Multi-Objective Screening of Bio-Based Phase Change Materials for Building Envelopes Using Surrogate Models Across Italian Climates

Bio-based phase change materials (PCMs) can increase transient heat storage in lightweight building envelopes, but their performance depends on the climate, transition properties, layer design, and assumptions used to translate thermal loads into carbon and cost indicators. Although PCM optimization, machine learning surrogates, and lifecycle assessment have each been studied extensively, comparatively few studies combine them while explicitly separating simulation-derived thermal outputs from scenario-dependent environmental and economic post-processing and benchmarking bio-based candidates against paraffin on a common wall area basis. This study develops a simulation-based screening framework for a south-facing office wall model using 18,000 EnergyPlus cases, climate-specific machine learning surrogates, TreeSHAP interpretation, NSGA-II optimization, and scenario-based lifecycle carbon and cost accounting. XGBoost achieved pooled held-out R2 values of 0.974 for occupied discomfort degree-hours and 0.978 for total annual thermal demand. For Palermo, the directly re-simulated balanced configuration (Tm = 24.8 °C, Lh = 178 kJ/kg, 22 mm thickness, intermediate position) reduced occupant discomfort by 42.6% and the modeled single-zone total thermal demand by 6.0%. Under the central all-electric scenario (SCOP = SEER = 3.0, grid factor = 0.233 kg CO2eq/kWh, 25 years), scenario-based net lifecycle carbon was −22.1 kg CO2eq/m2 for the analyzed south wall with an 8.0-year environmental payback, compared with −7.4 kg CO2eq/m2 and 19.2 years for RT28 paraffin. Energy savings did not recover the additional investment; the incremental lifecycle cost was +24.1 EUR/m2. The theoretical contribution is a transparent, climate-dependent screening logic that couples surrogate interpretation with explicit evidence boundaries; the applied outcome is a palmitic–capric target-property region prioritized for laboratory validation rather than a deployment-ready product. Only directly re-simulated configurations are used for quantitative applied thermal claims; surrogate-only Pareto points are retained as exploratory screening candidates and are not interpreted as validated optima. The numerical results are specific to the modeled south-wall, single-zone boundary; whole-building and cross-regional application requires local recalibration and direct validation. By linking passive comfort, carbon accounting, material innovation, and responsible pre-experimental selection, the workflow is relevant to the decarbonization objectives represented by SDGs 7, 9, 11, 12, and 13.