Energy and Buildings 34 (2002) 345–355
Study of moisture in buildings for hot humid climates
Franck Lucas*,1, Laetitia Adelard1, François Garde1, Harry Boyer1
Laboratoire de Génie Industriel, IUT de Saint Pierre, Université of Reunion Island, 40 avenue de Soweto,
Saint-Pierre, Reunion Island, 97 410, France
Received 5 June 2001; accepted 15 August 2001
Abstract
Humidity in buildings generates many disorders or disadvantages. A dry-bulb temperature of the air relatively low, strong moisture and
wall surface temperatures very low characterize the interior conditions of the highland dwellings in Reunion Island, during the southern
winter. This causes many disorders related to phenomena of condensation on walls: deterioration of the envelope, odor of mould. It is thus,
significant to precisely know the evolution of the moisture in a building to avoid any disorder on the frame. In this study we will expose a
series of experiments carried out on real residences in order to highlight main parameters of the problem. On the basis of these results,
numerical simulations were used to extrapolate the behavior of this building on unusual climatic sequences, holding account various
improvements of its constitution. A curative study and a preventive study were carried out on two different types of residences. The aim is to
propose solutions to prevent deteriorations of the coatings due to the surface condensation. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Humidity; Condensation; Expérimentations; Numerical simulations; Humid climate
1. Introduction
In moderate climate, hydrous transfers in buildings are
relatively well controlled as they are generally in a welldefined direction. Indeed, interior climatic conditions are
controlled by air-conditioning systems, which ensure a
constant temperature. In certain cases, humidity is also
controlled precisely In wet tropical climate a majority of
residences is not air-conditioned and the interior temperature and humidity are free floating. Moreover, in the highlands of Reunion Island, the dry-bulb temperature of the air
decreases appreciably with altitude to go down below the
usual values of comfort. Low dry-bulb temperature of air,
strong moisture and wall surface temperatures very low
characterize the interior conditions of the highland dwellings, during the southern winter. Taking into account the
external conditions the transferred moisture quantities are
significant. It follows of many disorders related to phenomena of condensation on wall. The buildings’ owners confronted with these problems have significant maintenance
and restoration costs, and must work out preventive and
*
Corresponding author. Tel.: þ33-2-62-96-28-90;
fax: þ33-2-62-96-28-99.
E-mail address: [email protected] (F. Lucas).
1
Tel.: þ33-2-62-96-28-91; fax: þ33-2-62-96-28-99.
curative solutions. They then initiated this study and the
carried out reflection takes into account the economic constraints they are confronted with. This paper proposes to
present the methodology used and the results obtained. After
a reviewing concerning moisture in buildings, we will
present the study undertaken in order to cure the problems
on existing dwellings. This first study is composed of a series
of measurements carried out to examine buildings having
undergone degradations with an aim of identifying the
causes of the damages. We will then present the tools used
for simulations and the results, which validate some
improvements concerning the thermal design of the envelope and the associated systems, in order to cure the problems. In the third part, we will present the study carried out
for a building project with a timber structure and therefore
with preventive purposes.
2. Outline relating to humidity in buildings
Humidity in dwellings has consequences not only on the
comfort and health of occupants but also on perenniality
of the coatings and the frame. Condensation of water contained in air occurs when the relative humidity reached a
limiting value known as saturation. Condensation can appear
in the form of droplets in suspension in air (fog) or on a
cold material support. The presence of fog in a dwelling
0378-7788/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 7 7 8 8 ( 0 1 ) 0 0 1 1 5 - 3
346
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
This equation imposes a good knowledge of the material
properties constitutive of the wall and applies badly to
heterogeneous or strongly hygroscopic materials. Several
methods of wall design are based on this equation: Dewpoint method, Glaser diagram. Their objective is to evaluate
the possibility of condensation of the water vapor during its
migration through the wall. These methods are intended for
the study of the wall in steady state conditions. In moderate
climates, as the building are more often heated, the migration of vapor is from outside to inside. In tropical climate,
most of the time, outside and inside conditions vary without
any control and consequently, the direction of the vapor flow
is not so well defined as the experiment will show it.
