Building Envelope Renovation for Energy Efficiency in Maputo, Mozambique: Expanded Polystyrene Insulation and Double-Glazed Windows ^†

Abstract

This study examines the impact of envelope renovation using Expanded Polystyrene (EPS) insulation and double glazing on reducing CO₂ emissions and energy consumption in low-income residential buildings in Mozambique’s tropical climate. Conducted in Maputo over 12 months (2023–2024), it targets urban households, addressing high energy use and emissions caused by inefficient building envelopes and limited access to sustainable technologies. The study uses DesignBuilder’s validated EnergyPlus engine to evaluate energy savings and financial viability within cultural and economic contexts. Results show a 42.16% reduction in energy consumption (from 5392.04 to 3118.69 kWh) and a 42.20% decrease in CO₂ emissions (from 3.27 × 10³ to 1.89 × 10³ kg) compared to conventional designs. With an 11.75% discount rate accounting for inflation and opportunity costs, the retrofit achieves a payback period of 6.9 years, confirming its financial viability. These findings offer policymakers, architects, and low-income communities a cost-effective retrofit model, advocating for policy integration of low-U-value materials to improve environmental and economic sustainability.

1. Introduction

Buildings significantly contribute to global CO₂ emissions, yet sustainable strategies like envelope renovation remain underutilised in rapidly urbanising regions such as Mozambique. With only 29% of Mozambique’s population having access to electricity [1,2,3], the construction sector drives emissions, exacerbated by financial constraints and limited awareness of sustainable practices. Envelope renovation, using lightweight, cost-effective expanded polystyrene (EPS) insulation with high thermal performance, effectively reduces cooling demands and CO₂ emissions in tropical climates [4]. However, while global research on envelope renovation is extensive, its application to Mozambique’s low-income housing is limited, as prior studies often neglect cultural and economic contexts.

Mozambique’s rising energy demands, driven by rapid urbanisation, energy-intensive consumption, industrial growth, and deforestation, have increased its carbon footprint. Fossil CO₂ emissions reached 7.795 Mt in 2022, a 10.19% increase from 2021 and 1.947 Mt higher than 2020, reflecting a consistent upward trend from 2000 to 2022 [5,6] (see Figure 1). This trajectory highlights the urgent need for energy-efficient building designs, such as envelope renovation, to mitigate emissions growth.

This study examines the retrofitting of a typical 80 m² single-story house in Maputo’s low-income informal settlements using 95 mm EPS roof insulation and double-glazed windows to address cooling-dominated energy demand. Unlike urban-focused studies [7], this study incorporates local cultural factors (e.g., traditional building preferences) and economic constraints (e.g., limited retrofit financing), evaluates energy and environmental impacts, and proposes a scalable model for sustainable building design in developing nations. While promising, retrofitting faces challenges such as material costs and local availability, warranting further investigation. This study applies established principles of EPS and double-glazing to evaluate energy efficiency in low-income Maputo housing, addressing local constraints.

2. Literature Review

2.1. EPS in Sustainable Construction

Expanded polystyrene (EPS) is a lightweight closed-cell insulation material with a thermal conductivity of 0.424 W/m·K at a thickness of 95 mm [8]. Its low production energy and absence of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) enhance its environmental friendliness [9]. Transitioning to its application, EPS offers a transformative alternative in Mozambique, where concrete dominates, effectively reducing cooling demands [10]. This approach contrasts with prior studies reporting a 30% CO₂ reduction, which lacked Mozambique-specific data, thus highlighting the need for this study. Building on this, existing research lacks data on EPS insulation in Mozambique’s low-income housing, a gap this paper addresses with a culturally adapted model.

2.2. Energy and Environmental Benefits

EPS can reduce HVAC loads by up to 30% in tropical climates, thereby stabilising indoor temperatures [11]. In Sri Lanka, EPS-based concrete panels achieved energy savings of 40% [12]. In Mozambique, where cooling accounts for 60% of residential energy use [3], EPS can significantly lower CO₂ emissions, which is aligned with global sustainability goals [13].

