Buildings are increasingly expected to contribute to both energy system flexibility and resilience during disturbances in energy supply. Structural thermal mass is often considered a key mechanism supporting these objectives by stabilising indoor temperatures and enabling short-term thermal energy storage. However, most existing studies rely on simulation approaches, while experimental evidence under real winter conditions remains limited. This study experimentally investigates the influence of structural thermal mass on building thermal behaviour during winter heating interruptions using two nearly identical full-scale residential buildings constructed with different wall technologies: a masonry system and a lightweight timber-frame structure. The buildings have identical geometry, orientation and internal layout, enabling a controlled comparison of their thermal response. Experiments were conducted during winter cold spells under two boundary conditions: with external blinds closed, limiting solar gains, and with blinds open, allowing solar radiation to enter the interior spaces. When solar gains were minimized, the masonry building cooled more slowly and maintained higher indoor temperatures, with an average temperature difference of 0.87 °C compared with the lightweight building. When solar gains were present, indoor temperature trajectories became nearly identical and the average temperature difference decreased to 0.08 °C, which was not statistically significant. These results show that the influence of structural thermal mass on winter thermal resilience and building-level energy flexibility depends strongly on boundary conditions. While higher thermal mass improves short-term thermal stability during heating interruptions, its contribution to solar heat storage and winter energy flexibility appears limited under typical winter conditions.