EN ISO 13788:2012

Hygrothermal performance of building components and building elements - Internal surface temperature to avoid critical surface humidity and interstitial condensation - Calculation methods (ISO 13788:2012, Corrected version 2020-05)

ISO 13788:2012 gives simplified calculation methods for:
      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.
      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.
      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.

Wärme- und feuchtetechnisches Verhalten von Bauteilen und Bauelementen - Raumseitige Oberflächentemperatur zur Vermeidung kritischer Oberflächenfeuchte und Tauwasserbildung im Bauteilinneren - Berechnungsverfahren (ISO 13788:2012, korrigierte Fassung 2020-05)

Diese Norm gibt vereinfachte Verfahren an zur Berechnung
a) der raumseitigen Oberflächentemperatur von Bauteilen oder Bauelementen, unterhalb der bei der
gegebenen Innentemperatur und relativen Luftfeuchte ein Befall durch Schimmel wahrscheinlich ist —
das Verfahren kann auch eingesetzt werden um das Risiko dafür zu beurteilen, dass weitere Probleme im
Zusammenhang mit der Tauwasserbildung auf der raumseitigen Oberfläche auftreten können.
b) zum Zwecke der Beurteilung des Risikos der Tauwasserbildung im Bauteilinneren infolge von
Wasserdampfdiffusion. Bei dem angewendeten Verfahren wird eine Reihe wichtiger physikalischer
Phänomene, einschließlich der folgenden nicht berücksichtigt:
 Schwankungen der Materialeigenschaften in Abhängigkeit vom Feuchtegehalt;
 kapillare Saugwirkung und Transport von Feuchte in der flüssigen Phase in Baustoffen;
 Luftbewegung aus dem Gebäudeinneren in das Bauteil durch Spalte oder in Lufträumen;
 hygroskopisches Verhalten von Baustoffen.
Folglich gilt das Verfahren nur, wenn die Wirkung dieser Phänomene vernachlässigbar ist.
c) der Zeit, die in einer Schicht zwischen zwei Schichten mit hohem Wasserdampf-Diffusionsdurchlasswiderstand
befindliches und aus einer beliebigen Quelle stammendes Wasser zum Trocknen benötigt,
sowie des Risikos der Tauwasserbildung an anderer Stelle im Inneren des Bauteils während des
Trocknungsvorgangs.

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, Version corrigée 2020-05)

L'ISO 13788:2012 donne des méthodes de calcul simplifié permettant de déterminer:
1.    La température de surface intérieure minimale d'un composant ou d'une paroi de bâtiment nécessaire pour rendre improbable le développement de moisissures, compte tenu de la température et de l'humidité relative intérieures. Cette méthode peut également être utilisée pour évaluer le risque d'autres problèmes de condensation superficielle intérieure.
2.    Le risque de condensation dans la masse dû à la diffusion de vapeur d'eau. La méthode utilisée ne tient pas compte d'un certain nombre de phénomènes physiques importants, parmis lesquels la variation des propriétés de matériaux avec la teneur en humidité; l'absorption capillaire et le transfert d'humidité à l'état liquide dans les matériaux; la perméabilité à l'air des parois via des espaces ou des lames d'air; la capacité hygroscopique des matériaux.
3.    Le temps mis par l'eau, provenant de toute source, se trouvant dans une couche située entre deux couches de forte résistance à la vapeur, pour sécher, et le risque de condensation dans la masse survenant ailleurs dans le composant au cours du processus de séchage.

Higrotermalno obnašanje sestavnih delov stavb in elementov stavb - Notranja površinska temperatura za preprečevanje kritične vlage na površini konstrukcije in kondenzacije v konstrukciji - Računska metoda (ISO 13788:2012)

Ta mednarodni standard določa poenostavljene računske metode za: a) Notranjo površinsko temperaturo sestavnega dela stavbe ali elementa stavbe, pod katerim je verjetna rast plesni zaradi notranje temperature in relativne vlažnosti. Metoda se lahko uporabi tudi za oceno tveganja težav v zvezi s kondenzacijo na drugih notranjih površinah. b) Oceno tveganja kondenzacije v konstrukciji zaradi difuzije vodne pare. Uporabljena metoda ne upošteva številnih pomembnih fizikalnih pojavov, med katerimi so: – raznolikost lastnosti materiala z vsebnostjo vlage; – kapilarno vsesavanje in prenos tekoče vlage v materialih; – premikanje zraka iz notranjosti stavbe v sestavni del skozi luknje ali v zračnih prostorih; – kapaciteta materialov za higroskopsko vlago. Posledično se metoda uporablja samo takrat, ko so učinki navedenih pojavov lahko zanemarljivi. c) Čas, ki je potreben, da se voda iz katerega koli vira v plasti med dvema plastema z visoko upornostjo vodni pari posuši, in tveganje za nastanek kondenzacije v konstrukciji kjer koli drugje v sestavnem delu med postopkom sušenja.

