Assessing the Potential of Phase-Change Materials in Energy Retrofitting of Existing Buildings in a Mediterranean Climate

Abstract

The European Community has prioritized reducing energy consumption and improving energy efficiency in the building sector, along with ensuring increasingly high standards of thermal comfort, as key goals over recent decades. Given the impact of climate change, the rising frequency of extreme weather events, and the rapid shifts in peak demand during both winter and summer, buildings must efficiently respond to sudden and extreme temperature fluctuations while maintaining optimal indoor comfort. Phase-change materials (PCMs), which can adapt their thermophysical properties in response to external conditions, may offer a solution for enhancing building resilience to climate change. This paper evaluates the benefits of integrating various PCMs with plasterboard in the energy retrofit of a multi-family complex in a Mediterranean climate. The study examines the application of a PCM with a melting temperature of 25 °C at three different thicknesses (74.2 mm, 37.1 mm, and 20.8 mm) to external walls, ceilings, and both walls and ceilings simultaneously. Among the various applications, using the PCM on walls alone maximized heating savings as thickness increased (26.6%), while ceiling application maximized cooling energy savings (17.5%). Combined solutions offered the most balanced seasonal benefits, leading to the greatest overall energy reductions (24.1%).

1. Introduction

The resilience of the built environment to climate change and associated extreme events has been gaining significant global attention in recent years [1,2]. The UN General Assembly resolution 71/276 defines ‘resilience’ as the ability of a system, community, or society exposed to hazards to resist, absorb, adapt, transform, and recover from those effects in a timely and efficient manner. This concept has become a cornerstone of the UN Sustainable Development Goals (SDGs). Similarly, the European Commission’s European Green Deal identifies climate-resilient and low-carbon buildings as critical for achieving energy neutrality by 2050 [3].

Projections from the Intergovernmental Panel on Climate Change (IPCC) indicate that the average global temperature is expected to rise by more than 1.5 °C within the next 20 years compared to the pre-industrial period [4]. This increase will significantly boost energy demand for HVAC systems, potentially doubling in most European countries [5,6]. The growing demand for electricity, particularly during increasingly frequent heatwaves—as observed in recent decades—could lead to widespread blackouts and grid failures, turning many buildings unsuitable for maintaining indoor comfort [7].

Adaptive technologies in buildings, which can reversibly modify their characteristics in response to external stimuli, offer promising solutions for developing buildings that can flexibly respond to climate fluctuations while ensuring better comfort conditions [8,9]. Among these, passive thermal energy storage (TES) systems are particularly effective at reducing energy consumption and enhancing indoor comfort, increasing the flexibility of building systems and decreasing dependency on energy sources [10,11].

The Use of PCMs in the Built Environment

Among the various TES technologies available or under investigation, latent heat storage using phase-change materials (PCMs) offers notable advantages over sensible heat storage systems [12]. PCMs have a higher storage capacity and more stable thermal behaviour during charging and discharging processes [13]. PCMs are materials capable of storing and releasing energy in the form of latent heat through a phase change, often occurring between solid and liquid states at a specific temperature, typically chosen to fall within the comfortable temperature range for buildings [14]. As thermal storage systems, PCMs can absorb excess heat within buildings, enhancing thermal inertia and improving energy performance [15]. When PCMs are below their melting point, they remain solid and behave like other materials; as temperatures rise, they absorb and transfer heat according to their conductivity, density, and heat capacity.

Once the ambient temperature reaches their melting point, they undergo a phase change at a nearly constant temperature, absorbing heat until they fully melt. Due to their high latent heat capacity in a low-temperature range, PCMs can store significant thermal energy during the day by melting, thereby reducing indoor air temperature fluctuations caused by solar and internal gains [16]. At night, the stored energy is released as the material solidifies again; this process can be enhanced by nighttime ventilation, which lowers internal temperatures and aids in the solidification of the PCM [17].

PCMs are typically categorized into three types: organic, inorganic, and eutectic. Organic PCMs, such as paraffin, fatty acids, and polyethylene glycol (PEG), are available in various melting temperature ranges, are chemically stable, and do not exhibit supercooling, but they have lower conductivity (approximately λ = 0.2 W/(mK), which slows their heat exchange capability. In contrast, inorganic PCMs offer higher conductivity and lower volume change but often experience supercooling and are corrosive, making their use in building applications more challenging (Figure 1).

Numerous studies have explored the use of PCMs as TES materials in various applications, including building envelopes [19,20,21] and HVAC systems [22,23,24].

