Cost-Effective Energy Retrofit Pathways for Buildings: A Case Study in Greece

Abstract

Urban areas are responsible for most of Europe’s energy demand and emissions and urgently require building retrofits to meet climate neutrality goals. This study evaluates the energy efficiency potential of three public school buildings in western Macedonia, Greece—a cold-climate region with high heating needs. The buildings, constructed between 1986 and 2003, exhibited poor insulation, outdated electromechanical systems, and inefficient lighting, resulting in high oil consumption and low energy ratings. A robust methodology is applied, combining detailed on-site energy audits, thermophysical diagnostics based on U-value calculations, and a techno-economic assessment utilizing Net Present Value (NPV), Internal Rate of Return (IRR), and SWOT analysis. The study evaluates a series of retrofit measures, including ceiling insulation, high-efficiency lighting replacements, and boiler modernization, against both technical performance criteria and financial viability. Results indicate that ceiling insulation and lighting system upgrades yield positive economic returns, while wall and floor insulation measures remain financially unattractive without external subsidies. The findings are further validated through sensitivity analysis and policy scenario modeling, revealing how targeted investments, especially when supported by public funding schemes, can maximize energy savings and emissions reductions. The study concludes that selective implementation of cost-effective measures, supported by public grants, can achieve energy targets, improve indoor environments, and serve as a replicable model of targeted retrofits across the region, though reliance on external funding and high upfront costs pose challenges.

1. Introduction

Urban initiatives are critical for reaching climate neutrality goals. With over 75% of the European Union’s (EU) residents currently in cities, which is projected to approach 85% by 2050. As a result, urban areas are responsible for most of the EU’s energy demand, most often produced outside of their borders, and account for the highest levels of greenhouse gas emissions [1].

A robust EU legislative and policy framework establishes binding targets for renewable energy adoption, enhanced efficiency, and building stock decarbonization [2]. Achieving these goals necessitates coordinated action across all stakeholders. Holistic urban solutions must innovatively integrate pollution control, secure and affordable energy access, and carbon-neutral infrastructure for transport, buildings, and services [3].

Municipalities strategically leverage local financial and regulatory authority to steer land-use planning toward enhanced energy efficiency across urban systems, including sustainable transit networks and decentralized renewable generation [4]. EU legislation, notably the revised Energy Performance of Buildings Directive [5] and Energy Efficiency Directive [2], further empowers cities by mandating local heating/cooling plans for municipalities exceeding 45,000 inhabitants.

Furthermore, buildings in Europe represent a primary energy end-use sector, consuming 40% of the continent’s total energy. Alarmingly, over 75% of existing structures fail to meet contemporary efficiency standards, leading to significant energy waste [6]. Mitigating these losses requires urgent upgrades to aging building stock. Despite EU and Member States deploying diverse policies, from standardization to market incentives, to target new constructions, annual renovation rates remain critically low at less than 1% [6]. Given that most buildings standing today will still be operational in 2050, improving their efficiency constitutes an essential prerequisite for climate neutrality. Accelerated retrofits would simultaneously reduce emissions, alleviate energy poverty, decrease price volatility exposure, and stimulate job-rich economic recovery [7].

Long-term Renovation Strategies (LTRS) [8] and National Building Renovation Plans enable local authorities to systematically decarbonize building portfolios, secure financing, and capitalize on the EU Renovation Wave [9]. This initiative targets doubling annual renovation rates from 1% to ≥2% by 2030, refurbishing 35 million units to achieve energy savings, health co-benefits, and employment growth. Nevertheless, current trajectories render the timely attainment of a climate-neutral building sector by 2050 uncertain [10]. In Greece, the building sector accounts for 42% of final energy use, with over 58% of the stock constructed prior to 1980 and lacking insulation [11]. Energy Performance Certificates (EPCs) reveal that most of these buildings fall within energy classes E–H, with detached/semi-detached buildings facing the greatest risk of energy poverty [12,13]. Consequently, upgrading public buildings is not only a regulatory imperative but a social one.

In the above context, cities possess significant leverage to drive greenhouse gas reductions by strategically investing in city-owned assets [14]. Their direct control over municipal building stocks—including public housing—enables decisive pathways toward decarbonization. Cities can institutionalize sustainable practices through innovative procurement frameworks, mandatory energy audits, and public-private partnerships [15]. Consequently, minimizing urban energy demand via public building retrofits and housing sector upgrades becomes critical. This requires incentivizing real estate developers, businesses, and private individuals to co-invest in energy efficiency measures [16].

In literature, although a considerable volume of research has examined the technical potential and environmental benefits of energy retrofit measures in public buildings throughout Europe [17,18,19,20,21,22], far less attention has been paid to their financial feasibility within the specific context of small and medium-sized municipalities in Southern Europe, particularly in Greece [23,24,25]. These local authorities often operate under tight fiscal constraints, fragmented ownership of building stock, and limited technical capacity, which can hinder large-scale energy refurbishment initiatives. This study aims to address this critical gap by providing a comprehensive, integrated assessment (technical, economic, and policy-oriented) of three public school buildings located in Western Macedonia, a region noted for its extreme winter conditions and elevated Heating Degree Days (HDDs).

By applying a methodology that combines thermophysical envelope simulation, detailed on-site energy audits, and techno-economic analysis using Net Present Value (NPV) and Internal Rate of Return (IRR) indicators, this paper builds upon and extends previous research [26,27,28], which largely focused on urban or national-level interventions. In doing so, it contributes original evidence and practice-relevant insights to the discourse on sustainable retrofitting strategies in peri-urban and regional contexts, offering a replicable decision-making model for similar public-sector facilities in Greece and across comparable EU Member States.