2.3. Barriers to Adoption

Low awareness and a preference for traditional materials, such as concrete, hinder EPS adoption in Mozambique [14]. Cultural resistance, particularly in rural areas, requires tailored awareness campaigns. This study addresses these barriers by proposing community-driven microfinancing models.

2.4. Building Performance in Tropical African Climates

Energy-efficient designs in sub-Saharan Africa focus on passive cooling, natural ventilation, and local materials to lower energy consumption in hot and humid climates [15]. Kajjoba, Wesonga, Lwanyaga, Kasedde, Olupot and Kirabira [10] emphasise thermal comfort through building orientation and shading, achieving up to 30% savings in low-income areas. The approach highlights site analysis and renewable energy integration for the tropical zone [16]. These strategies align with EPS retrofits in Maputo but need adaptation for higher coastal humidity, which enhances cooling effectiveness.

3. Materials and Methods

3.1. Study Context and Building Selection

This study focuses on a 12-month investigation (2023–2024) in Maputo, Mozambique (25.92° S, 32.58° E), where cultural preferences emphasise concrete construction and community-driven design. The research centres on an 80 m² single-story residential building with concrete walls and single-glazed windows, a typology selected for its prevalence, representing 70% of Maputo’s housing stock [17,18]. This choice aligns with the city’s urban dynamics and robust data availability. Through participatory design, the study integrates low-income residents’ daily activities and cultural practices, enabling a comprehensive analysis of environmental and economic priorities.

3.2. Simulation Model

DesignBuilder v7.0, with its integrated EnergyPlus engine, was selected for this study due to its robust validation in tropical climate simulations and its ability to model complex building physics, including the specific thermal properties of EPS and double-glazing [19]. Figure 2 provides an axonometric view of the base case building model, demonstrating a compact design optimised for Maputo’s tropical climate.

• Building Envelope:

Table 1. U-values and layer composition of the opaque building envelope for the base case model.

Component	Layers (From Exterior to Interior)	Total Thickness (mm)	U-Value (W/m2·K)
External Walls	Cement plaster (20 mm), Hollow blocks (225 mm), Cement plaster (10 mm)	255	1.862
Internal Walls	Cement plaster (12 mm), Hollow blocks (200 mm), Cement plaster (12 mm)	224	1.408
Roof (Pitched)	Cement plaster (20 mm), Reinforced concrete (319 mm), Ceramic tile (20 mm)	359	3.218
Doors	Plywood (3 mm), Foam-core plywood (34 mm), Plywood (3 mm)	40	0.230
Floor (ground)	Ceramic tile (10 mm), Concrete slab (150 mm), Compacted soil (304.8 mm)	474.8	1.508

lists the thermal characteristics of opaque building envelope elements, revealing high U-values (e.g., 3.218 W/m²·K for roofs) and highlighting the need for retrofit interventions.

• Retrofit Measures:

In cooling-dominated climates like Maputo’s, roofs are the primary source of unwanted heat gain, accounting for up to 60% of the total due to direct solar radiation [20,21]. Therefore, a preliminary analysis of retrofit strategies indicated that exterior wall insulation offered a poor cost-to-benefit ratio and was excluded from this study. Instead, the strategy prioritised roof insulation, which provides the most significant reduction in cooling loads for the lowest financial investment, a critical consideration for low-income households. To evaluate this approach, five roof configurations (R1–R5, with 5–95 mm EPS insulation) were assessed, as detailed in

Table 2. Thermal properties of the simulated roof retrofit configurations with Expanded Polystyrene (EPS) insulation.

Configuration	Layers (From Exterior to Interior)	Insulation Thickness (mm)	Total Thickness (mm)	U-Value (W/m2·K)
R-0 (Base Case)	Cement plaster, Reinforced concrete, Ceramic tile	0	359	3.218
R-1	Cement plaster, Reinforced concrete, EPS, Ceramic tile	95	454	0.424
R-2	Cement plaster, Reinforced concrete, EPS, Ceramic tile	75	434	0.520
R-3	Cement plaster, Reinforced concrete, EPS, Ceramic tile	50	409	0.725
R-4	Cement plaster, Reinforced concrete, EPS, Ceramic tile	25	384	1.197
R-5	Cement plaster, Reinforced concrete, EPS, Ceramic tile	5	364	1.962

and Figure 3.

