Integrated Approach of Retrofitting an Existing Residential Building to a Nearly Zero Energy Building with Simultaneous Seismic Upgrading

1.1. Primary Energy and Energy Efficiency Police

During the last decades (1984–2018), international primary energy has grown by 49% and CO₂ emissions by 43%, with an average annual increase of 2% and 1.8% respectively [1]. The review by Pérez-Lombard et al. [1] on the energy consumption of buildings and the associated data has shown that the rapidly growing world energy use of the last decades has already raised concerns over supply difficulties, depletion of energy resources and heavy environmental impacts.

On a global scale, commercial and residential buildings use almost 40% of the total primary energy and are responsible for 24% of greenhouse gas emissions, according to the IEA [2]. The residential sector alone, as shown in Fig. (1), uses 23.1% of the world's energy.

The IEA [3] gathered data on the total energy consumption by sector in Cyprus. As shown in Fig. (2), the residential sector starts with 12% of the total consumption for the period 1990-2004, gradually increases to 19% between 2004-2006, and stabilizes at 21% for the period 2006-2018.

Until now, the energy upgrade policy applied is mainly concerned with new buildings and not with existing buildings. Laustsen [4] describes and analyzes an approach to encourage energy efficiency in new construction regulations. Based on his study, policy proposals for improving how energy efficiency is addressed in building codes and other new construction laws are provided. He also argues that because structures have a lengthy lifespan of 50-100 years or more, considerable maintenance and enhancement will be required over their tenure. Refurbishments or enhancements are required because some sections of the structures, such as roofs, windows, boilers, air conditioning systems, and so on, will need to be replaced. Refurbishment is also required when the building's construction, equipment, or organization become insufficient. Laustsen [4] noted that since climatic factors have such a major influence on building standards and because there are different approaches to developing building rules, comparing these energy restrictions for new structures in different nations or areas is challenging. Simultaneously, it would be interesting to compare the code to the general optimum for the energy efficiency level required. One way to compare building regulations is to look at the lowest overall costs over time using life cycle analysis (LCA). In this comparison, the expenses of the investments must be weighed against the economic advantages to the building's owner or user in terms of savings. For structures in general, a 30-year term is fair to consider since this is a normal interval before a new building requires the first significant refurbishment and corresponds to maximum mortgage or loan lengths in many nations. According to CYS EN 15459: 2007 [5], the external surface life of the building blocks is 30 years.

Sassu et al. [6] proposed that the constructive management of existing constructions should take into account the structural safety, the energy cost, and the comfort conditions of the building. In particular, in the case of masonry buildings, the authors present a brief review of state-of-the-art structural and energy retrofitting techniques for existing masonry buildings and propose a definition of a new synthetic performance parameter P presented through an example and a more generalized approach. They suggest that this new parameter is capable of representing the retrofitting improvement in structural safety and thermal insulation for each masonry wall.

Subsequently, Mistretta et al. [7] concentrated on improving the thermal insulation of masonry buildings. Six example retrofitting interventions were characterized by increased thermal resistance, bending moment, and shear structural strength. The conclusion of the problem of retrofitting a single unitary masonry wall has been analyzed in a cost analysis framework, taking into account both structural and thermal performances. The unitary economic (€/m2) and ecological (kg CO2/m2) costs of retrofitting have been analyzed in order to obtain regression functions that allow direct comparison of various interventions. The main results presented are a synthesis of alternative masonry building retrofitting solutions. The authors mention that several European countries have recently implemented political initiatives to support long-term renovation, but currently, there is no international standard method for this kind of analysis. Finally, with regard to economic and ecological costs for the evaluation of the structural and thermal retrofitting strategies, the authors propose an integrated approach for the thermal insulation improvement of masonry buildings.

Belleri and Marini [8] investigated a framework for quantifying the impact of seismic events on building environmental impact assessments. The framework was a selected building that could have been located in different seismic zones. They assessed the building's environmental impact in terms of carbon footprint in two scenarios: after an energy refurbishment alone and after a combined intervention focusing on energy refurbishment and seismic retrofit. The main result of the study was that, in the case of only energy refurbishment and when the building is located in a high-seismicity region, an expected additional annual embodied equivalent carbon dioxide is presented due to seismic risk, which nearly equals the annual operational carbon dioxide after thermal refurbishment.

