As environment-friendly building materials, earth materials are attracting significant attention because of their favorable hygrothermal properties. In this study, the earth materials in northwest Sichuan were tested and curves of thermal conductivity and water vapor permeability with relative humidity were obtained. The function curves and constants of the two coefficients were substituted into the verified nonstationary model of heat and moisture transfer in rammed earth walls and indoor air

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... or calculation. The difference in the calculation results when the hygrothermal parameters are functions and constants were analyzed, and the influence of the non-constant hygrothermal parameters on the heat and moisture transfer in rammed earth walls, was obtained. The test results show that thermal conductivity is linearly related to moisture content, and water vapor permeability has a small variation in the relative humidity range of 0–60% and increases exponentially above 60%. The calculation results indicate that the non-constant hygrothermal parameters have little influence on the internal surface temperature of the rammed earth walls and Mianyang City's indoor air temperature and humidity during the summer and winter. The heat transfer on the internal surface will be underestimated by using a non-constant for the hygrothermal parameter when the moisture content of the wall is low, and vice versa. In hot-humid areas or seasons with large differences in temperature and humidity between indoors and outdoors, non-constant hygrothermal parameters have a more obvious effect on heat transfer on the internal surface of the wall. The results of this study demonstrate the necessity of parameter testing.