SIST EN ISO 13788:2002

SLOVENSKI STANDARD
SIST EN ISO 13788:2002
01-marec-2002
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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 13788:2001)
Wärme- und feuchtetechnisches Verhalten von Bauteilen und Bauelementen -
Raumseitige Oberflächentemperatur zur Vermeidung kritischer Oberflächenfeuchte und
Tauwasserbildung im Bauteilinneren - Berechnungsverfahren (ISO 13788:2001)
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 13788:2001)
Ta slovenski standard je istoveten z: EN ISO 13788:2001
ICS:
91.120.10 Toplotna izolacija stavb Thermal insulation
91.120.30 =DãþLWDSUHGYODJR Waterproofing
SIST EN ISO 13788:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 13788:2002

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SIST EN ISO 13788:2002
EUROPEAN STANDARD
EN ISO 13788
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2001
ICS
English version
Hygrothermal performance of building components and building
elements - Internal surface temperature to avoid critical surface
humidity and interstitial condensation - Calculation methods
(ISO 13788:2001)
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 13788:2001) Tauwasserbildung im Bauteilinneren -
Berechnungsverfahren (ISO 13788:2001)
This European Standard was approved by CEN on 18 October 2000.
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 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 Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, 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
© 2001 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 13788:2001 E
worldwide for CEN national Members.

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SIST EN ISO 13788:2002
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EN ISO 13788:2001
Contents
Page
Foreword 3
Introduction 3
1Scope 4
2 Normative references 4
3 Definitions, symbols and units 5
4 Input data for the calculations 7
5 Calculation of surface temperature to avoid critical surface humidity 10
6 Calculation of interstitial condensation 12
Annex A (informative) Classes of internal humidity load 21
Annex B (informative) Examples of calculation of the temperature factor at the internal surface to avoid
critical surface humidity 22
Annex C (informative) Examples of calculation of interstitial condensation 26
Annex D (informative) The assessment of the risk of condensation on window frames 31
Annex E (informative) Relationships governing moisture transfer and water vapour pressure 32
Annex F (informative) More advanced calculation methods 34
ANNEX ZA (normative) Normative references to international publications with their corresponding
European publications 35
ANNEX ZB (informative) Informative references to international publications with their corresponding
European publications 36

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EN ISO 13788:2001
Foreword
The text of EN ISO 13788:2001 has been prepared by Technical Committee CEN/TC 89
"Thermal performance of buildings and building components", the secretariat of which is held by
SIS, in collaboration with Technical Committee ISO/TC 163 "Thermal insulation".
This European Standard shall be given the status of a national standard, either by publication of
an identical text or by endorsement, at the latest by January 2002, and conflicting national
standards shall be withdrawn at the latest by January 2002.
This standard is one of a series of standards, which specify test methods for the thermal and
moisture related properties of building materials and products.
The European publications to be used instead of the International Standards listed in clause 2 are
given in normative annex ZA, which is an integral part of this European Standard.
The annexes A, B, C, D, E, F and ZB are informative. Annex ZA is normative.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy,
Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the United
Kingdom.
Introduction
Moisture transfer is a very complex process and the knowledge of moisture transfer mechanisms,
material properties, initial conditions and boundary conditions is often insufficient, inadequate
and still under development. Therefore this standard lays down simplified calculation methods,
based on experience and commonly accepted knowledge. The standardisation of these
calculation methods does not exclude use of more advanced methods. 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 standard deals with critical surface humidity and interstitial condensation, and does not
cover other aspects of moisture, e.g. ground water, precipitation, built-in moisture and moisture
convection, which can be considered in the design of a building component.

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EN ISO 13788:2001
1Scope
This standard gives 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 surface condensation problems.
b) The assessment of the risk of interstitial condensation due to water vapour diffusion. The
method used assumes built-in water has dried out and does not take account of a number of
important physical phenomena including:
- the dependence of thermal conductivity on moisture content;
- the release and absorption of latent heat;
- the variation of material properties with moisture content;
- capillary suction and liquid moisture transfer within materials;
- air movement through cracks or within air spaces;
- the hygroscopic moisture capacity of materials.
Consequently the method is applicable only to structures where these effects are negligible.
2 Normative references
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of
any of these publications apply to this European standard only when incorporated in it by
amendment or revision. For undated references the latest edition of the publication referred to
applies.
EN 12524 Building materials and products – Hygrothermal properties - Tabulated design
values
ISO 6946 Building components and building elements – Thermal resistance and thermal
transmittance - Calculation method
ISO 9346 Thermal insulation - Mass transfer - Physical quantities and definitions
ISO 10211-1 Thermal bridges in building construction – Calculation of heat flows and surface
temperatures - Part 1: General methods
ISO 10456 Building materials and products – Procedures for determining declared and
design thermal values
ISO 12572 Hygrothermal performance of building materials and products - Determination of
water vapour transmission properties