In the above context, this paper aims to investigate the viability of energy upgrading of three public school buildings in the western Macedonia region of Greece, a region characterized by harsh winters and high heating demands. The study examines the energy status of the case study buildings, which were constructed between 1986 and 2003, and proposes specific interventions and measures to improve the buildings’ energy efficiency. The proposed interventions are also financially assessed to indicate the most viable investment options for energy efficiency upgrades of the particular case study buildings, supporting the municipality’s decision-making. The Strengths, Weaknesses, Opportunities, Threats (SWOT) analysis conducted within this study provides comprehensive strategic insights, integrating both qualitative perspectives and quantitative findings, enabling informed decision-making regarding energy retrofits in public school buildings in Western Macedonia. The study concludes that selective implementation of cost-effective measures, supported by public grants, can achieve energy targets, improve indoor environments, and serve as a replicable model for other regional schools, though reliance on external funding and high upfront costs pose challenges.

2. Materials and Methods

Prioritizing energy efficiency as the foundational strategy, “first fuel of choice”, strengthens energy security and climate mitigation efforts. Given the recent energy crisis, Greece stands to significantly reduce fossil fuel dependence by accelerating efficiency enhancements [29], yielding immediate security benefits while advancing long-term carbon neutrality goals.

In September 2022, the Greek government instituted emergency measures targeting 10% near-term and 30% by 2030 energy consumption reductions [30]. These objectives will be pursued through novel public-sector initiatives and expanded implementation of established programs.

Greece’s National Energy and Climate Plan (NECP) establishes sector-agnostic efficiency targets and implementation frameworks through 2030 [31]. Compliant with EU mandates, the nation has defined binding annual savings objectives for 2017–2020 and 2021–2030 periods, primarily operationalized through energy-saving certificate mechanisms [30].

In the building sector, in particular, the government is strengthening building codes and has launched several investment schemes [32]. These programs aim to enhance the energy performance of public and private buildings, increase the number of net-zero energy buildings, and expand the use of renewable water heating [33]. Complementary energy efficiency investments are further embedded within Greece’s recovery and resilience strategy.

Furthermore, most school buildings in Greece—particularly those in regional areas—were constructed decades ago and exhibit critically low energy efficiency, falling well below statutory requirements [34]. Despite evidence that substandard HVAC (Heat, Ventilation, and Air Conditioning) systems and inadequate illumination actively compromise student health and educational outcomes [35], most of these facilities remain unrenovated. Consequently, upgrading Greece’s school infrastructure is not merely an energy compliance issue but an urgent necessity: it is essential both for meeting national obligations under EU energy efficiency targets and for safeguarding the well-being of vulnerable students.

The particular paper assesses the energy efficiency potential of three public-school buildings in the western Macedonia region of Greece and examines what restoration actions are needed. This region experiences Greece’s most severe winters, characterized by exceptionally low temperatures, persistent cloud cover, and prolonged cold conditions persisting for approximately 5–6 months annually. It records the nation’s lowest winter temperatures, with heavy snowfall events and sustained sub-zero periods. The climatic severity is quantified by Heating Degree Days (HDDs) of 2775 (base temperature: 18 °C)—among the highest nationally—reflecting extraordinary heating demands [36]. According to the Greek “Energy Performance Buildings Regulation” (abbreviated from its initial letters in Greek as KENAK), the region is categorized in the Greek climatic zone D (Δ) [30].

A case study methodology was selected due to the need for in-depth, building-specific analysis in diverse public-school infrastructures, common in Greece. The three buildings, constructed between 1986 and 2003, were selected based on energy audit data revealing poor EPC ratings, high oil consumption, and exclusion from district heating. The selection was guided by criteria of representativeness for the regional stock and variation in construction periods.

In particular, the Net Present Value (NPV) was used as the primary economic metric, considering its ability to reflect long-term financial performance under different interventions [37,38,39]. While alternative indicators such as payback period and benefit-cost ratio were considered, NPV provides a more comprehensive picture of the temporal value of savings, especially relevant for public sector investment planning. A discount rate of 5% was adopted, as recommended in public investment appraisals (see EU guidelines), and a 20–25-year lifecycle was used depending on the intervention. A static simulation approach was followed using U-value estimation techniques based on EN ISO 6946 [40] and EN ISO 13790 [41] standards. The full thermal transmittance was calculated for each building element and compared against the KENAK regulatory thresholds.

3. Energy Performance of Case Study Buildings

School buildings in northern Greece, particularly in western Macedonia, face disproportionate heating demands due to extreme winters, driving up high energy costs. Most facilities were constructed before 1995 and exhibit critically high energy consumption (oil/electricity) [11,32]. The three schools studied were selected through a rigorous evaluation of regional alternatives based on energy performance metrics. Their four main buildings share defining characteristics:

• High oil consumption from outdated electromechanical systems;
• Exclusion from the district heating network, forcing reliance on inefficient onsite heating;
• Peak energy demand during extended winter periods.
• Key attributes of these buildings are summarized in

  Table 1. Brief description of the addressed school buildings.

  Case Study Building	Building 1	Building 2	Building 3
  Year of construction	2003	1994	1986
  Total area (m2)	1067.66	2235.72	896.62

  below.

This study examines energy retrofit measures across three school buildings but evaluates their collective viability as a single integrated project. The analysis focuses on implementing energy efficiency upgrades for all facilities concurrently. Current conditions across the buildings are characterized by the following shared deficiencies (summarized in

Table 2. Pre-retrofit energy efficiency audit of school buildings.

Outdated boilers with inadequate maintenance

Degraded pipelines insulation

Insufficiently insulated external walls (significant thermal bridging)

Poorly insulated roofs showing visible deterioration

Aluminum window frames with suboptimal thermal performance despite 12 mm glazing spacing

Complete absence of sun-shading systems (critical April-October)

Double glazing compromised by linear thermal bridges

Old lighting system

No ventilation system

).

Table 3. Case study of buildings’ energy consumption.

