EN ISO 12631:2017

SLOVENSKI STANDARD
01-september-2017
1DGRPHãþD
SIST EN ISO 12631:2013
7RSORWQH]QDþLOQRVWLREHãHQLKIDVDG,]UDþXQWRSORWQHSUHKRGQRVWL,62

Thermal performance of curtain walling - Calculation of thermal transmittance (ISO
12631:2017)
Wärmetechnisches Verhalten von Vorhangfassaden - Berechnung des
Wärmedurchgangskoeffizienten (ISO 12631:2017)
Performance thermique des façades-rideaux - Calcul du coefficient de transmission
thermique (ISO 12631:2017)
Ta slovenski standard je istoveten z: EN ISO 12631:2017
ICS:
91.060.10 Stene. Predelne stene. Walls. Partitions. Facades
Fasade
91.120.10 Toplotna izolacija stavb Thermal insulation of
buildings
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 12631
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2017
EUROPÄISCHE NORM
ICS 91.060.10; 91.120.10 Supersedes EN ISO 12631:2012
English Version
Thermal performance of curtain walling - Calculation of
thermal transmittance (ISO 12631:2017)
Performance thermique des façades-rideaux - Calcul du Wärmetechnisches Verhalten von Vorhangfassaden -
coefficient de transmission thermique (ISO Berechnung des Wärmedurchgangskoeffizienten (ISO
12631:2017) 12631:2017)
This European Standard was approved by CEN on 27 February 2017.

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, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 12631:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 12631:2017) 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 January 2018 and conflicting national standards shall
be withdrawn at the latest by January 2018.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association.
This document is part of the set of standards on the energy performance of buildings (the set of EPB
standards) and has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association (Mandate M/480, see reference [EF1] below), and supports essential
requirements of EU Directive 2010/31/EC on the energy performance of buildings (EPBD, [EF2]).
In case this standard is used in the context of national or regional legal requirements, mandatory
choices may be given at national or regional level for such specific applications, in particular for the
application within the context of EU Directives transposed into national legal requirements.
Further target groups are users of the voluntary common European Union certification scheme for the
energy performance of non-residential buildings (EPBD art.11.9) and any other regional (e.g. Pan
European) parties wanting to motivate their assumptions by classifying the building energy
performance for a dedicated building stock.
This document supersedes EN ISO 12631:2012.
According to the CEN-CENELEC Internal Regulations, the national standards organizations 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, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
the United Kingdom.
References:
[EF1] Mandate M480, Mandate to CEN, CENELEC and ETSI for the elaboration and adoption of
standards for a methodology calculating the integrated energy performance of buildings and
promoting the energy efficiency of buildings, in accordance with the terms set in the recast of the
Directive on the energy performance of buildings (2010/31/EU) of 14th December 2010
th
[EF2] EPBD, Recast of the Directive on the energy performance of buildings (2010/31/EU) of 14
December 2010
Endorsement notice
The text of ISO 12631:2017 has been approved by CEN as EN ISO 12631:2017 without any modification.
INTERNATIONAL ISO
STANDARD 12631
Second edition
2017-06
Thermal performance of curtain
walling — Calculation of thermal
transmittance
Performance thermique des façades-rideaux — Calcul du coefficient
de transmission thermique
Reference number
ISO 12631:2017(E)
©
ISO 2017
ISO 12631:2017(E)
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved

ISO 12631:2017(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and subscripts . 3
4.1 Symbols . 3
4.2 Subscripts . 3
4.3 Superscripts . . 3
5 Description of the methods . 3
5.1 Output of the method . 3
5.2 General description . 4
5.3 Geometrical characteristics . 4
5.3.1 Main principles . 4
5.3.2 Internal depth . 6
5.3.3 Boundaries of curtain wall structures . 7
5.3.4 Cut-off planes and partitioning of thermal zones.10
6 Methodologies for the calculation of curtain wall transmittance .10
7 Single assessment method .12
7.1 Output data .12
7.2 Calculation time intervals .12
7.3 Input data .12
7.3.1 Geometrical characteristics .12
7.3.2 Thermal characteristics .16
7.4 Calculation procedure .18
7.4.1 Applicable time interval .18
7.4.2 Calculation of thermal transmittance .18
8 Component assessment method .19
8.1 Output data .19
8.2 Calculation time intervals .19
8.3 Input data .19
8.3.1 Geometrical characteristics .19
8.3.2 Thermal characteristics .23
8.4 Calculation procedure .26
8.4.1 Applicable time interval .26
8.4.2 Calculation of thermal transmittance .26
9 Report .27
9.1 Contents of report .27
9.2 Drawings .28
9.2.1 Section drawings .28
9.2.2 Overview drawing of the whole curtain wall element .28
9.3 Values used in the calculation .28
9.4 Presentation of results .28
Annex A (normative) Input and method selection data sheet — Template .29
Annex B (informative) Input and method selection data sheet — default choices .31
Annex C (normative) Regional references in line with ISO Global Relevance Policy .33
Annex D (normative) Linear thermal transmittance of junctions .34
ISO 12631:2017(E)
Annex E (normative) A method for calculating the thermal effect of screws using a 2D
numerical method and the procedures specified in ISO 10077-2 .41
Annex F (normative) Ventilated and unventilated air spaces .44
Bibliography .47
iv © ISO 2017 – All rights reserved

ISO 12631:2017(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.
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. www .iso .org/ directives
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received. www .iso .org/ patents
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on 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 the following
URL: http:// www .iso .org/ iso/ foreword .html
ISO 12631 was prepared by the European Committee for Standardization (CEN) Technical Committee
CEN/TC 89, Thermal performance of buildings and building components, in collaboration with ISO
Technical Committee ISO/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).
This second edition cancels and replaces the first edition (ISO 12631:2012) which has been technically
revised. The necessary editorial revisions were made to comply with the requirements for the EPB set
of standards.
In addition, the following clauses and subclauses of the previous version have been technically revised:
— Annex G and Annex H were deleted and moved to the technical report;
— Tabulated values in Annex D were checked and revised where necessary.
ISO 12631:2017(E)
Introduction
This document is part of a series of standards aiming at international harmonization of the methodology
for the assessment of the energy performance of buildings, called “set of EPB standards”.
All EPB standards follow specific rules to ensure overall consistency, unambiguity and transparency.
All EPB standards provide a certain flexibility with regard to the methods, the required input data and
references to other EPB standards, by the introduction of a normative template in Annex A and Annex B
with informative default choices.
For the correct use of this document a normative template is given in Annex A to specify these choices.
Informative default choices are provided in Annex B.
The main target groups of this document are manufacturers of curtain wallings.
Use by or for regulators: In case the document is used in the context of national or regional legal
requirements, mandatory choices may be given at national or regional level for such specific
applications. These choices (either the informative default choices from Annex B or choices adapted to
national / regional needs, but in any case following the template of this Annex A) can be made available
as national annex or as separate (e.g. legal) document (national data sheet).
NOTE 1 So in this case:
— the regulators will specify the choices;
— the individual user will apply the document to assess the energy performance of a building, and thereby use
the choices made by the regulators.
Topics addressed in this document can be subject to public regulation. Public regulation on the same
topics can override the default values in Annex B of this document. Public regulation on the same topics
can even, for certain applications, override the use of this document. Legal requirements and choices
are in general not published in standards but in legal documents. In order to avoid double publications
and difficult updating of double documents, a national annex may refer to the legal texts where national
choices have been made by public authorities. Different national annexes or national data sheets are
possible, for different applications.
It is expected, if the default values, choices and references to other EPB standards in Annex B are not
followed due to national regulations, policy or traditions, that
— national or regional authorities prepare data sheets containing the choices and national or regional
values, according to the model in Annex A. In this case a national annex (e.g. NA) is recommended,
containing a reference to these data sheets;
— or, by default, the national standards body will consider the possibility to add or include a national
annex in agreement with the template of Annex A, in accordance to the legal documents that give
national or regional values and choices.
Further target groups are parties wanting to motivate their assumptions by classifying the building
energy performance for a dedicated building stock.
More information is provided in the Technical Report accompanying this document (ISO/TR 52022-2).
The design and construction of curtain wall systems is complex. This document 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 document, can be
used for comparison of the thermal transmittance of different types of curtain wall or as part of the
vi © ISO 2017 – All rights reserved

