Research on Energy-Saving Renovation of Building Envelope Structures in Rural Areas of Central Plains of China

Rural residential buildings in China’s Central Plains region suffer from poor envelope thermal performance, resulting in severe thermal discomfort and excessive energy consumption for both winter heating and summer cooling. This study presents a systematic three-in-one envelope retrofit strategy (roof, exterior walls, and windows, with supplementary door replacement) tailored to the region’s cold climate zone (GB 50176-2016, Zone IIB; HDD18 = 2309 °C·d, CDD26 = 131 °C·d), which experiences hot summers and cold winters and thus requires both effective winter insulation and summer heat protection, utilizing cost-effective materials suitable for rural construction. A typical brick-concrete rural residence in Anyang was selected as the case study building. Field measurements of indoor thermal conditions were conducted over 72 h in winter to characterize baseline performance. An Ecotect simulation model was developed and calibrated against measured data using actual hourly meteorological observations from the Anyang National Meteorological Station for the monitored period; the CSWD Typical Meteorological Year (TMY) file was used for the annual simulation. To address concerns regarding discontinued software, key annual load results were cross-validated against an independent EnergyPlus v22.2 model using identical geometry and envelope inputs, yielding agreement within 5%. The proposed retrofit scheme retains the existing 240 mm solid brick walls, adds external insulation consisting of 100 mm EPS panels for walls and 50 mm XPS panels installed at ceiling level within the attic (without disturbing the existing asbestos-cement roof sheeting, in accordance with strict asbestos-handling protocols) for roofs, and replaces single-glazed windows with 6 + 12A + 6 insulated hollow glass units. The results demonstrate that the optimized envelope significantly reduces overall heat transfer coefficients: wall U-value decreases from 1.79 to 0.32 W/(m2·K), roof U-value from 2.46 to 0.48 W/(m2·K), and window U-value from 6.40 to 2.40 W/(m2·K). The passive adaptability index (PAI)—the annual proportion of free-running hours within the fixed 18–28 °C screening band specified in Table 4.3.1 of GB/T 50785-2012—improves from 0.41 to 0.68, and annual heating and cooling energy consumption is reduced by 50.4% (49.6% when the supplementary door replacement is excluded; (123 − 62)/123 = 49.6%). Under this fixed-band screening criterion, indoor operative temperature lies within the band for 68% of annual hours, compared with 41% in the baseline; these figures are fixed-temperature-range screening results rather than a formal adaptive-comfort evaluation (Section 3.3 and Section 6.3). A Glaser method condensation analysis confirms no interstitial condensation risk in the externally insulated EPS wall assembly. This study provides validated, region-specific technical parameters and demonstrates that a coordinated three-component envelope retrofit can achieve over 50% energy savings while substantially improving indoor thermal conditions as measured by the fixed 18–28 °C screening band in rural Central Plains buildings; a formal adaptive thermal comfort evaluation was not conducted and is identified as future work (Section 3.3 and Section 6.3). The material-cost-based simple payback is approximately 5.5 years (CNY 19,500 ÷ CNY 3550/year); including estimated rural labor and scaffolding costs (CNY 4000), the full project payback is approximately 6.6 years (CNY 23,500 ÷ CNY 3550/year). The findings offer practical guidance for large-scale rural building energy retrofitting programs in cold climate zones of China with transitional characteristics.