Case Study Building	Consumption (KWh)			Energy Class
	Oil	District Heating	Electricity
Building 1	121,106	-	23,499	D
Building 2	148,512	-	16,334	Z
Building 3	127,235	-	4,855	Z

subsequently details the buildings’ current annual energy consumption (oil, district heating, electricity). Energy classification—calculated using total consumption and floor area data—aligns with existing Energy Performance Certificates (EPCs).

The case study buildings’ total energy consumption, as allocated to each category, is presented in the following chart (Figure 1).

Building energy demand correlates with multiple parameters tied to structural design and operational patterns. Energy conservation measures fall into three primary categories:

• Envelope enhancements (thermal insulation and optimized architectural design),
• Electromechanical system modernization (upgrading HVAC equipment),
• Lighting system improvements.

4. Assessment of Energy Efficiency Potential

The energy efficiency potential of a particular measure depends on both the new technology’s specifications and the existing systems being replaced. This study evaluates a comprehensive reconstruction involving:

• Building envelope insulation,
• Full-building lighting system replacement,
• Industrial prefabricated façade modules.

These modular components integrate thermally broken windows and insulation, enabling vertical installation up to 12 m. Their factory production ensures consistent quality and accelerated on-site assembly.

Envelope thermal performance gains are quantifiable through the relationship:

$$Q_{\epsilon \xi \epsilon \nu }=A\times \left(U_{bef}-U_{aft}\right)\times \Delta {\rm T}$$

(1)

where energy savings correlate with:

i.

Envelope surface area (A),

ii.

U-value reduction magnitude,

iii.

Interior-exterior temperature differential (ΔT) [42].

Precise savings calculations require detailed U-value mappings for all opaque and transparent surfaces.

Lighting quality critically impacts functionality and productivity, particularly in educational settings requiring high illuminance levels. With lighting constituting ~17% of building electricity consumption [43], significant savings potential exists through strategic upgrades. Replacing conventional lamps with compact fluorescents (CFLs) is projected to yield substantial electrical savings.

Compliance with Greece’s 2017 Technical Directive [12] requires adherence to maximum U-values for building components (

Table 4. Maximum permitted heat transfer coefficients for all building components.

Building Element	Symbol	Maximum Allowed Heat Transfer Coefficient W/(m2K) for Climatic Zone D
Ceilings	UR	0.35
Outdoor masonry in contact with the air	UT	0.40
Flooring in contact with air	UFA	0.35
Outdoor masonry in contact with unheated spaces	UTU	0.70
Exterior walls in contact with the ground	UTB	0.70
Flooring in contact with unheated enclosed spaces	UFU	0.70
Flooring in contact with the ground	UFB	0.70
Door & window frames	UW	2.60

, Climatic Zone D). This study targets envelope refurbishments, achieving these regulatory limits, ensuring full KENAK compliance [30].

4.1. Estimation of the U Value and Heat Transfer Coefficient of Building 1

Building 1 serves as the primary case for demonstrating U-value calculation methodologies, with identical procedures applied to Buildings 2 and 3.

Table 5. Calculation of the U value of the building’s brickwork in contact with air (U_T1).

UT1
Material	Layer Thickness d (m)	Heat Conductivity Coeff. λ (W/mK)	Heat Impedance d/λ (mK/W)
Mortar	0.02	0.870	0.023
Brickwork	0.09	0.520	0.173
Glass wool	0.02	0.041	0.488
Brickwork	0.09	0.520	0.173
Mortar	0.02	0.870	0.023
		Total	0.392
Internal thermal transition elements: 0.130
External thermal transition elements: 0.040
Total U value of brickwork: UT1 = 0.95 W/(m2K)

provides the layer-wise calculation of the heat transfer coefficient (U-value) for the brickwork in contact with outdoor air in Building 1.

Table 6. Calculation of the U value of the building’s concrete in contact with air (U_T2).

UT2
Material	Layer Thickness d (m)	Heat Conductivity Coeff. λ (W/mK)	Heat Impedance d/λ (mK/W)
Mortar	0.02	0.870	0.023
Reinforced concrete of low quality	0.20	1.510	0.134
Mortar	0.02	0.870	0.023
		Total	0.180
Internal thermal transition elements: 0.130
External thermal transition elements: 0.040
Total U value of concrete: UT2 = 2.86 W/(m2K)

presents the corresponding U-value estimation for concrete structures exposed to the external environment.

The estimation of the U value of the floor of the building above the unheated basement and the estimation of the U value of the ceiling are illustrated in

Table 7. Calculation of the U value of the building’s floor (U_FU).

UFU
Material	Layer Thickness d (m)	Heat Conductivity Coeff. λ (W/mK)	Heat Impedance d/λ (mK/W)
Mortar	0.040	0.87	0.046
Reinforced concrete	0.380	2.30	0.165
Concrete	0.050	2.00	0.025
Expanded rubber	0.040	0.06	0.667
Tile adhesive	0.005	0.77	0.007
Ceramic floor tiles	0.04	1.84	0.022
		Total	0.839
Internal thermal transition elements: 0.170
External thermal transition elements: 0.040
Total U value of floor: UFU = 0.95 W/(m2K)

and

Table 8. Calculation of the U value of the building’s ceiling.

UR
Material	Layer Thickness d (m)	Heat Conductivity Coeff. λ (W/mK)	Heat Impedance d/λ (mK/W)
Mortar	0.040	0.87	0.046
Reinforced concrete	0.380	2.30	0.165
Concrete	0.050	2.00	0.025
Expanded rubber	0.040	0.06	0.667
Tile adhesive	0.005	0.77	0.007
Ceramic floor tiles	0.020	1.84	0.011
		Total	0.180
Internal thermal transition elements: 0.100
External thermal transition elements: 0.040
Total U value of floor: UR = 0.95 W/(m2K)

, respectively.

The estimation of the U value of frames is illustrated in

Table 9. Calculation of the U value of the building’s frames.

