Space heating accounts for a large part of building energy consumption. Lowering the heating temperature set-point (Tsp) is expected to be a feasible approach for energy efficiency. In this paper, eight globally typical cities are selected, and the energy-saving mechanism and variation trends of lowering heating Tsp are investigated under different working conditions (climate conditions, construction completion year and inner heat sources). The results show that significant energy-saving effects even appear in the relatively warm-winter cities. The energy-saving mechanism is dominated by two different categories of heating hours including the temperature-difference saving (TDS) hours and the behavioral saving (BS) hours. The contribution of TDS and BS to the whole annual heating energy saving amount (HSA) depends on the reducing level of heating hours. The HSA of lowing Tsp is mainly affected by TDS influence. After coupling the consideration of different factors, with the decreasing annual HSA of buildings, the dominance of the TDS influence mechanism shrinks gradually while the annual heating energy saving ratio (HSR) increases. This work provides the analysis method for building heating energy saving potential evaluation and reference for the establishment of standards and residents’ behavioral energy saving in different climatic zones.

Windows have a great impact on building energy consumption, and the thermal performance of window frames directly affects its energy-saving potential. In this paper, a novel method is proposed to optimize the thermal performance of commercially available broken-bridge aluminum window frames, by incorporating radiant insulation panels (RIPs) into the window frame cavity. A typical aluminum alloy window frame heat transfer model is theoretically analyzed and validated, and the effects of key design parameters on the equivalent thermal conductivity (ETC) of the cavity radiation heat transfer and the heat transfer coefficient (U-factor) of window frames are quantitatively analyzed by a finite element simulation method using the THERM software. Moreover, the RIP, the insulation material filling, and low surface emissivity on the thermal performance of the window frame are compared and analyzed. The results show that the RIP is better placed in the middle, the width and quantity of RIPs are negatively correlated with the U-factor, while the surface emissivity of RIPs is positively correlated with the U-factor. Adding RIPs in the cavity can reduce the U-factor of the window frame by more than 7.43%, slightly lower than 8.97% for the filling type, but significantly higher than 0.81% for the low-emissivity type. Inserting RIPs is a simple and effective way to reduce the U-factor of the window frame and have a great potential of use.

Envelope-parameter optimization for residential buildings is strongly climate-dependent, yet previous studies have mainly focused on single parameters or specific regions, limiting cross-climate comparison of parameter priorities. This study establishes a unified DeST-based residential building model to compare the effects of external wall insulation thickness and external surface solar absorptivity on heating load, cooling load, annual total load, and operational carbon emissions across five representative climate zones in China. Yinchuan, Beijing, Chengdu, Guangzhou, and Kunming were selected as case cities. Single-factor analysis, two-parameter coupling analysis, supplementary sensitivity analysis, and load-to-carbon translation were conducted under consistent building prototypes, operating boundaries, and evaluation indices. The results show that the effects of the two parameters differ markedly across climate zones, reflecting a transition in parameter dominance under different heating–cooling load structures. Over the tested ranges, insulation thickness changes annual total load by up to approximately 10%, whereas solar absorptivity changes it by only 0.17–1.91%. Mechanistically, insulation reduces winter heat loss but may suppress summer heat dissipation, while solar absorptivity directly affects exterior-surface solar heat gains. Further carbon analysis shows that load-optimal configurations do not necessarily correspond to carbon-optimal configurations. These findings provide quantitative support for climate-adaptive and low-carbon envelope design.