ISO/FDIS 10292

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

Applies to glass, coated glass and materials opaque in the far infrared wavelengths. Gives the fundamental rules for calculating the thermal transmittance in the glazing central area (edge effects are not included). The rules are intended to enable the heat loss through glazing in a building to be estimated from the U values and, together with heat losses through the opaque elements of the building, are used to determine the capacity of the heating or cooling plant. In addition, U values for other purposes (e.g. condensation on glazing surfaces, seasonal heat loss through glazing) can be calculated using the same procedure.

Titre manque

General Information

• Status: Not Published
• ICS: 81.040.20 - Glass in building
• Technical Committee: ISO/TC 160 - Glass in building
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 22-Apr-2026
• Completion Date: 22-Apr-2026

Relations

• Revises: ISO 10292:1994 - Glass in building — Calculation of steady-state U values (thermal transmittance) of multiple glazing
• Effective Date: 15-Jun-2024

Overview

ISO/FDIS 10292:2026, Glass in building - Determination of thermal transmittance (U value) - Calculation method is an international standard developed by ISO for the precise calculation of the thermal transmittance, or U value, of glass and glazing materials used in building applications. This document is a crucial reference for architects, engineers, and building professionals focused on energy efficiency, thermal performance, and product selection in the construction industry.

The standard outlines the general rules and standardized procedure for calculating U values in the central area of glass and glazing combinations, covering uncoated glass, coated glass, and opaque materials in the far infrared range. It enables professionals to estimate heat loss through glazing and, when combined with estimates for opaque building components, helps determine heating and cooling system capacity. Notably, ISO/FDIS 10292 purposefully omits edge effects, window frames, solar radiation, and certain specialty configurations like vacuum insulating glass (VIG).

---

Key Topics

• Scope and Applicability

  • Applies to flat, parallel glass with or without coatings and to specific glass types not transparent in the far infrared spectrum.
  • Excludes vacuum insulating glass (reference ISO 10291 or ISO 19916-1 for VIG), edge effects, window frames, solar gains, and bars such as Georgian bars.

• Calculation Principles

  • Establishes formulas for calculating the U value, representing steady-state heat transfer per temperature difference between environments on either side of the glass.
  • Considers the thermal conductance of glass, gas spaces, and material layers but not the thermal bridging at edges.
  • Incorporates key variables such as thickness, gas properties (thermal conductivity, density, viscosity, specific heat), and surface emissivity.

• Boundary and Design Values

  • Details standardized boundary conditions for declared U values:
    • External/internal heat transfer coefficients
    • Surface emissivity values
    • Gas properties at set temperatures (e.g., 10°C)
    • Mean temperatures and temperature differences
  • Provides guidance for adapting calculations to specific building situations, including orientations and unusual internal or external conditions.

• Reporting and Documentation

  • Specifies information to be included for product comparison and design:
    • Glass and material thicknesses
    • Gas type and concentration
    • Surface coatings and their positions
    • Emissivity and transfer coefficients
    • Inclination, heat flow direction, and any deviations from standard conditions
  • Encourages inclusion of cross-sectional schematics and clear rounding rules for reported U values.

---

Applications

ISO/FDIS 10292 is essential for:

• Product Assessment and Certification: Enables transparent comparison of thermal performance between different glass products and designs.
• Building Energy Efficiency: Accurate U value calculation informs the design of energy-efficient buildings and selection of appropriate glazing solutions to minimize heat loss or gain.
• HVAC System Design: Supports precise sizing of heating and cooling systems by providing reliable data on glazing heat loss.
• Regulatory Compliance: Assists in meeting building codes and green building certifications that demand specific energy performance levels.
• Condensation Risk Evaluation: Facilitates assessment of condensation potential on glazing surfaces by calculating surface temperatures.

---

Related Standards

• ISO 6946: Building components and building elements - Thermal resistance and thermal transmittance - Calculation method.
• ISO 10077-1: Thermal performance of windows, doors and shutters - Calculation of thermal transmittance - General.
• ISO 15099: Thermal performance of windows, doors and shading devices - Detailed calculations.
• ISO 10291 / ISO 19916-1: Vacuum insulating glass - U value determination methodologies.
• ISO 20589: Glass in building - Determination of emissivity.
• ISO 52016-1, ISO 10211: Energy performance of buildings - Calculation procedures relevant to heat transfer, including glazing systems.

