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Hygrothermal analysis of a stabilised rammed earth test building in... David Allinson , Matthew Hall *
Energy and Buildings 42 (2010) 845–852
Contents lists available at ScienceDirect
Energy and Buildings
journal homepage: www.elsevier.com/locate/enbuild
Hygrothermal analysis of a stabilised rammed earth test building in the UK
David Allinson a,*, Matthew Hall b
a
b
Department of Civil and Building Engineering, Loughborough University, Leicestershire LE11 3TU, UK
Nottingham Centre for Geomechanics, Division of Materials, Mechanics and Structures, Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 21 July 2009
Received in revised form 4 December 2009
Accepted 17 December 2009
This paper describes the analysis of the hygrothermal behaviours of stabilised rammed earth (SRE) walls
used in a building in the UK. The analysis was achieved by computer simulation using WUFI Plus v1.2
whole building hygrothermal analysis software. To validate the model, an unoccupied test room in an
unheated SRE building was monitored for 10 months. The hygrothermal properties of the SRE material
were measured in the laboratory. It is shown that the SRE walls significantly reduced the amplitude of
relative humidity fluctuations in the room air and reduced the frequency of high humidity periods at the
wall surface. By adapting the model to represent an occupied and conditioned space, it is demonstrated
that SRE walls have the potential to reduce the energy demand for humidification/dehumidification
plant.
ß 2010 Elsevier B.V. All rights reserved.
Keywords:
Stabilised rammed earth
Hygrothermal
Whole building simulation
1. Introduction
Hygrothermal behaviour describes the interrelated heat and
moisture transfer between a material and its environment. In this
paper we consider the walls of a room in a building constructed
from stabilised rammed earth (SRE). The SRE walls were
manufactured from moist earth materials mixed with cement
and compacted into formwork. At the outside surfaces, the walls
interact with the outside environment through the exchange of
heat (convection and radiation) and moisture (wind driven rain
and vapour exchange). Similarly, at the inside surfaces the walls
interact with the indoor environment through the exchange of heat
and water vapour (see Fig. 1). The temperature and relative
humidity of the indoor air can be greatly influenced by its
interaction with the walls [1] and the transport and storage of heat
and water in the wall depends on the properties of the construction
materials.
Some materials have high thermal mass and can absorb, store
and release large amounts of heat, helping to maintain stable
indoor air temperatures, e.g. dense concrete and natural stone.
Similarly, some materials have high ‘hygric mass’ and can absorb,
store and release significant amounts of water, helping to maintain
a stable indoor air relative humidity, e.g. desiccants and clays. SRE
materials have been identified as having a high thermal mass and a
high hygric mass [2]. They also have the advantage that the inside
* Corresponding author. Tel.: +44 0 115 846 7132.
E-mail addresses: [email protected] (D. Allinson),
[email protected] (M. Hall).
0378-7788/$ – see front matter ß 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.enbuild.2009.12.005
surfaces are traditionally left uncovered (i.e. no internal plaster,
paint, wallpaper etc.) and are therefore in direct contact with the
room air. More significantly, the hygrothermal properties of SRE
can be adjusted through modification of the pore structure by
changing the particle size distribution (PSD) [2,3]. The modification of soil PSD is already practiced for enhancing density and
strength in earth building materials but, in the absence of reliable
research data, is not yet commercially practiced to modify thermal
and/or hygric properties. Based on prior experimental evidence [2],
the authors hypothesise that the optimisation of SRE hygrothermal
functional properties could be used to build walls that offer
building-integrated, passive air conditioning to the indoor
environment.
The use of exposed thermal mass in buildings is well known and
commonly accounted for at the design stage using calculation tools
and building simulation. It is used chiefly to suppress indoor air
temperature variation and reduce peak cooling loads. Rammed
earth walls are known to possess high thermal and hygric mass, the
latter being for passive suppression of indoor relative humidity
variation, air quality, condensation and mould growth. The focus of
the analysis in this paper is to better understand and quantify how
the hygric mass of SRE can improve indoor environmental
conditions when hygrothermal behaviour is considered.