General Information

• Status: Published
• Publication Date: 18-Dec-2012
• Withdrawal Date: 29-Jun-2013
• ICS: 91.060 - Elements of buildings
• ICS: 91.060.01 - Elements of buildings in general
• ICS: 91.120.10 - Thermal insulation of buildings
• ICS: 91.120.30 - Waterproofing
• Technical Committee: CEN/TC 89 - Thermal performance of buildings and building components
• Drafting Committee: CEN/TC 89/WG 10 - Moisture
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 19-Dec-2012
• Completion Date: 19-Dec-2012
• Directive: 305/2011 - Regulation (eu) No 305/2011 of the European Parliament and of the Council of 9 march 2011 laying down harmonised conditions for the marketing of construction products and repealing council directive 89/106/eec
• Directive: 89/106/EEC - Construction products
• Ref Project: SIST EN ISO 13788:2013 - Hygrothermal performance of building components and building elements - Internal surface temperature to avoid critical surface humidity and interstitial condensation - Calculation methods (ISO 13788:2012)

Relations

• Replaces: EN ISO 13788:2001 - Hygrothermal performance of building components and building elements - Internal surface temperature to avoid critical surface humidity and interstitial condensation - Calculation methods (ISO 13788:2001)
• Effective Date: 08-Jun-2022

Overview

EN ISO 13788:2012 (Corrected version 2020-05) - published by CEN/ISO - defines simplified hygrothermal calculation methods for building components and elements. The standard helps determine the internal surface temperature and assess the risk of critical surface humidity, mould growth, and interstitial condensation caused by water vapour diffusion. It also provides a procedure to estimate the drying time for a wetted layer trapped between high vapour resistance layers and the risk of condensation elsewhere during drying.

Key topics

• Surface humidity and mould risk
  • Calculation of the internal surface temperature below which mould or other surface condensation problems are likely, given interior temperature and relative humidity.
• Interstitial condensation due to vapour diffusion
  • Simplified monthly-step methods to assess vapour diffusion condensation risk inside multi-layer building components during heating, cooling and cold-store conditions.
• Drying of wetted layers
  • Estimation of the time required for a wet layer (between two high vapour resistance layers) to dry and whether interstitial condensation will occur elsewhere during drying.
• Input and boundary data
  • Material/product properties, external and internal boundary conditions, and surface resistances required for calculations.
• Limitations and assumptions
  • The methods assume moisture transport by vapour diffusion only and typically use monthly climate data. They do not account for:
    • Variation of material properties with moisture content
    • Capillary suction and liquid moisture flow within materials
    • Air movement from the interior into components (convective transport)
    • Hygroscopic moisture storage in materials
  • Consequently, results are more reliable for lightweight, airtight constructions with low moisture storage capacity.

Practical applications

• Design checks to avoid mould and internal condensation on walls, roofs and floors
• Early-stage compliance assessments for building envelopes and retrofit projects
• Quick screening of interstitial condensation risk before applying more detailed hygrothermal simulation
• Evaluating drying potential of assemblies after construction moisture or accidental wetting

Who should use this standard

• Architects and building envelope designers
• Building physicists and consultants specializing in hygrothermal performance
• Insulation and façade engineers
• Contractors performing design validation or remedial assessments

Related standards

• ISO 6946 - Thermal resistance and transmittance calculation methods
• ISO 9346 - Hygrothermal vocabulary for mass transfer
• ISO 15927-1 - (Referenced for moisture/climate data handling)

Note: Where convective moisture transport, capillary flow or hygroscopic storage are important, use more advanced hygrothermal simulation methods (e.g., transient coupled heat–moisture models). Keywords: hygrothermal, ISO 13788, interstitial condensation, surface humidity, mould growth, vapour diffusion, thermal insulation.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2013
+LJURWHUPDOQRREQDãDQMHVHVWDYQLKGHORYVWDYELQHOHPHQWRYVWDYE1RWUDQMD
SRYUãLQVNDWHPSHUDWXUD]DSUHSUHþHYDQMHNULWLþQHYODJHQDSRYUãLQLNRQVWUXNFLMHLQ
NRQGHQ]DFLMHYNRQVWUXNFLML5DþXQVNDPHWRGD,62
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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.

