Rammed earth (RE) is a low-tech recyclable building material with good heat storage and moisture absorption performance that can better maintain the stability of the indoor thermal environment and improve indoor comfort. With innovations in and the development of new technology, the field of rammed earth construction technology is gradually expanding. However, deficiencies in the thermal insulation of traditional rammed earth structures make it impossible for them to meet China’s building energy codes in cold regions. This study constructs a comprehensive evaluation index of the thermal performance of rammed earth walls that is based on the heat transfer mechanism, optimizing the thickness of the boundary conditions of the building interior’s design temperature, as well as the energy demand and economic efficiency. This research also offers a new design for the thermal insulation of rammed earth construction by combining the building energy savings design code with WUFI Pro software. This study demonstrates that the optimum thickness of rammed earth construction in Beijing is about 360 mm, the thickness of extruded polystyrene board (XPS) is 50 mm (for public buildings) and 70 mm (for residential buildings), and the structural form of external insulation offers the highest performance benefit. In addition, this work also evaluates the risk of condensation inside composite rammed earth construction, finding that there is a risk of condensation on the exterior side of the wall and at the interface between the insulation panels and rammed earth wall, thus requiring an additional moisture-proof layer. In this study, thermal mass and insulation are fully considered and a design strategy for rammed earth construction given quantitatively, providing a theoretical basis for the application of rammed earth materials in cold regions.

The thermal performance of buildings in the south of China focuses on thermal mass design, while in the north it favors thermal insulation design, which makes it impossible to achieve a balance between the thermal mass and insulation. Here, a comprehensive evaluation index is developed to measure the thermal performance of a building’s external envelope, which aims to find out the optimal range of the wall thickness under the influence of the thermal mass and insulation, and to seek the correct balance between a building’s energy consumption and the thermal performance of walls. In this paper, four dimensions, namely the heat transfer coefficient, thermal inertia index, attenuation degree, and delay time, are discussed, and the weight coefficients of each subfactor are calculated and isotropically treated to create comprehensive evaluation indicators. Then the distribution laws of the composite index values of common building materials in different climatic zones are examined. The result shows that the correlation coefficient (R²) between M and building energy consumption is about 0.7736–0.8215, which is higher than 0.3494–0.384, the heat transfer coefficient, and is more accurate in predicting building energy demands. Furthermore, through the analysis of the thermal improvement rate and the building energy-saving rate, the suitable wall thickness of commonly used building materials in different climate zones is determined, and the application prospects of the research results are described. With the above research findings, the thickness ranges of walls can be determined at the initial period of building design by combining regional environmental factors and material characteristics to provide a reference for building energy-saving design.