Other advanced building modelling tools such as ESP-r, Energy
Plus and TAS are restricted to heat storage and transfer and do not
simulate the transient coupled behaviour of simultaneous heat and
mass transport/storage. However, following the leading research
conducted by Hens, Künzel and Karagiozis, Pedersen et al., and
others, there are an increasing number of computer models that
will model heat and mass transfer in building materials [4]. The
D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
846
Fig. 1. Hygrothermal behaviour of external building fabric, e.g. a wall (from [22]).
commercially available PC software tool WUFI Plus v1.2 was
chosen for the analysis in this paper. The extensively validated
software was developed by the Fraunhofer Institute for Building
Physics in Germany, for ‘‘. . .the calculation of transient internal
climatic conditions and heat losses by combination of energetic wholebuilding simulation with hygrothermal component calculation’’ [5].
While others have looked at whole building hygrothermal
simulation (e.g. [6–8]) the technique has never, to the authors’
knowledge, been applied to SRE materials.
2. Test building and data collection
The SRE building, shown in Fig. 2, is located in a village near to
Market Harbrough in the county of Leicestershire, UK. It was
constructed by Earth Structures (Europe) Ltd. [9] using wellestablished, patented SRE formwork and pneumatic compaction
techniques developed in Australia by the parent company Earth
Structures Pty Ltd. The internal dimensions of the isolated test
room are given in Fig. 3. The walls of the room are of solid cavity
wall construction with the inner and outer leaves made of 175 mm
SRE and a 50 mm layer of extruded polystyrene insulation. The SRE
Fig. 2. The south façade of the test room in the SRE building.
Fig. 3. Schematic of the test room in the SRE building.
was manufactured from a blend of MOT type 1 crushed ironstone
quarry waste and grit sand with 7 wt% white Portland cement
incorporating a non-pore blocking hydrophobic chemical admixture that retains the majority of vapour permeability whilst
significantly reducing capillary potential (i.e. liquid water absorption). The south wall contained the window (650 mm wide by
960 mm high) and a floor-to-ceiling timber door. The ceiling
consisted of a layer of hardboard with 75 mm of low density glass
fibre batt insulation, whilst the floor was a 125 mm thick concrete
slab with no insulation or vapour barrier.
Tinytag1 sensors were used to record the temperature and
relative humidity of the air inside the room, and in the adjoining
building, at 30 min intervals for the period 4/7/2008 to 1/4/2009.
The data from the adjoining room was averaged for each hour and
used as a boundary condition in the computer model. The ‘Ultra 2’
sensors incorporate data logging in a small portable battery
operated unit with a stated accuracy of 0.5 8C and 3% RH. They
were positioned above the door and above head height on the back
wall of the building, as shown in Fig. 3. In the computer simulation, air
temperature and relative humidity is represented by a single node in
the middle of the room while in reality spatial variations, due to both
temperature stratification (air temperature increasing with height)
and boundary layers (air temperature decreasing across a thin layer
close to the wall surfaces), complicate the measurement of
representative values. Furthermore, sensors could not be placed in
locations were they would interfere with the use of the building
which precluded lower wall positions and suspension from the ceiling
D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
in the centre of the room. Therefore, the sensors were calibrated in the
laboratory and the difference between air temperature measured in
the centre of the room and air temperature measured above 2 m
height when attached to the wall was found to be less than 5%
difference. It was concluded that locating the sensor toward the top of
a wall was a good compromise when sensor placement is restricted,
as was the case in this investigation.
The computer modelling period was 1/4/2008 to 1/4/2009 and
historical weather data were obtained from a personal weather
station, located approximately 7 km due south of the SRE building
[10]. These data consisted of observations of temperature,
pressure, relative humidity, wind speed, wind direction, rainfall
and solar radiation, recorded at approximately five minute periods.