Double-glazed windows (6 mm glass/13 mm air gap/wood frame), as detailed in

Table 3. Comparative thermal properties of the base case and retrofitted window systems.

Parameter	Base Case: Single Glazing Aluminium Frame	Proposed Retrofit: Double Glazing Wood Frame
Glazing Configuration	6 mm glass	6 mm glass/13 mm air gap/6 mm glass
Frame Material	Aluminium	Wood
U-Value (W/m2·K)	6.121	2.540
Solar Heat Gain Coeff. (SHGC)	0.810	0.698
Visible Transmittance (VT)	0.881	0.781

, were selected to reduce solar heat gain in Maputo’s tropical climate. Transitioning from this, their air gap, with a Solar Heat Gain Coefficient (SHGC) of 0.698, provides superior thermal insulation compared to single-glazed windows, thereby minimising the cooling energy demand [22]. Furthermore, utilising locally available materials, the wood frame enhances insulation while reducing costs and environmental impact.

• ClimateData:

Climate data were obtained from the EnergyPlus Weather (EPW) database using Climate Consultant 6.0. Maputo’s tropical climate, characterised by high humidity (65–80%) and 2750 annual sunlight hours (62.7% of potential daylight), combined with high energy costs and construction practices compatible with EPS, makes it an ideal case study for EPS simulation. The abundant sunlight emphasises the considerable solar energy potential [23], guiding the choice of solar-dependent technologies and installations [24]. Additionally, high humidity and seasonal variations, affected by the proximity to the Indian Ocean and local weather patterns, highlight the importance of ventilation strategies, particularly cross-ventilation, in improving indoor environmental quality. The interaction between dry-bulb temperature and relative humidity further informs the development of energy-efficient solutions tailored to Maputo’s climate.

• Occupancy Schedules and Internal Heat Gains

Occupancy schedules and internal heat gains were modelled to reflect realistic usage patterns (

Table 4. Modelled occupancy schedules and internal heat gains for the simulated household in Maputo.

Room	Weekday Schedule	Weekend Schedule	Equipment and Heat Gains	Notes
Double Bedroom	9:00 PM–6:30 AM	10:00 PM–9:00 AM	70 W/person (sensible), 45 W/person (latent), 5 W/m2	Same as weekday
Main Bedroom	8:00 PM–6:30 AM	10:00 PM–9:00 AM	Fans: 50 W, 3 W/m2	Same as weekday
Living Room	8:00 AM–10:00 PM, 4:00 PM–12:00 AM	4:00 PM–12:00 AM	Fans: 50 W, TV: 120 W (4 h), Computer: 150 W (2 h)	Same as weekday
Kitchen-Dining	5:00 AM–7:00 AM, 1:00–2:00 PM, 8:00–9:00 PM	9:00–10:00 AM, 2:00–4:00 PM, 8:00–10:00 PM	Cooking: 150 W (2 h), Refrigerator: 150 W, 5 W/m2	Same as weekday

). Occupancy varied daily, weekly, and seasonally, with peak numbers at night during weekdays (four occupants) and reduced presence during the day. Internal heat gains included occupant metabolic heat (70 W/w/person sensible, 45 W/w/person latent), lighting (5 W/m²), and appliances (for example, refrigerator: 150 W continuously).

Given the defined occupancy and heat gains, the configuration and performance of the HVAC system are critical for assessing energy efficiency.

• HVAC Systems and Controls

The cooling system widely used in Maputo’s low-income housing was modelled using locally available and cost-effective technologies, such as ceiling fans and natural ventilation. The model includes a simplified Heating, Ventilation, and Air Conditioning (HVAC) system with basic 50 W fans, designed to meet the affordability constraints of low-income communities. These specifications provide adequate cooling, which is essential for Maputo’s hot climate, and establish a baseline for assessing the impact of insulation. The following section validates the model by testing the effectiveness of EPS insulation and analysing its performance.

• Model Validation

To ensure the reliability of the base case model, it was validated using 12 months (2023–2024) of electricity consumption data from a comparable residence in Maputo, as shown in

Table 5. Monthly breakdown of measured versus simulated energy consumption data used for model validation.

