Effect of Non-Isothermal Conditions on Hygrothermal
Transcription
Effect of Non-Isothermal Conditions on Hygrothermal
Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. EFFECT OF NON-ISOTHERMAL CONDITIONS ON HYGROTHERMAL BEHAVIOUR OF A HEMP CONCRETE BUILDING ENVELOPE Anh Dung Tran Le1, Driss Samri2 , Chadi Maalouf3, Mourad Rahim1, Omar Douzane1, Geoffrey Promis1, Thierry Langlet1 1 Laboratoire des Technologies Innovantes (LTI), Université de Picardie Jules Verne Avenue des Facultés, 80025 Amiens Cedex 1, France 2 Cerema, Direction territoriale Sud-Ouest, Rue Pierre Ramond CS 60013 33166 Saint Médard en Jalles Cedex, France 3 GRESPI, Faculté des Sciences, Université de Reims, BP1039, 51687, Reims, France ABSTRACT The objective of this work is to study the effect of taking into account the non-isothermal conditions on the prediction of hygrothermal behavior of a hemp building envelope and on the hygrothermal comfort in building. At the beginning, two mathematical models (non isothermal and isothermal models) to describe the coupled heat and mass transfer in porous materials have been investigated. Both models are one-dimensional and analyzed, using finite difference technique with an implicit scheme. Then, numerical results are compared to experimental data. The comparison has shown that the model which disregards the effect of temperature on the sorption cannot predict correctly one-dimensional heat and moisture transfer through the building envelope. Finally, the application of both models at whole building level for predicting the hygrothermal comfort conditions has been conducted. INTRODUCTION Temperature and relative humidity are important parameters influencing perceived indoor air quality and human comfort. High moisture levels can damage construction and inhabitant’s health. High humidity harms materials, especially in case of condensation and it helps moulds development increasing allergic risks. Consequently, several researchers have studied the use of various hygroscopic materials to moderate indoor humidity levels. The material that absorbs and desorbs water vapor can be used to moderate the amplitude of indoor relative humidity and therefore to participate in the improvement of the indoor quality and energy saving (Olalekan and Simonson, 2006; Tran Le et al., 2010; Colinart et al., 2012). Vegetal fiber materials are an interesting solution as they are eco materials and have low embodied energy. Hemp concrete is one of these materials which is more and more recommended by the eco-builders for its low environmental impact. The physical properties of hemp concrete has been measured by many authors ( Rahim et al., 2015, Collet et al., 2008; Cerezo, 2005; Evrard and De Herde, 2010; Pretot et al., 2014). It is highlighted that the one presents high moisture buffering capacity and a good compromise between insulation and inertia materials (Maalouf et al., 2014). MATHEMATICAL MODELS Heat and moisture transport in porous building materials Mechanisms of moisture transport in a single porous building material have been extensively studied (Philip and De Vries, 1957; Kunzel, 1995; Mendes et al., 2003). Most of the models have nearly the same origin; the main difference among them is related to particular assumptions used. In this article, the model that takes into account liquid and vapor moisture transport is used (Mendes et al., 2003). Forms of moisture transport depend on the pore structure as well as on the environmental conditions. The liquid phase is transported by capillarity whereas the vapor phase is due to the gradients of partial vapour pressure. With these considerations, the mass conservation equation becomes: ∂θ ∂ ∂T ∂ ∂θ = DT + Dθ (1) ∂t ∂x ∂x ∂x ∂x with the following boundary conditions respectively for the external (x=0) and internal (x=L) surfaces of the wall: ∂T ∂θ − ρl DT + Dθ = hM , e ρ v, a,e − ρv, s,e ∂ x ∂x x = 0,e ( ) (2) ∂T ∂θ − ρl DT + Dθ = hM ,i ρ v, s,i − ρ v, a ,i ∂x ∂x x = L,i ( ) (3) where the subscript a represent the adjacent air and s the solid surface of the material, while the subscripts e and i correspond respectively to the external and internal neighbouring environment (a) or solid surface (s). One dimension of the