## 1 Introduction
Rural buildings in the Tibet plateau are generally built of adobe blocks, earth, stone, wood, brick, and concrete block, and the thermal performance of the envelope is generally inadequate. It is difficult to obtain an ideal indoor thermal environment in winter only by habitual construction technology or passive solar technology [24; 20; 25; 26]. Generally, in the heat transfer analysis of the building envelope, the outdoor air temperature is regarded as the external boundary condition, and the heat transfer modification coefficient is adopted to modify the heat transfer amount of the envelope caused by solar radiation. The heat transfer coefficient of all orientation envelopes is usually constant in building thermal design [27]. However, for areas with high solar radiation intensity, there is the considerable difference of solar radiation intensity in different orientations. If the same heat transfer coefficient is applied to the wall design for all orientations, though this can simplify the thermal design and construction of the wall, it neglects the influence of solar radiation difference in different directions on wall heat transfer, and is not effectively conducive to the optimization of the energy-efficient wall. [PERSON] et al. [28] proposed the design method of \"no-uniformity insulation envelope\" based on the difference of solar radiation thermal action in the separate direction of the envelope to improve the indoor thermal environment. Nevertheless, the above method of no-uniformity insulation envelope configuration was built on a single-space solar building. The heat transfer process of multi-space solar buildings is more complicated than that of single-space buildings. It was indicated that the external boundary thermal conditions varied with the orientation, and an open thermodynamic system which had energy exchanges with the outer surroundings and adjacent spaces in multi-space buildings [29; 30]. In other words, in the multi-space solar building, there existed simultaneously diversified indoor and outdoor thermal boundary conditions, and heat transfer between the indoor and outdoor, and between adjacent indoor spaces. Obviously, thermal resistance change of exterior walls in different directions has diverse effects on the indoor thermal environment. The renovation that effectively combines the optimization of the wall thermal performance and the indoor space-arrangement is a useful idea to improve the indoor thermal environment in the passive solar building in winter. However, related research has been scarcely reported. In this paper, we present results from the field survey and measurement designed to obtain the characteristics of residential buildings and the climatic characteristics in the Lhasa area. Based on the local climatic and building feature, the mathematical models of the multi-space solar buildings were established. The coupling influences of indoor space-arrangement and exterior wall thermal resistance on the indoor thermal environment of multi-space solar building were analyzed. In view of the fact that the indoor air temperature is a key parameter for characterizing the indoor thermal environment, in order to simplify the analysis and evaluation, in this study, the indoor air temperature was adopted as an indoor thermal environment evaluation parameter. The result of this study can provide reference for the optimization of solar building thermal design.