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EN ISO 13788:2001
1
ISO 15927-1 Hygrothermal performance of buildings – Calculation and presentation of
climatic data - Part 1: Monthly means of single meteorological elements
3 Definitions, symbols and units
3.1 Terms and definitions
For the purposes of this standard, the terms and definitions given in ISO 9346 and the following
apply.
3.1.1
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 air temperature and the external air temperature,
calculated with a surface resistance at the internal surface R :
si

si e
f (1)
Rsi

i e
Methods of calculating the temperature factor in complex constructions are given in
ISO 10211-1.
3.1.2
design temperature factor at the internal surface
minimum acceptable temperature factor at the internal surface:

si,min e
f (2)
Rsi,min

i e
3.1.3
minimum acceptable temperature
lowest internal surface temperature before mould growth starts
3.1.4
internal moisture excess
rate of moisture production in a space divided by the air change rate and the volume of the space:
v= v - v = G/(n V)  (3)
i e
3.1.5
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 = d (4)
d
3.1.6
relative humidity
ratio of the vapour pressure to the saturated vapour pressure at the same temperature:
p
(5)
p sat

1
To be published

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EN ISO 13788:2001
3.1.7
critical surface humidity
relative humidity at the surface that leads to deterioration of the surface, specifically mould
growth
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 kg/m
a
interface
2.
R thermal resistance m K/W
3
. .
R gas constant for water vapour = 462 Pa m /(K kg)
v
T temperature K
2
.
U thermal transmittance of component or element W/(m K)
3
V internal volume of building m
2. .
Z water vapour resistance with respect to partial m s Pa/kg
p
vapour pressure
2
Z water vapour resistance with respect to humidity s/m
v
by volume
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 m
d
thickness
t time s
3
w moisture content mass by volume kg/m
. .
water vapour permeability of material with kg/(m s Pa)
p
respect to partial vapour pressure
. .
water vapour permeability of air with respect to kg/(m s Pa)0
partial vapour pressure
3
v humidity of air by volume kg/m
3
v internal moisture excess, v – v kg/m
i e
internal vapour pressure excess, p – p Pap
i e
relative humidity of air -
.
thermal conductivity W/(m K)
water vapour resistance factor -
Celsius temperatureC
minimum acceptable surface temperatureC
si,min

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EN ISO 13788:2001
3.3 Subscripts
c condensation n interface
cr critical value s surface
e external air sat value at saturation
ev evaporation se external surface
i internal air si internal surface
min minimum value T total over whole component or element
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 Obtained from EN 12524 or determined in
thermal resistance accordance with ISO 10456.
R
Water vapour resistance factor Obtained from EN 12524 or determined in
water vapour diffusion-equivalent air accordance with ISO 12572.
s
d
layer thickness
Thermal conductivity, and water vapour resistance factor, , are applicable to homogenous
materials and thermal resistance, R, and water vapour diffusion-equivalent air layer thickness, s ,
d
primarily to composite products or products without well-defined thickness.
For air layers, R is taken from ISO 6946; s is assumed to be 0,01 m, independent of air layer
d
thickness and inclination.
4.2 Climatic conditions
4.2.1  Location
Unless otherwise specified, the external conditions used shall be representative of the location of
the building.
4.2.2 Time period
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, shall be used.
For calculations of the risk of surface condensation on low thermal inertia elements such as, for
example, windows and their frames, the mean annual minimum temperature on a daily basis and
corresponding relative humidity shall be used.

