SIST EN ISO 13788:2013

SLOVENSKI STANDARD
SIST EN ISO 13788:2013
01-maj-2013
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SRYUãLQVNDWHPSHUDWXUD]DSUHSUHþHYDQMHNULWLþQHYODJHQDSRYUãLQLNRQVWUXNFLMHLQ
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Hygrothermal performance of building components and building elements - Internal
surface temperature to avoid critical surface humidity and interstitial condensation -
Calculation methods (ISO 13788:2012)
Wärme- und feuchtetechnisches Verhalten von Bauteilen und Bauelementen -
Raumseitige Oberflächentemperatur zur Vermeidung kritischer Oberflächenfeuchte und
Tauwasserbildung im Bauteilinneren - Berechnungsverfahren (ISO 13788:2012)
Performance hygrothermique des composants et parois de bâtiments - Température
superficielle intérieure permettant d'éviter l'humidité superficielle critique et la
condensation dans la masse - Méthodes de calcul (ISO 13788:2012)
Ta slovenski standard je istoveten z: EN ISO 13788:2012
ICS:
91.120.10 Toplotna izolacija stavb Thermal insulation
91.120.30 =DãþLWDSUHGYODJR Waterproofing
SIST EN ISO 13788:2013 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 13788:2013

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SIST EN ISO 13788:2013


EUROPEAN STANDARD
EN ISO 13788

NORME EUROPÉENNE

EUROPÄISCHE NORM
December 2012
ICS 91.060.01; 91.120.10 Supersedes EN ISO 13788:2001
English Version
Hygrothermal performance of building components and building
elements - Internal surface temperature to avoid critical surface
humidity and interstitial condensation - Calculation methods
(ISO 13788:2012)
Performance hygrothermique des composants et parois de Wärme- und feuchtetechnisches Verhalten von Bauteilen
bâtiments - Température superficielle intérieure permettant und Bauelementen - Raumseitige Oberflächentemperatur
d'éviter l'humidité superficielle critique et la condensation zur Vermeidung kritischer Oberflächenfeuchte und
dans la masse - Méthodes de calcul (ISO 13788:2012) Tauwasserbildung im Bauteilinneren -
Berechnungsverfahren (ISO 13788:2012)
This European Standard was approved by CEN on 28 December 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 13788:2012: E
worldwide for CEN national Members.

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SIST EN ISO 13788:2013
EN ISO 13788:2012 (E)
Contents Page
Foreword . 3

2

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SIST EN ISO 13788:2013
EN ISO 13788:2012 (E)
Foreword
This document (EN ISO 13788:2012) 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 2013, and conflicting national standards shall be withdrawn at
the latest by June 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 ISO 13788:2001.
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.
Endorsement notice
The text of ISO 13788:2012 has been approved by CEN as a EN ISO 13788:2012 without any modification.

3

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SIST EN ISO 13788:2013

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SIST EN ISO 13788:2013
INTERNATIONAL ISO
STANDARD 13788
Second edition
2012-12-15
Hygrothermal performance of
building components and building
elements — Internal surface
temperature to avoid critical
surface humidity and interstitial
condensation — Calculation methods
Performance hygrothermique des composants et parois de
bâtiments — Température superficielle intérieure permettant d’éviter
l’humidité superficielle critique et la condensation dans la masse —
Méthodes de calcul
Reference number
ISO 13788:2012(E)
©
ISO 2012

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SIST EN ISO 13788:2013
ISO 13788:2012(E)

COPYRIGHT PROTECTED DOCUMENT
© 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
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – All rights reserved

