## 1 Introduction  
Relative air humidity is one of the important environmental parameters closely related to daily life and productivity [1]. Many evidences have indicated that high or low indoor humidity environments are closely related to many health problems [2; 3; 4], energy consumption [5] and the durability of the building envelope [6; 7]. Therefore, it is essential to maintain the indoor humidity environment steady within an acceptable range of 40-70% [8]. It is well known that porous materials have excellent adsorption properties. For example, hygroscopic building materials can absorb moisture in a high humidity environment and desorb it in a low humidity environment, and thus they can adjust the indoor relative humidity without energy consumption. These materials have drawn increasing interest of researchers. The humidity level in a building depends on a combination of factors, such as moisture sources, ventilation and air movement, sources and sinks, heating, insulation, external conditions, as well as building materials and occupant behaviour. Among these, the moisture buffering performance of the materials used in a building is an important factor. Researchers have carried out extensive studieon the microstructure, the adsorption/desorption isotherm, and moisture absorption performance of composite hygroscopic materials. [PERSON] et al. [9] investigated specific surface area of different bentonite considering them strictly relating to microporosity. Several studies have investigated the hygrothermal properties of unfired clays and shown their potential for passive buffering of relative humidity fluctuations in building physics applications [10; 11; 12; 13; 14]. Standard evaluation methods for the assessment of the moisture buffering performance of building materials have been issued in Japan, America and China. The moisture buffering characteristics of hygroscopic materials used for interior finishing or furnishing materials in buildings can improve thermal comfort and save energy by reducing the operating hours and/or the size of HVAC systems [15; 16; 17; 18; 19]. [PERSON] et al. [20] investigated the hygroscopic adsorption/desorption and thermal properties of expanded vitrified small hollow ball (EVSB) and normal expanded perlite particle (NEPP) mortars. [PERSON] et al. [21; 22] used the moisture buffer value (MBV) testing method to investigate the factors influencing the buffering capacity of plywood in two different facilities. [PERSON] et al. [23] moulded a series of wood fibre, sepiolite and expanded perlite (WSE)-based mortars, and tested the hygroscopic performance, thermal and mechanical properties of these mortars. They pointed out that WSE-based mortars have good moisture adsorption and desorption properties. Moisture buffering is the dynamic process of the interaction between the hygroscopic materials and the indoor humid air, and it is related to the building structure characteristics. In previous research, priority was given to analysis and evaluation the moisture absorption ability of the specimen of hygroscopic building materials. The NORDTEST Project [24], Japanese Industrial Standard (JIS) A1470-1 [25], and ISO/DIS 24353 [26] have proposed experimental methods to evaluate the moisture buffering performance. However, all these methods assume a well-mixed room air and only tested a material sample in a small climatic oven without ventilation. These evaluation methods are limited to the material level. The moisture buffering performance is often disregarded by building designers and engineers. The moisture buffer capacity of the wall covering materials, as well as furniture and textiles inside the buildings, define the hygroscopic inertia level of a room, which can play an important role in the reduction of RH peaks. There are few studies about the application of hygroscopic materials for building envelopes and its impact on the indoor environment.