Research on an Improved Adaptive Optimization Calculation Method for Dynamic Heat Flux of Building Envelope Based on IFDM-RKF

As the boundary between indoor and outdoor spaces, the heat flux of a building envelope is a crucial factor influencing the indoor thermal environment and human thermal comfort, and also an important indicator reflecting the impact of outdoor meteorological factors on the indoor environment. In scenarios involving rapid assessment of existing buildings and engineering projects, the dynamic thermal performance of the building envelope are often affected by factors such as outdoor weather fluctuations, window–wall coupling, wall heat storage, and thermal bridging. To address this issue, this study proposes a dynamic heat flux calculation method that accounts for hysteresis. Simultaneously, the heat conduction equation of the implicit finite difference method (IFDM) and boundary conditions based on wall energy balance are used to optimize the wall surface temperature. An adaptive step size control strategy (Runge–Kutta–Fehlberg) is introduced in the time step setting. Results show that the heat flux R² of the proposed dynamic heat flux calculation method is 0.9207, and the optimized R² is 0.9435, both within an acceptable range for engineering applications. Studies have shown that the simplified framework derived from the heat flux analysis of building envelopes retains the characteristics of wall heat storage and delayed heat release, while effectively solving the window–wall coupling problem and significantly reducing the reliance on computationally expensive numerical methods. This method therefore provides an efficient and scalable technical pathway for thermal performance assessment and energy-retrofit decision support for existing building envelopes.