Regarding their application in building envelopes as passive thermal storage systems, Berardi et al. demonstrated that applying PCM to walls and floors can reduce internal temperature fluctuations by approximately 5 °C during summer, resulting in energy savings of 29% to 59%, depending on climatic conditions [25].

However, Arumugam et al. [26] found that PCM integration is not beneficial for type A climates (Köppen Climate Classification) but recommended PCM integration on the building’s external surface for type B climates. Similarly, they found that placing PCMs in the middle or innermost layers of the building envelope yielded better results for type C and D climates.

The effectiveness of PCMs was shown to be highly dependent on the climate, with greater day/night temperature swings, typical of arid climates, proving more favourable for the PCM regeneration cycle [27]. Since PCM activation is influenced by several factors, including external temperatures, building envelope properties, internal loads, and the PCM’s melting point, selecting the appropriate melting temperature is crucial [28].

A melting point that is too low can result in underutilization of the material during warmer months, while a melting point that is too high may render the PCM ineffective during transitional seasons. Berardi et al. [13] suggested that the optimal melting temperature should be determined based on the average ambient temperature throughout the year.

The most critical aspect of using PCMs in the Mediterranean climate is assessing the rate of the solidification process, as high night temperatures could block the discharge process, making the PCMs storing capacity ineffective [12,17]. The integration of PCMs with night natural ventilation is found to improve PCM activation by between 22.4% and 26.1% in the warm and temperate Mediterranean climate, enhancing their energy saving potential [29,30,31].

This paper aims to assess the potential of using PCMs for the energy retrofit of typical existing buildings with heavy masonry and a low window-to-wall ratio (WWR) in the Mediterranean climate, in terms of energy consumption for heating and cooling, discomfort hours, and cost-effectiveness for different PCM thicknesses and envelope applications.

2. Methods

2.1. Case Study

The case study focuses on a typical archetype from the existing Italian building stock, as identified in the TABULA project (Typology Approach for BUiLding stock energy Assessment) [32]. Each national typology within the project consists of a set of model residential buildings that reflect common energy characteristics. These model buildings serve as a typological abacus, enabling the assessment of energy-saving potential through upgrades to building envelopes and thermal systems.

Each class corresponds to a specific historical period, representing distinct dimensional and construction typologies that are significant from an energy perspective. For this study, class 6, which includes multi-family, multi-story buildings constructed between 1976 and 1990, was selected. This class is characterized by buildings that adhere to the first legislative energy efficiency requirements and represents a substantial portion of the existing building stock. The envelope thermophysical properties and HVAC characteristics of the building were derived from those specific to the type of building described in the TABULA project for class 6. The building selected is a typical three-story, in-line structure with two identical flats per floor (Figure 2).

Each flat has a net floor area of 75 m² and a heated volume of 210 m³. The building has an S/V ratio of 0.45 and a window-to-wall ratio (WWR) of 16.53%. The internal layout of the apartments includes a 24 m² living room, a 7.4 m² kitchen, two bedrooms (one double and one twin), and two bathrooms. The building envelope is characterized by:

• External walls: made of weakly insulated perforated brick blocks (U-value = 0.8 W/m²K);
• Walls to unheated zone: made of uninsulated blocks (U-value = 0.75 W/m²K);
• Indoor partitions: made of perforated brick blocks (U-value = 2.10 W/m²K);
• Floor and ceiling: made of 30 cm hollow clay floor (U-value = 1.51 W/m²K);
• Window frames: double glazing (3:6:3) with metal frame without thermal break and air gap (U-value = 3.7 W/m²K and g_gl,n = 0.75).

The heating system is a self-sufficient system for each flat consisting of a standard boiler installed in a non-conditioned room with an average seasonal efficiency (η_H) of 0.88 that feeds water heaters (T_u = 80 °C) in each room. For summer air-conditioning, the use of split-type air-conditioners with a SEER of 3 was assumed.

2.2. BES Parameters

Building energy performance with PCM integration was analysed through a series of annual dynamic energy simulations using EnergyPlus v. 8.9. These analyses were conducted in the city of Bari, which falls under the Csa classification (warm and temperate climate) according to the Köppen system.

Each simulation series followed three steps: first, the base case, representing the baseline building without PCMs, was modelled. Next, the building energy performance was evaluated with PCMs applied only to the external walls. Then, PCMs were applied to the ceiling, and finally, the performance of the building was assessed with PCMs applied to both the external walls and the ceiling.