They were averaged for each hour and then converted into a WUFI
weather file format.
The weather file was used as the outer boundary condition for
the south, west and east walls. The solar radiation measurements
were applied directly to these walls. This consisted of only the
direct and diffuse radiation, since ground reflected radiation is
largely insignificant compared to these terms and so is ignored by
the software. Direct and diffuse radiation were not separated as
high humidity periods were of particular interest in this study and
those are typified by cloudy days such that the solar radiation is
entirely diffuse and was assumed to be isotropic. The only window
was on the south façade and therefore exposed to the total solar
radiation during the peak sunlight hours.
847
Table 1
Hygrothermal properties of the SRE material.
Property
Dry density
Porosity
Specific heat capacity
Dry state thermal conductivity
Vapour diffusion resistance
Water content at 80% RH
Free water saturation
Moisture heat conductivity supplement
Symbol
Value
Units
r
1900
0.295
868.0
0.643
14.34
61.5
253.47
4.39
kg/m3
m3/m3
J/kg K
W/m K
–
kg/m3
kg/m3
%/% RH
n
cp
l
m
w80
wf
b
3. Determining the functional properties of the SRE material
Before starting the hygrothermal analysis, a range of detailed
material functional properties must be known or measured. While
the database included with the software contains some common
building materials, and properties can be found in the literature
[11], there is little data available on earth materials. Also, previous
work by the authors has shown that the hygrothermal material
properties, especially those related to moisture transport and
storage, can vary considerably between different SRE mix types [2]
and so it was necessary to characterise the specific SRE material
used for the building. Samples were manufactured from the same
materials and mix design that was used in the construction [12].
The ironstone waste (2/3 volume), sand (1/3 volume) and cement
(7 wt%) were gravimetrically proportioned and a mixture of water
with admixture (1.5% Rheomix 790) were added to achieve the
optimum moisture content (15%) which was determined experimentally using the Proctor light compaction method after BS
BS1377-4:1990 [13]. Samples, comprising 1 l cylinders with
105 mm diameter, 1/3 l discs with 105 mm diameter, and a
300 mm square slab of depth 53 mm, were manufactured using
identical compaction energies (596 kJ/m3), achieved by repeated
dropping of a known weight from a given height (dynamic
compaction or ramming). The 1 l cylinders were manufactured in
three layers while the slab and disc were rammed in a single layer.
All were cured for a minimum of 28 days at 23 8C and 75% RH.
Testing and measurement included specific thermal and hygric
properties (described below) and a summary of the averaged
results is included in Table 1.
3.1. Thermal properties
Thermal conductivity measurements were carried out on the
300 mm 300 mm 53 mm SRE slab using the heat flow meter
method after ISO 8301:1991 [14] and using a Hilton B480 heat flow
meter apparatus. To determine the relationship between the
moisture content of the SRE sample and its thermal conductivity,
tests were carried out at a number of moisture contents in
accordance with ISO10051:1996 [15]. Similar tests have been
Fig. 4. Measured moisture storage isotherms for the SRE material.
reported by the authors elsewhere [16]. The software requires a
‘moisture-induced heat conductivity supplement’ that was calculated to be 4.39 kg/kg, from the percentage increase in thermal
conductivity per percentage increase in moisture content. The
specific heat capacity of the SRE material was calculated to be
868 J/kgK, using the mass-proportioned specific heat capacities of
its constituent parts and the equation given in [17].
3.2. Hygric properties
Moisture storage isotherms (equilibrium moisture content vs
relative humidity) were measured using the method described in
BS EN ISO 12571:2000 [18]. The equilibrium moisture content was
measured at five points on each isotherm as well as at saturation.
The resulting moisture storage isotherm is shown in Fig. 4. The
average moisture content at each relative humidity was entered as
a table in the software.
