EN ISO 12631:2012

SLOVENSKI STANDARD
01-marec-2013
1DGRPHãþD
SIST EN 13947:2007
7RSORWQH]QDþLOQRVWLREHãHQLKIDVDG,]UDþXQWRSORWQHSUHKRGQRVWL,62

Thermal performance of curtain walling - Calculation of thermal transmittance (ISO
12631:2012)
Wärmetechnisches Verhalten von Vorhangfassaden - Berechnung des
Wärmedurchgangskoeffizienten (ISO 12631:2012)
Performance thermique des façades-rideaux - Calcul du coefficient de transmission
thermique (ISO 12631:2012)
Ta slovenski standard je istoveten z: EN ISO 12631:2012
ICS:
91.060.10 Stene. Predelne stene. Walls. Partitions. Facades
Fasade
91.120.10 Toplotna izolacija stavb Thermal insulation
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 12631
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2012
ICS 91.120.10 Supersedes EN 13947:2006
English Version
Thermal performance of curtain walling - Calculation of thermal
transmittance (ISO 12631:2012)
Performance thermique des façades-rideaux - Calcul du Wärmetechnisches Verhalten von Vorhangfassaden -
coefficient de transmission thermique (ISO 12631:2012) Berechnung des Wärmedurchgangskoeffizienten (ISO
12631:2012)
This European Standard was approved by CEN on 30 September 2012.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.

This European Standard exists 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2012 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 12631:2012: E
worldwide for CEN national Members.

Contents Page
Foreword .3
Foreword
This document (EN ISO 12631:2012) has been prepared by Technical Committee CEN/TC 89 “Thermal
performance of buildings and building components”, the secretariat of which is held by SIS, in collaboration
with Technical Committee ISO/TC 163 "Thermal performance and energy use in the built environment".
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by April 2013, and conflicting national standards shall be withdrawn at the
latest by April 2013.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 13947:2006.
According to the CEN/CENELEC Internal Regulations, the national standards organisations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech
Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

INTERNATIONAL ISO
STANDARD 12631
First edition
2012-10-01
Thermal performance of curtain
walling — Calculation of thermal
transmittance
Performance thermique des façades-rideaux — Calcul du coefficient de
transmission thermique
Reference number
ISO 12631:2012(E)
©
ISO 2012
ISO 12631:2012(E)
©  ISO 2012
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56  CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
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Published in Switzerland
ii © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Contents Page
Foreword . iv
Introduction . v
1  Scope . 1
2  Normative references . 1
3  Terms, definitions, symbols and units . 2
3.1  Terms and definitions . 2
3.2  Symbols and units . 3
3.3  Subscripts . 3
3.4  Superscripts . 4
4  Geometrical characteristics . 4
4.1  Main principles . 4
4.2  Internal depth . 6
4.3  Boundaries of curtain wall structures . 6
5  Cut-off planes and partitioning of thermal zones . 9
5.1  Rules for thermal modelling . 9
5.2  Cut-off planes of the geometrical model . 9
6  Calculation of curtain wall transmittance . 9
6.1  Methodologies . 9
6.2  Single assessment method . 10
6.3  Component assessment method . 16
6.4  Thermal transmittance of a curtain wall built of different elements . 21
7  Input data . 21
8  Report . 22
8.1  Section drawings . 22
8.2  Overview drawing of the whole curtain wall element . 23
8.3  Values used for calculation . 23
8.4  Presentation of results . 23
Annex A (informative) Guidance for calculating the thermal transmittance U of curtain walling
cw
using the two methods . 24
Annex B (informative) Linear thermal transmittance of junctions . 25
Annex C (normative) A method for calculating the thermal effect of screws using a 2D numerical
method and the procedures specified in ISO 10077-2:2012 . 33
Annex D (normative) Ventilated and unventilated air spaces . 36
Annex E (informative) Component method: Calculation example . 39
Annex F (informative) Single assessment method: Calculation example . 45
Annex G (normative) Parallel routes in normative references . 48
Bibliography . 49