Moreover, the steady state methods do not consider the
cycles of condensation/evaporation. It is then difficult to
evaluate generated damages, knowing that a small quantity
of condensation remains tolerable. In the simulation codes,
the term of water vapor diffusion through the envelope is
often neglected in the hydrous balance of a zone. Its
influence is indeed weak comparison with the quantities
exchanged through openings or by ventilation.
Nomenclature
j
m_
p
H
P
x
T
w
h
mass flow rate (kg/m2 s)
mass flow (kg/s)
permeability (kg/m Pa s)
relative humidity (%)
pressure (Pa)
distance (m)
temperature (k)
humidity ratio (kg/kgdryair)
exchange coefficient (W/Km2)
Subscripts
v
vapour
air
air in the zone
ae
outside air
as
dry air
surf
surface
sat
saturation
c
convection
i
subscript of the zones
2.2. Surface condensation
is rare. It is generally confined in specific parts and over
short periods related to the occupant activity. Natural or
mechanical ventilations are intended to fight efficiently
against these internal contributions. Condensation on a
material support occurs when the temperature of this one
is lower than the air dew point temperature of the zone. This
case worries the designers by the degradations involved on
the support. The caused disorders are generally deteriorations of the interior coatings (yellowish, black spots and then
separation of paintings). Phenomena of corrosion of the
metal structure can appear in the event of cracks in the
coating. The hydrous transfers depend on the following
phenomena:
Diffusion of water vapor through the envelope of the
room;
Surface condensation of the water vapor;
Absorption and water vapor desorption by hygroscopic
materials of the room;
Airflow transfers with the outside or the other zones of the
building;
Diffusion of vapor in the air;
Production of vapor dependent on occupants and their
activities;
Injection or withdrawal of moisture by HVAC system.
It happens when the temperature of a wall is lower than
the dew point temperature, we have
m_ vcond ¼ Sjcond
(2)
The rate of vapor condensation depends on the difference
in partial pressure of vapor between the air of the room and
the air on the surface of the wall and can be expressed by
[1]
jcond ¼ $ðPvair Pvsurf Þ ¼ $ðPvair Pvsurf; sat Þ;
$ ¼ 7:4 109 hc
(3)
where, for walls
hc ¼ 1:079 DT 0:33 and for tilted roofs;
hc ¼ 1:135 DT 0:33
(4)
with [2]
Pvair ¼
H
Pv
100 air; sat
3928:5
(5)
Tair þ 231:667
3928:5
¼ f ðTsurf Þ ¼ 140974 105 exp
Tsurf þ 231:667
Pvair; sat ¼ 140974 105 exp
Pvsat; surf
(6)
2.1. The water vapor diffusion through a wall
This depends on the difference of partial pressure of vapor
on both sides of the wall and the permeability of material
following the law
jvdif
dPv
¼ p
dx
(1)
2.3. The hygroscopic behavior of materials
The hygroscopic behavior of materials is not always taken
into account in building thermal simulations. However, it
constitutes in certain case a significant element of moisture
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
exchanges in a room. One can distinguish two types of
models,
the detailed models, based on the diffusion law of the water
vapor through the walls of the envelope and in materials
of the room. These models require a perfect knowledge of
the building constitution and the characteristics of the
materials used. This data are often difficult to evaluate
in the project phases as well as for the existing buildings.
the simplified models simulate the behavior of a fictive
volume representing materials of the room reacting with
the moisture of the air. This fictive volume called buffer,
generally consists of two parts: a surface element reacting
with the moisture of the zone and a heart exchanging with
the surface element. The buffer is characterized by a
reduced number of coefficient. They are evaluated either
summarily by qualifying more or less the behavior of the
hygroscopic room [3] or, more precisely, by giving a
description of the materials of the room and their properties [4]. Another type of models consists in defining two
buffers one reacting quickly with the moisture of the air
and the other slowly [5].
For the curative study, we studied uninhabited residences
consisting of little hygroscopic materials (metal roof and
wall in coated breeze blocks) so, we will not consider the
phenomena of absorption or desorption due to hygroscopic
materials. On the other hand, for the preventive study, the
structure of buildings being made of timber we chose to
carry out simulations using TYPE 56 of TRNSYS [4]. This
multi-zone building model includes a hygroscopic buffer
model.
