EN ISO 6946:2007

SLOVENSKI STANDARD
01-junij-2008
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SIST EN ISO 6946:1997
SIST EN ISO 6946:1997/A1:2003
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Building components and building elements - Thermal resistance and thermal
transmittance - Calculation method (ISO 6946:2007)
Bauteile - Wärmedurchlasswiderstand und Wärmedurchgangskoeffizient -
Berechnungsverfahren (ISO 6946:2007)
Composants et parois de bâtiments - Résistance thermique et coefficient de
transmission thermique - Méthode de calcul (ISO 6946:2007)
Ta slovenski standard je istoveten z: EN ISO 6946:2007
ICS:
91.060.01 Stavbni elementi na splošno Elements of buildings in
general
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 6946
NORME EUROPÉENNE
EUROPÄISCHE NORM
December 2007
ICS 91.060.01; 91.120.10 Supersedes EN ISO 6946:1996
English Version
Building components and building elements - Thermal
resistance and thermal transmittance - Calculation method (ISO
6946:2007)
Composants et parois de bâtiments - Résistance thermique Bauteile - Wärmedurchlasswiderstand und
et coefficient de transmission thermique - Méthode de Wärmedurchgangskoeffizient - Berechnungsverfahren (ISO
calcul (ISO 6946:2007) 6946:2007)
This European Standard was approved by CEN on 7 December 2007.
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 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 Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 6946:2007: E
worldwide for CEN national Members.

Contents Page
Foreword.3

Foreword
This document (EN ISO 6946:2007) has been prepared by Technical Committee ISO/TC 163 "Thermal
performance and energy use in the built environment" in collaboration with Technical Committee CEN/TC 89
"Thermal performance of buildings and building components", the secretariat of which is held by SIS.
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 June 2008, and conflicting national standards shall be withdrawn at
the latest by June 2008.
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 ISO 6946:1996.
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, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.
Endorsement notice
The text of ISO 6946:2007 has been approved by CEN as a EN ISO 6946:2007 without any modification.

INTERNATIONAL ISO
STANDARD 6946
Second edition
2007-12-15
Building components and building
elements — Thermal resistance and
thermal transmittance — Calculation
method
Composants et parois de bâtiments — Résistance thermique et
coefficient de transmission thermique — Méthode de calcul

Reference number
ISO 6946:2007(E)
©
ISO 2007
ISO 6946:2007(E)
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ii © ISO 2007 – All rights reserved

ISO 6946:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and units . 1
3.1 Terms and definitions. 1
3.2 Symbols and units. 2
4 Principles. 2
5 Thermal resistances . 3
5.1 Thermal resistance of homogeneous layers . 3
5.2 Surface resistances. 3
5.3 Thermal resistance of air layers. 4
5.4 Thermal resistance of unheated spaces . 6
6 Total thermal resistance . 7
6.1 Total thermal resistance of a building component consisting of homogeneous layers. 7
6.2 Total thermal resistance of a building component consisting of homogeneous and
inhomogeneous layers. 7
7 Thermal transmittance . 11
Annex A (normative) Surface resistance. 12
Annex B (normative) Thermal resistance of airspaces . 15
Annex C (normative) Calculation of the thermal transmittance of components with tapered layers . 18
Annex D (normative) Corrections to thermal transmittance. 22
Bibliography . 28