ISO 12631:2012(E)
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 12631 was prepared by the European Committee for Standardization (CEN) Technical Committee
CEN/TC 89, Thermal performance of buildings and building components, the secretariat of which is held by
SIS, in collaboration with ISO Technical Committee TC 163, Thermal performance and energy use in the built
environment, Subcommittee SC 2, Calculation methods, in accordance with the Agreement on technical
cooperation between ISO and CEN (Vienna Agreement).
iv © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Introduction
The design and construction of curtain wall systems is complex. This International Standard specifies a
procedure for calculating the thermal transmittance of curtain wall structures.
Curtain walls often contain different kinds of materials, joined in different ways, and can exhibit numerous
variations of geometrical shape. With such a complex structure, the likelihood of producing thermal bridges
across the curtain wall envelope is quite high.
The results of calculations, carried out following the procedures specified in this International Standard, can be
used for comparison of the thermal transmittance of different types of curtain wall or as part of the input data
for calculating the heat used in a building. This International Standard is not suitable for determining whether
or not condensation will occur on the structure surfaces nor within the structure itself.
Two methods are given in this International Standard:
 single assessment method (see 6.2);
 component assessment method (see 6.3).
Guidance on the use of these two methods is given in Annex A. Calculation examples for these two methods
are given in Annex E and Annex F.
Testing according to ISO 12567-1:2010 is an alternative to this calculation method.
The thermal effects of connections to the main building structure as well as fixing lugs can be calculated
according to ISO 10211:2007.
The thermal transmittance of the frame, U , is defined according to ISO 10077-2:2012 or EN 12412-2:2003
f
together with Annex A. The thermal transmittance of glazing units, U , is defined according to the documents
g
listed in Table G.1 which does not include the edge effects. The thermal interaction of the frame and the filling
element is included in the linear thermal transmittance  which is derived using the procedures specified in
ISO 10077-2:2012.
INTERNATIONAL STANDARD ISO 12631:2012(E)

Thermal performance of curtain walling — Calculation of
thermal transmittance
1 Scope
This International Standard specifies a method for calculating the thermal transmittance of curtain walls
consisting of glazed and/or opaque panels fitted in, or connected to, frames.
The calculation includes:
 different types of glazing, e.g. glass or plastic; single or multiple glazing; with or without low emissivity
coating; with cavities filled with air or other gases;
 frames (of any material) with or without thermal breaks;
 different types of opaque panels clad with metal, glass, ceramics or any other material.
Thermal bridge effects at the rebate or connection between the glazed area, the frame area and the panel
area are included in the calculation.
The calculation does not include:
 effects of solar radiation;
 heat transfer caused by air leakage;
 calculation of condensation;
 effect of shutters;
 additional heat transfer at the corners and edges of the curtain walling;
 connections to the main building structure nor through fixing lugs;
 curtain wall systems with integrated heating.
2 Normative references
The following referenced documents are indispensable for the application 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 6946:2007, Building components and building elements — Thermal resistance and thermal
transmittance — Calculation method
ISO 7345:1987, Thermal insulation — Physical quantities and definitions
ISO 12631:2012(E)
ISO 10077-1:2006, Thermal performance of windows, doors and shutters — Calculation of thermal
transmittance — Part 1: General
ISO 10077-2:2012, Thermal performance of windows, doors and shutters — Calculation of thermal
transmittance — Part 2: Numerical method for frames
ISO 10211:2007, Thermal bridges in building construction — Heat flows and surface temperatures — Detailed
calculations
ISO 10291:1994, Glass in building — Determination of steady-state U values (thermal transmittance) of
multiple glazing - Guarded hot plate method
ISO 10292:1994, Glass in building — Calculation of steady-state U values (thermal transmittance) of multiple
glazing
ISO 10293:1997, Glass in building — Determination of steady-state U values (thermal transmittance) of
multiple glazing - Heat flow meter method
ISO 12567-1:2010, Thermal performance of windows and doors — Determination of thermal transmittance by
hot box method — Part 1: Complete windows and doors
EN 673:2011, Glass in building — Determination of thermal transmittance (U value) — Calculation method
EN 674:2011, Glass in building — Determination of thermal transmittance (U value) — Guarded hot plate
method
EN 675:2011, Glass in building — Determination of thermal transmittance (U value) — Heat flow meter
method
EN 12412-2:2003, Thermal performance of windows, doors and shutters — Determination of thermal
transmittance by hot-box method — Part 2: Frames
3 Terms, definitions, symbols and units
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7345:1987, ISO 6946:2007 and
Annex G apply.
NOTE Clause 4 includes descriptions of a number of geometrical characteristics of glazing units, frame sections and
panels.
2 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
3.2 Symbols and units
Table 1 — Symbols and units
Symbol Quantity Unit
A area m
T thermodynamic temperature K
U thermal transmittance W/(m ·K)
l length m
d depth m
 heat flow rate W
 linear thermal transmittance W/(m·K)
∆ difference
Σ summation
 emissivity
3.3 Subscripts
cw curtain walling
d developed
e external
eq equivalent
f frame
f,g frame/glazing
FE filling element
g glazing
i internal
j joint
m mullion
m,f mullion/frame
m,g mullion/glazing
n normal
p panel (opaque)
s screw
t transom
t,f transom/frame
t,g transom/glazing
tot total
TJ thermal joint at a connection between two filling elements
W window
ISO 12631:2012(E)
3.4 Superscripts
* definition of areas for length-related treatment of thermal joints (see 6.2.2.3)
4 Geometrical characteristics
4.1 Main principles
The main principles of curtain walling are shown in Figures 1 and 2.