2.4. The airflow transfers
The air transfers with outside or with the other zones
intervene in the moisture balance of the zone in the form
m_ vaeraul ¼ m_ as ðwae wi Þ
(7)
2.5. The vapor diffusion in the air
It is generally neglected compared to the quantities
exchanged by airflow transfers.
2.6. Contributions due to the occupants
The releases of vapor due to occupants and their activity
are significant and can reach up to 20 l of water per day for a
family of four people. This load appears in the moisture
balance of the zone in a term giving the vapor generation
rate.
2.7. Contributions due to air conditioning systems
The most common type of HVAC system used in Reunion
are split-system functioning with 100% of recycled air. The
347
moisture exchanges due to HVAC are evaluated according to
the evolution of the air on the exchanger.
The heating of air is done with constant absolute humidity
and does not modify the moisture balance of the zone. The
split-systems used carry out a cooling, with generally a
dehumidification of the air through the cold battery. The
quantity of condensates extracted from the zone will then be
determined by
m_ vSTA ¼ m_ as eðwb wi Þ with e ¼
wi we Ti Te
¼
wi wb Ti Tb
(8)
3. Curative study
3.1. Introduction
This work is intended to study the problem of condensation on existing buildings. A series of measurements was
carried out to highlight the various sites of surface condensation, the process of condensation and the repercussions concerning the users of the dwellings. This phase will have to
validate some assumptions required for mathematical modelling. Instrumentation was thus, carried out on empty dwellings. Moreover, users were interviewed in order to determine
their behaviors and their reaction to the problems encountered. This study will lead to prescriptions and evaluation of
improvements, which can be applied to existing residences.
3.2. Series of measurements
3.2.1. The buildings under study
The study relates to two groups of dwellings located in the
highlands of Reunion Island at ‘‘L’Entre-Deux’’ (400 m of
altitude) and ‘‘Le Tampon’’ (800 m) (see Fig. 1). The studied
residences do not comprise heating or cooling system. The
majority of the instrumented residences are uninhabited and
not furnished. The studied rooms are isolated from other
rooms of the dwelling by closing the gates of communication. The walls are in hollow blocks of 20 cm covered with
painted coating mortar outside and inside. The roof consists
of a ceiling in plasterboards of 1.2 cm, a air layer, glass wool
insulator for the building in ‘‘l’Entre-Deux’’ (no insulation
for the building in ‘‘Le Tampon’’) and a corrugated iron.
The experimental campaign was conducted during the
end of the coldest period of the year corresponding to the
southern winter. The two sites are characterized by different
climatic conditions. ‘‘Le Tampon’’ is generally colder and
wetter than the site of ‘‘l’Entre-Deux’’. The average temperature and relative humidity are given in Table 1.
The most damaged rooms of the buildings were instrumented in a precise way. The characteristics of the air of the
rooms (dry-bulb temperature, resultant temperature, relative
humidity) were noted down as well as the temperatures of
surface of the walls, floor and ceiling. The main damages
being located on the ceiling, we measured the characteristics
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F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
Fig. 1. Buildings under study: Le Tampon and l’Entre-Deux.
Table 1
Climatic conditions of the two sites.
Le Tampon
L’Entre-deux
Temperature
Humidity
Temperature
Humidity
Mean
Minimum
Maximum
19.9
70.4
19.2
68.4
12.1
33.5
12.3
33.9
25.4
92.0
27.5
93.6
of the air layer inside the ceiling and the surface temperatures of the various parts of the frame. The complete
instrumentation requiring the drilling of the ceiling and
the installation of a heavy hardware, the living rooms and
the bathrooms of the inhabited residences were instrumented
in a reduced way by small autonomous sensors of air
temperature and relative humidity.
