SIST EN 673:2026/oprA1:2026
(Amendment)Glass in building - Determination of thermal transmittance (U value) - Calculation method
Glass in building - Determination of thermal transmittance (U value) - Calculation method
This document specifies a calculation method to determine the thermal transmittance of glass with flat and parallel surfaces.
This document applies to uncoated glass (including glass with structured surfaces, e.g. patterned glass), coated glass and materials not transparent in the far infrared which is the case for soda lime glass products, borosilicate glass, glass ceramic, alkaline earth silicate glass and alumino silicate glass. It applies also to multiple glazing comprising such glasses and/or materials. It does not apply to multiple glazing which include in the gas space sheets or foils that are far infrared transparent.
The procedure specified in this document determines the U value (thermal transmittance) in the central area of glazing.
The edge effects due to the thermal bridge through the spacer of an insulating glass unit or through the window frame are not included. Furthermore, energy transfer due to solar radiation is not taken into account. The effects of Georgian and other bars are excluded from the scope of this document.
NOTE EN ISO 10077 1:2017 provides a methodology for calculating the overall U value of windows, doors and shutters [1], taking account of the U value calculated for the glass components according to this document.
Also excluded from the calculation methodology are any effects due to gases that absorb infrared radiation in the 5 to 50 µm range.
The primary purpose of this document is product comparison, for which a vertical position of the glazing is specified. In addition, U values are calculated using the same procedure for other purposes, in particular for predicting:
- heat loss through glass;
- conduction heat gains in summer;
- condensation on glass surfaces;
- the effect of the absorbed solar radiation in determining the solar factor [2].
Reference can be made to [3], [4] and [5] or other European Standards dealing with heat loss calculations for the application of glazing U values determined by this standard.
Reference can be made to [6] for detailed calculations of U values of glazing, including shading devices.
Vacuum Insulating Glass (VIG) is excluded from the scope of this document. For determination of the U value of VIG, please refer to EN 674 or ISO 19916-1.
A procedure for the determination of emissivity is given in EN 12898.
The rules have been made as simple as possible consistent with accuracy.
Glas im Bauwesen - Bestimmung des Wärmedurchgangskoeffizienten (U-Wert) - Berechnungsverfahren
Verre dans la construction - Détermination du coefficient de transmission thermique, U - Méthode de calcul
Steklo v gradbeništvu - Določanje toplotne prehodnosti (vrednost U) - Računska metoda
General Information
- Status
- Not Published
- Public Enquiry End Date
- 27-Feb-2026
- Technical Committee
- STV - Steklo, svetloba in razsvetljava v gradbeništvu
- Current Stage
- 4020 - Public enquire (PE) (Adopted Project)
- Start Date
- 18-Dec-2025
- Due Date
- 07-May-2026
- Completion Date
- 10-Apr-2026
Relations
- Effective Date
- 23-Sep-2025
Overview
SIST EN 673:2026/oprA1:2026 is a European Standard established by CEN for the determination of thermal transmittance (U value) of glass in building applications. The document specifies a standardized calculation method for establishing the U value, focusing on glass panes with flat and parallel surfaces. This procedure is vital for comparing products, assessing building energy performance, and ensuring regulatory compliance across various construction projects involving glass.
This standard applies to a wide range of glass products:
- Uncoated and coated glass, including patterned and structured surfaces
- Glass and materials not transparent in the far infrared, such as soda lime glass, borosilicate glass, glass ceramics, alkaline earth silicate glass, and alumino silicate glass
- Multiple glazing units using these types of glass and materials
Vacuum insulating glass (VIG) and glazing units incorporating far-infrared-transparent foils are not covered by this standard.