Key
1 structure fixing bracket
A-A vertical section
Figure 1 — Principle of curtain walling construction: unitised construction
4 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Key
1 structure fixing bracket
A-A vertical section
Figure 2 — Principle of curtain walling construction: stick construction
ISO 12631:2012(E)
4.2 Internal depth
The internal depth is defined as shown in Figure 3.

Key
1 internal
2 external
d internal depth of mullion or transom
i
Figure 3 — Internal and external developed area, internal depth
4.3 Boundaries of curtain wall structures
4.3.1 General
To evaluate the thermal transmittance of façades, representative reference areas should be defined. The
following subclauses define the various areas.
6 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
4.3.2 Boundaries of a representative reference element
The boundaries of the representative reference element shall be chosen according to the principles shown in
Figure 4.
Key
— — — — Boundaries of the representative element
Figure 4 — Boundaries of a representative reference element of a façade
ISO 12631:2012(E)
4.3.3 Curtain wall areas
The representative reference element is divided into areas of different thermal properties (sash, frame,
mullion, transom, glazing units and panel sections) as shown in Figure 5.

Key
1 mullion
2 transom
3 sash and frame
4 glazing
5 panel
Figure 5 — Areas with different thermal properties
8 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
5 Cut-off planes and partitioning of thermal zones
5.1 Rules for thermal modelling
In most cases, the façade can be partitioned into several sections by using cut-off planes so that the thermal
transmittance of the overall façade can be calculated as the area-weighted average of the thermal
transmittance of each section. The necessary input data (thermal properties of each section) can be evaluated
by measurement, two-dimensional finite element or finite difference software calculation or by tables or
diagrams. In general there are two possibilities:
 the single assessment method (see 6.2);
 the component assessment method (see 6.3).
The partitioning of the façade shall be performed in such a way as to avoid any significant differences in
calculation results of the façade treated as a whole and the heat flow rate through the partitioned façade.
Appropriate partitioning into several geometrical parts is achieved by choosing suitable cut-off planes.
5.2 Cut-off planes of the geometrical model
The geometrical model includes central elements (glazing units, spandrel panels etc.) and thermal joints
(mullion, transom, silicone joint etc.), which connect the different central elements. The geometrical model is
delimited by cut-off planes.
Curtain walling often contains highly conductive elements (glass and metals) which implies that significant
lateral heat flow is possible. Cut-off planes shall represent adiabatic boundaries, which can be either:
 a symmetry plane, or
 a plane where the heat flow through that plane is perpendicular to the plane of the curtain wall, i.e. no
edge effect is present (e.g. at least 190 mm away from the edge of a double glazing unit).
Cut-off planes may be positioned only where there is a clear adiabatic situation (i.e. the heat flow is
perpendicular to the plane). Figure 6 shows adiabatic lines (in the middle of the glass or panel far enough from
the frame) where the heat flow will be perpendicular to the glass panes.
Cut-off planes do not necessarily fall at the same place as the geometrical boundaries of a unitised element
(i.e. through the frame). The middle of a frame might not be an adiabatic boundary. This might be due to
asymmetric geometrical shape of the frame, asymmetric material properties (e.g. different conductivity of sub-
components at each side of the frame), or asymmetric connection of panels in a symmetric frame (e.g. a
frame that connects a spandrel panel and a glazing unit, or two glazing units with different thermal properties).
6 Calculation of curtain wall transmittance
6.1 Methodologies
Two methods of calculating the thermal transmittance of curtain wall systems are specified: the single
assessment method and the component assessment method.
The single assessment method (see 6.2) is based on detailed computer calculations of the heat transfer
through a complete construction including mullions, transoms, and filling elements (e.g. glazing unit, opaque
panel). The heat flow rate (between two adiabatic lines) is calculated by modelling each thermal joint between
two filling elements (opaque panel and/or glazing unit) using two-dimensional or three-dimensional finite
element analysis software. By area weighting the U-values of thermal joints and filling elements, the overall
façade U-value can be calculated. This method can be used for any curtain walling system (i.e. unitised
systems, stick systems, patent glazing, structural sealant glazing, rain screens, structural glazing).
ISO 12631:2012(E)
The component assessment method (see 6.3) divides the representative element into areas of different
thermal properties, e.g. glazing units, opaque panels and frames. By area weighting the U-values of these
elements with additional correction terms describing the thermal interaction between these elements
(-values), the overall façade U-value can be calculated. This method can be used for curtain walling systems
such as unitised systems, stick systems and patent glazing. Structural silicone glazing, rain screens and
structural glazing are excluded from the component assessment method.
For the purposes of this International Standard, the term “filling element” is any façade component that has a
one-dimensional heat flow in the absence of edge effects (the flat surface being perpendicular to the heat flow
direction). Examples are glazing units and spandrel panels.