The analyses focused on a central flat within the building with N—S exposure, assuming adiabatic conditions for the floor and ceiling. The occupancy was set at 0.054 persons/m², totalling four occupants. Internal heat gains were set at 9 W/m² for the living area and 3 W/m² for the sleeping area, according to UNI/TS 11300-1 (2019) [33]. Lighting was modelled with a normalized power density of 2 W/m²-100 lux, and the infiltration rate was set at 0.5 h⁻¹. For indoor climate control, the temperature set points were 26 °C in summer and 20 °C in winter, with the HVAC system set to be continuously available. Natural ventilation rates ranged from 5 to 12 air changes per hour, activated based on internal temperatures within the 21 °C to 25 °C range.

To prevent overheating or overcooling, a differential of 1 °C between indoor and outdoor temperatures was maintained in the simulations. The internal loads, occupancy, ventilation, and HVAC settings remained constant across all simulations, with only the building envelope characteristics varying according to the application of PCMs.

2.3. PCM Characteristics

The PCM evaluated in this study is BioPCM©, produced by Phase Change Solution Inc. (Greensboro, NC, USA). It is an organic PCM derived from plant extracts, encapsulated in small cells and available in roll form. BioPCM© comes in various thicknesses (M27, M51, M91, and M182) and with different melting temperatures (Q19, Q21, Q23, Q25, etc.) (Figure 3).

In the material’s name, the “M” value indicates the heat storage capacity in Btu per square foot, while the “Q” value represents the BioPCM’s melting point temperature. For this study, based on literature recommendations, a PCM with a melting temperature of 25 °C was selected (Figure 4a). Additionally, three different material thicknesses—M51, M91, and M182—were examined. The key properties of the PCM used are outlined in

Table 1. BioPCM Q25 thermophysical properties.

	M51	M91	M182
Thickness (s)	2.08 cm	3.71 cm	7.42 cm
Thermal conductivity (λ)	0.2 W/m K
Density (ρ)	235 Kg/m3
Specific heat (c)	1.97 kJ/Kg K
Melting temperature (Q)	25 °C
Enthalpy (h)	Figure 3

. The material was applied as the innermost layer of the exterior perimeter masonry and the ceiling, covered by gypsum plasterboard.

To assess the PCM’s enthalpy temperature dependent behavior a one-dimensional conduction finite difference model (CondFD) was implemented in the simulation as the conduction transfer function transformation (CTF) model. EnergyPlus provides two distinct approaches for the discretization of the conduction equation: fully implicit and semi-implicit Crank–Nicholson [34].

The Crank–Nicholson scheme, semi-implicit and based on an Adams–Moulton solution approach, was used. The Crank–Nicholson scheme considers the second order in time coupled with an enthalpy/temperature function (HTF) to account for phase-change energy accurately. The implicit formulation for the internal node used is shown in the Equation (1):

$$c_p\rho ∆x{\displaystyle \frac{T_i^{j+1}-T_i^j}{∆t}}={\displaystyle \frac{1}{2}}\left({\lambda }_W{\displaystyle \frac{T_{i+1}^{j+1}-T_i^{j+1}}{∆x}}+{\lambda }_E{\displaystyle \frac{T_{i-1}^{j+1}-T_i^{j+1}}{∆x}}+{\lambda }_W{\displaystyle \frac{T_{i+1}^j-T_i^j}{∆x}}+{\lambda }_E{\displaystyle \frac{T_{i-1}^j-T_i^j}{∆x}}\right)$$

(1)

where c_p is the specific heat capacity, ρ is the density, and Δx is the spacing between the nodes in the modelled domain. The temporal time step was given as Δt. The node temperature is represented by T, while the thermal conductivity is represented by λ. Also, i is the node modelled, whereas i − 1 and i + 1 are the neighbouring nodes. Thus, λ_w indicates the thermal conductivity for the interface between the i node and i + 1 node, while λ_E is the thermal conductivity for the interface between the i node and i − 1 node. The fully implicit scheme in Equation (1) is solved based on an Adams–Moulton solution approach which considers the first order in time (Equation (2)).

$$c_p\rho ∆x{\displaystyle \frac{T_i^{j+1}-T_i^j}{∆t}}=\left({\lambda }_W{\displaystyle \frac{T_{i+1}^{j+1}-T_i^{j+1}}{∆x}}+{\lambda }_E{\displaystyle \frac{T_{i-1}^{j+1}-T_i^{j+1}}{∆x}}\right)$$

(2)

The previous and next time steps are represented by j and j + 1, respectively. The resulting linear equations were solved using an iterative Gauss–Seidel method with underrelaxation for increased stability according to Equation (3).

