It excludes combined edge effects (spacer/frame thermal bridges) and is intended for estimating heat loss through glazing for heating/cooling sizing, condensation risk assessment and seasonal energy accounting. U values are expressed in Watts per square metre kelvin [W/(m2·K)].
Key topics and technical requirements
- Scope and definition: U value defined as steady‑state heat transfer rate per temperature difference for glazing central area (edge effects not included).
- Calculation basis: Combines surface heat transfer coefficients, gas‑space conductance (conduction + convection), and radiation conductance. Uses fundamental dimensionless numbers (Grashof, Prandtl, Nusselt) to assess convection in gas layers.
- Radiation and emissivity:
- Uses corrected emissivity for coated surfaces; Annex A prescribes spectral reflectance measurement and conversion tables.
- For ordinary glass, corrected emissivity = 0.837; radiation conductance reference = 4.4 W/(m2·K).
- Gas properties: Annex A lists thermal conductivity, density, viscosity and specific heat for common fills (air, argon, krypton, SF6) and prescribes how to treat gas mixtures.
- Reference values and defaults:
- External heat transfer coefficient he = 23 W/(m2·K) (for comparison)
- Internal heat transfer coefficient hi = 8 W/(m2·K)
- Glass thermal resistivity r = 1 mm·K/W
- Mean glazing temperature Tm = 283 K, ΔT = 15 K
- Practical rules: Guidance for vertical, horizontal and inclined spaces; special handling when convection is suppressed or enhanced; iterative solution for multi‑space glazing; U values quoted to one decimal.
Applications
- Sizing heating and cooling plant by estimating glazing heat losses
- Window and façade thermal performance calculations for building energy modelling
- Predicting condensation risk on glazing surfaces
- Comparing glazing options (coatings, gas fills, multi‑layer designs) for energy efficiency
- Producing standardized U‑value data for product datasheets and certification
Who uses this standard
- Façade engineers, window and glazing manufacturers
- Architects and building envelope consultants
- HVAC and energy‑modelling professionals
- Building code assessors and certification bodies
- Test laboratories measuring glazing thermal performance
Related standards
ISO 10292 was prepared by ISO/TC 160 (Glass in building). Users often apply it alongside other building thermal and window performance standards and national regulations for U‑values and energy compliance.
Keywords: ISO 10292, U value, thermal transmittance, multiple glazing, glass in building, glazing U values, emissivity, gas fill, heat transfer coefficients, window thermal performance.
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