3.2.2. Experimental results
In Fig. 2, the sites of condensation are characterized by a
negative difference between temperature of surface and dew
point temperature of air. Condensation occurs at the coldest
hours of the day. On the corrugated iron, condensation
occurs almost throughout the night between 7:00 pm and
8:00 am. It appears on steel structure very early in the
morning when it is cooled by conduction with iron sheet
and when the temperature of the surrounding air is minimum. Under certain colder climatic conditions, condensation can appear on the higher face of plasterboard, on outside
walls, and on the lower face of plasterboard (in absence of
insulation in the roof). Let us note that the frontages exposed
to the south and the west have more significant duration of
condensation because they do not benefit quickly from the
solar contributions.
Fig. 2. Visualization of surface condensation in the roof.
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
Fig. 3. Sites of condensation.
The principal sites of condensation are identified and presented in Fig. 3 by order of importance in the diagram below:
Fig. 4 shows the evolution of vapor pressure for the indoor
and the outdoor conditions. One can notice that the sign of
349
vapor pressure gradient, between inside and outside,
expressed in Eq. (1), is changing along the period. The
diffusion of vapor is from outside to inside during the day
and reverse at night. Anyway, the vapor pressure gradient is
weak and thus, a very few quantity of vapor is transferred by
diffusion. This can justify the assumption to neglect the
vapor diffusion through walls used in simulation codes.
Moreover, we can notice that the position of vapor barrier
is consequently not well defined in this climate.
The relations given above allow determining the condensed vapor mass flux. Fig. 5 gives the evolution of the
vapor mass flux during 1 day. The average daily quantity of
condensed vapor by (m2) of roof surface is about 0.04 kg.
The streaming of condensates depends on the quantity
of water, on the slope and on the surface quality of the sheet.
Experiments give the maximum quantity of water being
able to condense on a sloping sheet before the streaming
(see Fig. 6). Let us notice that for some vertical parts of
the roof the streaming can appear for a quantity of water of
60 g/m2. For a slope of 308, (current slope of roofs) the
streaming intervenes for 100 g/m2. If one compares this
value with the average quantities of condensate observed
Fig. 4. Evolution of vapor pressure of indoor and outdoor humid air.
Fig. 5. Daily evolution of condensed vapor mass flux.
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F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
Fig. 6. Maximum quantity of condensed vapor before streaming.
during the experiments, one can suppose that the streaming
appears only at the time of exceptional climatic conditions.
Noted damages on buildings can be explained partly by the
occasional streaming of condensation on the wall. Moreover,
the accumulation of water in an absorbent insulation decreases its thermal qualities and creates a significant cold
bridge. The condensation is increased on the parts of the
frame than those of poorly insulated.
The ventilation of the air layer of the roof by a circulation
of external air is usually used along the coastal line to ensure
the cooling of the buildings. This technique was reproduced
in certain buildings located in altitude to drain off the
humidity from the roof. By comparing the dew point temperature of the external air with the temperatures of the two
principal sites of condensation of the roof (internal surface
of iron sheet and steel structure), in the Fig. 7, one notes that
the phenomena of condensation persist. As the Fig. 8 shows,
ventilation of roof air layer with external air or inhabited
room air does not bring a solution to the problems of
condensation. One can suppose that this step makes the
problem worse by continuously feeding the lower surface of
the sheet with external air close to condensation. In the case
of non-ventilated ceilings, the humidity ratio and thus, the
dew point temperature of the air decreases as moisture
condenses and tends towards a limit value corresponding
to equilibrium. As equilibrium is reached condensation
ceases. If the roof is ventilated, the dew point temperature
of air is determined by the external conditions and thus,
condensation will persist as long as the surface temperature
of the sheet will be lower than the dew point temperature. In
this case, the quantity of condensates may be more significant. In the highlands of Reunion, a ventilated air layer
should be used carefully and only in order to dry possible
infiltration of water trough the roof.
The presence of insulation in the ceiling modifies the
phenomena of condensation appreciably. Indeed, as the sheet
exchanges less energy with the other parts of the roof, the
sheet cools faster and reaches more quickly a temperature
Fig. 7. Ventilation of air layer with external air.