Giresini et al. [9] proposed a methodology to quantitatively assess the improvement of seismic and energy performance of masonry buildings through retrofitting interventions. The framework was an analysis of three masonry façades with and without LCA. The results show the necessity of always considering LCA for a reliable assessment: some retrofitting interventions are the most expensive in the construction phase, but they are also the most convenient in economic terms and in the amount of CO₂eq emissions.

Also of interest is the research by Fiore and Donnarumma [10], who developed a decision-making process for drawing up a public administration strategy with the aim of optimizing school services in small municipalities. Their approach provides two levels of analysis. In the first level, a preliminary analytical phase of knowledge is developed, which leads to the definition of alternative intervention solutions and the evaluation criteria. The second level consists of the phases of constructing the building and solving the multi-criteria matrix, as well as the discussion of the results and the final choices of the decision-makers. An integrated multi-criteria approach is proposed to support the decision-makers involved, and in particular the local administrations to analyze the possible alternative interventions, taking into account the multiplicity of dimensions of the problem. Finally, they propose the adoption of three evaluation criteria: location, energy-environmental sustainability, and cost. The selection of these criteria aims to promote strategies oriented towards the three main dimensions: social, environmental, and economic.

It would be interesting for the states to make an energy upgrade policy for existing buildings based not only on energy consumption but also on the life expectancy of the building as an amortization time of the investment. At present, the existing buildings account for the largest percentage of the building stock for each country.

1.2. Nearly Zero Energy Building Definition

The definition of zero energy building (ZEB) has acquired considerable international attention during the last decade and is seen as the future goal of building design or renovation. The scientific community started with the main goal of developing a common understanding and an international definition framework of the ZEB.

Torcellini et al. [11] proposed that the definition of the zero-energy goal influences the choices designers make to attain it and whether they can claim success. The ZEB definition might emphasize demand-side or supply-side tactics, as well as whether fuel switching and conversion accounting are acceptable for achieving a ZEB target.. Torcellini et al. [11] studied the design impacts of four well-documented definitions:

• Net-zero site energy
• Net-zero source energy
• Net-zero energy costs
• Net-zero energy emissions

These definitions were applied to a set of low-energy buildings for which extensive energy data were available, and the large difference between definitions highlights the key characteristics of each definition.

Torcellini et al. [11] also suggested that ‘a net zero-energy building (ZEB) is a residential or commercial building with greatly reduced energy needs through efficiency gains such that the balance of energy needs can be supplied with renewable technologies.

Kilkis [12] advises that ‘a new concept was developed that is aimed to better quantify the environmental impact of buildings. They show that a net-zero energy building may or may not be a net-zero impact building; therefore, engineers, architects and decision-makers must recognize that the harmful emissions and global warming issues cannot be fully addressed by a simple net-zero energy building concept. The exergy dimension of the balance must be absolutely taken into account in order to fully reveal the magnitude of the problem and, at the same time, draw solution roadmaps.

According to Laustsen [4], ZEB can be defined in various ways, as follows:

• “Zero Net Energy Buildings are buildings that, over a year, are neutral, meaning that they deliver as much energy to the supply grids as they use from the grids. Seen in these terms, they do not need any fossil fuel for heating, cooling, lighting or other energy uses although they sometimes draw energy from the grid.”
• “Zero Stand Alone Buildings are buildings that do not require connection to the grid or only as a backup. Stand-alone buildings can autonomously supply themselves with energy, as they have the capacity to store energy for night-time or wintertime use.”
• “Plus Energy Buildings are buildings that deliver more energy to the supply systems than they use. Over a year, these buildings produce more energy than they consume.”
• “Zero Carbon Buildings are buildings that, over a year, do not use energy that entails carbon dioxide emission. Over the year, these buildings are carbon neutral or positive in the term that they produce enough CO₂ free energy to supply themselves with energy.”