---

By following ISO/FDIS 10292, professionals ensure consistent, reliable, and internationally recognized methods for determining glazing U values - a fundamental component in advancing energy-efficient building design and transparent product evaluation. Keywords: U value calculation, glazing thermal transmittance, glass in building, ISO glass standards, energy performance.

Standards Content (Sample)

FINAL DRAFT
International
Standard
ISO/TC 160
Glass in building — Determination
Secretariat: BSI
of thermal transmittance (U value)
Voting begins on:
— Calculation method
2026-04-22
Voting terminates on:
2026-06-17
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 160
Glass in building — Determination
Secretariat: BSI
of thermal transmittance (U value)
Voting begins on:
— Calculation method
Voting terminates on:
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols, dimensionless numbers and subscripts . 2
5 Basic formulae . 3
5.1 General .3
5.2 U value .3
th
5.3 Radiative heat transfer coefficient of k space h .4
r,k
5.4 Gas conductance, Λ .5
g
5.4.1 General .5
5.4.2 Heat flow direction . . .5
6 Basic material properties. 6
6.1 Emissivity .6
6.2 Gas properties .7
7 External and internal heat transfer coefficients . 8
7.1 External heat transfer coefficient, h .8
e
7.2 Internal heat transfer coefficient h .9
i
7.3 Design values .11
8 Declared values: standardized boundary conditions .11
9 Expression of the results .12
9.1 U values . 12
9.2 Intermediate values . 12
10 Report .12
10.1 Information included in the report . 12
10.2 Identification of the glazing . 12
10.3 Cross section of the glazing . 13
10.4 Results . . 13
Annex A (normative) Iteration procedure for an insulating glass unit with more than one gas
space . . 14
Annex B (informative) Determination of gas properties at different temperatures .16
Bibliography . 17

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 160, Glass in building.
This second edition cancels and replaces the first edition (ISO 10292:1994), which has been technically
revised.
The main changes are as follows:
— changes to some of the gas properties;
— introduction of a linear approximation for determining gas properties at different temperatures;
— modification of the calculation of the internal heat transfer coefficient for vertical glazing for consistency
with ISO 6946;
— provision of more details on heat transfer coefficients for glazing at angles other than vertical, for
consistency with ISO 6946 and ISO 10077-1;
— additional clarification provided on the iteration procedure for an insulating glass unit with more than
one gas space.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
For the purposes of this document, the term glass has been used in the context of a material or surface
property, whereas the term glazing has been adopted to refer to either monolithic glass or an insulating
glass unit.
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 (see ISO 9050).
Reference can be made to ISO 6946, ISO 52016-1 and ISO 10211 or other European Standards dealing with
heat loss calculations for the application of glazing U values determined by this document.
Reference can be made to ISO 15099 for detailed calculations of U values of glazing, including shading
devices.
A procedure for the determination of emissivity is given in ISO 20589.
The rules have been made as simple as possible consistent with accuracy.

v
FINAL DRAFT International Standard ISO/FDIS 10292:2026(en)
Glass in building — Determination of thermal transmittance
(U value) — Calculation method
1 Scope
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.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 20589, Glass in building — Determination of the emissivity
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/