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SIST EN ISO 13788:2013
ISO 13788:2012(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions, symbols and units . 1
3.1 Terms and definitions . 1
3.2 Symbols and units . 3
3.3 Subscripts . 4
4 Input data for the calculations . 4
4.1 Material and product properties . 4
4.2 External boundary conditions . 4
4.3 Internal boundary conditions . 6
4.4 Surface resistances . 6
5 Calculation of surface temperature to avoid critical surface humidity .7
5.1 General . 7
5.2 Determining parameters . 7
5.3 Design for avoidance of mould growth, corrosion or other moisture damage. 7
5.4 Design for the limitation of surface condensation on low thermal inertia elements . 8
6 Calculation of interstitial condensation . 9
6.1 General . 9
6.2 Principle . 9
6.3 Limitation of sources of error .10
6.4 Calculation .10
6.5 Criteria used to assess building components .16
7 Calculation of drying of building components .16
7.1 General .16
7.2 Principle .17
7.3 Specification of the method .17
7.4 Criteria used to assess drying potential of building components .17
Annex A (informative) Internal boundary conditions .18
Annex B (informative) Examples of calculation of the temperature factor at the internal surface to
avoid critical surface humidity .20
Annex C (informative) Examples of calculation of interstitial condensation .24
Annex D (informative) Example of the calculation of the drying of a wetted layer .34
Annex E (informative) Relationships governing moisture transfer and water vapour pressure .37
Bibliography .40
© ISO 2012 – All rights reserved iii

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SIST EN ISO 13788:2013
ISO 13788:2012(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 13788 was prepared by Technical Committee ISO/TC 163, Thermal performance and energy use in
the built environment, Subcommittee SC 2, Calculation methods in cooperation with CEN/TC 89, Thermal
performance of buildings and building components.
This second edition cancels and replaces the first edition (ISO 13788:2001), which has been
technically revised.
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SIST EN ISO 13788:2013
ISO 13788:2012(E)

Introduction
Moisture transfer is a very complex process and the knowledge of moisture transfer mechanisms, material
properties, initial conditions and boundary conditions is often limited. Therefore this International
Standard lays down simplified calculation methods, which assume that moisture transport is by vapour
diffusion alone and use monthly climate data. The standardization of these calculation methods does
not exclude use of more advanced methods. If other sources of moisture, such as rain penetration or
convection, are negligible, the calculations will normally lead to designs well on the safe side and if a
construction fails a specified design criterion according to this procedure, more accurate methods may
be used to show that the design will pass.
This International Standard deals with:
a) the critical surface humidity likely to lead to problems such as mould growth on the internal surfaces
of buildings,
b) interstitial condensation within a building component, in:
— heating periods, where the internal temperature is usually higher than outside;
— cooling periods, where the internal temperature is usually lower than the outside;
— cold stores, where the internal temperature is always lower than outside.
c) an estimate of the time taken for a component, between high vapour resistance layers, to dry,
after wetting from any source, and the risk of interstitial condensation occurring elsewhere in the
component during the drying process.
This International Standard does not cover other aspects of moisture, e.g. ground water and ingress of
precipitation.
In some cases, airflow from the interior of the building into the structure is the major mechanism for
moisture transport, which can increase the risk of condensation problems very significantly. This
International Standard does not address this issue; where it is felt to be important, more advanced
assessment methods should be considered.
The limitations on the physical processes covered by this International Standard mean that it can
provide a more robust analysis of some structures than others. The results will be more reliable for
lightweight, airtight structures that do not contain materials that store large amounts of water. They
will be less reliable for structures with large thermal and moisture capacity and which are subject to
significant air leakage.
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SIST EN ISO 13788:2013

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SIST EN ISO 13788:2013
INTERNATIONAL STANDARD ISO 13788:2012(E)
Hygrothermal performance of building components and
building elements — Internal surface temperature to avoid
critical surface humidity and interstitial condensation —
Calculation methods
1 Scope
This International Standard gives simplified calculation methods for:
a) The internal surface temperature of a building component or building element below which mould
growth is likely, given the internal temperature and relative humidity. The method can also be used
to assess the risk of other internal surface condensation problems.
b) The assessment of the risk of interstitial condensation due to water vapour diffusion. The method
used does not take account of a number of important physical phenomena including:
— the variation of material properties with moisture content;
— capillary suction and liquid moisture transfer within materials;
— air movement from within the building into the component through gaps or within air spaces;
— the hygroscopic moisture capacity of materials.
Consequently, the method is applicable only where the effects of these phenomena can be considered
to be negligible.
c) The time taken for water, from any source, in a layer between two high vapour resistance layers to
dry out and the risk of interstitial condensation occurring elsewhere in the component during the
drying process.
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 9346, Hygrothermal performance of buildings and building materials — Physical quantities for mass
transfer — Vocabulary
ISO 15927-1, Hygrothermal performance of buildings — Calculation and presentation of climatic data —
Part 1: Monthly means of single meteorological elements
3 Terms and definitions, symbols and units
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9346 and the following apply.
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SIST EN ISO 13788:2013
ISO 13788:2012(E)