Month	Energy Monthly Consumption (kWh)		Difference (kWh) (mi − si)	Difference %
	Actual (kWh) Measured (mi)	Simulated (kWh) (si)
January	311.61	293.08	18.53	5.48%
February	291.27	310.44	−19.17	−6.59
March	185.78	189.04	−3.26	−1.22%
April	365.82	370.69	−4.87	−1.97
May	383.36	384.72	−1.36	−0.36
June	1100.16	1106.61	−6.45	−0.59
July	1196.18	1228.94	−32.76	−2.74
August	627.86	625.79	2.07	0.33
September	282.96	274.64	8.32	2.93
October	315.47	326.38	−10.91	−3.47
November	144.42	145.68	−1.26	−0.87
December	132.20	135.97	−3.77	−2.10
Annually	5337.09	5392.04
Mean (measured values m˜)	401.17	550.98
(NMBE)	−4.65
(CV(RMSE))	3.64%

. The measured data, including monthly cooling loads (kWh) from utility bills and Mozambique’s Credelec energy metering system, are displayed in Figure 4. The base case model achieved a Mean Bias Error (MBE) of 4.1% and a Coefficient of Variation of Root Mean Square Error (CV (RMSE)) of 3.4%, confirming its accuracy through Equations (1) and (2). The retrofitted model was subjected to a similar validation process to verify its predictive reliability.

$$CV(RMSE)={\displaystyle \frac{\sqrt{{\sum }_{i=1}^{N_i}\left[\right(M_i-S_i){]}^2/N_i}}{\frac{1}{N_i}{\sum }_{i=1}^{N_i}{M}_i}}$$

(1)

$$MBE={\displaystyle \frac{{\sum }_{i=1}^{N_i}\left(M_I-S_i\right)}{{\sum }_{i=1}^{N_i}{M}_i}}$$

(2)

3.3. Cost–Benefit and Environmental Analysis

The economic viability was assessed using the initial EPS insulation costs, energy savings (Equations (3) and (4)), HVAC cost reductions, discounted payback period (DPP) (Equation (5)), and net present value (NPV) (Equation (6)). The environmental impact was measured through CO₂ emission reduction rates (Equation (7)), which were compared with the base case and retrofit scenarios to evaluate EPS’s contribution to sustainability objectives.

$$Energy saving \left(\%\right)={\displaystyle \frac{Energy saving \left(\mathrm{kWh}\right)}{Energy used \left(base-case\right)}}\times 100\%$$

(3)

$$Energy saving \left(\mathrm{kWh}\right)=Energy used \left(base-case\right)-energy used \left(retrofit\right)$$

(4)

$$DPP=i_{NPV\left(i\right)=0}$$

(5)

$$NPV=\sum _{i=0}^T{\displaystyle \frac{CF}{(1+r{)}^i}}-i_0\ge 0$$

(6)

$$RCDE={\displaystyle \frac{CDEPOP-DEPMP}{DEPMP}}\times 100\%$$

(7)

4. Results

4.1. Individual Component Impacts

Retrofitting windows with 6 mm/13 mm double-glazing reduced CO₂ emissions by 9.17% (from 3.27 × 10³ to 2.97 × 10³ kg), due to a decreased U-value (2.54 W/m²·K) and Solar Heat Gain Coefficient (SHGC), which measures solar radiation admitted through a window of 0.698, as shown in Table 3. This led to a 9.23% decrease in annual energy consumption, from 5392.04 to 4894.22 kWh (

Table 6. Impact of individual retrofit strategies on annual energy and emissions performance.

Design Scenario	Energy Consumption (kWh)	Energy Consumption Reduction (%)	CO2 Emissions (×103 kg)	Energy Consumption Reduction (%)
Base Case	5392.04	-	3.27	-
Double-Glazed Windows Only	4894.22	9.23	2.97	9.17
95 mm EPS Roof Insulation Only	3767.28	30.13	2.28	30.27

).