ISO 12631:2017(E)
input data for calculating the heat used in a building. This document 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 document:
— single assessment method (see Clause 7);
— component assessment method (see Clause 8).
Guidance on the use of these two methods is given in Clause 6. Calculation examples for these two
methods are given in ISO/TR 52022-2.
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.
The thermal transmittance of the frame, U , is defined according to ISO 10077-2 or EN 12412-2 together
f
with Annex D. The thermal transmittance of glazing units, U , is defined according to ISO 10291,
g
1)
ISO 10292, ISO 10293 (or see Subjects 1, 2 and 3 in Table C.1 ) which do not include the edge effects. The
thermal interaction of the frame and the filling element is included in the linear thermal transmittance
Ψ which is derived using the procedures specified in ISO 10077-2.
Table 1 shows the relative position of this document within the set of EPB standards in the context of
the modular structure as set out in ISO 52000-1.
NOTE 2 In ISO/TR 52000-2, the same table can be found, with, for each module, the numbers of the relevant
EPB standards and accompanying technical reports that are published or in preparation.
NOTE 3 The modules represent EPB standards, although one EPB standard could cover more than one module
and one module could be covered by more than one EPB standard, for instance, a simplified and a detailed method
respectively.
1) See Table C.1 for alternative references in line with ISO Global Relevance Policy.
ISO 12631:2017(E)
Table 1 — Position of this document (in casu M2–5), within the modular structure of the set of
EPB standards
Building
Overarching Technical Building Systems
(as such)
Hu- De- Do- Build-
De- Ven- mid- hu- mes- ing
Descrip- Heat- Cool- Light- PV,
Submodule Descriptions scrip- tila- ifi mid- tic automa-
tions ing ing ing wind
tions tion cati- ifica- Hot tion and
on tion water control
sub1 M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11
1 General General General
Common terms
Building
and definitions;
a
2 energy Needs
symbols, units
needs
and subscripts
(Free) Maxi-
indoor mum
3 Applications conditions load
without and
systems power
Ways to Ways to
Ways to express express express
4 energy perfor- energy energy
mance perfor- perfor-
mance mance
Building Heat Emis-
categories and transfer sion
5 ISO 12631
building bound- by trans- and
aries mission control
Heat
Building transfer Distri-
occupancy by infil- bution
and operating tration and
conditions and venti- control
lation
Aggregation of
Storage
energy services Internal
7 and
and energy heat gains
control
carriers
a
The shaded modules are not applicable.
viii © ISO 2017 – All rights reserved