energy conservation equation with coupled temperature and moisture for a porous media is considered and the effect of the - 2222 - Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. adsorption or desorption heat is added. This equation is written as: ρ 0 Cp m ∂T ∂ ∂T = λ ∂t ∂x ∂x ∂ ∂T + Lv ρ l DT ,v + ∂ x ∂x ∂ ∂θ Dθ ,v (4) ∂x ∂x where Cpm is the average specific heat which takes into account the dry material specific heat and the contribution of the specific heat of liquid phase. + Cpm = Cp0 + Cpl ρl θ ρ0 (5) λ is thermal conductivity depending on moisture content. , Indoor conditions , Outdoor conditions , , Convective heat and mass transfer Convective heat and mass transfer , , Heat and mass transfer Figure 1 Schematic representation of heat and moisture transfer through the wall. Figure 1 shows the boundary conditions that take into account convective heat transfer, radiative heat transfer and heat associated to phase changes, as expressed in the right side of Eqs. (6) and (7) for the external and internal surfaces, respectively. −λ ∂T ∂T ∂θ − Lv ρl DT ,v + Dθ ,v = hT ,e Ta,e − Ts,e ∂x ∂x ∂x x=0,e ( ( + L v h M ,e ρ ve , a , e − ρ ve , s ,e ) ( ( ) Effect of temperature on the sorption characteristics Up to now, many studies have been carried out to measure the general shape of the isothermal sorption characteristic. However, the physics related to the isothermal sorption curves are still disputed. The researches showed that the sorption capacity of materials depends on the temperature (Carsten and Clorius, 2004; Ait Oumeziane, 2014; Navi and Herger, 2005). Increasing temperature will entail that the isosteric moisture content be reached in equilibrium with a higher relative humidity. The study of its effect on the hgrothermal behavior for the bio-based materials is new (Carsten and Clorius, 2004). Concerning the hemp concrete, Ait Ouméziane (2014) showed that taking into account its influence is necessary. In this article, we use two models describing the relation between sorption characteristics at different temperatures: Milly's model (Milly, 1982) and Poyet's model (Poyet and Charles, 2009). Milly's model is based on the effect of temperature on the intrinsic properties of water to establish the sorption curves. The temperature dependent sorption characteristics are expressed as: Cϕ (T2 −T1 ) ϕ2 (θ , T2 ) = ϕ1(θ , T1)e (6) ∂T ∂T ∂θ − λ − Lvρl DT ,v + Dθ ,v = hT ,i Ts,i −Ta,i ∂x ∂ x ∂x x=L,i + L v h M ,i ρ ve , s ,i − ρ ve, a ,i ) Pvs (T ) ∂ϕ (8) ρl ∂θ The equations contain several parameters that are themselves function of the state variables. The special interests of the model are the dependencies of moisture content, moisture transport coefficient, thermal conductivity ,etc. upon the relative humidity and temperature. This makes possible to take into account the temperature-sorption dependence into the model, which will be presented in the next subsection. Dθ = π (9) where the coefficient Cϕ is given as: 1 ∂ϕ (10) ϕ ∂T However, Poyet (2009) showed that this consideration is not sufficient to well predict the hygrothermal behavior of concrete. Thus, the authors propose one another model based on the differential heat of sorption, which is written as: Cϕ = ) (7) In the case lacking a data base of moisture transport coefficients, simplified models should be used. The use of simplified mathematical models has varying effects on accuracy and has been discussed in (Mendes et al., 2003). In this article, the effect of temperature gradient on moisture diffusion is neglected which is generally accepted. The moisture diffusion coefficient related to moisture content gradient is evaluated as: qst (θ ) P (T ) ϕ(T2,θ ) = ϕ(T1,θ ) sat 1 e Psat(T2) Ml T2 −T1 R T1T2 (11) where: Ml: molar mass of water [kg.mol-1]; R: ideal gas constant [J.mol-1 K-1]; qst : isosteric heat [J.kg-1], which is calculated from two sorption isotherms at two different temperatures (T1 and T2). - 2223 - Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. Air model In order to model heat and mass transfer in the room, we used the nodal method, which consider the room as a perfectly mixed zone. The energy equation and the mass balance equation for room air can be written as: ∂T ρi c pV = ΦWest − Φ East + Φ South − Φ North ∂t +ΦBottom − ΦTop + ΦSource (12) V H: enthalpy of moist air (KJ/kg) Acceptability scale ranges from +1 (clearly acceptable) to -1 (clearly unacceptable). The empirical coefficients a and b have been calculated for polluted and unpolluted air with five building materials at two different loading levels. For clean air and air polluted with carpet and sealant, the constants a, b and the acceptability function are shown in Table 1. These formulas have been used to predict the PD and the indoor air acceptability. ∂ρ i = QmWest − QmEast + QmSouth − QmNorth ∂t +QmBottom − QmTop + QmSource Table 1 a, b and acceptability for the clean air and air polluted air (based on Fang et al., 1998). (13) where: ρi : air density (kg.m-3) Φ: Heat flux (W) Qm : Air flow rate (kg.s-1) Φsource : Heat source power (W) w : Humidity ratio (kg.kg-1). Prediction of percentage perceived discomfort (PD) and indoor air acceptability (Acc) The literature shows that indoor humidity has a strong effect on the acceptability and freshness of indoor air. Toftum (1998) found that 1°C change in air temperature had the same effect on acceptability and freshness as 121 and 130 Pa change in vapour pressure, respectively. For thermal sensation, a 1°C change in air temperature corresponded to 231 Pa change in vapour pressure. Investigations showed that complaints about bad indoor air quality increased with a higher enthalpy (H) of the air (Hens, 2011). The enthalpy of moist air is the sum of the enthalpy of the dry air and the enthalpy of the water vapour. The enthalpy of moist air can be described as: H=1.008T+w (2500.8+1.805T) (14) Where: T: dry-bulb temperature of air-vapour mixture (°C); w: humidity ratio, kg of water vapour/kg of dry air. The statistical relation found between enthalpy and number of dissatisfied (with 20% dissatisfied is handled as the limit) looks like (Hens, 2011): exp[− 0.18 − 5.28(−0.0333H + 1.662)] PD = (15) 1 + exp[− 0.18 − 5.28(−0.0333H + 1.662)] Fang (1998) studied 40 subjects which were facially exposed to air supplied through a diffuser under laboratory conditions. The subjects assessed the acceptability of polluted and unpolluted air at different humidity and temperatures. Based on the response of the subjects, the acceptability of air can be expressed as function of the enthalpy of air: Acceptability= a. H + b (16) Where: a and b: empirical coefficients Case study Clean air a -0.033 b 1.662 Air polluted with carpet Air polluted with sealant -0.023 0.966 -0.013 0.263 Acceptability Acceptability= 0.033H + 1.662 Acceptability= 0.023H + 0.966 Acceptability= 0.013H + 0.263 In order to solve the previous equation system, the numerical solution is based on the finite difference technique with an implicit scheme. To solve this system of equations, we used the Simulation Problem Analysis and Research Kernel (SPARK) which is especially suited to solve efficiently differential equation systems (Sowell and Haves, 2001; Mendonça, 2004; Wurtz, 1995). NUMERICAL STUDY AND EXPERIMENTAL VALIDATION AT WALL LEVEL Experiment and simulation conditions This section concerns the validation of the numerical model by comparing the simulation results with experimental ones. For this, we use the results obtained from the experimental facility developed at the laboratory by Samri (2008). One side of tested wall was submitted to various outdoor conditions of temperature and relative humidity using a climate chamber, while other side of the wall was in contact with the laboratory ambient where temperature and relative humidity are relatively constant. The test wall was instrumented with sensors which are connected to an acquisition system to measure the hygrothermal profiles. It consists of a hemp concrete wall with 30 cm of thickness. The wall was subjected to cyclic step-changes in relative humidity and temperature: 20°C/50% RH during 24h followed of 10°C/80% RH during 24h; 40°C/45% RH during 24h and finally finished with 20°C/50% during 24h (see Figure 2). - 2224 - Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. In addition, to test the effect of using the different sorption curves, for each model, three cases have been considered: • Adsorption curve • Desorption