Water vapour transmission was measured through the 1/3 l disc
SRE samples by applying a humidity gradient across the faces of the
discs using the wet cup method after BS EN ISO 12572:2001 [19]. The
water vapour resistance factor, m, was calculated as the ratio of the
vapour permeability of air to the vapour permeability of the
material. The average value of m for the 4 test samples was 14.34.
The software will approximate the liquid transport coefficient
from the water absorption coefficient and the moisture content at
80% relative humidity. The former was measured by partial
immersion after BS EN ISO 15148:2002 [20] and latter was
estimated from the averaged moisture storage isotherm.
4. The hygrothermal model of the building
The building model consisted of a single zone, representing the
room, and a single attached zone adjacent to both the partition
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D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
wall and ceiling, representing the SRE building. An optional
climate, generated from the temperature and relative humidity
measurements in the adjoining SRE building, was used to describe
the climate of the attached zone and the external climate was
generated from the historical weather data. Ventilation rate was
estimated to be 1.5 air changes per hour and the walls were all set
at 80% relative humidity at the start of the simulation period. The
model was run from 1/4/2008 00:00 to 1/4/2009 00:00 in 1 h time
steps.
The building was ‘leaky’ by design, having a loosely fitted door
and a louvered window that could not be sealed. It was in an
unsheltered position and the door and window were regularly
opened to aid ventilation. In the absence of a blower door air
tightness test, the figure of 1.5ACH was chosen as it represented
the expected upper limit for a leaky, single story, dwelling. To
assess the choice, sensitivity analysis of the simulation was carried
out by varying the air change rate about the selected mean value.
As could be anticipated, the magnitude of air change rate was
inversely related to the variance in diurnal air temperature swings.
By direct comparison with the measured temperature data, the
initial estimate of 1.5ACH was found to be the most suitable value
for simulating air infiltration in this test building. The additional
infiltration that resulted from door and window opening was not
included as it was unpredictable.
The temperature and relative humidity of the underside of the
floor slab were defined as an optional climate within the software
such that relative humidity beneath the slab was constant
throughout the year and the temperature varied sinusoidally
about a mean value, being highest in mid-summer. As the values
could not be easily measured, the model was back-calibrated
against the air temperature and humidity data recorded in the
room. For the relative humidity beneath the slab a value of 98% was
found, which seemed reasonable for a slab on ground. The
temperature profile beneath the slab was given a mean of 10 8C and
an amplitude of 6 8C. This also seemed reasonable as the average
temperature recorded in the room was 10 8C.
The resulting room air humidity and temperature profiles,
generated by the model, compared well with those measured on
site as shown in Figs. 5 and 6. There are a number of factors which
are believed to contribute to the minor discrepancies, above and
beyond those attributable to the mechanics of the model and its
accuracy settings (0.5 8C and 0.5% RH). Effects due to changing
patterns of ventilation due to periodic daytime opening of the
window and door by the building owner, as well as the relationship
between ventilation and wind speed/direction, were not considered.
The contribution of the contents of the room (e.g. coats, horse saddles,
boots etc.) to the thermal and hygric mass were not included. The
The moisture buffering of the room air by the SRE walls was
explored, using the model, by changing the vapour resistance at
the surface of the walls to represent different coverings:
plasterboard (gypsum board), painted plasterboard and metal
foil. These provided additional surface resistances described by a
vapour diffusion thickness of 0.1 m, 1 m and 100,000 m, respectively. The thermal properties of these walls was unchanged. The
results in Fig. 7 show that increasing the vapour resistance of the
wall surface by the application of wall coverings significantly
increased the magnitude of the fluctuations in indoor relative
humidity.
The number of hours per day that a wall surface is above 80%
relative humidity greatly increases the likelihood of mould growth
[21]. The frequency histogram in Fig. 8 indicates the average
number of hours each day (i.e. the number of hours per year
divided by 365 days) that the inside surface of the North wall is
within given relative humidity ranges. The surface of the plain SRE
Fig. 5. Test room: comparison of measured and simulated air humidity.