$$T_{i+1}^{j+1}=T_i^j+\left(T_i^{j+1}-T_i^j\right)F_{Relax}$$

(3)

F_Relax represents the relaxation factor for temperature. The variable specific heat value was used to simulate the phase-change process based on the updated node enthalpy in every iteration by using the input tabulated data of enthalpy/temperature pairs as in Equation (4)

$$c_p={\displaystyle \frac{h_i^{j+1}-h_i^j}{T_i^{j+1}-T_i^j}}$$

(4)

As the specific heat is not only dependent on the current state but also the previous state, the hysteresis physics present between the melting and freezing processes is determined as in Equation (5):

$$c_p=f(T_i^{j+1},T_i^j,{PhaseState}^{j+1},{PhaseState}^j)$$

(5)

3. Results

3.1. Energy Consumption

The initial set of simulations aimed to assess the effectiveness of using BioPCM with a melting temperature (Q) of 25 °C at different thicknesses and applications. The thicknesses evaluated were 2.08 cm (M51), 3.71 cm (M91), and 7.42 cm (M182).

The analyses considered applying the material first to the innermost layer of the external walls and then to the ceiling. The resulting reduction in energy consumption for heating and cooling was compared to the base case scenario.

Table 2. Heating and cooling energy demand variation based on different BioPCM Q25 thicknesses and applications.

	ID		Heating			Cooling
					Δ%			Δ%
	BL	Baseline	41.11	kWh/m2	-	12.86	kWh/m2	-
Wall	PCM 1	BioPCM M51 Q25	37.54	kWh/m2	−8.7%	10.76	kWh/m2	−16.4%
	PCM 2	BioPCM M91 Q25	35.29	kWh/m2	−14.2%	11.50	kWh/m2	−10.6%
	PCM 3	BioPCM M182 Q25	30.64	kWh/m2	−25.5%	11.68	kWh/m2	−9.2%
Ceiling	PCM 4	BioPCM M51 Q25	41.18	kWh/m2	0.2%	10.93	kWh/m2	−15.1%
	PCM 5	BioPCM M91 Q25	41.08	kWh/m2	−0.1%	10.80	kWh/m2	−16.0%
	PCM 6	BioPCM M182 Q25	40.90	kWh/m2	−0.5%	10.61	kWh/m2	−17.5%

Δ% percentage variation compared to baseline scenario.

presents the key results for the use of PCM in the walls and ceiling based on the application thickness.

As can be seen, applying BioPCM to the external walls leads to a reduction in heating energy consumption that increases with wall thickness. Specifically, compared to the base case, BioPCM M51 reduces consumption by 8.7%, BioPCM M91 by 14.2%, and BioPCM M182 by 25.5%. Since the PCM has a melting temperature of 25 °C, this reduction is primarily due to the decreased wall transmittance from adding the material as its thermal storage capacity is minimal at temperatures below 25 °C.

The thermal transmittance of the external walls improves as the thickness of the PCM increases compared to the base case thermal transmittance of 0.80 W/m² K. It decreases to 0.74 W/m² K with 2 cm of PCM, 0.70 W/m² K with 3.71 cm, and 0.62 W/m² K with 7.42 cm, resulting in a maximum thermal transmittance improvement of 35% from the original value.

Regarding cooling energy savings, thinner BioPCM layers on the walls provide greater energy savings. BioPCM M51 yields the highest reduction at 16.4% compared to the base case, followed by BioPCM M91 with a 10.6% reduction, and BioPCM M182 with a 9.2% reduction. This trend occurs because the increased material thickness enhances insulation, reducing nighttime heat loss during summer and thus decreasing cooling energy savings as thickness increases.

When BioPCM is applied to the ceiling, the energy savings for heating are negligible, regardless of the thickness used. The maximum reduction of 0.5% is achieved with the thickest BioPCM, but this is mainly due to the reduced heating volume from the thicker horizontal closure. In contrast, the cooling energy savings are significant, increasing proportionally with the thickness of the material, reaching a maximum reduction of 17.5% with BioPCM M182.

In the final set of simulations, the impact of using BioPCM M182 on the ceiling, which produced, according to the previous analysis, the greatest energy savings, was evaluated alongside different thicknesses of BioPCM Q25 applied to the innermost layer of the external walls (

Table 3. Energy demand for heating and cooling with the combined application of BioPCM M182 Q25 to the ceiling and different thicknesses to the walls.