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
351
Fig. 8. Ventilation of air layer with various air.
lower than the dew point temperature. Moreover, the roofs
are generally about 308 tilted. This slope allows the streaming of the condensed water and thus, prevents the formation
of a liquid film of which thermal resistance would have
opposed condensation. By combining the insulation of the
underface of the roof and the slope of the sheet, one produces
a condenser for atmospheric humidity similar to those used
in the hot and dry climates to recover water. The presence
of insulation in the roof has as a consequence, a more significant condensate mass on the sheet and for a longer period
with risks of streaming and accumulation in certain parts
of the roof. Let us note however, that the temperatures
of the other parts of the roof, located under insulation, raise
and then remain always higher than the dew point temperature. In particular, condensation does no longer appear
on the plasterboard lining of the ceiling. The insulation
remains, thus, an essential protection for interior surfaces of
housing.
3.3. Numerical simulation
The prescriptions of improvement were supported by a
phase of numerical simulations based on the use of thermal
building simulation software and on a weather data generator. These two tools will be briefly described below.
3.3.1. The tools
CODYRUN [6,7], is a multi-zone and multi-model software for the simulation of building thermal behavior. It is a
simulation code gathering both design and search aspects,
and adapted to various types of climates. In particular, for
our study, CODYRUN allows to choose models adapted to
wet tropical climate. This software integrates natural ventilation and moisture transfers. Hypothesis is made that
humidity is tracked by airflow transfers and generated by
internal loads in each zone. There is no interaction between
humid air and walls or furnishing. Based on the nodal
analysis, the resolution uses an implicit finished difference
method and the coupling iterations between the zones enable
to calculate the evolution of temperatures and the energy
exchanged. CODYRUN determine the characteristics of the
air of the zone as well as the wall surface temperatures. As
for the interpretation of the experimental results, a surface
temperature lower than the interior air dew point temperature will announce the appearance of condensation. The
absorption of materials and the variation of the water content
of the air due to condensation are not taken into account by
the software. Indeed, CODYRUN does not calculate the
quantities of condensate. We will thus, qualify the prescriptions by evaluating the duration of the condensation period.
Taking into account these assumptions, one can expect that
those will be slightly over-estimated.
RUNeole [8] is a new climate generator usable in the wet
tropical zones. The software is composed of three principal
modules allowing description, creation of mathematical
and physical models starting from the existing weather
databases, and exploitation of these models for the generation of new data. A temporal or space interpolation of
the databases is then possible. These models are filed
according to the sites and to the periods covered. Generally,
the use of the weather databases supports two complementary steps [9]. The first approach, used for the dimensioning
of the systems (mainly the HVAC systems) uses extreme
and current climatic sequences of short duration. This type
of sequences consists of standard days which one will
have associated a frequency of occurrence. Data generated
artificially by the software worked out by Van Paassen
[10] or Degelman [11] can also be used. The second approach integrates the use of years of reference and finds its
application in the evaluation of the average power consumption of a building. The weather data generators also
provide this type of data while being based on functions of
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F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
Table 2
Artificial climatic conditions for the two locations
Location
Type of days
‘‘Le Tampon’’
Extreme day of winter
Average day of winter
‘‘l’Entre-Deux’’
Extreme day of winter
Average day of winter
Temperature (8C)
Humidity (%)
Temperature (8C)
Humidity (%)
Temperature (8C)
Humidity (%)
Temperature (8C)
Humidity (%)
correlation, stochastic models and statistical distributions
of the various climatic variables [12].
For our study, in order to test the technical improvements
suggested to decrease the condensation and considering that
simulated buildings were poorly hygroscopic, we need
severe climatic data on short periods. The experimentation
did not occur during the coldest month of the year, i.e. July in
Reunion Island, we used the generator RUNeole for the
generation of artificial data following the system dimensioning approach explained above. The quantitative criteria for
the definition of extreme and average climatic sequences are
exposed to the following Table 2.
For the site of ‘‘l’Entre-Deux’’, the only available data
was for September. Initially, we used the temperature data to
adapt them to the monthly average of July (16 8C). The total
solar radiation was computed according to the thermal
amplitude, and to the clearness index. The diffuse radiation
was computed based on the clearness index and the function
of correlation determined [13] for the highlands of Reunion.