Fig. 7. Test room: the effect of wall coating on simulated air humidity.
Fig. 6. Test room: comparison of measured and simulated air temperature.
measurements of air temperature and relative humidity taken in the
room, and the adjoining SRE building, may not have been
representative of the air as a whole due to poor mixing (stratification)
and boundary layer effects. The historical weather data may not have
been entirely representative due to any local micro-climate
phenomena. Taking all of these factor into consideration, the results
were deemed to be acceptable and the model was used for further
predictive analysis.
5. Analysis of indoor moisture buffering
D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
Fig. 8. Test room: the effect of wall coating on the frequency of simulated humidity
at the inside surface of the north wall.
and plasterboard covered walls are in the higher ranges for
significantly less time than the painted plasterboard and foil
covered walls, and it is expected that this would reduce the
likelihood of mould growth significantly. This is further supported
by observational evidence of the test room over a period of one
year when no mould growth occurred on any of the indoor surfaces
despite the room having no mechanical ventilation.
It can be observed that the relative humidity at the wall surface
is always very high for the painted plasterboard and the foil
covered walls. This may be due to the fact that (i) the building is
unheated providing a high RH in the winter and (ii) the high
thermal mass of the wall makes the surface temperature much
lower than the mean air temperature in the summer. It is
important to note that the test building was unfurnished, and that
in a real occupied building the presence of furniture, carpet,
curtains etc. can reduce the RH.
849
Fig. 9. Unconditioned bedroom: simulated air humidity in summer.
Fig. 10. Unconditioned bedroom: simulated air humidity in winter.
6. Analysis of operational energy saving potential
To consider the potential of SRE materials to improve thermal
comfort and reduce energy demand in occupied buildings, the
model was adapted to represent a bedroom with daily occupation
under three different cooling, heating, dehumidification and
humidification scenarios. The bedroom model was created from
the model of the room by increasing its width to give a floor area of
12 m2, reducing the ventilation to 0.5 air changes per hour,
removing the external door, increasing the size of the south facing
window and upgrading it to double glazing. The new window was
2 m wide and had a U-value of 2.73 W/mK and a SHGC
(hemispherical) of 0.6. The insulation above the ceiling was
increased to 250 mm and a further 250 mm of expanded
polystyrene insulation was added beneath the floor slab. The
ceiling and floor slab were both made vapour impermeable. The
room was occupied by two adults from 22:00 h to 06:00 h the next
day providing a moisture load of 86 g/h during those 8 h. Results
for the SRE walls were compared with those covered in
plasterboard, painted plasterboard and metal foil, as discussed
in the following sub-sections.
6.1. Unconditioned scenario
The results were initially examined for the room with no
heating, cooling, humidification or dehumidification. Figs. 9 and
10 show the daily fluctuations in relative humidity for the first 7
days of August 2008 and the first 7 days of January 2009. It can be
seen that the SRE walls significantly reduce the periodic
Fig. 11. Unconditioned bedroom: simulated moisture fluxes.
fluctuations in relative humidity of the air in the room, during
both these summer and winter periods. Fig. 11 shows the rate of
moisture generated by the people in the room (kg/h) as well as the
moisture flux between the room air and the walls (kg/h). It can be
seen that the walls absorbed and desorbed moisture in harmony
with moisture generation such that the walls typically absorbed a
proportion of the generated moisture during occupation and
released some of that moisture while the room was unoccupied. It
D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
850
Fig. 12. Unconditioned bedroom: simulated air temperature.
Fig. 14. Constantly
dehumidification.
conditioned
bedroom:
simulated
latent
heat
of
should be noted that external weather conditions can modify this
behaviour as seen on 5/8/2008 when desorption rates from the
wall into the room air were significantly lower during a period of
lower temperature and higher relative humidity. Increasing the
diffusion resistance at the surface of the wall produced a
significant decrease in the rate of moisture transfer. From
Fig. 12, it can be seen that this moisture buffering has very little
effect on the temperature evolution of the inside air as the
temperature profiles generally overlap.
