When interior thermal mass helps—and when it hurts: a dynamic parametric analysis across climate, insulation, and passive use

• Internal thermal mass is analysed independently from envelope effects using dynamic simulation. • The interaction between climate, insulation level, and passive use governs inertia effectiveness. • Quantitative ranges define when internal mass is beneficial, neutral, or counterproductive. • Passive activation (solar gains and night-time ventilation) conditions the net energy impact. • Results provide operational thresholds to support climate-responsive low-energy design. The effect of thermal mass on building energy demand remains a debated issue, with studies reporting both significant benefits and potentially adverse effects depending on climatic and constructive factors. This paper addresses this gap by systematically evaluating the conditions under which interior thermal mass is beneficial, neutral, or counterproductive. Using dynamic energy simulations, the study analyzes a residential building model where thermal mass is located in interior partitions and intermediate floor slabs, ensuring that all of this capacity contributes to the conditioning of the indoor spaces. In a first phase, 36 scenarios are simulated, combining three climate zones, three insulation levels, and the presence or absence of passive strategies (solar gains and night-time cross ventilation). A second phase explores eight representative cases with varying thermal mass quantities (25% to 150%) to identify optimal values. Results show that interior thermal mass consistently reduces cooling demand (typically 10–40%, corresponding to approximately 0.5–8 kWh/m 2 ·yr depending on climate, insulation level, and night-time ventilation), but may increase heating demand by up to 13 kWh/m 2 ·yr (approximately 5–7%) in cold climates (e.g., E1) with low insulation and no solar gains. In contrast, under favourable conditions—when sufficient insulation and solar gains are available—thermal mass can lead to moderate reductions in heating demand, even in cold climates. These findings indicate that the energy impact of interior thermal mass is highly context-dependent, with no universal optimum, and governed by the combined effects of climate, insulation level, and passive activation strategies. The study provides practical design guidance for energy-efficient buildings and energy retrofits and calls for its explicit consideration in performance-based regulations, especially in Southern European climates.