Key Topics
Thermal Transmittance (U Value):
EN 673 provides a calculation methodology to determine the central U value of glass or multiple glazing units. This value represents the rate of heat transfer through glass, which is crucial for energy efficiency and indoor climate control. Key points of the methodology include:
- Focus is on the central area of the glazing
- Does not account for edge effects from spacers or window frames
- Excludes solar radiation energy transfer and the impact of decorative bars (such as Georgian bars)
- Applies a vertical orientation for standard product comparisons
Scope Limitations:
- Excludes glazing with far-infrared-transparent sheets/foils in gas spaces
- Excludes vacuum insulating glass - reference EN 674 or ISO 19916-1 for VIG procedures
- Edge and framing losses are not part of the calculation
Emissivity Determination:
- Procedures for determining coated surface emissivity are referenced in EN 12898
Calculation Guidance:
- Clarifies the treatment of internal heat transfer coefficients and suggests using standardized values for consistent results
Applications
SIST EN 673:2026/oprA1:2026 is widely used to:
- Compare the thermal insulation performance of different glass types and multiple glazing configurations
- Predict and limit heat loss through glazed sections of building envelopes, supporting low-energy and passive house design
- Estimate condensation risks on glass surfaces in varying climatic and load conditions
- Support the calculation of conduction heat gain during warmer periods
- Contribute data for determining solar factor in building simulations (in conjunction with other standards)
Product Selection and Compliance:
Manufacturers, architects, and specifiers use EN 673 to benchmark glass products, ensure compliance with building energy requirements, and achieve certification for energy-efficient construction.
Related Standards
SIST EN 673:2026/oprA1:2026 is part of a robust ecosystem of European standards for glass and glazing in buildings, including:
- EN ISO 10077-1: Methods for calculating thermal performance of windows, doors, and shutters
- EN 674 and ISO 19916-1: Determination of the U value for vacuum insulating glass
- EN 12898: Procedures for measuring glass surface emissivity
- ISO 6946: General principles for calculating thermal resistance and transmittance
These related documents ensure harmonized methodology and compatibility in energy calculations across the construction industry.
Keywords:
thermal transmittance, U value, glass in building, glazing thermal performance, EN 673, building energy efficiency, multiple glazing, flat glass standards, CEN, conductivity, architectural glass standards
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Frequently Asked Questions
SIST EN 673:2026/oprA1:2026 is a draft published by the Slovenian Institute for Standardization (SIST). Its full title is "Glass in building - Determination of thermal transmittance (U value) - Calculation method". This standard covers: This document specifies a calculation method to determine the thermal transmittance of glass with flat and parallel surfaces. This document applies to uncoated glass (including glass with structured surfaces, e.g. patterned glass), coated glass and materials not transparent in the far infrared which is the case for soda lime glass products, borosilicate glass, glass ceramic, alkaline earth silicate glass and alumino silicate glass. It applies also to multiple glazing comprising such glasses and/or materials. It does not apply to multiple glazing which include in the gas space sheets or foils that are far infrared transparent. The procedure specified in this document determines the U value (thermal transmittance) in the central area of glazing. The edge effects due to the thermal bridge through the spacer of an insulating glass unit or through the window frame are not included. Furthermore, energy transfer due to solar radiation is not taken into account. The effects of Georgian and other bars are excluded from the scope of this document. NOTE EN ISO 10077 1:2017 provides a methodology for calculating the overall U value of windows, doors and shutters [1], taking account of the U value calculated for the glass components according to this document. Also excluded from the calculation methodology are any effects due to gases that absorb infrared radiation in the 5 to 50 µm range. The primary purpose of this document is product comparison, for which a vertical position of the glazing is specified. In addition, U values are calculated using the same procedure for other purposes, in particular for predicting: - heat loss through glass; - conduction heat gains in summer; - condensation on glass surfaces; - the effect of the absorbed solar radiation in determining the solar factor [2]. Reference can be made to [3], [4] and [5] or other European Standards dealing with heat loss calculations for the application of glazing U values determined by this standard. Reference can be made to [6] for detailed calculations of U values of glazing, including shading devices. Vacuum Insulating Glass (VIG) is excluded from the scope of this document. For determination of the U value of VIG, please refer to EN 674 or ISO 19916-1. A procedure for the determination of emissivity is given in EN 12898. The rules have been made as simple as possible consistent with accuracy.