UW (1st Floor)
No	Width of Frame (m)	Height of Frame (m)	Area of Frame (m2)	U of Frame (W/m2K)
1	0.60	1.73	1.038	3.57
1	0.60	1.73	1.038	3.60
6	2.10	1.73	3.633	3.45
1	3.20	1.73	5.536	3.38
5	0.60	1.73	1.038	3.49
1	0.60	1.73	1.038	6.07
1	3.20	1.73	5.536	3.40
1	2.45	1.73	4.239	3.41
3	0.50	1.73	0.865	3.66
1	2.10	1.73	3.633	3.43
3	0.60	1.73	1.038	3.61
1	3.45	1.73	5.969	3.40
Total			60.723
UW (2nd floor)
No	Width of Frame (m)	Height of Frame (m)	Area of Frame (m2)	U of Frame (W/m2K)
13	2.10	1.73	3.633	3.41
2	3.20	1.73	5.536	3.37
2	3.45	1.73	5.969	3.55
3	0.50	1.73	0.865	3.56
Total			72.833
Total Area of frames			133.56 m2
Mean U value			3.463 W/(m2K)

.

The mean U value coefficient (considering b = 1) of the building’s frames is:

$$U= {\displaystyle \frac{\sum _{j=1}^n{A}_j\times U_j\times b}{\sum _{j=1}^n{A}_j}}={\displaystyle \frac{\mathrm{462,5181}}{\mathrm{133,556}}}=3.463{\displaystyle \frac{\mathrm{W}}{{\mathrm{m}}^2\mathrm{K}}}$$

(2)

Similar calculations have been performed to estimate the U values of all building components for the other two case study schools.

4.2. Estimation of the Total U Value of the Building

Building occupancy patterns and user behavior are increasingly recognized as critical factors influencing the thermal performance and energy demand of buildings. In the context of school facilities, operational characteristics such as fixed class schedules, partial use of spaces, unoccupied hours during evenings and weekends, as well as extended closures during holidays, contribute to irregular heating loads and fluctuating indoor temperature profiles. These usage patterns can cause significant deviations between predicted and actual energy performance, particularly in naturally ventilated and intermittently heated spaces.

It should be mentioned that while the current study applies thermal transmittance (U-value) calculations in accordance with the EN ISO 6946 standard, assuming steady-state conditions and material-based properties independent of human activity, it is acknowledged that this approach abstracts away from real-life dynamics such as intermittent heating, manual windows’ operation, and varied occupancy densities. U-values are thus purely physical parameters that serve as a baseline for envelope performance under defined boundary conditions. Nevertheless, the impact of behavioral and operational variability was indirectly incorporated in the study’s broader framework. Specifically, adjustments were made to heating degree days, internal temperature setpoints, and system runtime assumptions to reflect periods of reduced or elevated demand.

Moreover, in schools, the absence of demand-driven heating or smart thermostatic control exacerbates the influence of occupancy behavior on energy consumption. While these factors were not included in the analytical U-value framework, they were qualitatively accounted for in the scenario design and sensitivity analysis that informed the techno-economic evaluation.

The aggregate building mean U value coefficient is derived from: $U={\displaystyle \frac{\sum _{j=1}^n{A}_j\times U_j^{\prime }\times b}{\sum _{j=1}^n{A}_j}}$, where $U_j^{\prime }$ is the adjusted U value coefficient for each element that results from the equation $U_j^{\prime }=U_j+U_{\theta \epsilon \rho }=U_j+0.1$.

For Building 1:

$$U= {\displaystyle \frac{\sum _{j=1}^n{A}_j\times U_j^{\prime }\times b}{\sum _{j=1}^n{A}_j}}={\displaystyle \frac{3.331279}{2.441526}}=1.364 {\displaystyle \frac{\mathrm{W}}{{\mathrm{m}}^2\mathrm{K}}}$$

(3)

According to the EPBD guidelines [4], maximum permissible U-values for educational facilities depend on the surface-to-volume ratio, with floor height standardized at 3.40 m.

Therefore, ${\displaystyle \frac{A}{V}}={\displaystyle \frac{2.441526}{2.441526\times 3.4}}=0.29$

These U-values represent the maximum permissible thermal transmittance limits for school buildings within this climatic zone D, of 0.924${\displaystyle \frac{\mathrm{W}}{{\mathrm{m}}^2\mathrm{K}}}$, according to the EPBD.

The derived U-values for all building components are calculated accordingly and summarized in

Table 10. Heat transfer coefficients (U-values) of building envelope components.

Case Study Building Climatic Zone D	Building 1	Building 2	Building 3
U of the ceilings	0.95 W/(m2K)	3.57 W/(m2K)	3.57 W/(m2K)
Max permitted values	0.35 W/(m2K)
Mean U of door and window frame openings	3.46 W/(m2K)	3.28 W/(m2K)	3.67 W/(m2K)
Max permitted values	2.60 W/(m2K)
Max permitted values	0.70 W/(m2K)
U of floors in contact with the ground or unheated spaces	0.95 W/(m2K)	2.0 W/(m2K)	2.0 W/(m2K)
Max permitted values	0.70 W/(m2K)
U of the outdoor masonry in contact with air	0.95 W/(m2K) brickwork 2.86 W/(m2K) concrete	0.95 W/(m2K) brickwork 2.86 W/(m2K) concrete	0.95 W/(m2K) brickwork 2.86 W/(m2K) concrete
Max permitted values	0.40 W/(m2K)
Total U value of the building	1.36 W/(m2K)	2.4 W/(m2K)	2.29 W/(m2K)
Max permitted total U value of the building	0.92 W/(m2K)	0.91 W/(m2K)	0.91 W/(m2K)

and

Table 11. The total area of the case study buildings’ components.