Figure 6 — Thermal section representing the full curtain wall
6.2 Single assessment method
6.2.1 Thermal transmittance of glazing units and panels (filling elements)
The thermal transmittance of opaque panels U shall be evaluated according to ISO 6946:2007. The thermal
p
transmittance of glazing units U shall be evaluated according to the documents listed in Table G.1. In some
g
cases, there is a different filling element at each side of the thermal joint (mullion, transom), so that two
thermal transmittances have to be determined.
6.2.2 Determination of the heat flow through filling element / mullion or transom / filling element
connection
6.2.2.1 General
The total heat flow rate  of the complete connection shall be calculated using computer software that
tot
conforms to ISO 10211:2007 and ISO 10077-2:2012 or measured according to ISO 12567-1:2010 with the
10 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
filling elements positioned between the adiabatic lines. The modelling of screws (if present) in the two
dimensional calculation shall be performed according to Annex C.
Since the heat flow rate is determined between the two adiabatic boundaries, it represents the heat flow
through the filling elements, the thermal joint (e.g. mullion/transom) and also the lateral heat flow (edge
effects) of the interaction between the two filling elements.
Therefore,  represents the total heat flow rate that results from making a thermal joint between two filling
tot
elements and includes:
 heat flow rate straight through filling element 1 and filling element 2 (one dimensional heat flow
perpendicular to the surface of the filling element);
 heat flow rate through the thermal joint that is used to connect the two filling elements together (e.g. a
frame in a framed curtain wall, a silicone joint in case of structural glazing);
 lateral and edge heat-flows due to the thermal interaction between the filling elements and the thermal
joint and due to the edge constructions of the two individual filling elements (e.g. glass spacer).
As in most cases these different heat flows are difficult to separate, and to assign to a specific sub-component
of the thermal joint, it is appropriate to split the overall heat flow through a thermal joint into only three parts
(see Figure 7a):
 the heat flow rate  through filling element 1 without the presence of the thermal joint (i.e. the heat
FE1
flow derived from the centre U-value of filling element 1);
 the heat flow rate  through filling element 2 without the presence of the thermal joint (i.e. the heat
FE2
flow derived from the centre U-value of filling element 2);
 the heat flow rate  which is the additional heat flow rate due to making a thermal joint (which includes
TJ
direct and lateral heat flows of all joint edges and the thermal joint itself excluding the one dimensional
heat flow through the filling elements).
There are two ways of allowing for the additional heat flow rate  , which are equivalent and either approach
TJ
will yield the same result for the thermal transmittance of the curtain wall. The possibilities are:
 consider the heat flow rate  in terms of an area-related joint thermal transmittance U ;
TJ TJ
 consider the heat flow rate  in terms of a length-related linear joint thermal transmittance  .
TJ TJ
The thermal transmittance of the joint U or the linear thermal transmittance of the joint  includes, in one
TJ TJ
single parameter, all thermal bridging effects resulting from making a thermal joint between the filling
elements. This definition should not be compared with the frame thermal transmittance U (e.g. as defined in
f
ISO 10077-2:2012 or in the alternative method described in 6.3), which is solely the heat flow rate through the
frame excluding the lateral heat flow effects of panels and interaction with the frame. U should not be used
TJ
to assess condensation risk.
6.2.2.2 Determination of the area-related joint thermal transmittance U
TJ
The heat flow rate  , which represents the additional heat flow rate due to making a thermal joint between
TJ
two filling elements, can be calculated as:
 =  – (U A + U A ) T (1)
TJ tot FE1 FE1 FE2 FE2
ISO 12631:2012(E)
where
T is the temperature difference between internal and external environments used to simulate the heat
transfer.
The thermal transmittance of the joint U [see Figure 7a)] is calculated as:
TJ
U =  / (A T) (2)
TJ TJ TJ
where
A is the projected area of the thermal joint;
TJ
T is the temperature difference between the internal and external environment used for the simulation.
NOTE For the example in Figure 7a, the areas and U-values of the filling elements are defined as follows:
A = A , A = A , U = U , U = U
FE1
g FE1 p FE2 g p FE2
6.2.2.3 Determination of the linear joint thermal transmittance 
TJ
The definition of the filling element areas is different from the definition in Figure 7a) and is as specified in
Figure 7b). The calculation of  is according to Equation (3).

