Can buildings cool themselves? Discover how phase change materials (PCM) in building elements can cut energy use while improving indoor comfort in a warming climate.
Dr Eva Zavrl, Faculty of Mechanical Engineering, University of Ljubljana | LinkedIn profile
(Note: Opinions in the articles are of the authors only and do not necessarily reflect the opinion of the European Union)
The demand for cooling in buildings is rising rapidly, driven by climate change, urbanisation and increasing expectations for indoor thermal comfort. Today, the building sector accounts for around 40% of final energy consumption, with cooling representing a growing share that is projected to increase significantly in the coming decades. At the same time, overheating in buildings—especially in urban areas affected by the heat island effect—poses serious risks to health, well-being and productivity.
Conventional approaches to reducing energy demand in buildings have largely focused on improving insulation. While effective for heating, these strategies often overlook an important aspect of thermal performance: the ability of buildings to store and release heat. This is particularly critical in lightweight prefabricated buildings, which are increasingly used due to their cost-efficiency and lower environmental impact, but are highly susceptible to overheating due to low thermal mass.
PCMs offer a promising solution. By storing and releasing large amounts of latent heat during phase transitions, PCMs can stabilise indoor temperatures without relying heavily on active cooling systems. When integrated into building elements such as walls, ceilings or façades, they enable passive or hybrid (active-passive) cooling strategies that can significantly reduce electricity demand while improving thermal comfort.
However, despite their potential, the adoption of PCM technologies in practice remains limited. Key challenges include optimising their thermal properties for different applications, improving their performance in real conditions, and providing accessible tools and guidelines for designers and engineers. Addressing these gaps is essential to unlock the full potential of PCMs in the transition towards more energy-efficient and climate-resilient buildings.
This article explores recent advances in PCM-integrated building elements, focusing on their role in reducing cooling demand, improving indoor comfort, and supporting the development of practical design tools for real-world applications, and presents the ongoing project, funded by the Slovenian Research Agency (ARIS), titled ‘Open-Source Online Tool and Guidelines for Determination of Thermal Characteristics of Phase Change Materials in Building Elements’.
Europe’s building sector is entering a new phase. Energy renovation can no longer be judged only by winter performance: buildings must also remain comfortable during hotter and longer summers. The revised Energy Performance of Buildings Directive (EPBD) explicitly brings cooling demand and indoor environmental quality into the energy performance discussion [1]. The new Construction Products Regulation also reinforces the need for construction products to be considered within a clearer performance and sustainability framework [2]. In parallel, European and international comfort standards and guidance, including EN 16798-1, ISO 17772-1, ASHRAE Standard 55, CIBSE TM52 and CIBSE TM59, provide methods for assessing indoor comfort and overheating risk [3], [4], [5], [6], [7].
This shift matters for lightweight, prefabricated buildings. These buildings are attractive because they can reduce material use, speed up construction, and support lower-carbon building approaches. Their weakness is also well known: low thermal mass. Without enough capacity to absorb and release heat, indoor temperatures can rise quickly during summer days. The default response is often more active cooling. That solves the immediate comfort problem, but it also increases electricity demand, peak loads, and operating costs.
The project starts from a practical question: can building elements themselves help reduce overheating before mechanical cooling is needed?
Phase change materials (PCMs) are one answer. They store and release heat through a phase transition, usually from solid to liquid and back again. When the PCM melts, it absorbs heat. When it solidifies, it releases it. This makes it possible to add thermal storage to a building element as shown in Figure 1 without adding the weight and volume of traditional heavy construction.
Figure 1. Possible locations of PCM-enhanced building elements. PCMs can be integrated into walls, ceilings, floors, roofs, façades, glazing systems and building-integrated photovoltaic elements. This flexibility makes them relevant both for new construction and renovation, especially where additional thermal mass is needed without reducing usable space.
The principle is simple; the design is not. For PCMs to work well, their phase change temperature must match the building, the climate and the intended application. A PCM that melts too early may be fully charged before the hottest part of the day. A PCM that melts too late may not activate when needed. The same problem appears during night-time cooling: if the material does not solidify again, it starts the next day with reduced storage capacity.
Previous studies confirm that PCM melting temperature is one of the most important design parameters. Saffari et al. showed that the optimum melting temperature depends strongly on climate, with higher values performing better in cooling-dominated climates and lower values in heating-dominated climates [8]. Other studies have shown that PCM performance is also influenced by latent heat, density, thermal conductivity and the phase transition range [9]. In practice, this means that ‘choose a PCM close to comfort temperature’ is not a design method. It is a guess.
The scientific literature on PCMs is extensive, but building professionals still lack a clear route from ‘promising material’ to ‘specified building solution’. Many studies focus on one building type, one climate, one PCM product or one modelling workflow. Results are valuable, but they are difficult to transfer directly to another project.