3.1
thermal transmittance
U value
parameter of glazing which characterizes the heat transfer through the central part of the glazing, i.e.
without edge effects, and states the steady-state density of heat transfer rate per temperature difference
between the environmental temperatures on each side
Note 1 to entry: The U value is given in watts per square metre kelvin [W/(m · K)].
3.2
declared value
U value (3.1) obtained under standardized boundary conditions
4 Symbols, dimensionless numbers and subscripts
A constant −
c specific heat capacity of gas J/(kg · K)
d nominal thickness of material layer (glass or alternative glazing material) m
F volume fraction −
h heat transfer coefficient W/(m · K)
Λ
thermal conductance W/(m · K)
M number of material layers −
n exponent −
N number of spaces −
r thermal resistivity of glass (or alternative glazing material) m · K/W
P gas property −
s width of gas space m
T absolute temperature K
U thermal transmittance W/(m · K)
ΔT temperature difference K
ε corrected emissivity −
ε normal emissivity (perpendicular to the surface) −
n
ρ gas density kg/m
−8 2 4
σ Stefan-Boltzmann’s constant 5,67 × 10 W/(m · K )
μ dynamic viscosity of gas kg/(m · s)
λ thermal conductivity of gas in space W/(m · K)
ϑ temperature on the Celsius scale °C
Gr Grashof number
Nu Nusselt number
Pr Prandtl number
c convection
e external
i internal
th
j j material layer
th
k k space
g gas
m mean
n normal
r radiation
s space
t total
1, 2, . first, second, .
5 Basic formulae
5.1 General
The method of this standard is based on a calculation according to the principles outlined in Clause 5.
5.2 U value
The U value is given by Formula (1):
11 11
  (1)
Uh  h
et i
where
h is the external heat transfer coefficient, in W/(m · K);
e
h is the internal heat transfer coefficient, in W/(m · K);
i
Λ is the total thermal conductance of the glazing, in W/(m · K).
t
11N M
 dr (2)
 
jj
1 1
 h
ts,k
where
Λ is the thermal conductance of each gas space, in W/(m · K);
t
N is the number of spaces;
d is the thickness of each material layer, in m;
j
r is the thermal resistivity of each material (thermal resistivity of soda lime glass = 1,0 m · K/W),
j
in m · K/W;
M is the number of material layers;
th 2
h is the total heat transfer coefficient of the k space, in W/(m · K), given by Formula (3):
s,k
hh h (3)
sk,,rk gk,
where
th
h is the radiative heat transfer coefficient of the k space;
r,k
th
h is the heat transfer coefficient of the gas of the k space.
g,k
The thermal resistivity of components other than glass (e.g. interlayers in laminated glass) may be taken
into account in determining the U value. Data on thermal conductivity of components other than glass
should be obtained from the supplier or a default value taken from ISO 10456. In instances where the effects
are not considered significant or important, a simplified approach may be taken, i.e. ignoring the effects of
components other than glass.
NOTE In the absence of measured data from the manufacturer, a conventional value for the thermal conductivity
1)
of the interlayer can be taken from ISO 18958 .
For an insulating glass unit with more than one gas space, the U value shall be determined following the
iteration procedure in Annex A.
th
5.3 Radiative heat transfer coefficient of k space h
r,k
The radiative heat transfer coefficient of k space is given by Formula (4):
th
1
 
hT4 1 (4)
r,k m,k
 

 1,k2,k 
where
σ is the Stefan-Boltzmann’s constant;
T is the mean absolute temperature of the gas space, in K;
m,k
ε and ε are the corrected emissivities at T of the surfaces bounding the enclosed space between
1,k 2,k m,k
the panes.
1) Under preparation. Stage at the time of balloting: ISO/DIS 18958.

5.4 Gas conductance, Λ
g
5.4.1 General
The gas conductance is given by Formula (5):

k
 Nu (5)
gk,
s
k
where
s is the width of the space, in m;
k
λ is the thermal conductivity, in W/(m · K);
k
Nu is the Nusselt number, calculated using Formula (6):
n
Nu A GrPr (6)

where
A is a constant;
Gr is the Grashof number;
Pr is the Prandtl number;
n is an exponent.
The Grashof number and the Prandtl number are calculated using Formula (7) and Formula (8) respectively:
98, 1sT
Gr  (7)
T 
m
c
Pr  (8)

where
ΔT is the temperature difference between glass surfaces bounding the gas space, in K;
ρ is the density, in kg/m ;
µ is the dynamic viscosity, in kg/(m · s);
c is the specific heat capacity, in J/(kg · K);
T is the mean temperature, in K.
m
If Nu is less than 1, then the value unity i
...