ISO 6946:2007(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 6946 was prepared by Technical Committee ISO/TC 163, Thermal performance and energy use in the
built environment, Subcommittee SC 2, Calculation methods.
This second edition cancels and replaces the first edition (ISO 6946:1996), which has been technically revised.
It also incorporates the Amendment ISO 6946:1996/Amd.1:2003.
The following changes have been made to the first edition:
⎯ information on the calculation of heat flow rates has been transferred from the Introduction to the note in
Clause 4;
⎯ 5.3.3 provides an amended basis for slightly ventilated air layers;
⎯ 5.4.2 provides clarification of the applicability of Table 3;
⎯ 5.4.3 has been completely revised;
⎯ 6.2.1 provides a new text to allow calculation of a component that is part of a complete element; it also
clarifies exceptions and the limit of applicability;
⎯ Annex B provides additional data for other temperature differences across cavities; it also provides a
correction to the formula for radiation transfer in divided airspaces;
⎯ Annex C contains an additional shape;
⎯ D.2 has been completely rewritten to clarify the intentions, the former Annex E having been deleted
(national annexes can be attached to this International Standard giving examples in accordance with local
building traditions);
⎯ D.3 provides a revised procedure for mechanical fasteners, including recessed fasteners;
⎯ D.4 does not apply in cooling situations.
iv © ISO 2007 – All rights reserved

ISO 6946:2007(E)
Introduction
This International Standard provides the means (in part) to assess the contribution that building products and
services make to energy conservation and to the overall energy performance of buildings.
INTERNATIONAL STANDARD ISO 6946:2007(E)

Building components and building elements — Thermal
resistance and thermal transmittance — Calculation method
1 Scope
This International Standard provides the method of calculation of the thermal resistance and thermal
transmittance of building components and building elements, excluding doors, windows and other glazed units,
curtain walling, components which involve heat transfer to the ground, and components through which air is
designed to permeate.
The calculation method is based on the appropriate design thermal conductivities or design thermal
resistances of the materials and products for the application concerned.
The method applies to components and elements consisting of thermally homogeneous layers (which can
include air layers).
This International Standard also provides an approximate method that can be used for elements containing
inhomogeneous layers, including the effect of metal fasteners, by means of a correction term given in
Annex D. Other cases where insulation is bridged by metal are outside the scope of this International
Standard.
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 7345, Thermal insulation — Physical quantities and definitions
ISO 10456, Building materials and products — Hygrothermal properties — Tabulated design values and
procedures for determining declared and design thermal values
ISO 13789, Thermal performance of buildings — Transmission and ventilation heat transfer coefficients —
Calculation method
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 and ISO 10456 and the
following apply.
3.1.1
building element
major part of a building such as a wall, floor or roof
ISO 6946:2007(E)
3.1.2
building component
building element or a part of it
NOTE In this International Standard, the word “component” is used to indicate both element and component.
3.1.3
thermally homogeneous layer
layer of constant thickness having thermal properties which may be regarded as being uniform
3.2 Symbols and units
Symbol Quantity Unit
A area m
d thickness m
h surface heat transfer coefficient W/(m ⋅K)
R design thermal resistance (surface to surface) m ⋅K/W
R thermal resistance of airspace m ⋅K/W
g
R external surface resistance m ⋅K/W
se
R internal surface resistance m ⋅K/W
si
R total thermal resistance (environment to environment) m ⋅K/W
T
′
R upper limit of total thermal resistance m ⋅K/W
T
′′
R lower limit of total thermal resistance m ⋅K/W
T
R thermal resistance of unheated space m ⋅K/W
u
U thermal transmittance W/(m ⋅K)
λ design thermal conductivity W/(m⋅K)
4 Principles
The principle of the calculation method is as follows:
⎯ to obtain the thermal resistance of each thermally homogeneous part of the component;
⎯ to combine these individual resistances so as to obtain the total thermal resistance of the component,
including (where appropriate) the effect of surface resistances.
Thermal resistances of individual parts are obtained in accordance with 5.1.
The values of surface resistance given in 5.2 are appropriate in most cases. Annex A gives detailed
procedures for low emissivity surfaces, specific external wind speeds and non-planar surfaces.
Air layers may be regarded as thermally homogeneous for the purposes of this International Standard. Values
of the thermal resistance of large air layers with high emissivity surfaces are given in 5.3. Annex B provides
procedures for other cases.
The resistances of the layers are combined as follows:
a) for components consisting of thermally homogeneous layers, obtain the total thermal resistance in
accordance with 6.1 and the thermal transmittance in accordance with Clause 7;
2 © ISO 2007 – All rights reserved