This is the gap addressed by the project. Its first objective is to analyse how PCM characteristics have been determined in existing studies, especially for passive and active-passive building element systems. The focus is not only on the final PCM values, but on the decision-making process behind them: which parameters were considered, which modelling tools were used, whether experiments were performed, and how validation was carried out.
The outcome will be guidance for selecting PCM characteristics based on the type of application. For designers, this is the useful part. They do not need another isolated case study. They need a structured way to answer questions such as: What melting temperature range is suitable for a ventilated façade? How much storage capacity is needed for a lightweight room? When is a passive PCM layer enough, and when is air-based heat transfer enhancement required [10]?
| Design input | Why it matters for PCM performance |
| Outdoor conditions | Temperature cycles and solar radiation determine when the PCM can charge and discharge. |
| Building geometry | Room volume and envelope area influence the required storage capacity. |
| Orientation and solar gains | South- and west-facing elements may require different PCM strategies. |
| Envelope properties | Insulation, thermal resistance and layer position affect heat flow into the PCM. |
| Ventilation strategy | Night ventilation can improve PCM solidification and restore storage capacity. |
| PCM characteristics | Melting temperature, phase range, latent heat and encapsulation determine performance. |
| HVAC context | Existing cooling, heating or ventilation systems influence whether the PCM works passively or as part of a hybrid system. |
Table 1. Main design inputs for PCM selection in building elements.
A passive PCM layer can reduce temperature peaks, but it has one critical limitation: it depends on favourable boundary conditions. In a hot spell, especially when nights remain warm, the PCM may not release enough heat. The result is incomplete solidification and weaker performance the next day.
The project therefore gives particular attention to active-passive PCM systems. These systems combine the storage capacity of PCM with controlled heat transfer, often through ventilation. A typical example is shown in Figure 2, where a PCM layer is integrated into a ventilated building element. During the day, the PCM absorbs heat from solar gains and the indoor environment. At night, cooler air flows through a ventilated gap and helps remove stored heat, so the material can solidify more effectively.
Figure 2. Active-passive PCM cooling concept with night-time ventilation. During the day, the PCM absorbs excess heat and reduces indoor temperature peaks. At night, airflow through the building element enhances heat removal and supports PCM solidification, preparing the material for the next cooling cycle.
This is where the project becomes especially relevant for practice. Lightweight buildings and renovation projects often already include ventilation systems or façade/roof cavities that could support such strategies. Instead of treating PCM as an isolated product, the project treats it as part of a building element system.
Simulation is necessary for PCM design because performance depends on climate, geometry, construction layers and operating conditions. However, simulation alone is not enough. For building professionals and manufacturers, credibility comes from comparison with measured performance.
The project therefore includes experimental validation using an outdoor thermostatic test cell. PCM-enhanced building elements will be tested under real environmental conditions, with measurements of outdoor air temperature, solar radiation, surface temperatures, heat flux, indoor air temperature and operative temperature. These data will be used to validate and refine the computational model.
This step is essential. PCMs are sensitive to real operating conditions, including imperfect heat transfer, changing airflows and incomplete phase change. Validation helps avoid the common trap of over-promising performance based only on idealised simulations.
The most visible output of the project will be an open-source online tool for estimating PCM characteristics in building elements. This is the project’s strongest novelty for BUILD UP readers.
The tool is planned as a bridge between advanced thermal modelling and everyday design decisions. Users will enter basic information on climate conditions, building geometry, envelope properties, ventilation, intended application and PCM system type. The tool will then estimate suitable PCM characteristics, such as melting temperature, melting temperature range and storage capacity. It will also provide indications of expected effects on indoor operative temperature and energy demand.
This matters because existing online tools generally support ventilation sizing, heat pump selection or simple PCM sizing calculations, but they do not guide users through PCM-integrated building element design. As shown in Figure 3, the proposed tool goes further by linking system type, influential factors and PCM properties. It is also designed with future upgrades in mind, such as commercial PCM product databases, life-cycle assessment, cost analysis and self-learning optimisation.
Figure 3. Conceptual design of the proposed online PCM design tool. The tool will connect building inputs, environmental conditions and PCM-integrated element designs to recommend suitable PCM characteristics and estimate thermal performance. Its modular structure allows future upgrades, including product databases, life cycle and cost modules.
A tool that engineers love but practitioners ignore is a failed tool. The project avoids this by including stakeholder engagement from the design phase onwards. Lightweight prefabricated building manufacturers, renovation designers and expert institutions will be involved through interviews, surveys, focus groups and tool testing.
This is not a decorative work package. It is central to impact. Stakeholders can identify which inputs are realistic, which outputs are useful, and where the tool risks becoming too complex for early-stage design. Their feedback will help shape both the interface and the guidance behind it.