The 95 mm EPS roof insulation (R1) was chosen for its lowest U-value of 0.424 W/m²·K, which indicates superior thermal performance compared to other options. This led to a 30.13% reduction in cooling energy use (from 5392.04 kWh to 3767.28 kWh) and a 30.27% decrease in CO₂ emissions (from 3.27 × 10³ kg to 2.28 × 10³ kg). Although individual retrofits yielded significant savings, combining EPS insulation with double-glazed windows maximised the energy and CO₂ reductions.

4.2. Combined Strategy

Integrating 95 mm EPS insulation with double-glazed windows reduced energy consumption by 42.16% (from 5392.04 to 3118.69 kWh) and CO₂ emissions by 42.20% (from 3.27 × 10³ to 1.89 × 10³ kg), demonstrating the transformative potential of EPS (

Table 7. Performance of the combined retrofit strategy versus the base case.

Design Scenario	Energy Consumption (kWh)	Energy Consumption Reduction (%)	CO2 Emissions (×103 kg)	CO2 Emissions Reduction (%)
Base Case	5392.04	-	3.27	-
Combined Strategy (95 mm EPS Roof Insulation and Double-Glazed Windows)	3118.69	42.16	1.89	42.20

). This synergy highlights the importance of comprehensive retrofitting in a tropical climate.

4.3. Cost–Benefit Analysis

• Retrofit Investment and Financial Feasibility

The proposed retrofit, utilising EPS roof insulation and double-glazed windows, requires a total initial investment of 90,561.71 MZN.

Table 8. Breakdown of initial investment costs for the proposed retrofit (Exchange rate: 63.00 MZN = 1 USD).

Component	Unit Cost (MZN)	Quantity	Total Cost (MZN)	Total Cost (USD)
EPS Roof Insulation	507.02/m2	80 m2	40,561.60	644.00
Double-Glazed Windows	3800.77/unit	9	34,206.93	543.00
Labour (2 workers)	1128.04/worker	7 days	15,793.18	250.68
Total Initial Investment			90,561.71	1437.68

provides a detailed cost breakdown, which includes EPS roof insulation (507.02 MZN/m² for 80 m², amounting to 40,561.60 MZN), the supply and installation of nine double-glazed windows at 3800.77 MZN per window (totalling 34,206.93 MZN), and labour. The labour cost is calculated at 1128.04 MZN per day per worker. With two workers employed for 7 days, the total labour cost is 15,793.18 MZN.

Assuming Mozambique’s residential power price of 8.122 MZN/kWh, the retrofit is projected to yield annual energy savings of 13,196.30 MZN (USD 206.48, based on the given exchange rate). Using a discount rate of 11.75% to account for inflation and opportunity costs, the investment is calculated to have a simple payback period of 6.9 years and a Net Present Value (NPV) of 29,428.65 MZN over 10 years. These figures confirm the long-term financial viability of the project.

• Glazing Options: Total Cost of Ownership (TCO) and NPV

A comparison of glazing options highlights the long-term economic advantages of double glazing.

Table 9. Total Cost of Ownership and Net Present Value Comparison for Glazing Options (MZN, 20-Year Period).

Glazing Option	Initial Cost (MZN)	Annual Maintenance (MZN)	Replacement Cost (MZN)	TCO (Nominal, MZN)	NPV (5% Discount, MZN)
Single Glazing (6 mm)	10,000.00	500.00	5000.00 (Yr 10)	25,000.00	15,443.47
Double Glazing (6 mm/13 mm Air)	34,206.93	200.00	0.00 (No replacement)	38,206.93	36,670.81

presents the Total Cost of Ownership (TCO) and Net Present Value (NPV) for single and double-glazing systems over a 20-year horizon, using a 5% discount rate.

Although double glazing entails a significantly higher initial cost, its superior insulation properties lead to greater energy savings, eliminating replacement costs and lower maintenance, justifying the investment. A more favourable NPV compared to the single-glazing alternative demonstrates this.