ISO 12631:2017(E)
Table 1 (continued)
Building
Overarching Technical Building Systems
(as such)
Hu- De- Do- Build-
De- Ven- mid- hu- mes- ing
Descrip- Heat- Cool- Light- PV,
Submodule Descriptions scrip- tila- ifi mid- tic automa-
tions ing ing ing wind
tions tion cati- ifica- Hot tion and
on tion water control
sub1 M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11
Genera-
Solar heat
8 Building zoning tion and
gains
control
Load
dis-
Building patch-
Calculated
dynamics ing and
9 energy perfor-
(thermal oper-
mance
mass) ating
condi-
tions
Meas-
Measured
ured
Measured ener- energy
10 energy
gy performance perfor-
perfor-
mance
mance
Inspec-
11 Inspection Inspection
tion
Ways to express
12 BMS
indoor comfort
External
13 environment
conditions
Economic cal-
culation
a
The shaded modules are not applicable.
INTERNATIONAL STANDARD ISO 12631:2017(E)
Thermal performance of curtain walling — Calculation of
thermal transmittance
1 Scope
This document 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.
NOTE Table 1 in the Introduction shows the relative position of this document within the set of EPB
standards in the context of the modular structure as set out in ISO 52000-1.
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 6946, Building components and building elements — Thermal resistance and thermal transmittance —
Calculation method
ISO 7345, Thermal insulation — Physical quantities and definitions
ISO 9488, Solar energy — Vocabulary
ISO 10077-1, Thermal performance of windows, doors and shutters — Calculation of thermal transmittance
— Part 1: General
ISO 10077-2, Thermal performance of windows, doors and shutters — Calculation of thermal transmittance
— Part 2: Numerical method for frames
ISO 12631:2017(E)
ISO 10211, Thermal bridges in building construction — Heat flows and surface temperatures — Detailed
calculations
ISO 10291, Glass in building — Determination of steady-state U values (thermal transmittance) of multiple
glazing — Guarded hot plate method
ISO 10292, Glass in building — Calculation of steady-state U values (thermal transmittance) of multiple
glazing
ISO 10293, Glass in building — Determination of steady-state U values (thermal transmittance) of multiple
glazing — Heat flow meter method
ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values and
procedures for determining declared and design thermal values
ISO 12567-1, Thermal performance of windows and doors — Determination of thermal transmittance by
the hot-box method — Part 1: Complete windows and doors
ISO 52000-1:2017, Energy performance of buildings — Overarching EPB assessment —– Part 1: General
framework and procedures
EN 673, Glass in building — Determination of thermal transmittance (U value) — Calculation method
EN 674, Glass in building — Determination of thermal transmittance (U value) — Guarded hot plate method
EN 675, Glass in building — Determination of thermal transmittance (U value) — Heat flow meter method
EN 12412-2, Thermal performance of windows, doors and shutters — Determination of thermal
transmittance by hot-box method — Part 2: Frames
NOTE Default references to EPB standards other than ISO 52000-1 are identified by the EPB module code
number and given in Annex A (normative template in Table A.1) and Annex B (informative default choice in
Table B.1).
EXAMPLE EPB module code number: M5–5, or M5–5.1 (if module M5–5 is subdivided), or M5–5/1 (if
reference to a specific clause of the standard covering M5–5).
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6946, ISO 7345, ISO 9488,
ISO 52000-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
NOTE Clause 4 includes descriptions of a number of geometrical characteristics of glazing units, frame
sections and panels.
3.1
EPB standard
[3]
standard that complies with the requirements given in ISO 52000-1, CEN/TS 16628 and
[4]
CEN/TS 16629
Note 1 to entry: These three basic EPB documents were developed under a mandate given to CEN by the European
Commission and the European Free Trade Association (Mandate M/480), and support essential requirements of
EU Directive 2010/31/EU on the energy performance of buildings (EPBD). Several EPB standards and related
documents are developed or revised under the same mandate.
[SOURCE: ISO 52000-1:2017, definition 3.5.14]
2 © ISO 2017 – All rights reserved

ISO 12631:2017(E)
4 Symbols and subscripts
4.1 Symbols
For the purposes of this document, the symbols given in ISO 52000-1 and the following apply.
Symbol Quantity Unit
A area m
* 2
A Area as specified in figure 8 m
T thermodynamic temperature K
U thermal transmittance W/(m ·K)
l length m
d depth m
Φ heat flow rate W
Ψ linear thermal transmittance W/(m·K)
Δ difference
Σ summation
ε emissivity
4.2 Subscripts
For the purposes of this document, the subscripts given in ISO 52000-1 and the following apply.
cw curtain walling m,f mullion/frame
d developed m,g mullion/glazing
e external n normal
eq equivalent p panel (opaque)
f frame s screw
f,g frame/glazing t transom
FE filling element t,f transom/frame
g glazing t,g transom/glazing
i internal tot total
thermal joint at a connection be-
j joint TJ tween two filling elements
m mullion W window
4.3 Superscripts
Definition of areas for length-related treatment of thermal joints (see 7.3.1.2).
5 Description of the methods
5.1 Output of the method
The output of this document is the thermal transmittance of a curtain wall consisting of glazed and/or
opaque panels fitted in a frame.
ISO 12631:2017(E)
5.2 General description
In general, the thermal transmittance or U-value of the curtain walling is calculated as a function of
the thermal transmittance of the components and their geometrical characteristics, plus the thermal
interactions between the components.
— The calculation procedures depend on the composition of the product or assembly;
— Components may include (where appropriate): glazings, opaque panels, frames, mullions, transoms;
— Thermal interactions are lateral heat flow (linear thermal bridge effect) between adjacent
components and surface and cavity thermal resistances (thermal radiation and convection);
— The geometrical characteristics concern the sizes and positions of the components.
Throughout this document, where indicated in the text, Table C.1 shall be used to identify alternative
regional references in line with ISO Global Relevance Policy.
5.3 Geometrical characteristics
5.3.1 Main principles
The main principles of curtain walling are shown in Figures 1 and 2.
4 © ISO 2017 – All rights reserved

