Traditional houses have adapted to the local climates and cultures, with mud walls serving as a classic example. As the demand for a decarbonised society increases, the heat storage performance of traditional mud walls warrants re-evaluation to enhance their energy efficiency. This study quantitatively evaluated the differences in heat storage effects caused by variations in the layout and thickness of mud walls in a house. This study also identified design methods that achieve both energy efficiency and thermal comfort. A house model was created based on a survey of mud houses and simulated using a heat and ventilation network model calculation program. The simulation compared external and internal walls made of plasterboard and mud with thicknesses of 80 or 300 mm. The results showed that with no air conditioning, the thermal autonomy was high throughout the year, especially in winter, when thick mud walls were used for internal walls. With intermittent air conditioning, mud walls reduced the peak heating and cooling loads, compared with plasterboard walls, and the time spent within the predicted mean vote (PMV) comfort zone nearly doubled. In the case of 24-h continuous air conditioning, thick mud walls used for internal walls reduced the energy consumption for air conditioning. The findings of this study demonstrate that incorporating traditional mud walls as a passive temperature control method in modern housing designs can improve energy efficiency, enhance occupant comfort, and support the development of zero-emission buildings by reducing the dependency on heating, ventilation, and air conditioning (HVAC) systems.