Recently, the number of buildings with curved surfaces has been increasing. Although these buildings are not complex from a design perspective, they are thermally complex from an environmental engineering perspective. Computational fluid dynamics (CFD) is a tool used in environmental engineering analysis that requires advanced analysis techniques. Its analysis load is high, making its use impractical in design. Therefore, a coupled analysis method was developed using the two-dimensional heat flow calculation tool Hygrabe, energy simulation, and CFD. Using this method, the environmental analysis of a building with fill in Hokkaido, Japan, was performed and the accuracy of the analysis was verified. The results of the coupled analysis model were used to evaluate thermal insulation performance. The high thermostability of the fill contributes to the high degree of freedom of the exterior skin and is highly useful for design. The results show that the thermal performance of the building envelope with and without fill did not change the insulation performance. The results for January 15 were below −3.0 °C for all insulation performance levels but higher than the outside air temperature during the night.