3.1.1
monthly mean temperature
mean temperature calculated from hourly values or the daily maximum and minimum temperature
over a month
3.1.2
temperature factor at the internal surface
difference between the temperature of the internal surface and the external air temperature, divided by
the difference between the internal operative temperature and the external air temperature, calculated
with a surface resistance at the internal surface R :
si
θθ−
si e
f =
R
si
θθ−
ie
Note 1 to entry: The operative temperature is taken as the arithmetic mean value of the internal air temperature
and the mean radiant temperature of all surfaces surrounding the internal environment.
Note 2 to entry: Methods of calculating the temperature factor in complex constructions are given in ISO 10211.
3.1.3
design temperature factor at the internal surface
minimum acceptable temperature factor at the internal surface:
θθ−
si,min e
f =
R
si,min
θθ−
ie
3.1.4
minimum acceptable temperature
lowest internal surface temperature before mould growth may start
3.1.5
mean annual minimum temperature
mean of the lowest temperature recorded in each year of a set of at least ten years’ data
3.1.6
internal moisture excess
rate of moisture production in a space divided by the air change rate and the volume of the space:
Δν=ν −ν =G/(n)⋅V
ie
3.1.7
water vapour diffusion-equivalent air layer thickness
thickness of a motionless air layer which has the same water vapour resistance as the material layer
in question:s =μ⋅d
d
3.1.8
relative humidity
ratio of the vapour pressure to the saturated vapour pressure at the same temperature:
p
ϕ=
p
sat
3.1.9
critical surface humidity
relative humidity at the surface that leads to deterioration of the surface, specifically mould growth
3.1.10
heating period
external climate that leads to risk of condensation when a building is being heated, so that the internal
temperature and vapour pressure are higher than outside
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SIST EN ISO 13788:2013
ISO 13788:2012(E)

3.1.11
cooling period
external climate that leads to risk of condensation when a building is being cooled, so that the internal
temperature and vapour pressure are lower than outside
3.2 Symbols and units
Symbol Quantity Unit
2
D water vapour diffusion coefficient in a material m /s
2
D water vapour diffusion coefficient in air m /s
0
G internal moisture production rate kg/h
2
M accumulated moisture content per area at an interface kg/m
a
2
R thermal resistance m ·K/W
3
R gas constant for water vapour = 462 Pa·m /(K·kg)
v
T thermodynamic temperature K
2
U thermal transmittance of component or element W/(m ·K)
3
V internal volume of building m
2
Z water vapour diffusion resistance with respect to partial vapour pressure m ·s·Pa/kg
p
2
Z water vapour diffusion resistance with respect to humidity by volume s/m
v
d material layer thickness m
f temperature factor at the internal surface -
Rsi
f design temperature factor at the internal surface -
Rsi,min
2
g density of water vapour flow rate kg/(m ·s)
−1
n air change rate h
p water vapour pressure Pa
2
q density of heat flow rate W/m
s water vapour diffusion-equivalent air layer thickness m
d
t time s
3
w moisture content mass by volume kg/m
δ water vapour permeability of material with respect to partial vapour pres- kg/(m·s·Pa)
p
sure
δ water vapour permeability of air with respect to partial vapour pressure kg/(m·s·Pa)
0
3
ν humidity of air by volume kg/m
3
Δν internal moisture excess, ν – ν kg/m
i e
Δp internal vapour pressure excess, p – p Pa
i e
φ relative humidity -
λ thermal conductivity W/(m·K)
μ water vapour resistance factor -
θ Celsius temperature °C
θ minimum acceptable surface temperature °C
si,min
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SIST EN ISO 13788:2013
ISO 13788:2012(E)