curve • Mean sorption curve obtained from the average between adsorption and desorption. In this paper, the isosteric heat adsorption of hemp concrete has been determined by using the sorption isotherms at 10° and 23°C (Ait Oumeziane, 2014). For the both indoor and outdoor surfaces, the heat and mass transfer coefficients are 9 W.m-2.K-1 and 0.003 m.s-1, respectively. The time step is 240 seconds and the wall was discretized into 25 nodes according to one sensitive study conducted by Tran Le (2009). In order to facilitate the investigation, only the results obtained in the middle of the wall (point C) will be presented. Step-changes in relative humidity and temperature 80 80 60 60 50 40 T 40 HR 30 20 20 10 Relative humidity (%) Temperature (°C) 70 Isothermal hygrothermal behavior of hemp concrete wall: Isoth model The comparison between the variation of temperature and relative humidity at point C obtained from the simulation using the Isoth model and the one from the experimental measurement is presented in Figure 3 and Figure 4. 0 0 12 24 36 48 60 Time (h) 72 84 96 Figure 2 Experimental procedure : Step-changes in relative humidity and temperature. Table 2 Hygrothermal properties of hemp concrete (Ait Oumeziane, 2014) T at C Testing T at C Simulation_Adsorption T at C Simulation_Mean curves T at C Simulation_Desorption T outside 60 Density Thermal conductivity Specific heat capacity Water vapor permeability kg/m3 320 W/(m.K) 0.091 J/(kg.K) 1250 Kg/(m.s.Pa) 1.10-10 Temperature (°C) 50 40 30 20 10 0 The hygrothermal properties of hemp concrete measured by (Ait Oumeziane, 2014) are used for the simulation. Some basic hygrothermal data of hemp concrete is given in Table 2. It is noticed here that some authors have validated their models with constant coefficients using the results of this experimental case (Tran Le et al., 2014; Samri, 2008). However, the fact that a value of about 3.10-7 (m²/s) of mass transport coefficient associated to a moisture content gradient was used for their validations is not realistic. Therefore, this article focuses on a more completed model that takes into account the effect of the temperature-dependent sorption characteristics on the hygrothermal response of hemp concrete wall. The simulation has been realized for two following models: • Isoth: Using the isothermal sorption characteristics in the simulation • Non-Isoth: Taking into account the effect of temperature on the sorption curve by using the Poyet's model or Milly's model 20 40 60 Time (h) 80 100 Figure 3 Variation of temperature at point C – Isoth model and experimental measurement One can observe that the variation of temperature is very similar for the three studied cases (adsorption, desorption and average curve). The model gives a quite satisfactory prediction of temperature within the wall, despite some discrepancy between experimental and predicted values. Concerning the variation of relative humidity, the computed results did not fit to the experimental ones. This should be explained by the fact that the studied model neglected the effect of temperature on the moisture sorption capacity of the material, in which increasing temperature results in increase of the relative humidity at given water content. Therefore, the dependency of sorption characteristic on temperature has been taken into account in the physical model and the result will be presented in the following subsection. - 2225 - Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. experimental ones is 3% RH. This value is small compared to the accuracy of sensor inserted in the tested wall, which is ±1.5% of RH. RH at C Testing 100% RH at C Simulation_Mean curves RH at C Testing RH at C Simulation_Desorption 80% 100% RH at C Simulation_Adsoprtion RH outside Relative humidity (%) Relative humidity (%) RH at C Simulation_Adsoprtion 60% 40% 0 20 40 60 80 RH at C Simulation_Mean curves RH at C Simulation_Desorption 80% RH outside 60% 100 Time (h) 40% 0 Figure 4 Variation of relative humidity at point C – Isoth model and experimental measurement T at C Testing T at C Simula tion_Adsorption T at C Simula tion_Mea n curves T at C Simula tion_Desorption T outside Temperature (°C) 50 40 30 20 10 0 20 40 60 80 40 60 80 100 Time (h) Non-Isothermal hygrothermal behavior of hemp concrete wall: Non-Isoth model As mentioned above, both Milly's model and Poyet's model have been used to study the impact of non-isothermal conditions on the hygrothermal behavior of hemp concrete. Because the results obtained from the Milly's model is very close to those from Isoth model, they are not depicted here. This subsection focuses only on the results obtained by using Poyet's model and the comparison between its results and experimental data are shown in Figure 5 and Figure 6. 