prBAS EN 673:2026

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.