Contents Page
Foreword . 3

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.

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
ISO 13788:2012(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 [e-mail]
Web www.iso.org
Published in Switzerland
ii © ISO 2012 – All rights reserved

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 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.
iv © ISO 2012 – All rights reserved

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.
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.
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
2 © ISO 2012 – All rights reserved

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
D water vapour diffusion coefficient in a material m /s
D water vapour diffusion coefficient in air m /s
G internal moisture production rate kg/h
M accumulated moisture content per area at an interface kg/m
a
R thermal resistance m ·K/W
R gas constant for water vapour = 462 Pa·m /(K·kg)
v
T thermodynamic temperature K
U thermal transmittance of component or element W/(m ·K)
V internal volume of building m
Z water vapour diffusion resistance with respect to partial vapour pressure m ·s·Pa/kg
p
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
g density of water vapour flow rate kg/(m ·s)
−1
n air change rate h
p water vapour pressure Pa
q density of heat flow rate W/m
s water vapour diffusion-equivalent air layer thickness m
d
t time s
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)
ν humidity of air by volume kg/m
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
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

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).
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 method for estimating internal relative humidity from the external air temperature.
c) given as a constant φ when the internal relative humidity is kept constant e.g. by air-conditioning.
i
4.4 Surface resistances
4.4.1 Heat transfer
The value of R shall be taken as 0,04 m ⋅K/W.
se
For condensation or mould growth on opaque surfaces, an internal surface thermal resistance of
0,25 m ·K/W shall be taken to represent the effect of corners, furniture, curtains or suspended ceilings,
if there are no national standards.
The values of R given in Table 2 shall be used for the assessment of interstitial condensation, or surface
si
condensation on windows and doors.
Table 2 — Internal thermal resistances for the assessment of interstitial condensation, or
surface condensation on windows and doors
Direction of heat flow Thermal resistance
m2⋅K/W
Upwards 0,10
Horizontal 0,13
Downwards 0,17
6 © ISO 2012 – All rights reserved

ISO 13788:2012(E)
4.4.2 Water vapour transfer
The surface water vapour resistance is assumed to be negligible in the calculations in accordance with
this International Standard.
5 Calculation of surface temperature to avoid critical surface humidity
5.1 General
This clause specifies a method to design the building envelope to prevent the adverse effects of critical
surface humidity, e.g. mould growth.
NOTE Surface condensation can cause damage to unprotected building materials that are sensitive to
moisture. It can be accepted temporarily and in small amounts, e.g. on windows and tiles in bathrooms, if the
surface does not absorb the moisture and adequate measures are taken to prevent its contact with adjacent
sensitive materials.
There is a risk of mould growth when monthly mean surface relative humidities are above a critical
relative humidity, φ , which should be taken as 0,8 unless more specific information is available from
si,cr
National Regulations or elsewhere.
5.2 Determining parameters
Besides the external climate (air temperature and humidity), three parameters govern surface
condensation and mould growth:
a) the “thermal quality” of each building envelope element, represented by thermal resistance, thermal
bridges, geometry and internal surface resistance. The thermal quality can be characterized by the
temperature factor at the internal surface, f ;
Rsi
NOTE ISO 10211 gives a method for calculating weighting factors, when there is more than one inside