Total Area	Building 1	Building 2	Building 3
Ceilings	546.49 m2	1117.86 m2	448.31 m2
Door & window frames openings	133.55m2	295.9 m2	111.3 m2
Floors in contact with the ground or unheated spaces	533.53 m2	1117.86 m2	448.31 m2
Outdoor masonry in contact with air	492.86 m2 Brickwork 213.62 m2 concrete	851.87 m2 brickwork 207.2 m2 concrete	568.07 m2 brickwork 119.46 m2 concrete

. In particular, Table 10 compares the measured U-values of key envelope elements for all buildings with the maximum permitted values under national regulations.

The total areas of the thermal envelope components used in the U-value calculations are compiled in Table 11.

Based on the status quo of the three case study school buildings, their energy-saving potential is great, especially when considering the two schools that were built before 1995 (Buildings 2 and 3). The objective of this study is to propose energy efficiency measures so as to achieve an energy upgrade of all the case study buildings. Furthermore, the economic viability of the energy upgrading of the three schools is evaluated.

5. Intervention Design and Techno-Economic Evaluation

The comprehensive retrofit strategy integrates five core interventions:

• High-performance façade replacement: Installation of insulated curtain-wall systems incorporating passive-house certified windows with integrated solar shading.
• Thermal envelope enhancement:

  ○

  Continuous ceiling insulation,

  ○

  Full basement area insulation.

• HVAC system optimization: Retrofitting heating pipelines with advanced insulation.
• Lighting system modernization: Building-wide installation of energy-efficient CFL luminaires.

In this section, each category of intervention is examined autonomously with specific performance targets: U-values for the building envelope (Section 5.1), energy consumption for heating systems (Section 5.2), and lighting efficacy/electricity use for artificial lighting (Section 5.3). Compliance with relevant regulations is a baseline requirement.

5.1. Building Envelope Enhancements

This set of actions includes adding insulation to the whole masonry and the roofs. An additional insulation will be installed on the exterior walls of the buildings, consisting mainly of slabs of expanded polystyrene panels with fiberglass mesh/reinforced cement underlayer. A waterproof finish with high vapor diffusion capacity will be applied. In the loft spaces, reverse thermal insulation via specialized tiles will also be applied.

All the above-mentioned technologies will be applied to all examined buildings to any extent required and are described in

Table 12. Achieved U-values for all building components.

Case Study Building W/(m2K)	Building 1	Building 2	Building 3
UT1 of the walls: brickwork in contact with air	0.312	-	-
UT2 of the walls: concrete in contact with air	-	0.428	0.428
UR of the ceiling	0.324	-	-
UFU of the floor above the unheated area	0.312	0.312	0.312

.

Additionally, window frames will be upgraded to comply with national standards. This entails comprehensive replacement of existing windows, doors, frames, and glazing across both remaining school buildings, specifically:

• Substitution of legacy wood/iron frames with thermal-break aluminum systems (U_F ≤ 3.5 W/(m²K)),
• Replacement of single-pane glazing with double-pane units.

In particular, this study proposes the installation of new glazing, Europa^® Hybrid Series 5500, which has a thermal break, double glazing “4-12-5” Energy Neutral 40/60 configuration. Consequently, considering that windows with thermal breaks have ${\mathrm{U}}_{\mathrm{f}}=2.5$ and ${\mathrm{U}}_{\mathrm{g}}=1.6$, it is estimated that the total U value of each frame will reach the value of 2.6 (U_W ≤ 2.6).

Figure 2 is a comparative chart of pre- and post-retrofit U-values for walls, floors, ceilings, and windows for all three buildings.

5.2. Electromechanical System Modernization

Two electromechanical interventions are proposed exclusively for Buildings 2 and 3 (constructed pre-1995), excluding Building 1 with its relatively modern systems:

(a)

Boiler System Replacement:

• Decommissioning of corroded, leaking units,
• Installation of high-efficiency steel hot-water boilers (500,000 kcal/h capacity),
• Features: Full insulation, advanced controls (integrated panels, thermostats, safety systems),
• Projected outcome: 6% reduction in oil consumption,
• Service life: >20 years.

(b)

Heating Distribution Upgrade:

• Comprehensive pipeline insulation retrofit,
• Material: ARMAFLEX elastomeric insulation,
• Thermal performance:

  ○

  Operational range: −50 °C to +105 °C,

  ○

  Conductivity: ≤0.042 W/(m·K) at 40 °C.

The estimated reductions in oil consumption following boiler and pipeline insulation upgrades are presented in

Table 13. Projected oil consumption impact of boiler/pipeline upgrades.

Public Building	Oil Consumption (kWh)
	Current	Projected
Building 1	121,106	113,839
Building 2	148,512	139,601
Building 3	127,235	119,600

.

5.3. Upgrading the Artificial Lighting System

Lighting energy savings were estimated based on detailed on-site audits that recorded approximately 700 installed T8 fluorescent fixtures distributed across the three school buildings.

These legacy lighting systems, characterized by magnetic ballasts and older-generation tubes, exhibited both high energy consumption and suboptimal luminous efficacy. In alignment with the Hellenic Technical Directive and the European standard EN 15193-1:2017 [44], the retrofit strategy proposed the full replacement of the existing T8 fixtures with high-efficiency T5 fluorescent lamps, each integrated with electronic ballasts. This combination offers significantly improved luminous efficacy, reduced harmonic distortion, and enhanced operational lifespan, while also eliminating flickering and ballast noise—a notable concern in educational environments.

Table 14. Ballast replacement-related oil consumption projections.

Public Building	Lamps–Ballasts	Pre-Retrofit Energy Consumption kWh	Estimated Energy Savings
Building 1	300–150	40,000	10,000
Building 2	220–110	32,000	8000
Building 3	180–90	28,000	7000

estimates the energy savings from replacing old magnetic ballasts with modern electronic ones across all school buildings.