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EN ISO 13788:2001
NOTE This implies that there will be some condensation on one day in half the years.
4.2.3 Temperatures
The following temperatures shall be used for the calculations.
a) External air temperature as specified in 4.2.2.
b) Ground temperature adjacent to building components.
  The annual mean value of the external air temperature shall be used.
c) Internal air temperature.
Use values according to the expected use of the building. Internal air temperatures to be used
in this standard may be specified nationally.
4.2.4 Humidity conditions
a) To define the external air humidity conditions, use humidity by volume, v , or vapour pressure,
e
.
p
e
Monthly mean vapour pressure or humidity by volume may be calculated from the mean
temperature and relative humidity using equations (6) or (7).
pp () (6)
e e sat e
vv () (7)
e e sat e
Due to the non-linear relationship between temperature and saturation humidity, these
equations become inaccurate in hot climates.
For calculations of the risk of surface condensation on low thermal inertia elements such as,
for example windows and their frames, the external relative humidity corresponding to the
mean annual minimum temperature on a daily basis shall be used.
b) Humidity conditions in the ground
  Assume saturation ( = 1).
c) Internal air humidity
The internal air humidity can be derived:
1) by either of the expressions
p = p + p (8)
i e
v = v + v (9)
i e

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EN ISO 13788:2001
Take values of p and v according to the expected use of the building and multiply them
by 1,10 to provide a safety margin. Values to be used in this standard may be specified
nationally.
or
2) given as a constant when the internal relative humidity is known and kept constant e.g.
i
by air-conditioning. To provide a safety margin add 0,05 to the relative humidity.
NOTE 1 The introduction of a factor 1,10 (or a margin of 0,05 RH) is intended to
allow for inaccuracies in the method. The calculation method as described in this
standard is a steady state calculation. In reality, however, external air temperature
variations, changing solar radiation, hygroscopic inertia and intermittent heating can
influence surface humidity conditions. This is especially the case for a thermal bridge
area consisting of building materials with high thermal inertia. The factor does not
include the behaviour of the occupants, which can have a significant effect on
ventilation.
NOTE 2 Internal humidities can be classified in five humidity classes, see
annex A.
4.3 Surface resistances
4.3.1 Heat transfer
The values of R and R given in Table 2 shall be used for the assessment of mould growth and
se si
interstitial condensation.
Table 2 - Surface thermal resistances
Resistance
2
mK/W
External surface resistance R 0,04
se
Internal surface resistance R
si
On glazing and frames 0,13
All other internal surfaces 0,25
NOTE An internal surface resistance of 0,25 is taken to represent the worst case of
condensation risk in a corner.
4.3.2 Water vapour transfer
The surface water vapour resistance is assumed to be negligible in the calculations in accordance
with this standard.

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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 for mould growth at surface relative humidities above 0,8 for several days.
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
characterised by the temperature factor at the internal surface, f ;
Rsi
NOTE ISO 10211-1 gives a method for calculating weighting factors, when there is more
than one inside boundary temperature.
b) the internal moisture supply, see 4.2.4;
c) internal air temperature and heating system.
NOTE A lower room temperature is in general more critical. This is especially the case
for rooms with reduced, intermittent or no heating where water vapour may enter from
adjacent warmer rooms. The heating system will influence air movement and temperature
distribution in the rooms and therefore locally cooler areas of the building envelope may
become more critical.
5.3 Design for avoidance of mould growth
To avoid mould growth the relative humidity at the surface should not exceed 0,8 for several
days. 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, , or vapour pressure, p , at the surface. From this value, a minimum
sat sat
surface temperature and hence a required “thermal quality” of the building envelope (for a given
internal air temperature and expressed by f ) is established.
Rsi

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EN ISO 13788:2001
For each month of the year, go through the following steps:
a) define the external air 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) calculate the internal relative humidity from v or p (defined in 4.2.4) or take a constant
relative humidity for a conditioned environment, including the corrections for safety margin
defined in 4.2.4;
e) with a maximum acceptable relative humidity at the surface, = 0,8, calculate the minimum
si
acceptable saturation humidity by volume, v , or vapour pressure, p .
sat sat
v
i
v () (10)
sat si
0,8
or
p
i
p ()(11)
sat si
0,8
NOTE 1 The criterion  0,8 is selected with regard to the risk of mould growth. Other
si
criteria, e.g.  0,6 to avoid corrosion, can be used, if appropriate.
si
f) Determine the minimum acceptable surface temperature,, from the minimum acceptable
si,min
saturation humidity.
NOTE 2  The temperature as a function of saturation humidity can be found from equation
(E.10) or (E.11) in annex E. Another option is to prepare a table or a graph, based on
equation (E.8), indicating the relationship between p and , to find from p .
sat sat
g) From the minimum acceptable surface temperature,, assumed internal air temperature, si,min i
(see 4.2.3) and external temperature, , the minimum temperature factor, f , is calculated
e Rsi,min
according to equation (2).
The month with the highest required value of f is the critical month. The temperature factor
Rsi,min
for this month is f and the building element shall be designed so that f is always
Rsi,max Rsi,max
exceeded; i.e. ff .
Rsi Rsi,max
Examples of this procedure are given in annex B.
NOTE 3 For a given building design effective values of f can be derived:
Rsi
-1 -1
- for plane elements, from f = (U – R )/U
Rsi si
- Where multidimensional heat flow occurs, from a finite element or similar
programme in accordance with ISO 10211, Thermal bridges in building construction
– Calculation of heat flows and surface temperatures – Part 1: General methods, or
Part 2: Linear thermal bridges.