boundary temperature.
b) the internal moisture supply;
c) internal air temperature and the heating system and its settings.
5.3 Design for avoidance of mould growth, corrosion or other moisture damage
To avoid mould growth the monthly mean relative humidity at the surface should not exceed a critical relative
humidity φ , which should be taken as 0,8 unless more specific information is available from National
sicr
Regulations or elsewhere. Other criteria, e.g. φ ≤ 0,6 to avoid corrosion, can be used if appropriate.
sicr
The principal steps in the design procedure are to determine the internal air humidity and then, based
on the required relative humidity at the surface, to calculate the acceptable saturation humidity by
volume, ν , or vapour pressure, p , at the surface. From this value, a minimum surface temperature
sat sat
and hence a required “thermal quality” of the building envelope (for a given internal air temperature
and expressed by f ) is established.
Rsi
For each month of the year, go through the following steps:
a) define the external temperature in accordance with 4.2.3;
b) define the external humidity in accordance with 4.2.4;
c) define the internal temperature in accordance with national practice;
d) use the procedure defined in 4.3.2 to obtain the internal relative humidity;
ISO 13788:2012(E)
e) with a maximum acceptable relative humidity at the surface, φ = φ , calculate the minimum
si sicr
acceptable saturation vapour pressure, p
sat
p
i
p θ = (4)
()
satsi
φ
sicr
f) determine the minimum acceptable surface temperature, θ , from the minimum acceptable
si,min
saturation vapour pressure calculated in e);
NOTE The temperature as a function of saturation vapour pressure can be found from Formula (E.3) or
Formula (E.4). Another option is to prepare a table or a graph, based on Formulae (E.1) and (E.2), indicating
the relationship between p and θ to find θ from p .
sat i sat
g) from the minimum acceptable surface temperature, θ , assumed internal air temperature, θ
si,min i
(see 4.3.1) and external temperature, θ , the minimum temperature factor, f , is calculated
e Rsi,min
according to the Formula in 3.1.3.
The month with the highest required value of f is the critical month. The temperature factor for
Rsi,min
this month is f and the building element shall be designed so that f is always exceeded, i.e.
Rsi,max Rsi,max
f > f .
Rsi Rsi,max
Examples of this procedure are given in Annex B.
For a given building design effective values of f can be derived:
Rsi
— for plane elements, from f = 1 – R U;
Rsi si
— where multidimensional heat flow occurs, from a finite element or similar programme in accordance
with ISO 10211.
5.4 Design for the limitation of surface condensation on low thermal inertia elements
The assessment of surface condensation on low thermal inertia elements such as, for example, windows
and their frames, which show fast response to temperature changes, requires a different procedure.
Condensation on the inside surface of window frames can be an inconvenience if the water runs onto
adjacent decorations, and can cause corrosion in metal frames or rot in wooden ones by penetrating
joints, e.g. between the frame and glass. Because of their impermeable surface finish, mould growth is
rarely a problem on window frames. The maximum acceptable relative humidity at the frame surface is
therefore φ = 1.
si
Some intermittent condensation on window frames may be acceptable, however the procedure specified
below will limit this.
a) Define the external temperature as the average, taken over several years, of the lowest daily mean
temperature in each year.
b) Define the internal temperature according to national practice.
c) Use the procedure defined in 4.3.2 to obtain the internal relative humidity.
d) With a maximum acceptable relative humidity at the internal surface, φ = 1,0, calculate the
si
minimum acceptable vapour pressure, p
sat
ppθ = (5)
()
satsii
e) Determine the minimum acceptable surface temperature, θ , from the minimum acceptable
si,min
saturation vapour pressure.
8 © ISO 2012 – All rights reserved

