SIST EN 15026:2007

SLOVENSKI OSIST prEN 15026:2004

PREDSTANDARD
oct 2004
Hygrothermal performance of building components and building elements -
Assessment of moisture transfer by numerical simulation
ICS 91.120.30 Referenčna številka
OSIST prEN 15026:2004(en)
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

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EUROPEAN STANDARD
DRAFT
prEN 15026
NORME EUROPÉENNE

EUROPÄISCHE NORM

August 2004
ICS

English version
Hygrothermal performance of building components and building
elements - Assessment of moisture transfer by numerical
simulation
Performance hygrothermique des composants et parois de Wärme- und feuchtetechnisches Verhalten von Bauteilen
bâtiments - Evaluation du transfert d'humidité par und Bauelementen - Bewertung der Feuchteübertragung
simulation numérique durch numerische Simulation
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee CEN/TC 89.

If this draft becomes a European Standard, 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.

This draft European Standard was established by CEN 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, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2004 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 15026:2004: E
worldwide for CEN national Members.

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Contents Page
Foreword.3
Introduction .4
1 Scope .6
2 Normative references .7
3 Definitions, symbols and units.7
4 Model components and material properties .9
5 Boundary conditions.14
6 Benchmark example – Moisture uptake in a semi-infinite region .16
7 Documentation of input data and results.19
Annex A (informative) Design of Moisture Reference Years .22
Annex B (informative) Internal boundary conditions.23
Bibliography .24

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Foreword
This document (prEN 15026:2004) has been prepared by Technical Committee CEN/TC 89 “Thermal
performance of buildings and building components”, the secretariat of which is held by SIS.
This document is currently submitted to the CEN Enquiry.

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Introduction
This standard defines the practical application of hygrothermal simulation software used to predict one-
dimensional transient heat, air and moisture transfer in multi-layer building envelope components subjected to
non steady climate conditions on either side. In contrast to the steady-state assessment of interstitial
condensation by the Glaser method (as described in EN ISO 13788), transient hygrothermal simulation
provides more detailed and accurate information on the risk of moisture problems within building components
and on the design of remedial treatment. While the Glaser method considers only steady-state conduction of
heat and vapour diffusion, the transient models covered in this standard take account of heat and moisture
storage, latent heat effects, and liquid and convective transport under realistic boundary and initial conditions.
The application of such models has become widely used in building practice in recent years, resulting in a
significant improvement in the accuracy and reproducibility of hygrothermal simulation.
The following examples of transient, one-dimensional heat and moisture phenomena in building components
can be simulated by the models covered by this standard:
� drying of initial construction moisture;
� moisture accumulation by interstitial condensation due to diffusion and convection in winter;
� moisture penetration due to driving rain exposure;
� summer condensation due to migration of moisture from outside to inside;
� exterior surface condensation due to cooling by long wave radiation exchange;
� moisture-related heat losses by transmission and moisture evaporation.
Figure 1 shows the factors relevant to hygrothermal building component simulation. The standard starts with
the description of the physical model on which hygrothermal simulation tools are based. Then the necessary
input parameters and their procurement are dealt with. A benchmark case with an analytical solution is given
for the assessment of numerical simulation tools. The evaluation, interpretation and documentation of the
output form the last part. The post processing tools in Figure 1 are not part of this standard. As far as possible
references to publications dealing with these tools are given.
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Figure 1 – Flow chart for hygrothermal simulations

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1 Scope
This standard specifies a method for calculating the non steady transfer of heat and moisture through building
structures. The necessary equations are defined and a benchmark example given.
The hygrothermal simulation models covered by this standard take account of the following storage and one-
dimensional transport phenomena:
� heat storage in dry building materials and absorbed water;
� heat transport by moisture-dependent thermal conduction;
� heat transfer by vapour diffusion and air convection, including phase changes;
� moisture storage by vapour absorption and capillary forces;
� moisture transport by vapour diffusion and air convection;
� moisture transport by liquid transport (surface diffusion and capillary flow);

The models described in this standard account for the following climatic variables:
� internal and external temperature;
� internal and external humidity;
� solar and long wave radiation;
� precipitation (normal and driving rain);
� wind speed and direction;
� air pressure differences.