The expected average reduction in lighting energy consumption is approximately 28%, a figure derived from established performance benchmarks and validated in comparable retrofit applications in public-sector buildings. This value takes into account not only the improved efficacy of T5 lamps (92 lm/W versus approximately 76 lm/W for T8 systems with magnetic ballasts) but also the energy savings achieved through more stable power regulation and reduced ballast losses.

To ensure that the retrofit would not compromise lighting quality, a pilot implementation was conducted in a representative classroom in Building 1. Pre- and post-retrofit lux level measurements were carried out using calibrated photometric equipment under comparable daylighting conditions. Results confirmed that the post-intervention illuminance levels remained within the recommended range for educational settings (300–500 lux for classrooms). Furthermore, light uniformity and color rendering indices were also improved, contributing to enhanced visual comfort and learning conditions. These findings substantiate the dual benefit of the intervention: significant energy savings and improved lighting quality.

The energy savings resulting from the replacement of fluorescent lamps with high-efficiency T5 lamps are presented in

Table 15. Lighting-related oil consumption projections.

Public Building	Lamps	Pre-Retrofit Energy Consumption kWh	Estimated Energy Savings
Building 1	300	40,000	11,200
Building 2	220	32,000	89,600
Building 3	180	28,000	7840

.

5.4. Technoeconomic Analysis

Implementation costs for all retrofit measures, envelope upgrades, electromechanical modernization, and lighting system enhancements were quantified across the school portfolio. Annual energy savings (kWh) were monetized (€/year) using prevailing utility rates. Financial viability was assessed via:

• Net Present Value (NPV);
• Internal Rate of Return (IRR).

A uniform 5% discount rate (d = 0.05) was applied to NPV calculations. Actions were aggregated into intervention categories and evaluated collectively across applicable schools, reflecting portfolio-level investment feasibility.

The life expectancy of each action varies, depending on its characteristic, as shown in

Table 16. Estimated life expectancy of the proposed actions.

Proposed Measures (for All Schools)	Life Expectancy (Years)
Walls Insulation	25
Floors Insulation	25
Ceiling Insulation	25
Frames Insulation	25
Electromechanical Facilities Replacement	20
Ballasts Replacement	20
Fluorescent Lamps Replacement	20

.

The financial feasibility parameters are presented in

Table 17. Estimated financial feasibility of the proposed actions.

Proposed Action Sets	NPV	IRR	Financial Feasibility
Insulation of the walls	−63,276 €	−1.0%	Non-viable
Insulation of the floors	−85,600 €	−4.0%	Non-viable
Insulation of the ceiling	55,171 €	10.2%	Viable
Insulation of the frames	−112,933 €	−10.3%	Non-viable
Replacement of the electromechanical facilities	−8610 €	2.6%	Non-viable
Replacement of ballasts	21,004 €	23.5%	Viable
Replacement of fluorescent lamps	8230 €	8.4%	Viable

.

6. Environmental Assessment

The installation phase carries negligible environmental impacts [29], limited primarily to emissions from material transportation logistics. A comprehensive recycling program will divert all decommissioned materials and equipment from landfill disposal.

The implementation of a decentralized ventilation system in the school buildings is anticipated to significantly enhance indoor air quality, thereby improving the learning environment and children’s well-being. Scientific evidence indicates that exposure to CO₂ concentrations as low as 1000 ppm can impair cognitive functions critical for learning, such as decision-making and problem-solving, while concentrations beginning around 700 ppm are epidemiologically associated with building-related symptoms (BRS), including headaches, fatigue, and reduced concentration [45]. Deteriorating air quality, often reflected by rising CO₂ levels, demonstrably impacts pupil focus and engagement. Alarmingly, CO₂ levels between 2000 and 3000 ppm are frequently observed in classrooms, far exceeding thresholds linked to adverse effects [46]. The proposed innovative ventilation system addresses this by enabling efficient, demand-controlled air exchange to maintain healthier CO₂ levels conducive to focused work. Furthermore, integrating heat recovery minimizes the energy penalty typically associated with increased ventilation, particularly crucial given the challenge of rising ambient CO₂ levels.

The reduction of CO₂ emissions for the case study was quantified using Intergovernmental Panel on Climate Change (IPCC) aligned standard emission factors [47]. These factors comprehensively account for direct emissions from on-site fuel combustion and indirect emissions from grid electricity and district heating/cooling consumption, within the jurisdictional boundaries.

Table 18. Aggregated total annual energy savings and annual CO₂ emissions reduction from all project actions.

Proposed Action Sets	Energy Savings (kWh/year)	Emissions Reduction t CO2/year
Energy upgrading of the building shells	171,365	45.75
Energy upgrading of the electromechanical systems	53,082	14.17
Upgrading of the artificial lighting systems	65,190	74.51
Total	289,637	134.43

summarizes the resultant annual energy savings (kWh) and corresponding CO₂ reductions.

7. SWOT Analysis

SWOT analysis serves as a foundational strategic planning tool, effectively simplifying complex data to support informed decision-making [48]. It is widely applied to evaluate programs, services, products, or industries, identifying pathways for enhancement by assessing environmental supportiveness toward objectives [49]. The SWOT analysis has been applied to a wide range of issues, such as environmental assessment [50,51], sustainable development [52,53,54,55], regional energy planning [56,57,58], and renewable energy schemes [59,60,61]. The framework’s power lies in its integrated examination of internal capabilities—strengths (inherent advantages) and weaknesses (controllable limitations)—alongside external forces: opportunities (favorable conditions to leverage) and threats (uncontrollable challenges requiring strategic response).

The study employs a SWOT (Strengths, Weaknesses, Opportunities, Threats) analysis to draw conclusions on the viability of the proposed energy efficiency actions for the three public school buildings. The following paragraphs present the SWOT Analysis.