ISO 12631:2017(E)
Key
1 structure fixing bracket
Figure 1 — Principle of curtain walling construction: unitised construction
ISO 12631:2017(E)
Key
1 structure fixing bracket
Figure 2 — Principle of curtain walling construction: stick construction
5.3.2 Internal depth
The internal depth is defined as shown in Figure 3.
6 © ISO 2017 – All rights reserved

ISO 12631:2017(E)
Key
1 internal
2 external
d internal depth of mullion or transom
i
Figure 3 — Internal and external developed area, internal depth
5.3.3 Boundaries of curtain wall structures
5.3.3.1 General
To evaluate the thermal transmittance of façades, representative reference areas should be defined.
The following subclauses define the various areas.
5.3.3.2 Boundaries of a representative reference element
The boundaries of the representative reference element shall be chosen according to the principles
shown in Figure 4.
ISO 12631:2017(E)
Key
1 boundaries of the representative element
Figure 4 — Boundaries of a representative reference element of a façade
8 © ISO 2017 – All rights reserved

ISO 12631:2017(E)
5.3.3.3 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
ISO 12631:2017(E)
5.3.4 Cut-off planes and partitioning of thermal zones
5.3.4.1 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 Clause 7);
— the component assessment method (see Clause 8).
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.
5.3.4.2 Rules for thermal modelling
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 Methodologies for the calculation of curtain wall transmittance
Two methods of calculating the thermal transmittance of curtain wall systems are specified (see
Table 2):
— the single assessment method, and
— the component assessment method.
The single assessment method (see Clause 7) 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 boundaries) 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
10 © ISO 2017 – All rights reserved

ISO 12631:2017(E)
be used for any curtain walling system (i.e. unitised systems, stick systems, patent glazing, structural
sealant glazing, rain screens, structural glazing).
The component assessment method (see Clause 8) 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 (Ψ-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 document, 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.
Key
adiabatic boundary
Figure 6 — Thermal section representing the full curtain wall
Table 2 — Summary of the two different methodologies for determining U of curtain walling
cw
Single assessment method Component assessment method
Frames (joints) Frames
Definition and evaluation of areas according to 5.3 Definition and evaluation of areas according to 5.3
Evaluation of U or Ψ according to 7.3.2.2 Evaluation of U , U und U values according to
TJ TJ f m t
ISO 10077-2 (see 6.3.4) or EN 12412–2
Evaluation of Ψ / Ψ according to Table D.6
m,f t,f
of this document or ISO 10077-2
Glazing Glazing
ISO 12631:2017(E)
Table 2 (continued)
Single assessment method Component assessment method
Definition and evaluation of areas according to this Definition and evaluation of areas according to this
document document
Evaluation of U according to ISO 10077-1 or ISO 10291 Evaluation of U according to ISO 10077-1 or ISO 10291
g g
for measured value (GHP), ISO 10292 for calculation for measured value (GHP), ISO 10292 for measured
value, ISO 10293 for measured value (HFM), (or see value (HFM), (or see Subjects 1, 2 and 3 in Table C.1)
Subjects 1, 2 and 3 in Table C.1)
Evaluation of Ψ Ψ and Ψ according to Table D.1,
t,g m,g f,g
D.2, D.3 or D.4 of this document or ISO 10077-2
Panels Panels
Definition and evaluation of areas according to 5.3 Definition and evaluation of areas according to 4.2
Evaluation of U according to ISO 6946 Evaluation of U according to ISO 6946
p p
Evaluation of Ψ -values according to Table D.5 or
p
ISO 10077-2
Complete elements Complete elements
Calculation of complete elements according to For- Calculation of complete elements according to For-
mula (5) or (6) mula (12)
Complete curtain walling Complete curtain walling
Calculation of a façade built of different elements Calculation of a façade built of different elements
according to Formula (7) according to Formula (14)
7 Single assessment method
7.1 Output data
The output of the single assessment method according to this document is the transmission heat
transfer coefficient of a curtain walling shown in Table 3.
Table 3 — Output data
Description Symbol Unit Destination module Validity interval Varying
Thermal
...