Despite the many years of discussion of the ZEB definition, there are no clear specifications on what calculation methodology and system boundaries to use, what the level of energy efficiency is, or what renewables can be implemented; hence, Panagiotidou [13] concludes that although the term ZEB has been used for some time, there is still no globally agreed definition or pathway to implement it.

EU decided that Member States should apply a methodology for calculating the energy performance of buildings in accordance with the common general framework as defined by the Directive 2010/30/EU [14], but this methodology should be adopted at the national or regional level.

Furthermore, to the various versions of the ZEB designation, the nearly zero energy building (NZEB) today is fully applicable to national building codes and international standards. The definition of the NZEB adopted by the Republic of Cyprus, based on its national needs, concluded that the definition of the NZEB imposes the following limits, Directive 2010/31/EU [15]:

• For residential buildings, the limit of 100 kWh/m²/y primary energy consumption with at least 25% being covered by renewable energy sources.
• For non-residential buildings, the limit of 125 kWh/m²/y primary energy consumption with at least 25% being covered by renewable energy sources.

The above definition of the Nearly Zero Energy Building has been adopted in the current study.

1.3. Embodied Energy

It should be noted that the NZEB definition does not take into account the structure's Life Cycle Analysis (LCA), i.e., the embodied energy of a building from the construction to the demolition is not calculated. Even for the newly designed buildings, the whole footprint of the structure is not considered.

Marszal et al. [16] suggest that an additional 25 kWh/m²/y should be included in energy demand calculations so that embodied energy is considered. According to the Department for Communities and Local Government (UK) resource, the embodied energy required for the material manufacture, construction, refurbishment and demolition may account for as much as 50% of the operational emissions of an ultra-low energy dwelling over an 80-year timeframe [17]. As a result, there is increased interest in comparing the energy and carbon embodied in buildings constructed using various methods and materials.

Thormark [18] states that the overall use of energy during a building's lifecycle is a growing research field. The initial construction energy of a building is a major component of the overall energy consumption of the building, particularly for buildings with zero energy consumption.

Recycling reduces the initial manufacturing energy, i.e., by using and re-using recyclable materials. In a 50-year lifespan, the initial construction energy represents 40% of the total energy needed. About 37-42% of the original construction energy can be recovered through recycling [18]. Recyclability was initially around 15% of the total energy use over a 50-year life span, so the emphasis will be on the energy intensity of the materials as well as whether the recycling principles were included in its phase.

Most of the energy recovery, about 90%, can be achieved through recycling and burning recyclables. In addition, a very important measure to facilitate future recycling is to use recyclable materials as well as to avoid constructions that are difficult to disassemble or where materials contaminate the environment [18].

Furthermore, Thormark reported that maintenance accounts for about 12% of total initial construction energy.

Asif et al. [19], in their study of a 3-bedroom maisonette house in Scotland, reported the following:

• 61% of the total construction energy of a dwelling is about concrete
• 13% of the total construction energy of a dwelling relates to timber
• 14% of the total construction energy of a dwelling relates to ceramic tiles
• Concrete and mortar are responsible for 99% of the total carbon dioxide generated for the construction of a dwelling.

According to Sartori and Hestnes [20], compared to an equivalent conventional building, passive heat-insulated homes show an increase in construction energy while reducing the total energy needed to 66% over the 80-year lifespan of the building. A new version of the design and construction of the passive house is expected to achieve a total reduction of the total energy needed to 25%. In conclusion, it is observed that decreasing demand for building operation is the most important parameter for designing buildings that are energy efficient throughout their life cycle.

According to Kohler and Hassler [21], the life cycle analysis of buildings has focused mainly on environmental impacts, with the result that the consumption of mineral resources has not been taken into account. They have also noted that in the case of buildings, resource value assessment is an important criterion for their long-term management strategies. Therefore, there is an urgent need to evaluate an integrated method of measuring the value of mineral resources for buildings.

