Numerical investigation of the effect of non-isotherme sorption characteristics on hygrothermal behavior of two bio-based building walls M. Rahim n , A.D. Tran Le, O. Douzane, G. Promis, T. Langlet Laboratoire des Technologies Innovantes (LTI), University of Picardie Jules Verne, Avenue des, Facultés - Le Bailly, 80 025 Amiens Cedex 1, France article info Article history: Received 11 March 2016 Received in revised form 1 July 2016 Accepted 4 July 2016 Available online 5 July 2016 Keywords: Hygrothermal behavior Hemp concrete Rape-straw concrete Moisture buffer potential Temperature-dependent sorption abstract This article deals with the coupled heat and mass transfer in porous materials. To predict the humidity in porous materials, many simulation tools use the sorption isotherms. However, further research has shown that sorption capacity of material depends on temperature. The aim of this article is to investigate the effect of the temperature dependency of the sorption curves on the hygrothermal behavior of a two bio-based building envelope. The numerical model takes into account the effect of temperature on the sorption characteristics using Poyet's approach. According to results for static and dynamic outdoor conditions and for both hemp and rape-straw concrete walls, the effect of temperature on the sorption curve has a significant impact on the hygrothermal behavior of the studied walls. In addition, the effect of moisture buffering capacity of these materials on hygric indoor comfort in a room has been investigated. The simulation shows the both materials can reduce 80% of the amplitude variation of indoor relative humidity. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction To maintain the ambient building temperature on the comfort level, the air conditioning control system corrects continuously the injected heat. While, the relative humidity is frequently freely evolves because it is considered less important than the tem- perature. However, research has shown that relative humidity af- fects the perception of indoor air quality [1]. High relative hu- midity within or near a wall promotes the development of mites, mold, and leads an increasing allergic risks [2,3] and harms ma- terials [4,5]. Therefore, several researchers [6–9] have studied the use of various hygroscopic materials to moderate indoor humidity levels. These studies have shown that hygroscopic materials are able to moderate the indoor humidity levels and thus improve the hygrothermal comfort and perceived indoor air quality in build- ings [9–14]. Among those materials, bio-based concrete becomes popular in the field of building construction because it has a low environmental impact, corresponds well to High Environment Quality Buildings [15,16] and has good hygothermal proprieties [17,18]. Up to now, hemp lime concrete (HLC) merits a special attention and can be considered as the reference material because it has been extensively studied in many researches. Likewise, others concretes divided plant as rape-straw or flax shivs have been studied previously and the results showed that the hygrothermal proprieties of these materials are similar to that of hemp concrete [11,19,20]. These materials have low thermal conductivity ( E0.1 W m À1 K À1 ) [20] and can therefore be used as a material for building thermal insulation for reducing the heating energy consumption. In term of hygric proprieties, hemp and rape-straw concretes have high water vapor permeability and have a good moisture buffer capacity [12,19,21]. At whole building level, using materials with a high moisture capacity can effectively reduce the amplitude of the daily variation of humidity and, thus, may posi- tively affect HVAC (Heating Ventilation and Air-Conditioning) system energy consumption or indoor air quality [14,22,23]. The study of Tran et al. [14] showed that, compared to the constant ventilation rate case the use of coupled relative humidity venti- lation strategy and moisture buffering capacity of hemp concrete can reduce energy consumption about 15%. Several simulation and experimental studies have been done [24–30] to investigate the hygrothermal behavior at wall scale. Regarding the hemp concrete building envelop, the results show a strong coupling between temperature and relative humidity [26,28], and the appearance of condensation/evaporation of water vapor which is accompanied by the increase/decrease of tem- perature within the wall [25,27]. Simulation is a good tool to study the hygrothermal behavior of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jobe Journal of Building Engineering http://dx.doi.org/10.1016/j.jobe.2016.07.003 2352-7102/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: mourad.rahim@u-picardie.fr (M. Rahim). Journal of Building Engineering 7 (2016) 263–272