Buildings in low-carbon energy systems must balance short-term demand-side flexibility with maintaining acceptable indoor conditions during prolonged heating interruptions. This study investigates whether slab–ground coupling addresses or intensifies that tension. A full-scale experimental comparison of two geometrically identical single-family residential buildings, differing only in the presence of under-slab thermal insulation, was conducted during two winter heating outages (January–February 2026, Nowy Kisielin, Poland). A calibrated three-node reduced-order model supported interpretation of the temporal structure of the thermal response. The results reveal a performance crossover governed by a two-mode thermal dynamic. Over the first 6–12 h after heating shutdown, the ground-coupled configuration cools approximately 0.5–0.8 K faster than the insulated reference, corresponding to a 3–4% reduction in the heat retention index, a measurable short-term penalty. After approximately 24–48 h, the direction reverses: the ground-coupled building maintains progressively higher indoor temperatures, reaching a difference of +3.8 K after six days. An analytical expression derived from a lumped energy balance shows that the initial cooling rate is governed by the indoor–ground temperature gradient, not by total thermal capacity, providing a mechanistic explanation for the crossover. A contribution decomposition separates the roles of structural thermal mass and ground coupling, showing that structural mass dominates the first 48 h while ground-coupled storage controls behaviour beyond 96 h. These findings reveal a fundamental time-scale incompatibility: ground coupling reduces short-horizon thermal accessibility, constraining demand response, while substantially enhancing multi-day passive survivability. Building design and grid-integration strategies should account not only for thermal capacity, but for the alignment between storage response time and intended operational horizon.