6.2. Constantly conditioned scenario
As living spaces are usually conditioned to improve the thermal
comfort of the occupants, the models were re-run with the inner
climate carefully controlled to give constant year-round design
conditions of 18–20 8C and 40–60% relative humidity. Oversized
heating, cooling, humidification and dehumidification plant were
specified so that equipment limitations would not be a factor in the
analysis. The moisture flux between the walls and the room is
shown in Fig. 13 where the extent to which increased diffusion
resistance of the walls reduced the overall moisture buffering
capacity can be seen. By looking at the energy demand (latent heat,
kW) of the dehumidification, shown in Fig. 14, it can be seen that
the walls cannot effectively buffer the additional moisture
produced during occupation in the summer (cooling) and the
dehumidification plant operates during high humidity periods.
From Fig. 15 it can also be seen that periodic humidification of the
Fig. 15. Constantly conditioned bedroom: simulated latent heat of humidification.
room also occurred outside of the occupied period in the winter
(heating). The energy requirement for both humidification and
dehumidification increased significantly when the vapour diffusion resistance of the wall was increased. The annual energy
demand for conditioning the space is shown in Table 2. While the
heating and cooling loads are almost identical a significant saving
was made in the energy required for humidification and
dehumidification when compared with the non-moisture buffering case (metal foil covered walls).
6.3. Intermittently conditioned scenario
Fig. 13. Constantly conditioned bedroom: simulated moisture fluxes.
As it is common to only condition the indoor space periodically,
rather than constantly, the same results were examined for the case
where the temperature and relative humidity controls were only in
place between 06:00 and 08:00 h and then again between 16:00 and
22:00 h, each day. From Fig. 16, it can be seen that significant
amounts of moisture were desorbed from the walls at the start of the
morning conditioning period (the spike). From Fig. 17, it can be seen
that this corresponds with an increased energy demand from the
dehumidification equipment. In this way the moisture buffering
behaviour of the walls increased the energy demand for dehumidification as moisture absorbed during the unconditioned period was
desorbed into the room air when the conditioning was switched on.
From Table 3 it can be seen that again there was little difference
between the annual energy demand for heating and cooling and that
there was a significant reduction in the energy required for
D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
851
Table 2
Energy use and savings for the constantly conditioned scenario.
T 18–20 RH 40–50 all day
SRE
Plasterboard
Foil
Unpainted
Painted
Heating power
Cooling power
Latent heat humidification
Latent heat dehumidification
kWh
kWh
kWh
kWh
1313.7
135.0
1.9
41.2
1313.7
135.0
7.0
58.7
1313.8
135.0
26.2
86.2
1314.1
135.0
35.0
89.5
Total humidifying and dehumidifying
Total energy
kWh
kWh
43.1
1491.8
65.7
1514.4
112.4
1561.2
124.5
1573.5
Saving
Saving
Saving
Saving
Saving
Saving
%
%
%
%
%
%
0.0
94.5
54.0
65.4
0.0
5.2
0.0
79.9
34.4
47.2
0.0
3.8
0.0
25.3
3.6
9.7
0.0
0.8
0.0
0.0
0.0
0.0
0.0
0.0
(cooling)
(humidification)
(dehumidification)
(humidifying and dehumidifying)
(heating)
(total)
Table 3
Energy use and savings for the intermittently conditioned scenario.