TJ
The heat flow rate  can be calculated as:

TJ
* *
 =  – (U A + U A ) T (3)
TJ tot FE1 FE1 FE2 FE2
where
T is the temperature difference between inside and outside air used to simulate the heat transfer.

a)  Definition of the areas when using U (example: glazing, mullion, panel)
TJ
12 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
b)  Definition of the areas when using  (example: glazing, mullion, panel)
TJ
Figure 7 — Definition of the areas
The linear thermal transmittance of the joint  is calculated as:
TJ
 =  / (l T) (4)
TJ TJ TJ
where
T is the temperature difference between the internal and external environments used for the simulation.
6.2.2.4 Definitions of areas for other combinations
Figures 8 to 11 give further examples of how the curtain wall can be decomposed into parts for analysis by the
single assessment method. The area of the joint A is the largest of the projected areas between the two

TJ
filling elements. The length l is the length of the thermal joint connecting the filling elements.
TJ
ISO 12631:2012(E)
Key
A mullion A window area
m w
A curtain walling A frame area

cw f
A panel area A glazing area
p g
Figure 8 — Example 1: Framed curtain wall

Key
A area of thermal joint
T
A glazing area
g
Figure 9 — Example 2: Structural silicone glazing
14 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Key
TJ thermal joint
Figure 10 — Example 3: Structural glazing

Figure 11 — Example 4: Rain screen
6.2.3 Determination of the overall thermal transmittance of a curtain wall (U )
cw
6.2.3.1 Using the area-related thermal transmittance U
TJ
The overall thermal transmittance of the curtain wall element U is calculated as the area-weighted average
cw
of all the thermal transmittances of the joints, glazing units and panels.
AUAUA U
gg p p TJ TJ
U  (5)
cw
AA A
gp TJ
where the areas A and A are defined according to Figure 7a).
g p
6.2.3.2 Using the length-related linear thermal transmittance Ψ
TJ
The overall thermal transmittance of the curtain wall element U is calculated as the area-weighted average
cw
of all the thermal transmittances of glazing units and panels and the linear thermal transmittances of the joints.
**
AUAUl 
g g p p TJ TJ
U  (6)
cw
**
AA
gp
using the areas  A* and  A* as defined according to Figure 7b).
g p
ISO 12631:2012(E)
6.3 Component assessment method
6.3.1 General
The thermal transmittance of a single element of a curtain walling, U , shall be calculated using Equation (7).
cw
AUAUA UA UAUl Ψl Ψl Ψl Ψl Ψl Ψ
g g p p f f m m t t f,g f,g m,g m,g t,g t,g p p m,f m,f t,f t,f
(7)
U 
cw
A
cw
where
U , U are the thermal transmittances of glazing and panels;
g p
U , U , U are the thermal transmittances of frames, mullions and transoms;
f m t
 ,  ,  ,  are the linear thermal transmittances due to the combined thermal effects of glazing
f,g m,g t,g p
unit or panel and frame or mullion or transom;
 ,  are the linear thermal transmittances due to the combined thermal effects of frame-
,
m,f t,f
mullion and frame-transom
and the other symbols are defined in Clause 3.
The area of the curtain walling shall be calculated according to Equation (8):
A = A + A + A + A + A (8)
cw g p f m t
where
A is the area of curtain walling;
cw
A is the total area of glazing;
g
A is the total area of panels;
p
A is the total area of frames;
f
A is the total area of mullions;