The hygrothermal models described in this standard shall not be applied in cases where:
• convection takes place in a three-dimensional manner through holes and cracks;
• two-dimensional effects play an important part (e.g. rising damp, conditions around thermal bridges, effect
of gravitational forces);
• hydraulic, osmotic, electrophoretic forces are present;
• daily mean temperatures exceed 50 °C or in cases of fire;
• the effects of ice formation have to be assessed.
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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.
EN 12524 Building materials and products – Hygrothermal properties –Tabulated design values
EN ISO 7345 Thermal insulation – Physical quantities and definitions
EN ISO 9346 Thermal insulation – Mass transfer – Physical quantities and definitions
EN ISO 10456 Building materials and products – Procedures for determining declared and design thermal
values
EN ISO 12572 Hygrothermal performance of building materials and products – Determination of water
vapour transmission properties
1
EN ISO 15927-3 Hygrothermal performance of buildings – Calculation and presentation of climatic data
– Part 3: Calculation of a driving rain index for vertical surfaces from hourly wind and
rain data
ISO 9053 Acoustics – Materials for acoustical applications – Determination of airflow resistance
ISO 10051 Thermal insulation – Moisture effects on heat transfer – Determination of thermal
transmissivity of a moist material
3 Definitions, symbols and units
3.1 Definitions
For the purposes of this standard, the terms and definitions given in EN ISO 9346 and EN ISO 7345 apply.
Other terms used are defined in the relevant clause of this standard
3.2 Symbols and units
Symbol Quantity Unit
n
C
flow coefficient per unit area m³/(m² s Pa )
D
liquid diffusivity m²/s
l
2
I
total flux of incident solar radiation W/m
sol
P
pressure of the surrounding air Pa
a
∆P total air pressure difference Pa
a
P water pressure inside pores Pa
l
R gas constant of water vapour J/(kg K)
H2O
R
liquid moisture flow resistance of interface, m/s
l
T
thermodynamic temperature K

1) To be published.
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T air temperature of the surrounding environment K
a
T equivalent temperature of the surrounding environment K
eq
T
mean radiation temperature of the surrounding K
r
environment
T arbitrary reference temperature K
ref
T surface temperature K
s
c specific heat capacity at constant pressure of air J/(kg K)
p,a
c specific heat capacity at constant pressure of liquid J/(kg K)
p,l
water
c
specific heat capacity at constant pressure of solid J/(kg K)
p,s
matrix
d thickness of the layer I m
I
g density of moisture flow rate kg/(m² s)
g density of air mass flow rate kg/(m² s)
r
g density of liquid moisture flow rate kg/(m² s)
l
g available water due from to precipitation kg/(m² s)
p
g density of vapour flow rate kg/(m² s)
v
2
h convective heat transfer coefficient
W/(m ⋅K)
c
h specific enthalpy of liquid-vapour phase change J/kg
e
2
h effective heat transfer coefficient
W/(m ⋅K)
eff
2
h radiative heat transfer coefficient
r W/(m ⋅K)
n
flow exponent -
p
partial vapour pressure, Pa
v
p
saturated vapour pressure Pa
v,sat
2
q
density of heat flow rate W/m
2
q density of conduction heat flow rate W/m
cond
2
q density of convection heat flow rate W/m
conv
s suction Pa
∆s suction difference across interface Pa
s
mean suction at interface Pa
s equivalent vapour diffusion thickness m
d
v wind speed m/s
3
w moisture content kg/m
3
w water content kg/m
l
solar absorptance -
α
sol
permeability of vapour in air kg/m s Pa
δ
0
ε long-wave emissivity of the external surface -
thermal conductivity
λ W/(m⋅K)
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moisture conductivity for capillary water transport s
λ
m,l
relative humidity -
ϕ
diffusion resistance factor -
µ
liquid water density kg/m³
ρ
l
density of air kg/m³
ρ
a
density of solid matrix kg/m³
ρ
s
2 4
Stefan-Boltzmann constant
σ W/(m ⋅K )
s

