Glass in building — Determination of thermal transmittance (U value) — Calculation method

This document specifies a calculation method to determine the thermal transmittance (U value) 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 or materials, or both. It does not apply to multiple glazing which include in the gas space sheets or foils that are far infrared transparent.

Vacuum insulating glass (VIG) is excluded from the scope of this document. To determine the U value of VIG, reference can be made to ISO 10291 or ISO 19916-1.

The procedure specified in this document determines the U value 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 ISO 10077-1 provides a methodology for calculating the overall U value of windows, doors and shutters, 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 µm to 50 µm range.

ISO

ISO 14819-2:2021/Amd 1:2026

ISO 10292:2026

ISO 22354:2026

ISO 4259-2:2026

ISO 41002:2026

ISO 23402-1:2026

ISO 12480-5:2026

ISO 23247-5:2026

ISO 24131-4:2026

ISO 25247:2026

ISO 3630-8:2026

ISO 4259-1:2026