Assessment of thermal comfort in building with double-skin facades equipped with movable openings and phase-change materials during heat waves in Iran

The rapid escalation of global temperatures and the increasing frequency, duration, and intensity of heatwaves have become defining environmental challenges of the 21st century. These phenomena pose severe risks to the liveability of cities and the health, productivity, and comfort of building occupants, particularly in regions characterized by arid and semi-arid climates with prolonged hot seasons. Rising ambient temperatures amplify the cooling demand in urban buildings, driving excessive electricity consumption, intensifying peak loads on national energy infrastructures, and indirectly contributing to further carbon emissions. Simultaneously, the reduction in outdoor thermal comfort exacerbates the urban heat island (UHI) effect, compounding the difficulty of maintaining habitable indoor environments through conventional mechanical systems. In response to these intertwined challenges, the field of architecture and building science has increasingly emphasized passive and adaptive design strategies that enhance thermal resilience without relying exclusively on energy-intensive systems. Among such strategies, innovations in the building envelope which mediates the most critical exchange of heat and air between interior and exterior environments are recognized as having the highest potential for impact. The double-skin façade (DSF), in particular, has emerged as a promising solution due to its ability to provide shading, facilitate natural ventilation, and regulate solar gain dynamically. However, conventional DSF systems often face significant drawbacks in hot climates: during periods of intense solar radiation, the air cavity between the two layers can overheat, turning into a heat trap and driving unwanted warm air into the occupied space when ventilation openings are activated. This phenomenon limits the DSF’s cooling potential and can, paradoxically, increase indoor heat stress. To address this limitation, the present research introduces an adaptive movable DSF system integrated with Phase Change Materials (PCM) a hybrid passive strategy designed to absorb and store excess heat within the cavity during daytime and release it gradually during the cooler nighttime periods. PCM-based materials are well-documented for their high latent heat storage capacity, stability under repeated melting and solidification cycles, and compatibility with lightweight façade assemblies. By incorporating PCM panels into the ventilated cavity of a movable DSF, the system simultaneously enables thermal buffering and controlled shading, thereby mitigating the overheating of the air cavity and enhancing the effectiveness of nighttime natural ventilation. The primary objective of this study is to evaluate the thermal performance and comfort implications of such a hybrid DSF–PCM configuration under realistic heatwave conditions. The research adopts a parametric simulation approach, integrating advanced modelling tools including Rhinoceros 7, Grasshopper (with the Ladybug and Honeybee plugins), and EnergyPlus as the core thermal engine. This workflow allows seamless coupling between parametric geometry manipulation and high-resolution computational simulations. The simulation environment was designed to capture transient thermal behaviour under variable boundary conditions, representing the diurnal fluctuations typical of heatwave events. Five representative cities in southern Iran including: Ahvaz, Bandar Abbas, Zahedan, Bushehr, and Abadan were selected as case studies. These cities exhibit distinct climatic patterns but share the characteristic of prolonged exposure to extreme summer temperatures and high solar intensity. Heatwaves were defined according to national meteorological thresholds as periods of three or more consecutive days with maximum daily temperatures ≥ 35 °C. For each city, the most recent meteorological data from 2024 (in EPW format) were employed to ensure that the simulations reflect the evolving climatic conditions of the region. The reference model represents a single-zone residential space with internal dimensions based on the dimensional criteria defined in Part 4 of the Iran National Building Regulations (INBR, 2016b). The south-facing façade was modelled as the only external heat exchange surface, while the other walls, the floor, and the ceiling were treated as adiabatic. The movable DSF module containing PCM panels is designed as a portable system that can be temporarily installed over the window during the onset of extreme heat events and removed during moderate conditions to maintain daylight access. The performance evaluation relies on two internationally standardized comfort indices: Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD), as defined by ASHRAE Standard 55. These metrics allow the quantification of human thermal comfort based on operative temperature, humidity, air velocity, and mean radiant temperature. In parallel, transient thermal simulations were used to track variations in indoor air temperature, heat flux across the envelope, and cavity air temperature profiles throughout 24-hour heatwave cycles. The results indicate that integrating PCM into the movable DSF substantially improved the indoor thermal environment under all climatic conditions analysed. During daytime hours, the PCM layer absorbed significant amounts of latent heat, reducing the peak indoor operative temperature by an average of 2.5–3.2 °C compared with the base model without PCM. At night, as outdoor temperatures decreased, the stored heat was released gradually, which stabilized the thermal profile and reduced temperature swings. The combined shading and latent heat storage effects led to an average reduction in total cooling energy demand of approximately 22–28%, depending on the city and the intensity of the heatwave. Notably, the analysis revealed that while the movable shading component primarily reduced direct solar radiation and surface heat gain, the PCM layer was responsible for dampening temperature fluctuations within the façade cavity and maintaining more uniform indoor conditions. In addition, the results demonstrated the ability of the movable DSF to maintain acceptable daylight levels, as the PCM panels used were semi-translucent in their melted state, allowing diffuse light penetrate while reducing glare. From a broader perspective, these findings reinforce the importance of integrating smart materials with adaptive architectural envelopes in hot-climate design. The PCM–DSF system not only mitigates short-term thermal stress during extreme events but also contributes to long-term energy resilience by reducing peak cooling loads and supporting passive comfort maintenance. Its modular and portable nature further enhances its applicability for retrofitting existing buildings, offering a cost-effective strategy for upgrading thermal performance without major structural interventions. In conclusion, the study underscores that movable double-skin façades incorporating PCM represent a forward-looking, scalable solution for climate-adaptive architecture. By combining latent heat storage, dynamic shading, and natural ventilation, such systems enable buildings to better withstand and adapt to the escalating impacts of climate change.