Hygrothermal analysis of a stabilised rammed earth test building in... David Allinson , Matthew Hall *
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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 848 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 852 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. References [1] H. Janssen, S. Roels, Qualitative and quantitative assessment of interior moisture buffering by enclosures, Energy and Buildings 41 (4) (2009) 382–394. [2] M. Hall, D. Allinson, Analysis of the hygrothermal functional properties of stabilised rammed earth materials, Building and Environment 44 (9) (2009) 1935–1942. [3] G. Minke, Earth Construction Handbook: The Building Material Earth in Modern Architecture, Southampton, WIT Press, 2000. [4] H. Hens, Final Report Task 1: Modelling Common Exercises, Summary Reports, 1996 (Annex 24). [5] Fraunhofer Institute for Building Physics, WUFI, Available at: http://www. wufi.de/index_e.html, Accessed on 17 July 2009. 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