3.3 Subscripts
an annual m mean
c condensation n interface
cr critical value s surface
e external air sat value at saturation
ev evaporation se external surface
eq equivalent (outside temperature) si internal surface
i internal air T total over the whole component or element
min minimum value
4 Input data for the calculations
4.1 Material and product properties
For the calculations, design values shall be used. Design values in product or material specifications or
the tabulated design values given in the standards referred to in Table 1 may be used.
Table 1 — Material and product properties
Property Symbol Design values
Thermal conductivity λ Obtained or determined in accordance with
Thermal resistance R ISO 10456.
Water vapour resistance factor μ
Obtained from ISO 10456 or determined in accord-
Water vapour diffusion-equivalent air s
d
ance with ISO 12572.
layer thickness
Thermal conductivity, λ, and water vapour resistance factor, μ, are applicable to homogenous materials
and thermal resistance, R, and water vapour diffusion-equivalent air layer thickness, s , apply primarily
d
to composite products or products without well-defined thickness.
For air layers, R is taken from ISO 6946 and s is assumed to be 0,01 m, independent of air layer thickness
d
and inclination.
4.2 External boundary conditions
4.2.1 Location
Unless otherwise specified, the external conditions used shall be representative of the location of the
building, taking account of altitude where appropriate.
NOTE Unless other information is available (for example in national standards), it can be assumed that
temperature falls by 1 K for every 200 m increase in altitude.
4.2.2 Time period for climatic data
For the calculation of the risk of surface mould growth or the assessment of structures for the risk of
interstitial condensation, monthly mean values, derived using the methods described in ISO 15927-1, or
in national standards, shall be used.
In the absence of national data or standards, the mean monthly temperatures shall be those likely to
occur once in 10 years, obtained from local climate records. If these data are not available, 2 K may
be subtracted from the monthly mean temperatures for an average year for calculations in a heating
climate, or 2 K added to the monthly mean temperatures for an average year in a cooling climate.
4 © ISO 2012 – All rights reserved

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SIST EN ISO 13788:2013
ISO 13788:2012(E)

For calculations of the risk of surface condensation on low thermal inertia elements such as windows
and their frames, the average, taken over several years, of the lowest daily mean temperature in each
year shall be used in the absence of any national standards.
4.2.3 External temperature
The following temperatures shall be used for the calculations.
a) For calculations of walls exposed to the outside, the external air temperature as specified in 4.2.1
and 4.2.2 shall be used.
b) For calculation of solid ground floors or walls below the ground, incorporate 2 m of soil below the
floor in the calculation. The monthly mean temperatures in the ground below this may be estimated
with the following steps:
— Take the twelve monthly mean external air temperatures: θ
m
— Average these to give the annual mean external air temperature: θ
an
— For each month calculate the average of the θ and θ : (θ +θ )/2
m an an m
— Displace the calculated values by one month, so the January value becomes February etc.
— If necessary, more detailed calculation of ground temperature may be carried out with the methods
in ISO 13370.
c) For calculations of suspended floors algorithms for the calculation of monthly subfloor temperatures
from the internal and external monthly temperatures are given in Annex E of ISO 13370
d) For calculations of roofs the monthly mean equivalent outside temperature, θ , which takes
eq
account of solar gain and cooling by long wave radiation, should be used; θ can be calculated
eq
using the methodology given in ISO 13790. As a simplified case, θ can be taken by subtracting 2 K
eq
from every monthly mean external air temperature.
4.2.4 External humidity
4.2.4.1 External air
To define the external air humidity conditions, use vapour pressure, p .
e
Monthly mean vapour pressure may be calculated from the mean temperature and relative humidity
using Formula (1).
pp=ϕθ (1)
()
ee sate
For calculations of the risk of surface condensation on low thermal inertia elements such as windows
and their frames, the external relative humidity corresponding to the temperatures defined in 4.2.2
shall be used.
NOTE In some climates the relative humidity associated with the mean annual minimum temperature can be
assumed to be 0,85.
4.2.4.2 Humidity conditions in the ground
Assume saturation (φ = 1).
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SIST EN ISO 13788:2013
ISO 13788:2012(E)

4.3 Internal boundary conditions
4.3.1 Internal air temperature
Use values according to the expected use of the building.
NOTE Annex A gives a method for estimating internal air temperature from the external temperature.
4.3.2 Internal humidity
The internal air humidity can be either
a) obtained from
pp=+ Δp (2)
ie
Take values of Δp according to the expected use of the building.
Δp may be derived from the internal moisture excess, Δν, using
G
ΔΔpR==ν T RT (3)
vi vi
nV
Values of Δp for a range of building types may be found in Appendix A.
or
b) given as a monthly mean value φ when the internal relative humidity is known.
i
NOTE Annex A gives a m
...