The relative humidity is determined according to the absolute moisture calculated starting from the initial value of
temperature and moisture. The wind conditions were regarded as identical because corresponding to the winter
conditions. For the site of ‘‘Le Tampon’’, we got the temperature and relative humidity data for the experimentation
period. Since the radiation data were not available, we
interpolate the radiation data of the site ‘‘L’Entre-Deux’’
(altitude: 400 m) and of another site ‘‘La Plaine des Caffres’’
(2000 m). The temperature, moisture and radiation data
were then interpolated temporarily by using the same algorithm exposed previously to have the average and extremes
sequences during the coldest month.
3.3.2. The step used
The experimental phase initially enabled us to characterize the behavior of the two buildings vis-à-vis the problems
of condensation under real climatic conditions. We can now
define the improvements to be brought to this building type.
The goal of the simulations is to evaluate the risks of
condensation under more extreme climatic conditions and
to validate this suggested improvements. For each site, we
worked out two short climatic sequences using the weather
data generator RUNeole. One sequence is representative of a
Mean
Minimum
Maximum
11.7
90
14.4
70.6
12.5
90
16.2
72
9
78.0
11
60.0
10.4
70.0
10.0
48.0
15
99.0
21.7
90.0
13.3
99.0
24.5
90.0
average day of the coldest month (sequence AD: average
day) and the other sequence represent an extreme cold day of
this coldest month (sequence ED: extreme day). The building improvement programmes tested are based on the use of
mechanical ventilation in the room and on insulation of the
roof. An additional program envisages the insulation of the
exterior walls. Simulations were carried out following different scenarios of occupation, renewal of air and internal
loads during the day. Indeed, occupation of the residences
induced sensible and latent additional loads and variable
airflow during the day, according to the user behavior. The
description of an existing building implies some assumptions. The method used consists in validating the description
of the not modified building by comparing the simulations
based on a real weather file with the experimental data. Once
the building model is adjusted, the simulations based on
artificial climatic data will make it possible to test the
improvements of the building.
3.3.3. Results of simulations
Taking into account the assumptions of simulation, it is
difficult to quantify the condensation phenomena in terms of
mass. We can however, estimate the degree of improvement
obtained by the modifications made to ventilation and to the
frame. Thus, we will evaluate the duration of condensation
in a day. The importance of the phenomena of condensation
will be expressed as a percentage of time, calculated over 1
AD and ED. Comparing the surface temperatures of the
various walls with dew point temperature of the modelled
zone will highlight the appearance of condensation. The
results for the site one and two, located at 800 and 400 m of
altitude, are recapitulated in the following Table 3.
It thus, appears that the modifications suggested bring a
noticeable improvement and make it possible to eliminate
the risks of condensation on the walls.
4. Preventive study
4.1. Introduction
As we have seen it, condensation on the walls generates
significant problems of maintenance for the building owners.
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
353
Table 3
Results of simulations
Ventilation
Average
day
(AD)
Extreme
day
9ED)
No
Yes
Yes
No
Yes
Yes
Insulation
of the roof
No
Yes
Yes
No
Yes
Yes
Insulation
of the walls
No
No
Yes
No
No
Yes
In order to prevent degradations in new buildings and to
reduce the maintenance cost, a study was undertaken to
predefine the provisions to implement. Simulations were
carried out on a project of social housing in a very wet area
of Reunion Island.
4.2. The building under study
The study relates to a housing project of 40 social houses
established in ‘‘la Plaine des Palmistes’’ located at 1000 m
of altitude. The weather conditions of this site are characterized by an abundant pluviometry involving a strong
relative humidity. The average temperature and relative
humidity are 15.4 8C and 74.4%. The buildings are made
of timber and built on ground or crawl space. An example of
a housing on ground is presented in Fig. 9.
4.3. The simulations
The objective of the simulations is to bring supports to the
designers of the buildings confronted with the problems of
condensation. Thanks to the result of the curative study,
some improvements were made concerning mainly the
insulation of walls, roofs and the materials used. Through
these simulations, we try to evaluate the influence of the
ventilating systems or heating, the behavior of the glazings,
Rate of occurrence of condensation (%)
Site 1: ‘‘Le Tampon’’
Site 2: ‘‘L’Entre-Deux’’
On the walls
On the ceiling
On the walls
On the ceiling
60
8
0
69
34
0
60
0
0
70
4
0
68
12
0
71
21
0
71
0
0
71
17
0
the influence of the nature of the ground and the handling of
the cold bridges.