This document specifies a calculation method to determine the thermal transmittance of glass with flat and parallel surfaces. This document applies to uncoated glass (including glass with structured surfaces, e.g. patterned glass), coated glass and materials not transparent in the far infrared which is the case for soda lime glass products, borosilicate glass, glass ceramic, alkaline earth silicate glass and alumino silicate glass. It applies also to multiple glazing comprising such glasses and/or materials. It does not apply to multiple glazing which include in the gas space sheets or foils that are far infrared transparent. The procedure specified in this document determines the U value (thermal transmittance) in the central area of glazing. The edge effects due to the thermal bridge through the spacer of an insulating glass unit or through the window frame are not included. Furthermore, energy transfer due to solar radiation is not taken into account. The effects of Georgian and other bars are excluded from the scope of this document. NOTE EN ISO 10077 1:2017 provides a methodology for calculating the overall U value of windows, doors and shutters [1], taking account of the U value calculated for the glass components according to this document. Also excluded from the calculation methodology are any effects due to gases that absorb infrared radiation in the 5 to 50 µm range. The primary purpose of this document is product comparison, for which a vertical position of the glazing is specified. In addition, U values are calculated using the same procedure for other purposes, in particular for predicting: - heat loss through glass; - conduction heat gains in summer; - condensation on glass surfaces; - the effect of the absorbed solar radiation in determining the solar factor [2]. Reference can be made to [3], [4] and [5] or other European Standards dealing with heat loss calculations for the application of glazing U values determined by this standard. Reference can be made to [6] for detailed calculations of U values of glazing, including shading devices. Vacuum Insulating Glass (VIG) is excluded from the scope of this document. For determination of the U value of VIG, please refer to EN 674 or ISO 19916-1. A procedure for the determination of emissivity is given in EN 12898. The rules have been made as simple as possible consistent with accuracy.
SIST EN 673:2026/oprA1:2026 is classified under the following ICS (International Classification for Standards) categories: 81.040.20 - Glass in building; 91.040.20 - Buildings for commerce and industry. The ICS classification helps identify the subject area and facilitates finding related standards.
SIST EN 673:2026/oprA1:2026 has the following relationships with other standards: It is inter standard links to SIST EN 673:2026. Understanding these relationships helps ensure you are using the most current and applicable version of the standard.
SIST EN 673:2026/oprA1:2026 is associated with the following European legislation: EU Directives/Regulations: 305/2011; Standardization Mandates: M/135. When a standard is cited in the Official Journal of the European Union, products manufactured in conformity with it benefit from a presumption of conformity with the essential requirements of the corresponding EU directive or regulation.
SIST EN 673:2026/oprA1:2026 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
SLOVENSKI STANDARD
01-februar-2026
Steklo v gradbeništvu - Določanje toplotne prehodnosti (vrednost U) - Računska
metoda
Glass in building - Determination of thermal transmittance (U value) - Calculation method
Glas im Bauwesen - Bestimmung des Wärmedurchgangskoeffizienten (U-Wert) -
Berechnungsverfahren
Verre dans la construction - Détermination du coefficient de transmission thermique, U -
Méthode de calcul
Ta slovenski standard je istoveten z: EN 673:2024/prA1
ICS:
81.040.20 Steklo v gradbeništvu Glass in building
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
DRAFT
EUROPEAN STANDARD
EN 673:2024
NORME EUROPÉENNE
EUROPÄISCHE NORM
prA1
February 2026
ICS 81.040.20; 91.040.20
English Version
Glass in building - Determination of thermal transmittance
(U value) - Calculation method
Verre dans la construction - Détermination du Glas im Bauwesen - Bestimmung des
coefficient de transmission thermique, U - Méthode de Wärmedurchgangskoeffizienten (U-Wert) -
calcul Berechnungsverfahren
This draft amendment is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 129.
This draft amendment A1, if approved, will modify the European Standard EN 673:2024. If this draft becomes an amendment,
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for inclusion of
this amendment into the relevant national standard without any alteration.
This draft amendment was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary,
...
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