5. Discussion

5.1. Energy and Environmental Impact

The retrofit achieved a 42.16% reduction in annual energy consumption, decreasing from 5992.04 to 3118.69 kWh, surpassing typical tropical climate savings of 25–30% [10]. This performance exceeds findings from [12], where retrofits in similar tropical climates delivered 20–28% savings. The superior efficacy results from combining 95 mm EPS roof insulation (U-value: 0.424 W/m²·K) and double-glazed windows (U-value: 0.6–0.7 W/m²·K), effectively minimising cooling demands in Mozambique’s tropical climate. Conversely, European retrofits, as noted in [13], focus on heating and achieve higher U-values (0.8–1.2 W/m²·K), highlighting the suitability of low U-value materials for cooling-dominated regions. Regarding environmental impact, the retrofit reduced CO₂ emissions by 42.20%, from 3.27 × 10³ to 1.89 × 10³ kg, indicating a direct link between energy use and emissions.

This reduction outperforms comparative studies (

Table 10. Comparison of CO₂ Emission Reductions Across Studies.

Study	CO2 Reduction	Context	Insulation Type	Study Period
This Study	42.20%	Maputo, Mozambique	95 mm EPS	2023–2024
[8,9,10,12,16]	30–40%	General Tropical	Standard EPS	Not specified
[25]	40%	Tropical, Low-Income	90 mm EPS	2020–2021

), such as [25], which reported 30–35% CO₂ savings in tropical retrofits, emphasising the combined insulation and glazing strategy as a key driver of enhanced sustainability. The retrofit’s effectiveness is most notable in urban residential buildings in Mozambique, tested during peak summer months (December–February) with consistent occupancy patterns (4–6 residents, daytime cooling usage). However, generalising these results remains limited to similar tropical, low-income settings and assumes consistent occupant behaviour, which may vary and warrants further study. Building on these findings, the Combined Strategy Section shows that integrated insulation and glazing delivered higher energy savings than individual components alone (Table 6). Additionally, the Cost–Benefit Analysis estimates a payback period of 5–7 years, with sustained cost savings over 10 years, assuming stable electricity tariffs. These outcomes underscore the retrofit’s energy efficiency, environmental sustainability, and long-term cost-effectiveness.

5.2. Comparison with Other Strategies

Compared to HVAC upgrades or renewable energy systems, envelope renovation with 95 mm EPS roof insulation (R1, U-value 0.424 W/m²·K) and double-glazed windows (6 mm glass/13 mm air gap/wood frame) provides a cost-effective CO₂ reduction strategy [26]. These retrofits balance the cost and performance in Maputo’s cooling-dominated climate by leveraging local material availability and requiring minimal structural changes.

5.3. Financial Viability

The favourable payback period of 6.9 years for this retrofit at the lower end of the typical European range of 4–10 years is primarily driven by Mozambique’s high residential electricity prices. This strong economic incentive underscores the need for accessible financing models appropriate for the local economic context. In this regard, microfinancing schemes align with Mozambique’s community-based financial norms and present a more viable solution than individual loan models standard in Western studies. Successful implementations in similar economies support this approach; for instance, microfinance has been shown to eliminate obstacles for low-income households in Malaysia [25]. In contrast, government-backed loan structures are used in wealthier nations such as Japan [27] are less directly applicable to Mozambique’s circumstances.

5.4. Policy Recommendations

The findings advocate mandating low-U-value insulation (≤0.424 W/m²·K) and double-glazing in building codes, coupled with community-based retrofit programmes tailored to Mozambique’s urban–rural divide, similar to Brazil’s low-cost initiatives [28]. This approach supports the goals of the Paris Agreement and contrasts with China’s top-down policies, which prioritise urban megaprojects over rural needs [29]. Community-driven programmes can bridge Mozambique’s urban–rural gap by promoting scalable and sustainable development rooted in local values.

6. Conclusions

This study shows that including 95 mm of EPS insulation and double-glazed windows in Mozambican residential buildings cuts energy use and CO₂ emissions by 42.16% and 42.20%, respectively, with a payback time of 6.9 years. These findings highlight EPS as a scalable, cost-effective option for sustainable tropical architecture. However, the results are specific to the building type examined, and changes in materials or occupant behaviour could lead to different results, requiring further research. To overcome financial and cultural barriers, policymakers should incorporate EPS into building regulations, supported by subsidies and public awareness campaigns. Future research should involve long-term field studies to assess sustained performance and expand the model to other tropical areas. Using low-U-value materials and community-led initiatives will support sustainability and help tackle global tropical housing issues.