In order to consider the strongly hygroscopic character of
the buildings structure it is necessary to use a simulation
code integrating materials behavior. This model were not
integrated in CODYRUN at the date of the study. We used
TRNSYS of which the multi-zone building model (type 56)
take into account the quantities of water stored in materials
constituting the envelope or the furniture in the zone. Not
having experimental weather data for the studied site we
worked out artificial climatic sequences using the weather
generator RUNeole.
Considering the strongly hygroscopic behavior of the
buildings, we carried out simulations with average weather
data but over one long period. The hourly weather data file
used extends over 492 days comprising two southern winters, allowing taking into account the sorption and desorption cycling.
The housing description is based on the elements of the
file drawing project description. The panes of glass are
single and without solar protection. The walls are constructed according to the regulation of wood structure
buildings. The internal loads take into account the sensible
and latent heat releases related to the occupants and their
activities and a rate of air infiltration during the day.
For simulations with ventilating system, the fan will be
Fig. 9. Timber housing tested in the preventive study.
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F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
Table 4
Frequency of condensation on walls and glazing
Ventilation
Heating
Ground type
Frequency of condensation on. . .
Walls
1
2
Off
Off
Off
Off
3
Off
Off
4
On
Off
5
Off
20 8C
6
Off
22 8C
Platform
Not insulated
crawl space
Insulated
crawl space
Insulated
crawl space
Insulated
crawl space
Insulated
crawl space
Glazing
Wall
South West
(%)
Wall
NW
(%)
Wall
NE
(%)
Wall
SE
(%)
Door
(%)
Floor
(%)
Ceiling
NE
(%)
Ceiling
SW
(%)
SW
(%)
NW
(%)
NE
(%)
SE
(%)
13
15
12
14
13
15
13
15
13
13
10
14
13
15
13
14
16.0
16.2
15.9
16.2
15.9
16.2
16.1
16.3
13
12
13
13
13
10
13
13
15.9
15.7
15.8
15.9
0
0
0
0
4
0
0
0
7.7
7.7
7.7
7.7
10
9
10
10
13
10
11
11
15.4
15.3
15.4
15.5
2
2
2
2
11
2
3
3
14.5
14.5
14.5
14.6
started according to the moisture releases schedule in the
housing.
We defined six scenarios to evaluate the influence of the
different parameters. Scenario 1 and 2 evaluate the basic
dispositions (without system) for the case of a building on
the ground and on a crawl space. Scenario 3 analyses the
effect of 80 mm rockwool insulation of the crawl space. The
mechanical ventilation is tested with the scenario 4. Scenario 5 and 6 evaluate influence of heating with two different
temperature set points. The first set point is 20 8C, which did
not bring enough improvement on comfort and condensation. We defined a new set point temperature (22 8C), which
leads to wall surface temperatures higher than 19 8C.
4.4. Simulation results and discussion
The importance of condensation phenomena will be
expressed in duration of condensation reported to the duration of the simulation period and expressed as a percentage
(Table 4).
It appears that the different parameters appreciably modify the behavior of the building vis-à-vis the problems of
condensation. The best results are obtained with the use of
controlled mechanical ventilation; the rate of condensation
on walls is then close to 0%. Heating brings a solution to the
problems of condensation only if the set point temperature is
sufficient.
4.4.1. Condensation on pane glass
The results of the simulations, based on the use of single
glazing, show that condensation is inevitable whatever the
type of system used. This condensation is aesthetically
annoying but does not involve degradation of the support
as long as there is no streaming on the frontage. It is thus,
necessary to consider the evacuation towards the outside of
condensate. Let us note that condensation on the glazings is
strongly reduced by the use of a ventilation system and that
to make it disappear completely, it will be necessary to use
multiple glazing.
4.4.2. Influence of the ground
The unevenness of the ground requires in certain case the
use of a crawl space. The configurations ground and isolated
crawl space are equivalent. But the presence of a non isolated
crawlspace increases the phenomena of condensation on the
ground, and also on the walls. It then appears that crawl
spaces have to be insulated with at least 80 mm insulation.