			Heating			Cooling
					Δ%			Δ%
	BL	Baseline	41.11	kWh/m2	-	12.86	kWh/m2
W + C	PCM 6 + PCM 1	BioPCM M182 + M51	37.34	kWh/m2	−9.2%	10.65	kWh/m2	−17.2%
	PCM 6 + PCM 2	BioPCM M182 + M91	34.85	kWh/m2	−15.2%	10.69	kWh/m2	−16.9%
	PCM 6 + PCM 3	BioPCM M182 + M182	30.17	kWh/m2	−26.6%	10.80	kWh/m2	−16.0%

).

It was found that the combined use of PCMs on the ceiling and external walls balances the benefits for heating obtained by the solutions using only the external walls with those for cooling obtained by the solution using BioPCM M182 on the ceiling. In terms of heating energy consumption, the solution with the highest energy savings of 26.6% is the one with the thicker BioPCM on both the walls and the ceiling.

On the other hand, in terms of cooling energy demand, the scenario with the maximum reduction in consumption is the one with BioPCM M182 on the ceiling and BioPCM M51 on the walls. The total energy savings achieved by the three solutions increase with the thickness of the PCM applied to the walls, reaching 26.4% for BioPCM M51, 32.2% for BioPCM M91, and 42.6% for BioPCM M182. Figure 5 shows the comparison of energy consumption for cooling (Figure 5a), heating (Figure 5b), and total (Figure 5c) between the different solutions in descending order of energy consumption.

The solution with the combined use of BioPCM M182 on both the external walls and the ceiling is the one with the greatest energy savings for both heating and total, while the solution with the use of BioPCM M182 on the ceiling only is the one with the most significant reduction in cooling consumption.

3.2. Thermal Comfort Assessment

After assessing the energy consumption of each solution, an analysis of indoor thermal comfort was conducted following EN Standard 16798-1 (2019).

A long-term thermal comfort analysis using the Fanger index was carried out in accordance with ISO Standard 7730 (2005) [35]. This analysis involved evaluating the PMV (Predicted Mean Vote) and hourly PPD (Predicted Percentage of Dissatisfied) during the heating season (15 November–31 March) and the cooling season (1 May–30 September).

The analysis assumed a metabolic rate of 1.2 met for sedentary activity in residential buildings, a clothing level of 0.5 clo in summer and 1 clo in winter, and an air velocity of 0.10 m/s. The results were then compared with the IEQ category II limits for residential buildings as specified by UNI EN 16798-1 [36].

To compare the outcomes of the various simulations, two indices were used for the long-term assessment of overall thermal discomfort: the percentage of hours out of range (POR) and the average PPD (<PPD>) [37].

The POR, introduced by ISO 7730 and reiterated by UNI EN 16798-1, calculates the percentage of occupied hours (hi) during which the simulated PMV falls outside a specified comfort range for the given comfort category (Equation (6)).

$$POR={\displaystyle \frac{{\sum }_{i=1}^{Oh}\left({wf}_i·h_i\right)}{{\sum }_{i=1}^{Oh}\left(h_i\right)}}\in \left[\mathrm{0,1}\right]$$

(6)

The average PPD index (<PPD>), also introduced by ISO 7730 (method D), involves calculating the expected percentage of dissatisfied individuals (PPD) averaged over the occupied hours (hi) during a specified calculation period, as described in Equation (7) [38].

$$<PPD>\equiv {\displaystyle \frac{{\sum }_{i=1}^{Oh}\left({PPD}_i·h_i\right)}{{\sum }_{i=1}^{Oh}\left(h_i\right)}}$$

(7)

Table 4. Long-term thermal comfort assessment according to EN 7730.

			Cooling Period				Heating Period
			POR		<PPD>		POR		<PPD>
	BL	Baseline	4.9%		6.65		24.6%		8.38
Walls	PCM 1	BioPCM M51 Q25	3.2%	−34.7%	6.12	−8.0%	22.6%	−8.1%	8.35	−0.4%
	PCM 2	BioPCM M91 Q25	3.3%	−32.7%	6.11	−8.1%	23.1%	−5.9%	8.31	−0.9%
	PCM 3	BioPCM M182 Q25	4.7%	−4.1%	6.09	−8.4%	22.7%	−7.5%	8.20	−2.2%
Ceiling	PCM 4	BioPCM M51 Q25	3.1%	−36.7%	6.44	−3.2%	23.7%	−3.5%	8.22	−2.0%
	PCM 5	BioPCM M91 Q25	3.2%	−34.7%	6.31	−5.2%	23.6%	−3.9%	8.10	−3.3%
	PCM 6	BioPCM M182 Q25	3.3%	−32.7%	6.24	−6.1%	23.8%	−3.3%	8.05	−4.0%
W + C	PCM 6 + PCM 1	BioPCM M182 + M51	3.0%	−38.8%	6.12	−8.0%	23.1%	−6.1%	8.10	−3.4%
	PCM 6 + PCM 2	BioPCM M182 + M91	3.2%	−34.7%	6.06	−8.8%	22.8%	−7.3%	8.11	−3.3%
	PCM 6 + PCM 3	BioPCM M182 + M182	4.0%	−18.4%	5.98	−10.1%	23.2%	−5.5%	8.07	−3.7%