ISO/DISFDIS 10292:2025(en)
ISO/TC 160
Secretariat: BSI
Date: 2026-01-2304-08
Glass in building — Determination of thermal transmittance (U
value) — Calculation method
FDIS stage
ISO #####-#:####(X/FDIS 10292:2026(en)
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
© ISO #### 2026 – All rights reserved
ii
ISO/DISFDIS 10292:20252026(en)
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols, dimensionless numbers and subscripts . 2
5 Basic formulae . 3
5.1 General . 3
5.2 U value . 3
5.3 Radiative heat transfer coefficient of kth space h . 4
r,k
5.4 Gas conductance, Λ . 5
g
6 Basic material properties . 7
6.1 Emissivity . 7
6.2 Gas properties . 8
7 External and internal heat transfer coefficients . 9
7.1 External heat transfer coefficient, h . 9
e
7.2 Internal heat transfer coefficient h . 9
i
7.3 Design values . 13
8 Declared values: standardized boundary conditions . 13
9 Expression of the results . 13
9.1 U values . 13
9.2 Intermediate values . 14
10 Report . 14
10.1 Information included in the report . 14
10.2 Identification of the glazing . 14
10.3 Cross section of the glazing . 14
10.4 Results . 14
Annex A (normative) Iteration procedure for an insulating glass unit with more than one gas
space . 15
Annex B (informative) Determination of gas properties at different temperatures . 17
Bibliography . 18

iii
ISO #####-#:####(X/FDIS 10292:2026(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO'sISO’s adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 160, Glass in building.
This second edition cancels and replaces the first edition (ISO 10292:1994), which has been technically
revised.
The main changes are as follows:
— — changes to some of the gas properties;
— — introduction of a linear approximation for determining gas properties at different temperatures;
— — modification of the calculation of the internal heat transfer coefficient for vertical glazing for
consistency with ISO 6946;
— — provision of more details on heat transfer coefficients for glazing at angles other than vertical, for
consistency with ISO 6946 and ISO 10077--1;
— — additional clarification provided on the iteration procedure for an insulating glass unit with more than
one gas space.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
© ISO #### 2026 – All rights reserved
iv
ISO/DISFDIS 10292:20252026(en)
Introduction
For the purposes of this document, the term glass has been used in the context of a material or surface
property, whereas the term glazing has been adopted to refer to either monolithic glass or an insulating glass
unit.
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]. (see ISO 9050).
Reference can be made to [3], [4]ISO 6946, ISO 52016-1 and [5]ISO 10211 or other European Standards
dealing with heat loss calculations for the application of glazing U values determined by this
standarddocument.
Reference can be made to [6]ISO 15099 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 ISO 10291 or ISO 19916 1.
A procedure for the determination of emissivity is given in ISO 20589.
The rules have been made as simple as possible consistent with accuracy.
v
DRAFT International Standard ISO/DIS 10292:2025(en)

Glass in building — Determination of thermal transmittance (U value)
— Calculation method
1 Scope
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 and/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 (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 ISO 10077--1:2017 [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.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 20589, Glass in building — Determination of the emissivity
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/