T 18–20 RH 40–50; 06:00–08:00 16:00–22:00 h
SRE
Plasterboard
Foil
Unpainted
Painted
Heating power
Cooling power
Latent heat humidification
Latent heat dehumidification
kWh
kWh
kWh
kWh
1060.6
89.9
1.3
23.3
1060.8
90.0
4.6
14.0
1061.0
90.0
16.5
19.0
1061.2
90.0
22.3
22.0
Total humidifying and dehumidifying
Total energy
kWh
kWh
24.6
1175.2
18.6
1169.4
35.5
1186.5
44.3
1195.4
Saving
Saving
Saving
Saving
Saving
Saving
%
%
%
%
%
%
0.1
94.1
5.8
44.4
0.1
1.7
0.0
79.3
36.3
57.9
0.0
2.2
0.0
25.8
13.7
19.8
0.0
0.7
0.0
0.0
0.0
0.0
0.0
0.0
(cooling)
(humidification)
(dehumidification)
(humidifying and dehumidifying)
(heating)
(total)
Fig. 16. Intermittently conditioned bedroom: simulated moisture fluxes.
humidification. However the energy requirement for dehumidification is higher for the unmodified SRE walls when compared with the
metal foil covered walls. Interestingly, the plaster board walls show
a significant reduction in dehumidification energy and perform
slightly better overall. This suggests that the hygrothermal
properties of the plaster board are more optimised than SRE, for
the given scenario. Therefore, it may be possible to specify the
hygrothermal properties of exposed construction materials by using
predictive modelling techniques at the design stage. The SRE walls
still show a reduction in energy, compared with the metal foil
covered walls, when humidification and dehumidification are
considered together.
Fig. 17. Intermittently
dehumidification.
conditioned
bedroom:
simulated
latent
heat
of
7. Conclusions
The indoor climate of a room within a building, constructed
with SRE walls, was modelled using WUFI Plus v1.2 building
simulation software. The hygrothermal material properties of the
SRE building material were characterised by experimental testing.
The simulated temperature and relative humidity profiles of the
test room indoor air were successfully validated against those
recorded on site.
The simulated results showed that the SRE walls significantly
reduced the amplitude of relative humidity fluctuations during
both the summer and winter, when compared to walls covered
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D. Allinson, M. Hall / Energy and Buildings 42 (2010) 845–852
with materials that increased surface diffusion resistance (e.g.
unpainted plasterboard, painted plaster board, and aluminium
foil). The SRE walls also reduced the frequency of high humidity
periods at the wall surface and were therefore judged to be highly
beneficial in reducing mould growth in buildings.
The results of the model, when modified to represent an
occupied space, showed that the SRE walls were responsive to
periods of internal moisture gain, absorbing moisture for later
release. When the temperature and relative humidity of the
space were controlled constantly, this provided a significant
reduction in humidification and dehumidification energy demand when compared to the vapour impermeable wall, though
heating and cooling energy were not significantly different. When
the temperature and relative humidity of the space were
controlled intermittently, there was still an overall reduction
in humidification and dehumidification demand, but dehumidification demand for the plain SRE walls was higher than the
impermeable case. This was because the walls absorbed
significant amounts of water vapour while the plant was
inoperative and that moisture then had to be removed when
the dehumidification restarted.
It is evident that materials that absorb moisture can buffer
relative humidity changes (hygric mass) in the same way that
materials that absorb heat buffer temperature changes (thermal
mass). Covering materials with less permeable layers (e.g. paints)
dramatically reduces that moisture buffering behaviour. SRE
materials are good moisture buffering materials and are normally
left uncovered inside buildings and are therefore ideally suited to
passive humidity control. It is concluded that SRE walls have the
potential to improve thermal comfort, improve indoor air quality
and reduce the energy demand in buildings but care should be
taken with the system design including conditioning strategy and
ventilation.
Future work should focus on how SRE materials might be
intelligently optimised for moisture buffering to suit the given
scenario, and how improvements in thermal comfort due to SRE
walls may allow a relaxation of design conditions inside of
buildings and the possibility to further reduce energy.
Acknowledgements
The authors wish to acknowledge the support of the Engineering and Physical Sciences Research Council, Mr. Bill Swaney of
Earth Structures (Europe) Ltd. for giving access to, and technical
information on, the SRE test building, and Simon Smith at
www.NorthantsWeather.com for the use of weather data.
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