Romano et al. [22] presented in their study that the key to sustainability in the construction sector is the integrated approach to the issue. They referred to the need to create a new way of constructing structures in order to reduce environmental impact and energy consumption. More specifically, they proposed the equation of all parameters in terms of equivalent CO₂ emissions with the aim of guiding them to the sustainability of the construction sector, as required by the European targets.

Finally, Romano et al. [22] have pointed out that the reference price of CO₂ emissions compared to environmental impacts remains low. In addition, the large difference between energy prices and CO₂ emissions depends on taxes imposed on energy. Therefore, in order to understand the importance of external factors in the construction sector, the reference price of CO₂ emissions needs to change in order to better reflect environmental and social costs.

The European Commission's policy Directive 2012/27/EU [23], to reduce greenhouse gas emissions by 85-90% in 2050 compared to 1990 emissions sets the framework in which an ambitious refurbishment scenario can be implemented.

According to the International Energy Agency’s Energy in Buildings and Communities Program, it is predicted that in most industrialized countries by 2050, new buildings will have contributed to energy consumption at a rate of 10% - 20%, with the remaining 80-90% relating to the existing building stock [24].

According to Ma et al. [25], the worldwide replacement rate of existing buildings by new buildings is only about 1-3% per year. Since this is a very small rate and since the factor “new buildings” will not change the energy problem in the short term due to energy demand, the existing buildings are an important factor in the energy balance.

If renovations to existing buildings are targeting only energy upgrading through the upgrading of individual building elements (such as roofs, facades, windows or heating systems) without a comprehensive inclusion of all parameters, there is a high likelihood of an inefficient and extremely non-cost-effective solution, with no long-term reduction in energy consumption.

Wang et al. [26] observed that the “investment cost” criterion is ranked first in the evaluation criteria set to be used in decision-making for sustainable energy, with the second criterion being “CO₂ emissions” (as it focuses on environmental protection).

Ruparathna et al. [27], in their research on modern approaches to improve the energy efficiency of commercial and public buildings, found that there is a large number of such approaches. Therefore, they have suggested that a centralized integrated knowledge base is required to inform all relevant stakeholders in order to optimally decide on energy upgrading of buildings. It was suggested that further study is needed on the factors associated with approaches to energy upgrading of buildings, such as life cycle, implementation and costs.

1.4. Research Objective

Based on the literature review, a limited of the previous relevant studies investigated an integrated approach for retrofitting an existing structure taking into account both the aspects of the energy upgrade and the improvement of the structural performance. Considering the impact on national energy consumption, it would be extremely important to detect the optimal long-term coalition and scenario for the specific type of building, which could be achieved through both energy and seismic resistance upgrades. The current study presents and discusses three possible coalitions and scenarios for approaching the rehabilitation and energy upgrade of a particular type of existing reinforced concrete frame infilled with concrete masonry units (concrete blocks) buildings that represent approximately 3.57% of all residential buildings in Cyprus. In each scenario, all criteria and implementation actions are taken into account based on energy consumption, CO₂ footprint, and earthquake resistance, which are the characteristics of the building. It is investigated whether the extension of the life expectancy of the existing structure through earthquake resistance upgrade will have a positive or negative effect on the Life-cycle CO₂ footprint of the Structure. The purpose is to examine if life expectancy is an important factor in deciding to implement an optimal long-term integrated approach of upgrading an existing residential building to a Nearly Zero Energy Building. In order to achieve this goal, the CO₂ footprint of the building must be benchmarked between the following three coalitions:

(A) Existing building that is in operation and in which the user chooses not to make any energy or static/seismic upgrades. The final Building Lifetime is the initial expected lifetime of the building.

(B) The existing building that is in operation and in which the user chooses to have only an energy upgrade. The final Building Lifetime is the initial expected lifetime of the building.

(C) The existing building that is in operation and in which the user chooses to have an energy and static/seismic upgrade. The final Lifetime of the building operation is the existing age of the building before the intervention was made, adding the updated initial expected lifetime of the existing building.

The most effective approach coalition and scenario for retrofitting an existing residential building to become a Nearly Zero Energy Building is discussed, and the conclusions are listed. The parameter of the extension of the life expectancy of the structure refers to the most effective coalition of retrofitting an existing residential building to a Nearly Zero Energy Building.

