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[Display omitted] • Courtyard buildings are undergoing a spontaneous evolution, characterized by courtyards being roofed (CBR). • The CBR phenomenon is diverse and has a positive effect on the energy efficiency of existing buildings. • By harvesting solar energy and storing heat, CBR can reduce heat dissipation and shorten the heating period. • The light transmittance of the material and the heat storage space are key factors affecting energy efficiency of CBR. Courtyard building is an important type of vernacular architecture, widely distributed in different climate zones around the world. Traditionally, courtyards are characterized by opening to the sky, providing buffer spaces for ventilation and light for the surrounding buildings. However, in Hebei, China, local buildings are showing an evolution trend characterized by the courtyards being roofed (CBR). This spontaneous evolution presents the diversity of CBR technologies (CBRTs), including light-transmitting CBRTs (T-CBRTs) and non-light-transmitting CBRTs (NT-CBRTs). Based on field measurements and simulation technology, the improvement effect of CBR on building energy efficiency (EE) is analyzed, and the EE of different CBRTs is compared. The results show that, T-CBRT and NT-CBRT can reduce the annual total energy consumption of open courtyards by 19.97% and 15.41% respectively without upgrading the original envelope structure. The analysis of the impact mechanism shows that CBRT increases the temperature of the courtyard and the room by improving the solar energy collection efficiency and expanding the heat storage space, which effectively reduces the heat leakage caused by the envelope and ventilation, and shortens the heating time. The light transmittance of the roof material is the most critical parameter affecting the EE of T-CBRT, and the roof height largely determines the performance of NT-CBRT. Multi-layer glass or concrete panels are not the best choice for CBRT roofing materials, and plastic films or polycarbonate sheets with high light transmittance and anti-aging are recommended. CBR is considered as a promising development direction because it can reduce the economic burden of energy consumption for local farmers by improving the EE of buildings. [ABSTRACT FROM AUTHOR]