m
A is the total area of transoms.

t
6.3.2 Definition of areas
6.3.2.1 Glazed areas
The glazed area, A , or the opaque panel area, A , of a component is the smaller of the visible areas that can
g p
be seen from both sides (see Figures 12 and 13). Any overlapping of the glazed area by the gaskets is
ignored.
6.3.2.2 Total visible perimeter of the glazing or opaque panel
The total perimeter of glazing, l , or of an opaque panel, l is the sum of the visible perimeter of the glass
,
g p
panes (or opaque panels). If different perimeters are seen from each side, the perimeter is defined by the
interface of the area of the glazing and the frame (see Figure 12).
16 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Key
1 glass
Figure 12 — Illustration of the glazed area and perimeter
6.3.2.3 Areas of frames, mullions and transoms
For the definition of the areas see also Figures 13 and 14.
A / A is the internal projected mullion/transom area at the interface glazing/mullion/transom, equal to the
m,i t,i
area of the projection of the internal mullion/transom on a plane parallel to the wall;
A / A is the external projected mullion/transom area at the interface glazing/mullion/transom, equal to the
m,e t,e
area of the projection of the external mullion/transom on a plane parallel to the wall;
A / A is the mullion/transom area, equal to the larger of the two projected areas seen from either side;
m t
A is the internal projected frame area at the interface glazing/frame, equal to the area of the
f,i
projection of the internal frame on a plane parallel to the wall;
A is the external projected frame area at the interface glazing/frame, equal to the area of the
f,e
projection of the external frame on a plane parallel to the wall;
A is the frame area, equal to the larger of the two projected areas seen from either side.
f
A = max (A A )
;
m m,i m,e
A = max (A A )
;
t t,i t,e
A = max (A A )
;
f f,i f,e
A = A + A
w f g
A = A + A + A + A + A
cw t m f g p
In Figure 13 only a mullion is shown. The general principal is also applicable for transoms.
ISO 12631:2012(E)
Key
1 internal A curtain walling A frame area
cw f
2 external A panel area A glazing area
p g
3 sash (movable) A internal mullion area A external mullion area
m,i m,e
4 frame (fixed) A mullion
m
5 mullion/transom A external frame area
f,e
Figure 13 — Illustration of the various areas on mullion or transom sections, panels and glazing
Key
1 internal
2 external
3 sash (movable)
4 frame (fixed)
Figure 14 — Illustration of the various areas on frame-sash sections and glazing
6.3.2.4 Area of a module of curtain walling
If the internal or external frame has a complex geometrical shape, the frame section A is determined
f
according to Figure 14, and the mullion and transom section A and A are determined according to Figure 13.
m t
The total area A , of an element of curtain walling is the sum of the mullion/transom area, A / A , the frame

cw m t
area, A the glazing area, A and the panel area, A (see also Figure 13).
, ,
f g p
18 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
6.3.3 Thermal transmittance of glazing units and panels (filling elements)
The thermal transmittance of opaque panels U shall be evaluated according to ISO 6946:2007. The thermal
p
transmittance of glazing units U shall be evaluated according to the documents listed in Table G.1. In some
g
cases, there is a different filling element at each side of the thermal joint (mullion, transom), so that two
thermal transmittances have to be calculated.
6.3.4 Thermal transmittance of frames, mullions and transoms
The U values of the sash and frame sections can be evaluated according to EN 12412-2:2003,
f
ISO 10077-1:2006 or ISO 10077-2:2012. See also Annex B concerning the boundary conditions for the
calculation of U for frames which are integrated in the façade.
f
The U and U values for the transom and mullion sections can be evaluated according to EN 12412-2:2003