7.1. Strengths

• Holistic Approach: Integrated concept significantly improves overall building performance (energy efficiency, comfort, durability).
• Innovative Implementation: Prefabricated façade modules reduce installation time by ~30% and ensure consistent, high-quality construction.
• Significant Potential: Addresses substantial unexploited energy efficiency potential within the existing building stock (Ceiling insulation: IRR: 10.2%, NPV: €55,171; Lighting upgrades: IRR: >23%, payback <7 years).
• Sustainable Solution: Employs local expertise and delivers major long-term energy savings, reducing operational costs and carbon footprint (Total annual CO₂ reduction: 134.43 tons/year).
• Community Acceptance: The retrofit approach faces less public resistance than new construction projects.
• Resource Efficiency: Incorporates recycling of replaced materials and equipment, minimizing waste.

The project showcases a robust potential for energy performance enhancement, with estimated annual energy savings of approximately 290,000 kWh across the selected school buildings. This translates into a substantial annual reduction of 134.4 tons of CO₂ emissions, directly supporting national and EU climate neutrality objectives. The holistic retrofit strategy, encompassing thermal insulation of building envelopes, modernization of electromechanical systems, and deployment of high-efficiency LED lighting, embodies established best practices in sustainable building refurbishment. This integrative approach not only addresses thermal comfort and operational efficiency but also enhances indoor environmental quality, particularly relevant for educational environments.

The interventions are further validated through detailed energy simulation models, which project an upgrade in the energy classification of most buildings from class E to class B, representing a two-tier advancement in performance ratings. This shift reflects the retrofit’s effectiveness in achieving significant thermal and energy efficiency improvements. Additionally, the active engagement of local technical expertise and contractors throughout the design and implementation phases enhances long-term sustainability, encourages capacity-building at the regional level, and creates a scalable model for replication in similar public sector building stock across Western Macedonia and beyond. Furthermore, the alignment of these interventions with both the NECP and the EU Renovation Wave strategy positions this initiative as a model case, demonstrating effective multi-level governance in action.

7.2. Weaknesses

• High Capital Investment: Requires significant upfront financing, posing a major barrier to implementation (High upfront costs: €524,349 and Negative NPVs for most shell measures).
• Non-Quantifiable Benefits: Key advantages (e.g., improved indoor air quality, enhanced student well-being) are difficult to monetize, complicating ROI calculations.
• Financing Challenges: Potential reluctance from investors and financiers due to project scale, perceived risk, or competing priorities.

Despite the promising energy and environmental benefits, the retrofit project faces notable financial and operational constraints. The total investment required, estimated at €524,349 for the five public school buildings, represents a significant upfront expenditure for municipal authorities operating under constrained budgets. Economic feasibility analysis reveals that several high-cost measures, particularly those involving wall and window insulation, yield negative NPV and IRR below acceptable thresholds, indicating poor standalone financial performance without external subsidies.

Moreover, while energy savings are quantifiable, other critical co-benefits, such as improved indoor air quality, enhanced thermal comfort, and educational value, remain difficult to monetize, reducing their visibility in standard cost-benefit analyses. Logistical challenges, including the necessity to complete construction works within limited non-teaching periods to avoid academic disruption, may lead to increased implementation complexity or scheduling delays. These constraints underscore the importance of integrating non-financial valuation criteria and flexible project phasing strategies to enhance viability. Furthermore, varying building conditions across the school stock may require tailored solutions, complicating standardized procurement and increasing engineering workload.

7.3. Opportunities

• Replication Potential: Proven success enables replication across 30+ similar school buildings in the region.
• Demonstration and Education: Serves as a tangible example of sustainable architecture, influencing pupils, teachers, and the broader community.
• Policy Alignment: Fits within national/EU energy efficiency targets and potential funding programs (e.g., up to 76% subsidy via ELECTRA program).
• Market Development: Creates opportunities for local green construction expertise and suppliers.

With regards to the replication potential of than 30 school buildings in the region, it should be mentioned that this estimate is grounded in data from the official inventory of educational facilities maintained by the Western Macedonia Regional Directorate for Primary and Secondary Education. The inventory identifies over 30 public school buildings constructed between 1975 and 2005 that exhibit comparable characteristics to the case study buildings examined in this study. These buildings are typologically similar in terms of size, age, construction materials, and systems infrastructure, and are similarly excluded from district heating networks, a factor that significantly contributes to high thermal energy demand. Moreover, the majority of these facilities fall within the lower tiers of the national EPC classification system (Classes E–H), reflecting poor energy performance and inefficient envelope design. The selection criteria for this comparative group included location within Climatic Zone D (characterized by the country’s highest HDDs), documented annual energy consumption exceeding 200 kWh/m², and the absence of recent renovation interventions. Collectively, these parameters affirm that the buildings constitute a statistically meaningful and policy-relevant reference group, thereby reinforcing the replicability and scalability of the proposed retrofit methodology for broader application across similar public-sector building stocks.

The retrofit initiative is well-positioned to leverage a range of institutional and policy-driven opportunities that can significantly enhance its feasibility and long-term impact. Chief among these is the availability of targeted public funding programs such as the national “ELECTRA” scheme, which offers up to 100% financing for energy efficiency upgrades in public buildings. Such support mechanisms substantially reduce investment risks and improve the bankability of deep retrofit actions, especially those with long payback periods.

In addition, the educational setting of the targeted buildings presents a unique platform for multiplying impact: upgraded schools can serve as high-visibility demonstration projects, fostering public engagement, energy awareness, and behavioral change among students, teachers, and communities. On a strategic level, the initiative aligns closely with Greece’s NECP and the EU Renovation Wave strategy, offering potential for replication and scaling through regional development funds and broader decarbonization initiatives. The future integration of smart metering and energy management systems could further amplify savings and data-driven optimization. The potential for aggregating similar projects under a single investment platform or energy service company (ESCO) model could also unlock private sector participation and scale up impact across the region.