2.1. Software Applications

The software used for the current study's objectives are the iSBEM-cy and STAAD.PRO V8i. For the energy consumption calculation, the iSBEM-cy is a free proprietary software interface to the Simplified Building Energy Model (SBEM), which is designed for the purpose of indicating compliance with the Republic of Cyprus building regulations with regards to carbon emissions from domestic or non-domestic buildings. The purpose of i-SBEM-cy is to produce consistent and reliable evaluations of energy use in non-domestic or domestic buildings for Building Regulations Compliance and for Building Energy Performance Certification purposes. Although it may assist the design process, it is not primarily a design tool, i.e., it does not calculate internal temperatures. As i-SBEM-cy is a compliance procedure and not a design tool, if the performance of a particular feature is critical to the design, even if it can be represented in i-SBEM-cy, it is prudent to use the most appropriate modelling tool for design purposes. The latest version at the time of writing is iSBEMcy_V3_4a, which is available as a download from the Ministry of Energy, Trade and Industry- Energy Service website [28].

For the structure bearing capacity calculations, the STAAD.PRO v8i is a structural analysis software with linear and nonlinear static and dynamic analysis capabilities [29].

2.2. Existing Building

In this study, the structure that was studied belongs to the category of buildings, which concerns houses that have been constructed without energy efficiency or seismic standards in the Republic of Cyprus. According to the Statistical Services of the Republic of Cyprus (CYSTAT) [30], most of the houses that constitute the current Cyprus building stock were built in the 70s and 80s (Fig. 3). Specifically, 88.27% of the residential buildings were built without any thermal insulation, while 47.21% of those structures were constructed without any seismic code consideration since the first preliminary measures for seismic design were introduced in 1986, and the Cyprus Earthquake Code was implemented after January 1^st 1994. Consequently, most of the structures before 1986 were designed for gravity loads only [31].

The building that is investigated in the present study is one of the 15,347 identical houses that were designed and constructed in Cyprus between the years 1975 - 1985 for the refugee settlements. This specific type of structure was the solution provided by the Cyprus Government to resolve the problem of the urgent need for houses for the large number of homeless refugees after the military events of the summer of 1974 on the island. Specifically, the building is a single-story masonry structure with an inclined timber roof. In the 2011 population census [30], the total number of households in Cyprus was 433,212 and therefore, the specific type of building accounts for approximately 3.57% of households in Cyprus. This percentage is considered significant with regards to energy consumption, as this type of house has an estimated energy consumption of 1000 kWh/m²/y and CO₂ emissions of 293.74kg CO₂/m²/y. This corresponds to 0.293 Mt CO₂/y, which is 4.18% of the total CO₂ emissions of the country in 2011, based on the IEA (International Energy Agency) data [3], as shown in Fig. (4).

2.2.1. Structural Properties

The building under consideration is a detached single-story house located in Limassol, Cyprus. The structural system of the superstructure is hybrid, consisting of a reinforced concrete frame filled with concrete masonry units (concrete blocks). The foundation consists of isolated concrete footings connected with grade beams. The inclined roof is made of timber beams and purlins, covered by clay tiles. The house has an internal floor area of 65m² and was constructed in 1980 without any thermal insulation or seismic code provisions. At the time of this study, the building is 40 years old, with a remaining life expectancy of 10 years. From the macroscopic observation of the masonry and the exploratory sections, which were carried out with absolute respect and without causing damage to the building, the following conclusions emerged:

• The building is constructed with concrete blocks consisting of materials that seem to have lacked proper treatment and the correct content of cement proportions, and the exact compressive strength of the masonry system could not be calculated in this study because the samples were so fragile that it was not possible to do sampling. For this reason, the values for the mechanical properties of concrete block masonry used in the analyses were obtained from the relevant literature [32]. Specifically, the Modulus of Elasticity of the concrete masonry was taken equal to 2 GPa, while the compressive strength for new concrete blocks is taken within the range of 2 to 3 MPa and for the existing 0.1 MPa due to the pure condition of the existing concrete blocks.
• From site inspections and in-situ tests at the column members, it has been observed that the concrete is highly carbonized (7-8 cm), and its compressive strength ranges from 5-7 MPa.
• The steel reinforcement, apart from being heavily corroded, differs from the initially designed reinforcement, not in the cross-section size, but in layout. Specifically, the stirrups in columns and beams were placed with a spacing of 40 cm instead of 20 cm, which was indicated in the design drawings.