ISO 6946:2007(E)
b) for components having one or more thermally inhomogeneous layers, obtain the total thermal resistance
in accordance with 6.2 and the thermal transmittance in accordance with Clause 7;
c) for components containing a tapered layer, obtain the thermal transmittance and/or the total thermal
resistance in accordance with Annex C.
Finally, corrections are applied to the thermal transmittance, if appropriate, in accordance with Annex D, in
order to allow for the effects of air voids in insulation, mechanical fasteners penetrating an insulation layer and
precipitation on inverted roofs.
The thermal transmittance calculated in this way applies between the environments on either side of the
component concerned, e.g. internal and external environments, two internal environments in the case of an
internal partition, an internal environment and an unheated space. Simplified procedures are given in 5.4 for
treating an unheated space as a thermal resistance.
NOTE Calculation of heat flow rates are commonly undertaken using operative temperature (usually approximated to
the arithmetic mean of air temperature and mean radiant temperature) to represent the environment inside buildings, and
air temperature to represent the external environment. Other definitions of the temperature of an environment are also
used when appropriate to the purpose of the calculation. See also Annex A.
5 Thermal resistances
5.1 Thermal resistance of homogeneous layers
Design thermal values can be given as either design thermal conductivity or design thermal resistance. If
thermal conductivity is given, obtain the thermal resistance of the layer from
d
R = (1)
λ
where
d is the thickness of the material layer in the component;
λ is the design thermal conductivity of the material, either calculated in accordance with ISO 10456 or
obtained from tabulated values.
NOTE The thickness, d, can be different from the nominal thickness (e.g. when a compressible product is installed in
a compressed state, d is less than the nominal thickness). If relevant, it is advisable that d also make appropriate
allowance for thickness tolerances (e.g. when they are negative).
Thermal resistance values used in intermediate calculations shall be calculated to at least three decimal
places.
5.2 Surface resistances
Use the values in Table 1 for plane surfaces in the absence of specific information on the boundary conditions.
The values under “horizontal” apply to heat flow directions ± 30° from the horizontal plane. For non-planar
surfaces or for specific boundary conditions, use the procedures in Annex A.
ISO 6946:2007(E)
Table 1 — Conventional surface resistances
Surface resistance Direction of heat flow
m ⋅K/W Upwards Horizontal Downwards
R 0,10 0,13 0,17
si
R 0,04 0,04 0,04
se
NOTE 1 The values given are design values. For the purposes of declaration of the
thermal transmittance of components and other cases where values independent of
heat flow direction are required, or when the heat flow direction is liable to vary, it is
advisable that the values for horizontal heat flow be used.
NOTE 2 The surface resistances apply to surfaces in contact with air. No surface
resistance applies to surfaces in contact with another material.

5.3 Thermal resistance of air layers
5.3.1 Applicability
The values given in 5.3.1 to 5.3.3 apply to an air layer which
⎯ is bounded by two faces that are effectively parallel and perpendicular to the direction of heat flow and
that have emissivities not less than 0,8,
⎯ has a thickness (in the direction of heat flow) of less than 0,1 times each one of the other two dimensions,
and not greater than 0,3 m,
⎯ has no air interchange with the internal environment.
If the above conditions do not apply, use the procedures in Annex B.
NOTE Most building materials have an emissivity greater than 0,8.
A single thermal transmittance should not be calculated for components containing air layers thicker than
0,3 m. Instead, heat flows should be calculated by performing a heat balance (see ISO 13789).
5.3.2 Unventilated air layer
An unventilated air layer is one in which there is no express provision for air flow through it. Design values of
thermal resistance are given in Table 2. The values under “horizontal” apply to heat flow directions ± 30° from
the horizontal plane.
An air layer having no insulation between it and the external environment, but with small openings to the
external environment, shall also be considered as an unventilated air layer if these openings are not arranged
so as to permit air flow through the layer and they do not exceed
⎯ 500 mm per metre of length (in the horizontal direction) for vertical air layers,
⎯ 500 mm per square metre of surface area for horizontal air layers.
NOTE Drain openings (weep holes) in the form of open vertical joints in the outer leaf of a masonry cavity wall
usually conform with the above criteria and so are not regarded as ventilation openings.
4 © ISO 2007 – All rights reserved

