Passive thermal resilience and architectural decision hierarchy: empirical evidence from full-scale buildings under heatwaves and power outages

Thermal resilience under extreme climatic and infrastructural stress is shaped primarily by architectural decisions made before active systems are installed, yet passive strategies are commonly evaluated in isolation, without reference to the decision hierarchy that governs their reliability during disruption. This study presents an integrated empirical synthesis of passive thermal resilience and energy performance across three decision orders defined by reversibility and operational dependence, using full-scale experimental data from two research facilities in a temperate European climate. The results demonstrate that passive resilience follows a hierarchical structure in which reliability during prolonged extreme events decreases as operational dependence increases, while peak effectiveness under favourable conditions does not follow the same hierarchy. First-order decisions embedding structural thermal mass and floor-ground coupling maintained safe indoor conditions throughout severe summer and winter stress events without occupant intervention or energy input, and improved long-term carbon performance despite a bounded seasonal energy penalty. Third-order strategies achieved the largest reductions in peak indoor temperature only when night ventilation, external shading, and high thermal mass operated together. Night ventilation was most effective during nocturnal hours while external shading was most effective during daytime hours: their complementarity depended on structural thermal inertia established at the first decision order. Night ventilation also substantially increased diurnal temperature fluctuations in lightweight constructions. Occupant dependence is not an external source of uncertainty but a direct consequence of the decision order through which passive strategies are implemented. Second-order retrofit additions narrowed, but did not eliminate, the performance gap established by first-order structural decisions. The framework repositions thermal mass and insulation level as system-defining first-order parameters, enabling Pareto-based optimisation to inform the earliest design decisions. The timing of passive strategy implementation has direct consequences for thermal resilience, operational energy use, and long-term carbon performance. The integrated framework linking trajectory-based resilience assessment with architectural decision hierarchy provides an empirical basis for resilience-oriented design and performance-based regulation.