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EN ISO 13788:2001
5.4 Design for avoidance of surface condensation on lightweight constructions
In the case of lightweight constructions, which respond to temperature changes in periods much
less than a day, the following procedure shall be used.
a) Define the external temperature as the mean annual minimum temperature.
b) Define the external relative humidity as 0,95 and calculate the vapour pressure or humidity by
volume with equation 6) or 7).
c) Define the internal temperature according to national practice.
d) Convert v or p (defined in 4.2.2) into the internal relative humidity.
e) With a maximum acceptable relative humidity at the surface, = 1,0, calculate the minimum
s
acceptable saturation humidity, by volume, v , or vapour pressure, p .
sat sat
v ()v (12)
sat si i
or p ()p (13)
sat si i
f) Determine the minimum acceptable surface temperature, , from the minimum acceptable
si,min
saturation humidity.
NOTE The temperature as a function of saturation humidity can be found from equation
(E.10) or (E.11) in annex E. Another option is to prepare a table or a graph, based on
equation (E.8), indicating the relationship between p and , to find from p .
sat sat
g) From the minimum acceptable surface temperature , assumed internal air temperature, si,min i
(see 4.2.3) and external temperature, , the required temperature factor of the building
e
envelope, f , is calculated according to equation (2).
Rsi,min
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 assumes
that any built-in moisture has dried out.
The method should be regarded as an assessment rather than as 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, and is not suitable for calculation of drying out of built-in moisture.
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 twelve
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. Air movements through or within the
building elements are not considered.

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EN ISO 13788:2001
Moisture transfer is assumed to be pure water vapour diffusion, described by the following
equation
pp
0
g(14)
0
x s
d
-10
where = 2 10 kg/(msPa).
0
NOTE 1  depends on temperature and barometric pressure, but these influences are
0
neglected in this standard. Other equations for water vapour transfer are given in annex E.
The density of heat flow rate is given by

q(15)
d R
NOTE 2 The thermal conductivity, 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/. 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 briefly described in annex F.
6.3 Limitations and 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 through cracks or within 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.
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.
In building elements where there is air flow through or within the element the calculated results
can be very unreliable and great caution shall be used when interpreting the results.

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EN ISO 13788:2001
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 4.1 and 4.3.
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
d
element. Subdivide high thermal resistance elements, such as insulants, into a number of layers
2
each with the same thermal resistance not exceeding 0,25 mK/W; these subdivisions are treated
as separate material layers in all calculations.
Some materials, such as sheet metals, effectively prevent the passage of any water vapour and
therefore have an infinite value of . However, as a finite value of 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.
Calculate the accumulated thermal resistance and the water vapour diffusion-equivalent air layer
thickness from the outside to each interface n:
n
RRR (16)
n sej
j1
n
ss (17)
d,nd, j
1
j
The total thermal resistance and the water vapour diffusion-equivalent air layer thickness are
given by equations (18) and (19):
N
RRRR (18)
T sij se
j1
N
ss (19)
d,Td,
j
j1
6.4.2 Boundary conditions
Define internal and external temperature and humidity according to 4.2.
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 condensation is predicted.
If no condensation is predicted in the trial month, repeat the calculation with successive
following months until either:

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a) no condensation has been found in any of the twelve months, then report the component as
free from condensation; or
b) a month is found with condensation, this is the starting month.
If condensation is predicted in the trial month, repeat the calculation with successively earlier
months until either:
a) conden
...