ISO 6946:2007(E)
Table 2 — Thermal resistance of unventilated air layers with high emissivity surfaces
Thermal resistance
Thickness 2
m ⋅K/W
of air layer
Direction of heat flow
mm Upwards Horizontal Downwards
0 0,00 0,00 0,00
5 0,11 0,11 0,11
7 0,13 0,13 0,13
10 0,15 0,15 0,15
15 0,16 0,17 0,17
25 0,16 0,18 0,19
50 0,16 0,18 0,21
100 0,16 0,18 0,22
300 0,16 0,18 0,23
NOTE Intermediate values may be obtained by linear interpolation.

5.3.3 Slightly ventilated air layer
A slightly ventilated air layer is one in which there is provision for limited air flow through it from the external
environment by openings of area, A , within the following ranges:
v
2 2
⎯ > 500 mm but < 1 500 mm per metre of length (in the horizontal direction) for vertical air layers;
2 2
⎯ > 500 mm but < 1 500 mm per square metre of surface area for horizontal air layers.
The effect of ventilation depends on the size and distribution of the ventilation openings. As an approximation,
the total thermal resistance of a component with a slightly ventilated air layer may be calculated as
1500−−AA 500
vv
RR=+ R (2)
TT,u T,v
1000 1000
where
R is the total thermal resistance with an unventilated air layer in accordance with 5.3.2;
T,u
R is the total thermal resistance with a well-ventilated air layer in accordance with 5.3.4.
T,v
5.3.4 Well-ventilated air layer
A well-ventilated air layer is one for which the openings between the air layer and the external environment
are equal to or exceed
⎯ 1 500 mm per metre of length (in the horizontal direction) for vertical air layers,
⎯ 1 500 mm per square of metre of surface area for horizontal air layers.
The total thermal resistance of a building component containing a well-ventilated air layer shall be obtained by
disregarding the thermal resistance of the air layer and all other layers between the air layer and external
environment, and including an external surface resistance corresponding to still air (see Annex A).
Alternatively, the corresponding value of R from Table 1 may be used.
si
ISO 6946:2007(E)
5.4 Thermal resistance of unheated spaces
5.4.1 General
When the external envelope of the unheated space is not insulated, the simplified procedures in 5.4.2 and
5.4.3, treating the unheated space as a thermal resistance, may be applied.
NOTE 1 ISO 13789 gives general and more precise procedures for the calculation of heat transfer from a building to
the external environment via unheated spaces, which it is advisable to use when a more accurate result is required. For
crawl spaces below suspended floors, see ISO 13370.
NOTE 2 The thermal resistances given in 5.4.2 and 5.4.3 are suitable for heat flow calculations, but not for calculations
concerned with the hygrothermal conditions in the unheated space.
5.4.2 Roof spaces
For a roof structure consisting of a flat, insulated ceiling and a pitched roof, the roof space may be regarded
as if it were a thermally homogeneous layer with thermal resistance as given in Table 3.
Table 3 — Thermal resistance of roof spaces
R
u
Characteristics of roof
m ⋅K/W
1 Tiled roof with no felt, boards or similar 0,06
2 Sheeted roof, or tiled roof with felt or boards or similar 0,2
under the tiles
3 As 2 (above) but with aluminium cladding or other low 0,3
emissivity surface at underside of roof
4 Roof lined with boards and felt 0,3
NOTE The values in this table include the thermal resistance of the ventilated space and the thermal
resistance of the (pitched) roof construction. They do not include the external surface resistance, R .
se
The data in Table 3 apply to naturally ventilated roof spaces above heated buildings. If mechanically ventilated,
use the detailed procedure in ISO 13789, treating the roof space as an unheated space with a specified
ventilation rate.
5.4.3 Other spaces
When a building has an unheated space adjacent to it, the thermal transmittance between the internal and
external environments can be obtained by treating the unheated space together with its external construction
components as if it were an additional homogeneous layer with thermal resistance, R . When all elements
u
between the internal environment and the unheated space have the same thermal transmittance, R is given
u
by
A
i
R = (3)
u
()AUn+×0,33V
∑ e,kke,
k
where
A is the total area of all elements between the internal environment and the unheated space, in m ;
i
A is the area of element k between the unheated space and the external environment, in m ;
e,k
6 © ISO 2007 – All rights reserved

