Water vapor properties of two hemp wools manufactured with different treatments Florence Collet a, * , Laurent Marmoret b,1 , Hassen Beji b,1 a Université européenne de Bretagne, Laboratoire de Génie Civil et Génie Mécanique, Equipe Matériaux Thermo Rhéologie, France b Université de Picardie Jules Verne, Laboratoire des Technologies Innovantes, Equipe Phénomènes de Transfert et Construction Durable, France article info Article history: Received 22 December 2009 Received in revised form 17 May 2010 Accepted 19 June 2010 Available online xxxx Keywords: Adsorption isotherm Water vapor permeability Fibrous Insulating material abstract Hemp wools are environmentally friendly materials that are used for thermal insulation of building. These fibrous materials are the places of moisture transfer and storage that impact on thermal perfor- mances of the material. In this work, two kinds of hemp wool are studied: hemp wool with an organic binder (called HW1) and hemp-cotton wool with a polyester binder (called HW2). The investigations are based on water vapor sorption and on water vapor permeability measurements. The two kinds of hemp wool show similar sorption isotherm with quite high values of water content. The water vapor per- meability is quite high too. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In a context of sustainable development, environmental aware- ness induces to save energy and resources. The use of thermal insu- lation in building walls contributes to reduce energy needs while the use of renewable raw materials prevents extinction of re- sources [1,2]. Hemp fibers are suitable for insulations due to their thermal properties and some ecological features, i.e. biodegradabil- ity [3]. Nowadays, the European market of insulation materials is still characterized by the domination of inorganic fibrous materials and organic foamy materials [4]. Environmental and public health aspects play an increasing role. Insulation materials have to im- prove their performance, but they also have to be adaptive, friendly to the construction site personnel and cost effective. However, hemp fibers as a natural resource have a risk for microbial and other contaminants, and their quality should be monitored regu- larly. Careful procedures during harvesting, processing, manufac- turing, building and maintenance of buildings are required in order to avoid the risk of negative effects (i.e. molding) caused by moisture and free water. So, additives are added along the fi- bers. Like all materials showing open porosity, hemp wools store and transfer moisture. This moisture may reduce the thermal performance of the insulation and may alter the chemical compo- sition of the material (molding in humid conditions). More, simul- taneous heat and mass transfer and storage may lead to water condensation. This condensation damages the insulation and the structure of the building walls. To prevent this, a vapor barrier can be used. The need of vapor barrier may be studied thanks to a numer- ical simulation. If the dew point appears, one should use a vapor barrier. To do such numerical study, hydric properties of the materi- als are needed as input. Also, this paper presents hydric characteriza- tion of two kinds of hemp wools. One is made with an organic binder (called HW1) and the other one is made with a polyester binder (called HW2). The experimental study investigates water vapor sorption isotherm and water vapor permeability. The sorption curves are then compared and fitted with BET and GAB models in order to calculate the specific surface area of the material. More, results are compared with hydric properties of mineral wools. 2. Theory 2.1. Water vapor sorption 2.1.1. Generality Materials that show open porosity adsorb surrounding gas. The water vapor sorption isotherm relates the water content of the material to the ambient relative humidity at constant temperature. In the lower half of the range of relative humidity (0.05 < u < 0.35– 0.40), molecules are strongly sorbed in a monomolecular layer. With increasing relative humidity, molecules are sorbed in multi- molecular layers. Lastly, capillary condensation appears at higher relative humidity (Fig. 1). The amount of gas adsorbed by physisorption (monolayer, mul- tilayer and capillary condensation) depends on the surface area and on the porous structure of the material. The IUPAC [5] gives 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2010.06.069 * Corresponding author. Address: IUT Génie Civil, 3, rue du Clos Courtel, BP 90422, 35704 Rennes, France. Tel.: +33 2 23 23 40 56; fax: +33 2 23 23 40 51. E-mail address: florence.collet@univ-rennes1.fr (F. Collet). 1 Address: IUT Génie Civil, Avenue des facultés, le bailly, 80025 Amiens, France. Construction and Building Materials xxx (2010) xxx–xxx Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Please cite this article in press as: Collet F et al. Water vapor properties of two hemp wools manufactured with different treatments. Constr Build Mater (2010), doi:10.1016/j.conbuildmat.2010.06.069