ISO 13788:2012(E)
NOTE 1 The temperature as a function of saturation vapour pressure can be found from Formula (E.3) or
Formula (E.4). Another option is to prepare a table or a graph, based on Formulae (E.1) and (E.2) indicating
the relationship between p and θ to find θ from p .
sat i sat
f) From the minimum acceptable surface temperature θ , assumed internal air temperature, θ
si,min i
(see 4.3.1) and external temperature, θ , the required temperature factor of the building element,
e
f , is calculated according to the Formula in 3.1.3.
Rsi,min
Owing to the complex form and variety of materials used in window frames and the interactions between
the glass, frame and wall containing the window, heat flows and surface temperatures cannot, generally,
be calculated by simple one dimensional methods. Care therefore needs to be taken linking the minimum
acceptable surface temperature of the frame to the internal and external air temperatures.
Two, or if necessary three, dimensional finite element calculations on complete window systems including
the glazing, give surface temperatures that can be scaled to any combination of internal or external
temperatures. Calculations carried out with an insulation material, such as expanded polystyrene,
substituted for the glazing, used to obtain an equivalent thermal transmittance of the frame, do not give
accurate surface temperatures.
NOTE 2 Details of appropriate calculation methods are given in ISO 10077-2.
Various simplified methods have been developed to allow the calculation of realistic thermal
transmittances of complete windows taking account of multi-dimensional heat flows through the frame
and the spacer between the panes of double glazing. While these will give accurate heat flows, surface
temperatures will be seriously in error and they should not be used to estimate the risk of condensation.
6 Calculation of interstitial condensation
6.1 General
This clause gives a method to establish the annual moisture balance and to calculate the maximum
amount of accumulated moisture due to interstitial condensation. The method is an assessment rather
than an accurate prediction tool. It is suitable for comparing different constructions and assessing the
effects of modifications. It does not provide an accurate prediction of moisture conditions within the
structure under service conditions.
6.2 Principle
Starting with the first month in which any condensation is predicted, the monthly mean external
conditions are used to calculate the amount of condensation or evaporation in each of the 12 months of
a year. The accumulated mass of condensed water at the end of those months when condensation has
occurred is compared with the total evaporation during the rest of the year. One-dimensional, steady-
state conditions are assumed. The only effect of air movement considered is the presence of a continuous
air cavity, which is well ventilated to the outside as defined in ISO 6946. The effect of air movement
through the building component is not considered.
Moisture transfer is assumed to be pure water vapour diffusion, described by the following equation:
δ ΔΔp p
g==δ (6)
μ d s
d
−10
where δ = 2 × 10 kg/(m⋅s⋅Pa).
NOTE 1 δ depends on temperature and barometric pressure, but these influences are neglected in this
International Standard.
The density of heat flow rate is given by:
ΔΔθθ
q==λ (7)
dR
ISO 13788:2012(E)
NOTE 2 The thermal conductivity, λ, and the thermal resistance, R, are assumed constant and the specific heat
capacity of the materials not relevant. For parallel sided homogeneous materials, R = d/λ. Heat sinks/sources due
to phase changes are neglected.
NOTE 3 Calculation methods according to this principle are often called “Glaser methods”. More advanced
methods are specified in EN 15026.
6.3 Limitation of sources of error
There are several sources of error caused by the simplifications described in 6.2.
a) The thermal conductivity depends on the moisture content, and heat is released/absorbed by
condensation/evaporation. This will change the temperature distribution and saturation values
and affect the amount of condensation/drying.
b) The use of constant material properties is an approximation.
c) Capillary suction and liquid moisture transfer occur in many materials and this may change the
moisture distribution.
d) Air movements within building materials, gaps, joints or air spaces may change the moisture
distribution by moisture convection. Rain or melting snow may also affect the moisture conditions.
e) The real boundary conditions are not constant over a month.
f) Most materials are at least to some extent hygroscopic and can absorb water vapour.
g) One-dimensional moisture transfer is assumed.
h) The effects of solar and long-wave radiation are neglected except for roofs.
NOTE Due to the many sources of error, this calculation method is less suitable for certain building
components and climates. Neglecting moisture transfer in the liquid phase normally results in an overestimate of
the risk of interstitial condensation.
This International Standard is not intended to be used for building elements where there is airflow
through or within the element or where rain water is absorbed.
6.4 Calculation
6.4.1 Material properties
Divide the building element into a series of parallel-sided homogeneous layers and define the material
properties of each layer and the surface coefficients in accordance with 4.4.1 and 4.4.2. Each layer in
multi-layer products or components, including any products with facings or coatings, shall be treated
as an individual layer, taking full account of their respective thermal and moisture vapour transmission
properties. Calculate the thermal resistance, R, and the water vapour diffusion-equivalent air layer
thickness, s , of each individual layer of the building element. It is recommended that elements with a
d
thermal resistance greater than 0,25 m ⋅K/W are subdivided into a number of notional layers each with
thermal resistance not exceeding 0,25 m ⋅K/W; these subdivisions are treated as separate material
layers with interfaces between them in all calculations.
If the element contains a layer which is well ventilated to the outside, as defined in 5.3.4 of ISO 6946:2007,
take no account of all material layers between the cavity and outside.
Some materials, such as sheet metals, effectively prevent the passage of any water vapour and therefore
have an infinite value of μ. However, as a finite value of μ for a material is required for the calculation
procedure, a value of 100 000 should be taken for these materials. This can lead to the prediction of
negligibly small amounts of condensation, which should be disregarded as due to the inaccuracy of the
calculation method.
10 © ISO 2012 – All rights reserved

ISO 13788:2012(E)
Calculate the accumulated thermal resistance and the water vapour diffusion-equivalent air layer
thickness from the outside to each interface n.
n
′
RR=+ R (8)
nse ∑ j
j=1
n
′
ss= (9)
dd,,nj∑
j=1
The total thermal resistance and the water vapour diffusion-equivalent air layer thickness are given by
Formulae (10) and (11):
N
′
RR=+ RR+ (10)
Tsis∑ j e
j=1
N
′
ss= (11)
dT,,∑ d j
j=1
6.4.2 Boundary conditions for interstitial condensation
Define internal and external temperature and humidity according to 4.2.
If the element contains a layer which is well ventilated to the outside, assume the temperature and
vapour pressure in the cavity are the same as outside air. Assume the outside surface thermal resistance
is the same as the value for inside appropriate to the direction of heat flow, as defined in Table 2.
6.4.3 Starting month
Starting with any month of the year (the trial month), calculate the temperature, saturated vapour
pressure and vapour distributions through the component as specified in 6.4.4 and 6.4.5. Determine
whether any condensati
...