shows the evaluated indices for both the heating and cooling seasons for each scenario. During the summer, all configurations incorporating BioPCM achieve significant reductions in the percentage of discomfort hours, averaging below 30% compared to the base case. The most effective results are obtained with thinner BioPCM layers in all applications.

Specifically, the greatest reduction in discomfort hours—38.8%—is observed when BioPCM M182 is applied to the ceiling alongside BioPCM M51 on the walls. As the thickness of BioPCM increases, wall transmittance decreases, which diminishes the PCM’s ability to store heat, leading to an increase in discomfort hours compared to thinner solutions. This effect is particularly noticeable on hot days when indoor temperatures remain above the PCM’s solidification temperature.

In contrast, when examining the average PPD, which measures the percentage of dissatisfaction based on indoor climate conditions and their deviation from neutral comfort standards, it is evident that the average PPD decreases as BioPCM thickness increases for each application.

This improvement is directly related to the PCM’s thermal storage capacity, which helps stabilize and enhance indoor climate conditions by reducing temperature fluctuations and promoting thermal comfort. Applying BioPCM M182 to both the ceiling and walls results in a 10.1% reduction in average PPD compared to the base case.

A similar trend is observed during the heating season, although the variations are less pronounced than in summer. The average PPD decreases with increasing BioPCM thickness across all applications.

However, in some cases during winter, thicker PCM layers—being in solid form and having lower thermal conductivity—can inhibit the masonry’s ability to absorb sensible heat. This reduction in the thermal mass effect can cause a slight increase in discomfort hours compared to configurations with thinner PCM layers.

Additionally, the number of hours in which the indoor temperature exceeded the summer limit of 26 °C was analyzed. Results (Figure 6) indicate that all PCM solutions significantly reduce the number of hours where the internal temperature surpasses the limit specified for category II of UNI EN 16798-1. The reduction ranges from 34.2%, achieved with BioPCM M182 applied to both walls and ceiling, to 43.1%, observed with BioPCM M51 applied to both walls and ceiling and BioPCM M91 applied to the ceiling only.

3.3. Summer Free-Floating Analysis

To evaluate the effectiveness of the PCMs and to gain a deeper understanding of their performance during the cooling season—when their properties are most relevant due to the PCM melting temperature—several simulations were conducted in free-floating mode during the peak summer period (July 15 to August 15).

Table 5. Assessment of thermal comfort in free-floating mode according to UNI EN 16798.

			Category I EN 16798-1		Category II EN 16798-1		Category III EN 16798-1
	ID			Δ%		Δ%		Δ%
	BL	Baseline	458.0		338.5		162.0
Walls	PCM 1	BioPCM M51 Q25	428.0	−6.6%	302.0	−10.8%	146.7	−9.5%
	PCM 2	BioPCM M91 Q25	429.0	−6.3%	301.0	−11.1%	145.5	−10.2%
	PCM 3	BioPCM M182 Q25	430.0	−6.1%	290.0	−14.3%	145.0	−10.5%
Ceiling	PCM 4	BioPCM M51 Q25	410.0	−10.5%	274.1	−19.0%	133.8	−17.4%
	PCM 5	BioPCM M91 Q25	408.0	−10.9%	265.8	−21.5%	122.6	−24.3%
	PCM 6	BioPCM M182 Q25	391.0	−14.6%	221.4	−34.6%	112.6	−30.5%
W + C	PCM 6 + PCM 1	BioPCM M182 + M51	387.3	−15.4%	230.1	−32.0%	112.6	−30.5%
	PCM 6 + PCM 2	BioPCM M182 + M91	385.0	−15.9%	229.7	−32.2%	111.5	−31.2%
	PCM 6 + PCM 3	BioPCM M182 + M182	376.7	−17.8%	215.8	−36.2%	88.1	−45.6%

presents the discomfort hours according to the three categories outlined in EN 16798-1 for buildings without mechanical cooling systems.