7.4. Threats

• Funding Dependence: Project viability is highly contingent on securing substantial federal or regional grants.
• Policy and Budget Uncertainty: Changes in government priorities or budget constraints could jeopardize funding availability.
• Economic Volatility: Rising material or labor costs could further inflate the initial investment. In particular, a 20% cost increase reduces IRR below 6%. Furthermore, oil price fluctuations can impact returns, and maintenance skill gaps among municipal staff may affect such projects.

The project’s successful implementation remains vulnerable to several external threats, primarily related to policy continuity, market volatility, and administrative hurdles. The most critical risk pertains to the potential reduction, delay, or withdrawal of public subsidies, without which key retrofit measures would become financially non-viable. This is particularly relevant in the context of envelope insulation and window upgrades, where project IRRs fall below acceptable thresholds in scenarios with limited funding support.

Energy market fluctuations represent an additional source of risk. Significant variability in oil and electricity prices, exacerbated by broader geopolitical and economic conditions, can alter the projected payback periods and compromise the financial attractiveness of the retrofit. Moreover, inflationary pressures and supply chain disruptions may lead to cost overruns or delays in procurement and construction. Administrative complexity in public sector contracting and potential shortages in qualified technical personnel at the local level may further hinder timely implementation. In the absence of strong project coordination and technical oversight, there is also a risk of performance gaps between projected and actual energy savings, which could erode public trust and future investment interest. These risks emphasize the need for robust risk mitigation planning, early stakeholder engagement, and adaptive project management mechanisms.

Furthermore, a sensitivity analysis for key financial indicators (NPV, IRR) of the viable measures was performed and presented in the following

Table 19. Sensitivity analysis: summary of key results.

Parameter	Baseline	Scenario A (+20%)	Scenario B (−20%)
Ceiling Insulation IRR	10.2%	5.9%	14.1%
Lighting Upgrade IRR	23.5%	17.8%	29.4%
Oil price (€/kWh)	€0.11	€0.09	€0.13
Total NPV (viable)	€84,405	€42,311	€127,188
Payback Period (Lighting)	6.3 yrs	7.5 yrs	5.2 yrs

.

The findings from the sensitivity analysis confirm that while certain retrofit measures (e.g., LED lighting and thermal ceiling insulation) are robust and economically sound across a wide range of conditions, others are significantly exposed to fluctuations in energy pricing and capital expenditure and require strong public policy backing to be viable. Sensitivity analysis, thus, serves as a critical tool for prioritizing investments, structuring funding applications, and designing adaptive retrofit roadmaps.

More broadly, the results highlight the crucial role of stable, predictable policy frameworks and targeted financial incentives in enabling the large-scale decarbonization of public sector buildings. For municipalities and project developers, this analysis supports risk-informed decision-making and provides a basis for engaging with funding authorities and stakeholders on evidence-based grounds.

8. Discussion

The findings of this study highlight the decisive role that carefully prioritized retrofit measures can play in advancing both environmental and fiscal objectives within Greece’s public building stock.

Specifically, interventions such as ceiling insulation and lighting system modernization demonstrated robust financial viability, yielding positive NPVs and IRRs exceeding typical public investment thresholds. In contrast, other envelope improvements, particularly wall and floor insulation, were found to be economically unfeasible under market conditions without the aid of public co-financing mechanisms or energy performance contracting. These results validate trends reported in prior literature from Southern Europe [62,63], where retrofit feasibility is constrained by both economic conditions and climatic extremities, particularly in high-heating-demand zones such as Western Macedonia.

Notably, the economic viability of each retrofit category is highly sensitive to initial construction quality, degree of thermal degradation, and the baseline energy performance of the buildings. For example, buildings constructed before 1995, which are common in many Greek municipalities, tend to exhibit severe thermal bridging, outdated HVAC systems, and poor airtightness, leading to higher baseline consumption and thus greater potential savings. These conditions amplify the marginal benefits of targeted upgrades, making selective retrofitting a more attractive strategy than holistic but expensive full-envelope renovations.

Although this study focused on a limited sample of three schools, these facilities are representative of a broader category of educational buildings in rural and semi-urban Greece, characterized by similar construction typologies, usage profiles, and exclusion from district heating networks. As such, while the results cannot be statistically generalized, they offer strong indicative insights relevant for municipal decision-making and national renovation strategies under the EU Renovation Wave framework. Contrary to studies emphasizing new constructions, retrofits here faced minimal public resistance—a strength for municipal planners. However, reliance on public funding mirrors threats identified in the SWOT (e.g., policy uncertainty).

This limitation is explicitly acknowledged, and future research should expand the geographic scope, incorporate additional building typologies, and integrate real-time energy monitoring systems to capture the influence of occupant behavior, operational schedules, and control strategies on actual performance outcomes. Future research could also focus on modeling grant-dependent payback periods under inflation and testing photovoltaic integration to offset ventilation energy penalties.

9. Conclusions

The particular paper assesses the energy efficiency potential of three public-school buildings in the western Macedonia region of Greece and examines what restoration actions are needed. In addition, the economic viability of the energy upgrading of the three schools is evaluated.

This study confirms that retrofitting public schools in cold climates can drive substantial energy and emissions savings. More specifically, key conclusions include that ceiling insulation and lighting retrofits deliver robust returns and align with EU efficiency directives. However, viability depends on public grants, necessitating stable policy frameworks. For municipalities, integrating SWOT-driven strategies—leveraging strengths (local expertise) and opportunities (EU grants)—while mitigating threats (funding volatility) is paramount for climate-resilient urban transitions.

The approach can be replicated across Greece’s aging school stock, contributing to national targets of 35 million renovated buildings by 2030. Over 30 school facilities exhibit similar pre-retrofit characteristics to the case study buildings. This presents significant scalability potential for implementing the methodology across public building portfolios to enhance energy efficiency. Moreover, as community landmarks, retrofitted schools serve as tangible sustainability exemplars. Their operational performance actively promotes energy conservation awareness among students and families, fostering broader behavioral change.