Thermal Optimization of a Glazing–PCM Integrated Balcony Sunspace for Cold-Climate Residential Buildings

Modern residential balconies have the potential to function as passive solar buffer spaces, yet their thermal performance in cold-region high-rise apartments remains insufficiently explored. This study proposes a stepwise glazing–phase change material (PCM) integrated balcony sunspace strategy for a typical high-rise residential apartment in Xi’an, China. Dynamic simulations were conducted to evaluate the effects of balcony orientation, glazing system, PCM melting temperature, and PCM thickness on indoor thermal comfort and annual heating and cooling demand. First, an open living room-connected balcony was used as the benchmark case, and five glazing systems were compared under southwest-, south-, and southeast-facing orientations. Then, based on the optimal glazing system, a 9C–PCM–9C partition wall was introduced between the enclosed balcony sunspace and the living room. The results showed that the optimal glazing system increased the annual thermal comfort ratio by 4.15–5.24 percentage points and reduced annual heating and cooling demand by 13.78–15.80 kWh/m2 and 6.16–7.21 kWh/m2, respectively. Further PCM integration achieved additional improvements, with 21 °C + 30 mm identified as the optimal configuration for all orientations. Compared with the glazing-only case, it increased thermal comfort by 2.05–2.50 percentage points and reduced heating and cooling demand by 4.80–5.10 kWh/m2 and 1.64–1.76 kWh/m2, respectively. The findings provide a practical passive design strategy for climate-responsive balcony sunspaces in cold-region residential buildings.