It is a fact that in Cyprus, the construction during the period 1975-1985 was done without much care for the various practical, social, and political reasons described above. At the same time, we must accept that the infill masonry improves the frame system, and the bearing capacity of the system is estimated to be greater.

2.2.2. Energy Properties

The current condition of the building in terms of thermal insulation is described in Table 1. The U_value is the thermal transmittance of the construction element, given in W/m²K. It can be calculated using the combined method given in BS EN ISO 6946 for simple constructions. Constructions, such as cladding and steel frame constructions require more complicated calculation procedures, and an appropriate methodology should be followed. The C_m is the effective thermal capacity of an element (wall, floor, ceiling, etc) given in kJ/m²K. The T_Solar is the total solar energy transmittance defined as the time-average ratio of energy travelling through the un-shaded element to that incident upon it. The amount of visible solar energy that passes through a glazing system represented as a proportion of the visible solar energy incident on it, is known as the L_Solar (light transmittance). This value will be used for the daylighting calculations. The SSEER is the system seasonal energy efficiency ratio for cooling that takes account of the seasonal efficiency of the cold generator, thermal losses and gains to and from pipework and duct leakage. It does not include the energy used by fans and pumps. The combined cooling demand of all the zones served by a particular system divided by its SSEER gives the energy consumption of the cooling system. The SSEFF is the system seasonal efficiency for heading, taking account of the seasonal efficiency of the heat generator, thermal losses, and gains to and from pipework and duct leakage. It does not include the energy used by fans and pumps. The combined cooling demand of all the zones served by a particular system divided by its SSEFF gives the energy consumption of the heating system.

The estimated energy consumption and energy category label of the existing structure is 1000 kWh/m²/y, and the emissions are 293.74kg CO₂/m²/y (Fig. 5).

3.1. Seismic Analysis Data

The total mass of the building was found to be 81.18 tons. Specifically, the mass of the roof corresponds to 4.8 tons, and the mass of the building corresponds to 76.38 tons. The mass of the masonry was considered distributed in the 3D solid finite elements. Based on Eurocode 8 part 3 (1998-3:2005) [33], the value of the behavior factor q was taken to be equal to one since the structure’s seismic performance was to be assessed for the specified seismic action. According to the Eurocode 8 national annexes CYS EN 1998-1:2004, part 1 [34], national parameters have been determined for fundamental requirements of a structure after a seismic action, and for this structure, the stage “Limit State of Damage Limitation” is selected. The reason is that this stage ensures that the structure has a longer life expectancy of operation/use. Discontinuation of operation/use of the building is undesirable. Note that the cost of any investment, i.e., the energy consumption upgrade cost of the structure, should be paid back according to its operating time. The Limit State of Damage Limitation does not prevent only the overall damage in the whole structural system but also any local damage in the various structural elements. Non-load-bearing components may be damaged and can be easily and economically repaired. This is achieved by limiting the system's deformations (horizontal movements) to levels that are acceptable for the integrity of all its members, including non-load-bearing members.

