Structural thermal mass plays a central role in indoor temperature stability during heating interruptions; however, its accessibility across different time scales remains insufficiently understood. The effect of slab–ground coupling on short-term energy flexibility and long-term thermal resilience has received limited experimental attention. This study investigates the thermal behaviour of two geometrically identical full-scale residential buildings differing only in the presence or absence of under-slab insulation. Indoor temperature evolution was monitored during a six-day winter heating interruption and interpreted using reduced-order models. The results reveal a two-stage cooling behaviour. During the initial hours, the ground-coupled configuration exhibits faster cooling due to transient heat transfer from indoor air to the slab–ground subsystem, resulting in reduced buffering within the 6–10 h photovoltaic-relevant window. After approximately 24–48 h, the temperature trajectories intersect, with the ground-coupled building maintaining higher indoor temperatures. After six days, the temperature difference reaches approximately 3.8 K. Analysis shows that first- and second-order models are sufficient for the insulated configuration, whereas the ground-coupled building requires a higher-order formulation to reproduce the observed dynamics. This confirms that slab–ground interaction introduces an additional thermally active domain operating on longer time scales. These findings demonstrate that slab–ground coupling redistributes thermal buffering across time scales rather than uniformly increasing it. While it significantly enhances multi-day indoor temperature resilience during prolonged heating outages, it provides limited benefit for short-term thermal buffering within typical diurnal operation windows. The effectiveness of thermal storage therefore depends on its temporal accessibility.