ISO 6946:2007(E)
U is the thermal transmittance of element k between the unheated space and the external
e,k
environment, in W/(m ⋅K);
n is the ventilation rate of the unheated space, in air changes per hour;
V is the volume of the unheated space, in m ;
and the summation is done over all elements between the unheated space and the external environment,
except for any ground floor.
Where the details of the construction of the external elements of the unheated space are not known, the
values U = 2 W/(m ⋅K) and n = 3 air changes per hour are recommended.
e,k
NOTE 1 Examples of unheated spaces include garages, store rooms and conservatories.
NOTE 2 If there is more than one component between the internal environment and the unheated space, R is included
u
in the calculation of the thermal transmittance of each such component.
NOTE 3 Equation (3) is based on the procedure in ISO 13789 for the calculation of heat transfer through unheated
spaces.
6 Total thermal resistance
6.1 Total thermal resistance of a building component consisting of homogeneous layers
The total thermal resistance, R , of a plane building component consisting of thermally homogeneous layers
T
perpendicular to the heat flow shall be calculated by the following expression:
R = R + R + R + . R + R (4)
T si 1 2 n se
where
R is the internal surface resistance;
si
R , R . R are the design thermal resistances of each layer;
1 2 n
R is the external surface resistance.
se
When calculating the resistance of internal building components (partitions, etc.), or a component between the
internal environment and an unheated space, R applies on both sides.
si
If the total thermal resistance is presented as a final result, it shall be rounded to two decimal places.
NOTE The surface resistances are omitted in Equation (4) when the resistance of a component from surface to
surface is required.
6.2 Total thermal resistance of a building component consisting of homogeneous and
inhomogeneous layers
6.2.1 Applicability
6.2.2 to 6.2.5 provide a simplified method for calculating the thermal resistance of building components
consisting of thermally homogeneous and inhomogeneous layers. The method is not valid for cases where the
ratio of the upper limit of thermal resistance to the lower limit of thermal resistance exceeds 1,5. The method
is not applicable to cases where insulation is bridged by metal. For metal fasteners, the method can be used
as if there were no metal fasteners and the result corrected in accordance with D.3.
ISO 6946:2007(E)
NOTE 1 A more precise result is obtained by using a numerical method conforming to ISO 10211. This can be
particularly relevant where there is a significant difference between the thermal conductivity of materials in the layer
providing the predominant thermal resistance of the construction.
NOTE 2 The method described in 6.2.2 to 6.2.5 is not suitable for computing surface temperatures for the purposes of
evaluating the risk of condensation.
If part of a building element is to be assessed separately from the complete structure, its thermal resistance
shall be obtained using the method in 6.2.2 to 6.2.5, but with a surface resistance equal to zero on both sides
of it. This thermal resistance can then be used in a subsequent calculation to obtain the thermal transmittance
of the complete element.
NOTE 3 This is relevant when part of an element is sold as a separate item. Examples could include structural panels
and voided masonry units.
6.2.2 Total thermal resistance of a
...