The Effect of Different Bio-based Fiber Systems on the Comprehensive Performance of Thermal Insulation and Moisture-Regulating Rendering Mortar

Focusing on the need for passive moisture regulation in building materials, this paper develops a moisture-regulating mortar system reinforced with a composite of wood and bamboo fibers. It systematically investigates the mechanisms by which different fiber combinations influence the mortar’s physical capacity, pore structure characteristics, thermal performance, mechanical capacity, and moisture buffering capacity. Furthermore, a comprehensive performance evaluation model and the fiber-to-cement volume ratio (FTCR,R _fc ) parameter are introduced to conduct a correlation analysis. The results indicate that the introduction of fibers significantly reduces the dry density of the mortar by approximately 67% and creates a multi-scale pore structure, increasing the specific surface area to a maximum of 54.50 m²/g and lowering the thermal conductivity to 0.097 W/(m·K). In terms of moisture response, the maximum adsorption rate of the bamboo fiber system was approximately 25.8%–40% higher than that of the wood fiber system, and the moisture buffering value (MBV) reached as high as 0.99 g/(m²·RH). Correlation analysis indicated that wood fibers primarily enhance moisture regulation by refining pore size, whereas bamboo fibers achieve this by increasing specific surface area and pore connectivity. Comprehensive performance evaluation results show that the bamboo fiber composite mortar BP-1 achieved the highest overall performance value of 85.81, significantly outperforming both the wood fiber system and ordinary mortar. Further research revealed that while the introduction of diatomaceous earth can optimize the microporous structure, its filling effect disrupts the interconnected pore network, leading to a significant decrease in adsorption capacity and MBV. Overall, bamboo fiber achieves synergistic optimization of thermal-humidity and mechanical capacity by constructing a connected, multi-level pore structure, providing a new structural regulation pathway for the design of high-performance passive humidity-regulating mortars.