Under preparation. Stage at the time of publication: ISO/PRF 20589:2025.
3.1 3.1
thermal transmittance
U value
parameter of glazing which characterizes the heat transfer through the central part of the glazing, i.e. without
edge effects, and states the steady-state density of heat transfer rate per temperature difference between the
environmental temperatures on each side
Note 1 to entry: The U value is given in watts per square metre kelvin [W/(m · K)].
3.2 3.2
declared value
U value (3.1(3.1)) obtained under standardized boundary conditions
4 Symbols, dimensionless numbers and subscripts
4.1 Symbols
A constant −
c specific heat capacity of gas J/(kg · K)
d nominal thickness of material layer (glass or alternative glazing material) m
F volume fraction −
h —  heat transfer coefficient W/(m · K)
L 𝛬𝛬 —  also thermal conductance W/(m · K)
M number of material layers −
n exponent −
N number of spaces −
r thermal resistivity of glass (or alternative glazing material) m · K/W
P gas property −
s width of gas space m
T absolute temperature K
U thermal transmittance W/(m · K)
ΔT temperature difference K
ε corrected emissivity −
ε normal emissivity (perpendicular to the surface) −
n
ρ gas density kg/m
−8 2 4
σ Stefan-Boltzmann'sBoltzmann’s constant 5,67 × 10 W/(m · K )
μ dynamic viscosity of gas kg/(m · s)
λ thermal conductivity of gas in space W/(m · K)
ϑ temperature on the Celsius scale °C
Gr Grashof number
Nu Nusselt number
Pr Prandtl number
© ISO #### 2026 – All rights reserved
ISO/DISFDIS 10292:20252026(en)
c convection
e external
i internal
th
j j material layer
th
k k space
g gas
m mean
n normal
r radiation
s space
t total
1;, 2, . first, second etc., .
5 Basic formulae
5.1 General
The method of this standard is based on a calculation according to the principles outlined in Clause 5clause 5.
5.2 U value
The U value is given by Formula (1)Formula (1)::
(1)
1 1 1 1
= + +
𝑈𝑈 ℎ 𝛬𝛬 ℎ
e t i
(1)
where
h is the external heat transfer coefficient;
e
h is the internal heat transfer coefficient;
i
is the total thermal conductance of the glazing.
Λ L
t
(2)
h is the external heat transfer coefficient, in W/(m · K);
e
h is the internal heat transfer coefficient, in W/(m · K);
i
𝛬𝛬 is the total thermal conductance of the glazing, in W/(m · K).
t
𝑁𝑁
𝑀𝑀
1 1
=� +� 𝑑𝑑⋅𝑟𝑟 (2)
j j
𝛬𝛬 ℎ
t s, k
where
𝛬𝛬 is the thermal conductance of each gas space, in W/(m · K);
t
N is the number of spaces;
d is the thickness of each material layer, in m;
j
r is the thermal resistivity of each material (thermal resistivity of soda lime glass = 1,0 m · K/W),
j
in m · K/W;
M is the number of material layers;
th 2
h is the total heat transfer coefficient of the k space, in W/(m · K), given by Formula (3)
s,k
is the thermal conductance of each gas space;
Λ L
s
N is the number of spaces;
d is the thickness of each material layer;
j
r is the thermal resistivity of each material (thermal resistivity of soda lime glass = 1,0 m · K/W);
j
M is the number of material layers.
:
ℎ =ℎ +ℎ (3)
s,k r,k g,k
where
th
h is the total heat transfer coefficient of the k space;
s,k
th
h is the radiative heat transfer coefficient of the k space;
r,k
th
h is the heat transfer coefficient of the gas of the k space.
g,k
th
h is the radiative heat transfer coefficient of the k space;
r,k
th
h is the heat transfer coefficient of the gas of the k space.
g,k
The thermal resistivity of components other than glass (e.g. interlayers in laminated glass) may be taken into
account in determining the U value. Data on thermal conductivity of components other than glass should be
obtained from the supplier or a default value taken from ISO 10456 [11]. In instances where the effects are
not considered significant or important, a simplified approach may be taken, i.e. ignoring the effects of
components other than glass.
NOTE In the absence of measured data from the manufacturer, a conventional value for the thermal conductivity of
1)
the interlayer can be taken from ISO 18958 [10]. .
For an insulating glass unit with more than one gas space, the U value shall be determined following the
iteration procedure in Annex AAnnex A.
th
5.3 Radiative heat transfer coefficient of k space h
r,k
The radiative heat transfer coefficient of k space is given by Formula (4)Formula (4)::
th
(4)
1 1
−1
ℎ = 4𝜎𝜎( + − 1) 𝑇𝑇 (4)
r,k
m,k
𝜀𝜀 𝜀𝜀
1,k 2,k
1)
Under preparation. Stage at the time of balloting: ISO/DIS 18958.