According to the adaptive approach specified by UNI EN 16798-1, the indoor temperature limit is not constant but varies as a function of the daily weighted outdoor mean temperature over the seven days preceding the day being analyzed. For the city of Bari, the upper temperature limits range as follows: Category I from 25.5 °C to 27.3 °C, Category II from 26.5 °C to 28.3 °C, and Category III from 27.5 °C to 29.3 °C.

The results show a significant improvement in indoor thermal comfort for each category in all configurations (Figure 7).

Increasing reduction values are registered, respectively, with wall, ceiling, and combined wall/ceiling applications. Specifically, as the thickness of BioPCM increases, the hours of discomfort decrease pro-rata, with the exception of wall application, which shows a decrease in improvement in category I alone as the thickness increases. The maximum reduction values are recorded with both ceiling and wall application, and among these the BioPCM with the greatest thickness generates the maximum reduction of 17.8% in category I, 36.2% in category II, and 45.6% in category III (Table 5).

This is also verified by comparing the operating temperatures during a central week of the summer season from 20 to 26 July (Figure 8). The results show that PCMs can significantly reduce internal temperatures during the summer period.

The greater the thickness of PCM used, the greater the reduction achieved. The greatest reduction is achieved by the BioPCM M182 applied to both ceiling and walls, which manages to produce a maximum reduction in internal temperature of 4.4 °C compared to the base case.

In addition, it is evident that the PCMs, due to their thermal storage capacity, manage to reduce the amplitude of daily fluctuations in the internal temperature by stabilizing the minimum and maximum temperature peaks. It is also observed that when extremely high daytime temperatures coupled with a reduced daily temperature range occur, the effectiveness of PCMs in reducing temperatures during the hottest hours diminishes. This occurs because the PCMs do not fully complete their discharge cycle, leading to incomplete thermal release during the nighttime temperature drop.

As a result, the material’s thermal storage performance decreases, leading to smaller temperature reductions compared to the base case. This effect is most pronounced with thinner PCM layers, particularly when applied only to walls, where operating temperatures rise during the hottest hours.

3.4. Economic Benefit Evaluation

To evaluate the cost-effectiveness of the different strategies, the payback time (TR) was calculated by comparing the initial investment cost (Ic) with the total energy savings achieved by each solution relative to the base case. Additionally, the payback period was assessed considering state incentives for energy retrofitting of existing buildings, which offer a 70% reduction in the projected investment cost for multi-family building energy retrofitting actions.

The total energy savings were calculated as the sum of reductions in natural gas consumption during the heating season and electricity usage during the cooling season. To estimate the potential economic savings, the saved energy was multiplied by the respective average unit costs of EUR 0.24/kWh for electricity and EUR 0.11/kWh for natural gas (Eurostat, 2023). For the investment cost evaluation, the following unit prices were considered, including both material and installation costs: EUR 28.9/m² for BioPCM M51, EUR 46.8/m² for BioPCM M91, and EUR 87.8/m² for BioPCM M182. As shown in

Table 6. Evaluation of the payback period achieved by each solution.

		Money Saving										Payback PERIOD
		Heating				Cooling				Tot.		IC		TR	I*C		T*R
Walls	PCM 1	269.4	kWh/y	EUR	29.6	158.7	kWh/y	EUR	39.7	EUR	69.3	EUR	1997.0	28.8	EUR	599.0	8.6
	PCM 2	439.1	kWh/y	EUR	48.3	102.6	kWh/y	EUR	25.7	EUR	74.0	EUR	3238.0	43.8	EUR	971.0	13.1
	PCM 3	789.8	kWh/y	EUR	86.9	89.5	kWh/y	EUR	22.4	EUR	109.3	EUR	6070.0	55.6	EUR	1821.0	16.7
Ceiling	PCM 4	−5.2	kWh/y	EUR	−0.6	146.2	kWh/y	EUR	36.5	EUR	36.0	EUR	2121.0	59.0	EUR	636.0	17.7
	PCM 5	2.0	kWh/y	EUR	0.2	155.4	kWh/y	EUR	38.9	EUR	39.1	EUR	3439.0	88.0	EUR	1032.0	26.4
	PCM 6	16.0	kWh/y	EUR	1.8	169.8	kWh/y	EUR	42.4	EUR	44.2	EUR	6447.0	145.9	EUR	1934.0	43.8
W + C	PCM 6 + PCM 1	284.6	kWh/y	EUR	31.3	167.1	kWh/y	EUR	41.8	EUR	73.1	EUR	8446.0	115.6	EUR	2534.0	34.7
	PCM 6 + PCM 2	471.9	kWh/y	EUR	51.9	163.7	kWh/y	EUR	40.9	EUR	92.8	EUR	9687.0	104.3	EUR	2906.0	31.3
	PCM 6 + PCM 3	825.1	kWh/y	EUR	90.8	155.4	kWh/y	EUR	38.8	EUR	129.6	EUR	12,519.0	96.6	EUR	3756.0	29.0

