Experiemental Measurement Of The Effective Water Vapor Diffusion Coefficient Of Porous Building Materials By A Transient Technique Toward Energy-efficient Buildings

Energy consumption of the building industry in our country has increased year by year in recent years, so the analysis of building energy consumption and optimization of energy conservation is imperative.For the heat losses in buildings, the heat transmission loss in building palisade structure is as high as 70-80%.Porous materials, with a strong moisture transfer in typical building environment, are used mostly in building palisade structure, which has very important influence on the building envelope thermal performance, building energy consumption and indoor environment.The effective water vapor diffusion coefficient, as one of the important physical parameters for the characterization of porous building materials moisture transfer performance, is the necessary basic physical property data for the quantitative analysis of moisture transfer in building palisade structure, indoor thermal and humid environment. The reliability of its value is of great significance to the accuracy of the analysis results, while the standard steady state test method is time-consuming and of poor accuracy.Therefore, referring to the principle of transient test method for water vapor diffusion coefficient of porous building materials in reference literature, the calculation formula of water vapor diffusion coefficient and relative humidity is obtained based on the solution of one-dimensional mass diffusion inverse problem. Furthermore, the corresponding transient test platform is established adopting high precision capacitive humidity sensor.To test the feasibility of transient test method for the effective water vapor diffusion coefficient, the transient test results of effective water vapor diffusion coefficient of the samples in 25℃ are compared with the literature value and the steady state test results at the same time, with the B04 aerated concretes as objects. The results show that, the change rule with the relative humidity of transient test results and the effective water vapor diffusion coefficient of concrete material reported in the literature is consistent and is the same order of magnitude. In the same relative humidity, the goodness of fit between the transient test values and steady state test values is higher, with a maximum relative deviation being less than 30%. The transient test method apply to the measurement of effective water vapor diffusion coefficient under different relative humidity for porous building materials, with many advantages, such as the measurement time is short, high repeatability, easy operation, etc.In order to analyze the application scope of the transient test method, the applicable material type and relative humidity range are analyzed based on the principle of one-dimensional diffusion model, and the influence of sample size is mainly studied on the result of the transient test method in this article. Results indicate that, the transient test method is applicable to porous building materials which can be easily processed into a rectangular block, and whose averagy holes diameter is greater than 10"6 m. And the test range of the relative humidity is 1-80%RH, which is enough for the actual engineering application. According to the experimental results, the recommended sample size is that, the side length of diffusion cross section is not less than 8 times of reserved holes diameter, and the side length in the water vapor diffusion direction is not less than 2.5 times of length of diffusion cross section.Based on the research of the scope of application of the transient test method, the influence of pore structure of material and the temperature on effective water vapor diffusion coefficient are studied for the typical thermal insulation porous building materials B04, B05 and B06 aerated concrete block. The results show that the aerated concrete effective water vapor diffusion coefficient is influenced by pore size distribution and increases with the rise of temperature, rather than porosity. And it is worth noting that the influence on the water vapor diffusion of the aerated concrete pore structure and temperature is associated with relative humidity conditions, which is because the transfer mechanism water vapor in the aerated concretes under different relative humidity is different. The empirical formula of effective water vapor diffusion coefficient of the B04, B05 and B06 aerated concrete under different absolute temperature and relative humidity is fitted in this article, which can be used to predict three kinds of aerated concrete block effective water vapor diffusion coefficient.