60 20 100 Time (h) Figure 5 Variation of temperature at point C – NonIsoth model and experimental measurement As can be seen from Figure 5 , the numerical results are in accordance to the experimental results. In addition, they are very close to those obtained by using the Isoth model (by comparing Figure 5 with Figure 3). Figure 6 showed that compared to the final one, the Non-Isoth model results in significantly better prediction of the relative humidity variation in the tested wall. Concerning three cases studied, the results are dependent on which the sorption curve is used for the simulations. The results calculated with the adsorption allow a better prediction of the relative humidity variation than those for the model that uses desorption curve or an average sorption curve between adsorption and desorption. The maximum difference between the computed results for the model that uses adsorption curve and the Figure 6 Variation of relative humidity at point C – Non-Isoth model and experimental measurement It can be drawn a conclusion that it is necessary to take into account the non-isothermal conditions on the sorption curves in order to well predict the hygrothermal behavior of the wall submitted to the dynamic conditions of temperature and relative humidity. EFFECT OF NON-ISOTHERMAL CONDITONS AND MASS TRANSFER WITHIN THE BUILIDNG ENVELOPE ON THE HYGROTHERMAL COMFORT To investigate the effect of the dependency of sorption characteristic on temperature on the hygrothermal comfort at whole building level, the hygrothermal behaviour of a bedroom which has a space area of 20 m2 and a volume of 57 m3 has been studied. It is noted that the numerical model for the whole building level has been fully validated in (Tran Le et al., 2010).The plan of the studied room is depicted in Figure 7. The west and south facades are in contact with outdoor conditions while other walls are considered as internal partitions and in contact with heated space at 19°C temperature and 50 % relative humidity. External and internal walls are made of hemp concrete and have 40 cm and 20 cm thickness respectively. It is considered that no moisture diffusion occurs through the floor and the ceiling. The bedroom is equipped with a radiator and a PI controller that keeps operative temperature at 19 °C. The bedroom is ventilated at 0.5 Ach. Heat transfer coefficients are hT,e= 16 W/m2.K for the outdoor surfaces and hT,i= 4 W/m2.K for indoor surfaces. The mass convection coefficients were calculated by using the Lewis relation and considering the Lewis number equals to 1. The room is occupied from 23.00 pm to 7.00 am by two persons. The initial relative humidity and temperature in the walls and the studied room are respectively 50 % and 19 °C. - 2226 - Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. variation of temperature in this thin layer results in a small effect of temperature-sorption dependence on the indoor humidity. By comparing the Th model with other ones, the results show that moisture buffering of hemp concrete could considerably dampen the indoor relative humidity variation and thus notably reduce the enthalpy of moisture air. Following this, it reduces the PD values which are shown in Figure 9 because bad indoor air quality increased with a higher enthalpy. For example, at 71th day, during the occupied period, the maximum value of PD of the Non-Isoth model is 13.5% compared to 27 % of Th model, so a relative increase of 50% was observed. PD _Isoth model PD_Non-Isoth model PD _Th model Water vapour source (g/h) 400 320 20% 400 320 60% 240 160 40% 80 20% 160 10% 80 0% 0 70 71 71 72 73 74 73 74 75 Time (day) Figure 9 