4 Model components and material properties
4.1 Assumptions
The components of the hygrothermal model discussed in the following clauses were developed under the
following additional assumptions:
• constant geometry, no swelling and shrinkage;
• no chemical reaction, heat of sorption is neglected;
• no change in material properties by damage or ageing;
• local equilibrium between liquid and vapour without hysteresis;
• the temperature dependence of the moisture storage function is neglected;
• only the partial vapour pressure is used for the calculation of the vapour diffusion; the additional
influence of temperature and barometric pressure gradients are neglected;
• the formation of ice is not considered.
The development of the equations is based on the conservation of energy and mass (air and moisture). The
mathematical expression of the conservation laws are the balance equations. The conserved quantity
changes in time, only if there is a source or sink inside a small representative volume (control volume) or if it is
transported between neighbouring control volumes. Considering the restrictions of nearly constant pressure,
the heat storage of a control volume can be expressed by the specific heat at constant pressure from the solid
matrix and the liquid water inside the pore structure. Phase change between liquid and vapour, i.e. the release
of latent heat, is considered as part of the convective heat flow through the structure.
∂T ∂q
()c ⋅ ρ + c ⋅ w ⋅ = − (1)
p,s s p,l l
∂t ∂x
q = q + q (2)
cd cv
Conservation of mass can be used to determine the air flow rate. If buoyancy can be neglected the air flow
rate is calculated from the total pressure difference over the construction.
n
g = ρ C∆ P (3)
a a a
NOTE For a whole construction, the coefficients C and n can be measured according to EN 12114. The air flow
coefficient per area has to be calculated by division of the flow coefficient by the overall area of the building component.
The increase of the moisture content of a control volume is determined by the net inflow of moisture. The
moisture flow rate equals the sum of the vapour flow rate and the flow rate of liquid water.
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∂w ∂g
= − (4)
∂t ∂x
g = g + g (5)
v l
In an unsaturated porous building material the moisture flow is always a mixture of flow in the vapour and the
liquid state. An additional flow of moisture can also take place in the surface absorbed moisture. The term
liquid is used in the following clauses if there is a continuous path of liquid moisture in the volume under
consideration. The following equations are based on the assumption of a local equilibrium between the solid
matrix, the flowing phases and the absorbed moisture. This means that the temperature of each part inside a
control volume is equal and the moisture distribution between the absorbed moisture and the flowing phases
equals the equilibrium distribution. To describe the amount of vapour, different state variables can be used
which are related by a number of equations.
The relative humidity is defined by using the saturated vapour pressure or the humidity by volume
at saturation
p
v
ϕ = (6)
p (T )
v,sat
The liquid phase of water inside a building material can be described by the moisture content mass by volume.
Due to the capillary forces the pressure inside the water is different from the pressure of the surrounding air.
The difference is called suction.
s = P − P (7)
a l
The suction of the pore water is related to the relative humidity of the surrounding air by the Kelvin equation:
s = −ρ R T lnϕ (8)
l H2O
The relation between the state variables φ, p , s, T and the moisture content of a building material is defined by
v
the moisture storage function. The moisture storage function of a building material can be expressed as the
moisture content as a function of suction (suction curve), w(s), or as the moisture content as a function of the
relative humidity (sorption curve), w(φ). As a simplification, a constant reference temperature is used in
Equation (8) so that the temperature dependence can be neglected.
4.2 Transport of heat, air and moisture