t m
or ISO 10077-2:2012.
The U-value calculated according to ISO 10077-2:2012 does not take into account the effect of screws
connecting the internal to the external sections of mullions and transoms. The effect of screws shall be
included using Equations (9) and (10).
U = U + U (9)
m 0
U = U + U (10)
t 0
where
U / U is the thermal transmittance of the mullion/transom;
m t
U is the thermal transmittance of the mullion/transom calculated according to ISO 10077-2:2012
excluding the effect of metal connectors (screws);
∆U is the difference in the thermal transmittance of mullion/transom with and without screws.
Values for U are given in Table 2.
Table 2 — Values of ∆U for mullion and transom sections for stainless steel connectors
Diameter of stainless steel Distance between stainless steel
U
connectors connectors
W/(m ·K)
mm mm
 6 200 to 300 0,3
U depends on the distance between the connectors, the diameter and the materials used.
An alternative to using the values given in Table 2 is to measure ΔU using the procedures specified in
EN 12412-2:2003. In this case, ∆U is derived from the difference between measured values for specimens
with metal screws and those on the same specimen but using plastic screws (which are assumed to have a
negligible effect). ∆U can also be evaluated by a three dimensional calculation according to ISO 10211:2007
obeying the specific rules for cavities given in ISO 10077-2:2012.
A second alternative is to calculate the influence of the screws according to Annex C.
ISO 12631:2012(E)
It is common practice to produce "profile systems" comprising a large number of different frames, having a
wide range of geometric shapes but having similar thermal properties. This is because in these groups of
frames, the important parameters such as the size, material and design of the thermal break, are the same.
The thermal transmittance of a profile or profile combination of a "profile system" can be evaluated by:
 using the highest value of U or U /U of the profiles or profile combinations within the profile system, or
f m t
 using trend lines that show the relationship between U or U /U and defined geometrical characteristics.
f m t
In the latter case, the data points for the trend line are evaluated on selected profile cross-sections, taken from
the profile system in question. Detailed procedures are described in [2], [3] and [4] of the Bibliography.
6.3.5 Linear thermal transmittance
Values for the linear thermal transmittance of glazing units,  , are given in Table B.1, B.2, B.3 and B.4 or can
g
be calculated using ISO 10077-2:2012. In the case of single glazing,  in Equation (7) shall be taken as zero
g
(no spacer effect) because any correction is negligible.
Values for the linear thermal transmittance  of panels are given in Table B.5 or can be calculated using
p
ISO 10077-2:2012.
The interaction between the frame and the mullion or transom caused by the installation of the frame into the
rebate of the mullion or transom (Figure 15) is accounted for with the linear thermal transmittances  and
m,f
 .
t,f
Values for the linear thermal transmittances  and  describing the thermal heat flow due to the
m,f t,f
installation of a window in the façade are given in Table B.6 or Table B.7 or can be calculated using
ISO 10077-2:2012:
2D
 LUAUAU AU A (11)
m,f  m m f f p1 p1 p2 p2
2D
LUAUAU AU A (12)
t,f  t t f f p1 p1 p2 p2
where
2D
L is the thermal conductance of the section shown in Figure 13, in W/(mK), calculated using

ISO 10077-2:2012;
U is the thermal transmittance of the frame, in W/(mK), calculated using ISO 10077-2:2012;

f
U is the thermal transmittance of the mullion, in W/(mK), calculated using ISO 10077-2:2012;

m
U is the thermal transmittance of the transom, in W/(mK), calculated using ISO 10077-2:2012;

t
U is the thermal transmittance of panel 1, in W/(mK);

p1
U is the thermal transmittance of panel 2, in W/(mK).

p2
20 © ISO 2012 – All rights reserved

ISO 12631:2012(E)
Key
1 transom or mullion
2 frame (fixed)
3 sash (movable)
4 infill, panel
Figure 15 — Illustration of a window integrated in a transom or mullion section
6.4 Thermal transmittance of a curtain wall built of different elements
The calculation of U of the overall curtain wall built with different sizes or design of elements shall be
cw,tot
calculated as the area-weighted average thermal transmittance of all modules according to Equation (13).
(U A )

cw,j cw,j
U  (13)
cw,tot
A
 cw,j
where
U  A is the sum of the products of thermal transmittances and corresponding areas of the

cw,j cw,j
differ
...