Percentage of dissatisfied of the indoor air for the studied models Figure 10 presents the indoor air acceptability calculated for the "clean air" case during the occupation time. The Acc values of Th model are lower than those of Non-Isoth or Isoth models. This fact signifies that the indoor air quality is more clearly acceptable when the moisture exchange between the building envelope and surrounding air is taken into account. Concerning the effect of nonisothermal conditions on PD and Acc, it is very small and neglected. Acc_Isoth model Acc_Non-Isoth model Acc_Th model Water vapour source (g/h) 0 70 72 75 Time (day) 0,8 400 320 0,6 Acc Figure 8 Indoor relative humidity for the studied models Figure 8 compares the predicted indoor relative humidity simulated of three models during 5 days (from 70th to 75th day). It can be seen that the effect of temperature-sorption dependence on the indoor relative humidity is neglectable even if its effect is important on wall level, which has shown in the last section. It is because only the moisture buffering capacity of a thin layer at the indoor surface that exchanges the moisture with indoor air contributes principally to moderating the variation of indoor relative humidity. Therefore, a significantly reduced - 2227 - 240 0,4 160 0,2 80 0,0 0 70 71 72 73 74 75 Time (day) Figure 10 Acceptability of the indoor air for the studied models Water vapour source (g/h) Relative humidity (%) 80% Water vapour source (g/h) Simulations are run for three months in winter using Nancy weather data (in France) by considering three models: • Isoth and Non-Isoth (Poyet's approach) models that use adsorption curve • Th model which neglects the water vapour exchange between the building envelope and indoor/outdoor air (only heat transfer is considered and the envelope is fully coated with aluminum foil) Indoor RH _Isoth model Indoor RH _Non-Isoth model_ads Indoor RH _Th model Water vapour source (g/h) 240 PD (%) Figure 7: Plan of the studied room Water vapour source (g/h) 30% Proceedings of BS2015: 14th Conference of International Building Performance Simulation Association, Hyderabad, India, Dec. 7-9, 2015. CONCLUSION This article focuses on the development and use of a numerical model that takes into account the effect of temperature on the sorption characteristics in the transient modeling of coupled heat and mass transfer in porous materials. In order for this to be done, two models (Isoth and Non-isoth that uses the approaches proposed by Milly and Poyet) have been carried out and implemented in the simulation environment SPARK which is suited to solve efficiently differential equation systems. The numerical results were then compared to the experimental ones. The results showed that taking the influence of temperature on the sorption curves into account is necessary for better prediction of the hygrothermal behavior of a hemp concrete wall. Both Isoth and Non-isoth models predicted well the variation of temperature. However, only Non-isoth model that uses the Poyet's approach is adapted to study the variation of the relative humidity in the wall. At whole building level, it can be drawn a conclusion that that moisture buffering capacity of hemp concrete could considerably dampen the indoor relative humidity variation and thus notably reduce the percentage perceived discomfort and increase indoor air acceptability during occupied period. In addition, the effect of temperature-sorption dependence on the hygrothermal comfort is very small and could be neglected. H Enthalpie of mois air KJ.kg-1 hM Mass transfer convection coefficient m .s-1 hT Heat transfer convection coefficient W.K-1.m-2 Lv Heat of vaporization J.kg-1 T Temperature °C t Time s x Abscise m θ Moisture content m3 .m-3 λ Thermal conductivity W.m-1.K-1 ρ0 Mass density of dry material kg.m-3 ρl Mass density of water kg.m-3 ρv Mass density of vapor water kg.m-3 φ Relative humidity % ρi Air density kg.m-3 Φ Heat flux W Qm Air flow rate kg.s-1 Φsource Heat source power W w Humidity ratio kgkg-1 i Internal e External ACKNOLEGEMENT REFERENCES The authors wish to thank the reviewers for their constructive comments on this article. 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