The project’s impact is not limited to one PCM product or one building type. Its real value lies in creating a repeatable decision-making framework. If successful, it can reduce uncertainty for designers, shorten the route from research to specification, support manufacturers in product integration, and help renovation professionals assess PCM solutions before costly implementation.
For Europe, this is timely. The renovation wave needs solutions that improve comfort without locking buildings into higher cooling demand. PCM-integrated building elements will not replace good shading, ventilation, insulation or efficient HVAC systems. But when designed properly, they can become a useful layer in climate-resilient building design: storing heat when it is excessive, releasing it when conditions allow, and helping buildings stay comfortable with less mechanical cooling.
As European buildings face increasing risks of overheating and rising cooling demand, improving summer thermal performance has become a key challenge for both new construction and renovation. Phase change materials offer a promising way to enhance the thermal storage capacity of lightweight building elements, reduce indoor temperature peaks and improve occupant comfort while limiting reliance on mechanical cooling systems.
However, the successful implementation of PCM technologies depends on more than the choice of material itself. Designers need reliable methods to determine appropriate PCM characteristics, validated performance data and practical tools that support decision-making from the earliest design stages. The ARIS project addresses these needs by developing evidence-based guidelines, validated computational models and an open-source online tool for PCM specification. By bridging the gap between research and practice, the project aims to support the wider adoption of PCM-enhanced building solutions and contribute to more energy-efficient, climate-resilient and comfortable buildings across Europe.
[1] European Parliament and Council, ’Directive (EU) 2024/1275 on the energy performance of buildings,’ Official Journal of the European Union, 2024. [Online]. Available: https://eur-lex.europa.eu/eli/dir/2024/1275/oj
[2] European Parliament and Council, ’Regulation (EU) 2024/3110 laying down harmonised rules for construction products and repealing Regulation (EU) No 305/2011,’ Official Journal of the European Union, 2024. [Online]. Available: https://eur-lex.europa.eu/eli/reg/2024/3110/oj
[3] CEN, EN 16798-1:2019, Energy Performance of Buildings – Ventilation for Buildings – Indoor Environmental Input Parameters for Design and Assessment of Energy Performance of Buildings Addressing Indoor Air Quality, Thermal Environment, Lighting and Acoustics. Brussels, Belgium: European Committee for Standardization, 2019. [Online]. Available: https://standards.iteh.ai/catalog/standards/cen/4b4d0f4a-3a89-4a8d-8ef4-8fd6f9c6a4c6/en-16798-1-2019
[4] ISO, ISO 17772-1:2017, Energy Performance of Buildings – Indoor Environmental Quality – Part 1: Indoor Environmental Input Parameters for the Design and Assessment of Energy Performance of Buildings. Geneva, Switzerland: International Organization for Standardization, 2017. [Online]. Available: https://www.iso.org/standard/59621.html
[5] ASHRAE, ANSI/ASHRAE Standard 55-2023, Thermal Environmental Conditions for Human Occupancy. Atlanta, GA, USA: ASHRAE, 2023. [Online]. Available: https://www.ashrae.org/technical-resources/bookstore/standard-55
[6] CIBSE, TM52: The Limits of Thermal Comfort – Avoiding Overheating in European Buildings. London, UK: Chartered Institution of Building Services Engineers, 2013. [Online]. Available: https://www.cibse.org/knowledge-research/knowledge-portal/tm52-the-limits-of-thermal-comfort-avoiding-overheating-in-european-buildings
[7] CIBSE, TM59: Design Methodology for the Assessment of Overheating Risk in Homes. London, UK: Chartered Institution of Building Services Engineers, 2017. [Online]. Available: https://www.cibse.org/knowledge-research/knowledge-portal/tm59-design-methodology-for-the-assessment-of-overheating-risk-in-homes
[8] M. Saffari, A. de Gracia, C. Fernández and L. F. Cabeza, ‘Simulation-based optimization of PCM melting temperature to improve the energy performance in buildings,’ Applied Energy, vol. 202, pp. 420–434, 2017. [Online]. Available: https://doi.org/10.1016/j.apenergy.2017.05.098
[9] D. Abd El-Raheim, A. Mohamed, H. Abou-Ziyan and M. Fatouh, ’The essential properties governing the appropriate selection of phase change materials integrated into heavy structure buildings,’ Energy, vol. 266, Art. no. 126515, 2023. [Online]. Available: https://doi.org/10.1016/j.energy.2022.126515
[10] E. Zavrl, M. El Mankibi, M. Dovjak and U. Stritih, ’Enhancing performance of building elements with phase change materials for cooling with air-based systems,’ Journal of Energy Storage, vol. 51, Art. no. 104461, 2022. [Online]. Available: https://doi.org/10.1016/j.est.2022.104461