4.2.1 Heat transport
4.2.1.1 Heat transport inside materials
The transport equation for heat can be split into heat conduction and convective heat transport:
q = q + q (9)
cd cv
The heat transport through heat conduction is calculated with Fourier’s law and includes all processes where
the transport of heat is closely correlated to the temperature gradient. The second heat flow term accounts for
the effects of both air flow and moisture flow, including sensible and latent heat:
∂T
q = −λ(w,T ) ⋅ (10)
cd
∂x
q = c ⋅()T − T ⋅ g + c ⋅()T − T ⋅ g + h ⋅ g (11)
cv p,a ref a p,l ref l e v
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4.2.1.2 Heat transport across boundaries
The heat flow from the surrounding environment into the construction consists of convection, shortwave
radiation from the sun and longwave radiation exchange with sky and surrounding surfaces. Additionally heat
can be transported to or from the construction by air or liquid water transport across the boundary.
Air (and liquid moisture) flow into the construction:
q = h ⋅ (T − T )+ g ⋅ c ⋅()T − T + g ⋅ c ⋅()T − T + h ⋅ g (12)
eff eq surf air p,air air ref l p,l air ref e v
Air flow (and no liquid moisture flow) out of the construction:
( ) ()
q = h ⋅ T −T + g ⋅c ⋅ T −T + h ⋅ g (13)
eq s a p,a s ref e v
The effective heat transfer coefficient and the equivalent temperature are
h = h + h (14)
c r
1
T = T +()I α +()T − T h (15)
eq a sol sol r a r
h
NOTE The radiative and convective heat exchange has been lumped into an equivalent temperature. Other means of
accounting for these effects may be used.
4.2.2 Moisture transport
4.2.2.1 Moisture transport inside materials
Moisture is transported by capillary forces, air transport and diffusion. The transport equations can be
formulated using the partial vapour pressure and the suction as the driving potentials.
g = g + g (16)
v l
∂p g p
1
v a v
g = − δ (T ) + (17)
v 0
µ(ϕ) ∂x ρ R T
a H O
2
∂s
g = λ (s) (18)
l m,l
∂x
The temperature dependence of the liquid moisture conductivity may be neglected.
NOTE For the liquid transport alternative potentials such as relative humidity, moisture content and temperature may
be used, if the transport coefficients are transformed and the interfaces between two materials are handled in such a way
that the suction and the partial vapour pressure are still a continuous function across the interface.
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4.2.2.2 Moisture transport across material interfaces
The details of the contact between two layers of building materials can have a large influence on the liquid
moisture transport. Additional coatings, such as adhesives, can also modify the diffusive moisture transport.
Small air gaps between materials and the modification of pore structure at material interfaces, because of
chemical reactions products, reduce the capillary water transport across the interface. The influence of the
interface on the liquid moisture flow can be described by a resistance.
∆ s
g = (19)
l
R (s)
l

External interfaces
Coatings and paints can cause additional resistance for water uptake and drying. The impact on diffusion can
be described by an additional moisture resistance. For air flowing into the structure:
g p
δ
a v,a
0
()
g = ± p − p + (20)
v v,a v,s
s ρ R T
d,s a H O a
2
and for air flowing out of the structure:
g p
δ
a v,s
0
g = ±()p − p + (21)
v v,a v,s
s ρ R T
d,s a H O s
2
where s is the equivalent vapour diffusion thickness of the interface, in m.
d,s
The positive sign in equations (20) and (21) is used for the surface where the normal surface vector out of the
construction points into the direction of the negative x-coordinate and the minus sign for the other surface.
The uptake of driving rain is limited by the amount of water which can be absorbed by the material:
 ∂s
g =λ  (22)
m,l
l,max
∂x
 
s
so that:
g = min(g , g ) (23)
l p l,max
where g is the water available for absorption from precipitation.
p
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4.3 Material properties
4.3.1 Necessary material data set and measurement methods
Table 1 provides a list of relevant material pro
...