ISO 10292

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)
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© 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
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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)
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represent the latest information, which may be obtained from the patent database available at
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Any trade name used in this document is information given for the convenience of users and does not
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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.
ISO/FDIS 10292:2026(en)
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
ISO/FDIS 10292:2026(en)
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:
ISO/FDIS 10292:2026(en)
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
...