I_C: investment cost, T_R: payback period, I*_C: investment cost with government incentives (70%), T*_R: payback period with government incentives.

, PCMs remain a relatively expensive technological solution, which renders them less cost-effective. The most economically advantageous outcomes are observed in instances of wall applications, followed by ceiling applications, and then combined applications. The application of thinner layers in both wall and ceiling applications results in a shorter return on investment. In the absence of state incentives, the payback period (T_R) ranges from a minimum of 28.8 years for the BioPCM M51 applied to walls to a maximum of 145.86 years for BioPCM M182 applied to the ceiling. However, the cost/benefit ratio is significantly enhanced when government incentives are taken into account. The payback time (T*_R) is reduced to between 8.64 years and 43.76 years, thereby rendering the utilization of PCMs a more economically viable proposition.

4. Conclusions

The integration of PCMs into building materials, such as plasterboard, offers a relatively low-invasive approach to retrofitting, allowing existing buildings to be upgraded without the need for major structural alterations. This is particularly relevant for multi-family buildings, where large-scale refurbishment is not always feasible or cost-effective. The potential benefits of such an approach include not only reduced energy consumption but also reduced environmental impact, contributing to the European Community’s wider goals of sustainability and climate resilience. By systematically analyzing the performance of different thicknesses and applications of PCMs, this study provided valuable insights into their effectiveness and applicability for future energy retrofits in Mediterranean regions.

The applications of a PCM with a melting temperature of 25 °C at three different thicknesses (74.2 mm, 37.1 mm, and 20.8 mm) to external walls, ceilings, and both walls and ceilings simultaneously were analyzed.

The findings of this study, consistent with the existing literature, demonstrate the effectiveness of PCMs in enhancing indoor thermal regulation by moderating temperature peaks and minimizing daily fluctuations. By retrofitting buildings with PCM-enhanced plasterboard, it is possible to reduce the reliance on active heating and cooling systems, thereby lowering overall energy consumption. Furthermore, the ability of PCMs to absorb and store excess heat during the day and release it during cooler periods contributes to maintaining stable indoor temperatures, reducing thermal discomfort, and improving occupant well-being.

This is particularly true even in buildings with heavy masonry and characterized by low window-to-wall ratios . The application of BioPCM M182 to both the ceiling and external walls yielded significant thermal improvements, with a maximum temperature reduction of 4.4 °C compared to the baseline scenario. Furthermore, the study noted a substantial decrease in discomfort hours under a free-floating configuration. Specifically, discomfort hours were reduced by 17.6% for Category I, 36.2% for Category II, and 46.5% for Category III, underscoring the potential of PCMs in mitigating the adverse impacts of climate change on building performance.

Among the various PCM applications investigated, wall installation alone was found to maximize heating savings, with efficiency improving as PCM thickness increased, resulting in a savings of up to 26.6%. In contrast, applying PCM to the ceiling was most effective for cooling purposes, achieving savings of up to 17.5%. The combination of both wall and ceiling applications offered the most balanced seasonal benefits, leading to the highest overall energy reduction, measured at 24.1%. This highlights the versatility of PCM integration in addressing both heating and cooling demands across different seasons.

Moreover, this study emphasized the significance of pairing PCMs with hybrid cooling systems, particularly during summer months, to optimize the charge and discharge cycle of the material. It was observed that extended periods of extremely high daytime temperatures, coupled with reduced diurnal temperature variations, could impair the thermal storage capacity of the PCM, thereby diminishing its overall performance. These findings suggest that careful consideration must be given to the climatic context in which PCMs are deployed and that hybrid systems may be necessary to maintain the material’s efficacy under challenging environmental conditions. Nevertheless, the high cost of the material and restricted market accessibility continue to render PCMs a comparatively costly technological solution. The most cost-effective outcomes are observed in wall applications, followed by ceiling and combination applications. Reduced application thicknesses produce the shortest payback periods, making them the most cost-effective to use.