© ISO #### 2026 – All rights reserved
ISO/DISFDIS 10292:20252026(en)
where
σ is the Stefan-Boltzmann'sBoltzmann’s constant;
T is the mean absolute temperature of the gas space, in K;
m,k
ε and ε are the corrected emissivities at T of the surfaces bounding the enclosed space between
1,k 2,k m,k
the panes.
5.4 Gas conductance Lg, Λg
5.4.1 General
The gas conductance is given by Formula (5)Formula (5)::
𝜆𝜆
k
𝛬𝛬 =𝑁𝑁𝑁𝑁 (5)
g,k
𝑠𝑠
k
where
s is the width of the space, in m;
k
λ is the thermal conductivity, in W/(m · K);
k
Nu is the Nusselt number, calculated using Formula (6)
s is the width of the space;
k
λ is the thermal conductivity;
k
Nu is the Nusselt number.
:
𝑛𝑛
𝑁𝑁𝑁𝑁 =𝐴𝐴⋅ (𝐺𝐺𝑟𝑟⋅𝑃𝑃𝑟𝑟) (6)
where
A is a constant;
Gr is the Grashof number;
Pr is the Prandtl number;
n is an exponent.
The Grashof number and the Prandtl number are calculated using Formula (7) and Formula (8)
A is a constant;
Gr is the Grashof number;
Pr is the Prandtl number;
n is an exponent.
(7)
respectively:
3 2
9,81⋅𝑠𝑠 ⋅𝛥𝛥𝛥𝛥⋅𝜌𝜌
𝐺𝐺𝑟𝑟 = (7)
𝛥𝛥 ⋅𝜇𝜇
m
𝜇𝜇⋅𝑐𝑐
𝑃𝑃𝑟𝑟 = (8)
𝜆𝜆
where
ΔT is the temperature difference between glass surfaces bounding the gas space;
ρ is the density;
µ is the dynamic viscosity;
c is the specific heat capacity;
T is the mean temperature.
m
The Nusselt number is calculated using Formula (6).
ΔT is the temperature difference between glass surfaces bounding the gas space, in K;
ρ is the density, in kg/m ;
µ is the dynamic viscosity, in kg/(m · s);
c is the specific heat capacity, in J/(kg · K);
T is the mean temperature, in K.
m
If Nu is less than 1, then the value unity is used for Nu in Formula (5)Formula (5).
5.4.2 Heat flow direction
Where the heat flow direction from the horizontal plane is 0°, which is typical of vertical glazing, the following
values shall be used in Formula (6)Formula (6)::
A = 0,035
n = 0,38
Where the heat flow direction from the horizontal plane is not 0°, for example for glazing other than vertical
and upward heat flow, the heat transfer by convection is enhanced.
NOTE Glazing other than vertical is sometimes referred to as horizontal, angled or sloped glazing.
This effect shall be considered by substituting, in Formula (6)Formula (6),, the values of A and n given in
Table 1Table 1.
Table 1— Values of A and n for heat flow direction
Heat flow direction
from the horizontal
A n Glazing position
plane
o
degrees ( )
a a
0 0,035 0,38 Vertical Vertical
45 0,10 0,31
Other than vertical
90 0,16 0,28
a
Standardized boundary conditions for product comparison.
© ISO #### 2026 – All rights reserved
ISO/DISFDIS 10292:20252026(en)
For intermediate angles linear interpolation is satisfactory, however, the linear interpolation shall be between
the two nearest points.
When the direction of heat flow is downward, the convection shall be considered suppressed for practical
cases and Nu = 1 is substituted in Formula (5)Formula (5). .
o
For heat flow direction from the horizontal plane up to and including 30 , there is no upwards or downwards
heat flow.
6 Basic material properties
6.1 Emissivity
The corrected emissivities ε and ε of the surfaces bounding the enclosed spaces are required to calculate
1,k 2,k
the radiative heat transfer coefficient h in Formula (4)Formula (4).
r,k
For uncoated soda lime glass surfaces or for soda lime glass surfaces with coatings which have no effect on the
emissivity, the normal emissivity to be used is 0,89. The corrected emissivity shall be determined from the
normal emissivity in accordance with Formula (9)Formula (9).
(9)
2 3
𝜀𝜀 = 1,1887𝜀𝜀 − 0,4967𝜀𝜀 + 0,2452𝜀𝜀 (9)
n n n
With reasonable confidence the same value may be used for uncoated b
...