4.4.3. Influence of the controlled mechanical ventilation
The ventilation of the buildings can be ensured by natural
or controlled distribution. The air circulation in the case of
natural distribution is ensured by thermal buoying or by
overpressure due to the wind on walls. Fluxes not being
controlled, the quantities of air are generally lower or much
higher than the needs. It follows moistures or comfort
disorder because of very low temperatures in the dwellings.
Controlled mechanical ventilation ensures an adjusted airflow in housing. Then, direction and quantities of air in
circulation correspond to a precise dimensioning. Simulations show that the mechanical ventilation makes an unquestionable improvement since condensation almost
disappeared on the walls and on the ground. It remains
on the other walls (doors and glazing) but is strongly
decreased. The periods of condensation correspond to the
periods of strong moisture contribution in the room (hour of
meal). That reinforces the interest of the mechanical ventilation whose role is to evacuate these internal loads. If it
appears as an interesting solution against condensation,
ventilation poses the problem of thermal comfort of the
occupants. With ventilation, inside air temperature frequently goes down below 20 8C. The average is 18.8 8C
over the period of simulation with a minimum of 12.6 8C.
This is the reason why many users shut down the system or
seal the exhaust. Besides too significant cooling, users also
F. Lucas et al. / Energy and Buildings 34 (2002) 345–355
complain about noise disturbance or annual cost of ventilation. In order to reduce the disadvantages related to an
excessive circulation of air, the use of hydro-adjustable
ventilation appears essential. Indeed, this system adjusts
the flow extracted out of the kitchens and the bathrooms
to control the humidity of the building. It thus, avoids over
ventilation of housing while guaranteeing an effective evacuation of the moisture loads.
4.4.4. Influence of heating
Scenarios 5 and 6 evaluate the influence of a heating
system. Ventilation, in this case, will be ensured by natural
means. The improvement made by a heating with a 20 8C set
point being tiny, we carried out simulations with a set point
of 22 8C. This value gives similar results as the mechanical
ventilation, concerning the problems of condensation. A dry
air temperature of 20 8C is not sufficient to increase the
temperatures of surface, which can go down under 17 8C.
Condensation persists since the temperature of dew of the air
remains unchanged. Moreover, the conditions of comfort are
not improved because the resultant temperature remains low.
A heating with 22 8C ensures a relative comfort with surface
temperatures not going down below 19 8C. This solution is
possible only if the users can afford the annual energy
expenditure. The yearly consumption of the heating will
be about 7000 kWh. This expensive solution prevents condensation in the building and ensures comfort to the inhabitants.
5. Conclusion and outlines
Our approach to study condensation phenomena uses a
first experimental step. These experiments aim at defining
the significant points of an existing building envelope
regarding to condensation phenomena and validate the
useful data for the building description. The measurements
shows that simulations will have to target more particularly
the change of the roof surface temperatures and walls
directed to the south. It appeared also, that under rigorous
climatic conditions, the quantities of condensate were likely
to stream and that prescriptions concerning ventilation of
the roof air layer must be taken carefully as it can generate
an increase in condensation. We then elaborate some
improvement for the existing buildings that were evaluated,
thanks to simulations for severe climatic conditions, which
could hardly be met during experimentation. This first study
points out importance of ventilation and frame to avoid
condensation.
The second step consists in evaluating the risk of condensation in a project building located in a very humid area
and to propose some improvement if necessary. To consider
the hygroscopic material of the structure we use a simulation
code integrating a buffer storage model. We focus our
355
simulations on the influence of the system (ventilation or
heating), the ground and the panes of glass. To fight against
condensation, ventilation remains the best solution when
heating implies a substantial investment. Condensation on
floor will be avoided if insulated when the condensation on
glazing will be definitely reduces.
This approach allows to propose solution to building
owners confronted with condensation problems, and to
evaluate the improvements. Future work will have to define
more precisely the thermal regulation in order to avoid
condensation but also to ensure comfort in the highlands
of Reunion Island.
Acknowledgements
The authors thank the company SODEGIS for his assistance in the realization for this study.
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