3.2. Structural Analysis

The structure was analyzed two times. The loads of the first analysis was the gravity, live, wind and snow load as they are described in the Eurocodes and the building capacity was assessed for the various load combinations according to the same building code. The ledger in Fig. (7) shows the numbers of the limits of each color in the figure. Column A is the upper limit, and column B is the lower limit. As shown in Fig. (7), for the load combination of 1.35 G+1.5Q+0.9W+0.75S, the analysis indicates that the normal stress SYY of concrete blocks (solid element) in many areas has values between -0.11 N/mm² and +0.18 N/mm² (negative values are compressive and positive values are tensile), which is close to its maximum capacity, and in some others it has values between -0.98 N/mm² and -0.69 N/mm² which exceed the strength limits of the existing concrete block masonry. This is the most critical combination for gravity loads. After the completion of the assessment of the roof bearing capacity, it was considered necessary to replace it and statically upgrade it, i.e., remove the existing beams and replace them with greater cross-section. In the design of the construction, it was taken into account that for the energy upgrade of the roof, the permanent loads will increase by 10 kg/m². In addition, due to the inclination of the roof, it is not possible to achieve a rigid diaphragm at the level of the roof.

In the second analysis, a seismic action was added, with a peak ground acceleration of 0.25g. As shown in Fig. (8), the analysis indicates that for the load combination of G + 0.3 Q - Ez, the developed stresses at certain structural members of the superstructure reach values between -1.52 N/mm² and -1.17 N/mm^2, which far exceed the strength limits of the existing masonry concrete block.

3.3. Seismic Retrofitting of the Existing Building

The degree of intervention for the existing building mainly intended not only to restore the original life expectancy of the construction to 50 years but also to simultaneously upgrade the existing seismic capacity, thus giving a final life expectancy of 90 years since the building at the time of the upgrading is considered to be 40 years old. The fact that the seismic resistance upgrade was based on the EC-8 provisions ensures a longer life expectancy of operation/use of the structure. The upgrade of the existing building was achieved through interventions in the foundation, the superstructure, and the wooden roof system.

For the foundation, as Fig. (9) shows, a new R/C grade beam was constructed along the perimeter with dimensions 35cm x 110cm and double zone reinforcement. The characteristic compressive strength of the concrete used was 37 MPa. The new foundation beam was anchored to the existing beam and at the top level of the beams, a new ground slab (floor) was constructed with reinforcement mesh #Y12/200, acting also as a floor diaphragm.

The exterior masonry-infill walls of the superstructure (shell), as shown in Fig. (10), were replaced by new types of concrete masonry units of higher compressive strength and durability. Specifically, the superstructure’s structural system consists of load-bearing reinforced masonry, which is supported on the new peripheral foundation beam and was constructed without demolishing the old masonry.

The new concrete blocks are made of concrete with a characteristic compressive strength of 37MPa, while the overall compressive strength of the concrete blocks is 7.5 MPa. Both vertical and horizontal reinforcement are embedded in the masonry. The diameter of the vertical bars is Y14, with a spacing of 250mm, and the diameter of the horizontal steel bars is Y12, placed every 200mm.

Concerning the block filling material, a higher quality concrete could be used for higher compressive strength. Tayeh et al. [35] observed that the compressive strength increased by 50% and 65% for concrete comprising 2.5% hooked-end fiber and corrugated fiber, respectively, when compared to concrete with the same volume of straight fiber. Tayeh et al. [36] suggest that the utilization of available supplementary cementitious materials is a critical step in saving energy and materials, as well as lowering the cost of concrete, and they investigated the impact of sand grain size distribution, supplementary cementitious materials, and curing regimes on the compressive strength of ultrahigh-performance concrete.

The structural system of the roof was maintained the same, but the existing 2” x 6” main timber beams (rafters) were replaced with new, larger cross-section dimensions, i.e., 3” x 8”.

The upgraded structure was analyzed, and its load-bearing capacity was found to be at a sufficient level. Specifically, the analysis indicates that the stresses in almost all parts of the new structure do not exceed their capacities.

3.4. Energy Upgrade of an Existing Building

A sensitivity analysis for each proposed retrofit measure was calculated using the iSBEM-cy software. What is calculated is the whole CO₂ footprint of the final amount of the embodied energy of the structure and the CO₂ footprint amount of the energy consumption with and without the upgrade actions. The interventions in the shell of the building concerned the following:

• For the masonry, an installation of thermal insulation with stone wool 5 cm thick outside and inside the space was considered.
• For the roof, installation of thermal insulation with extruded polystyrene (XPS) 5cm thick externally and with thermal insulation 5 cm thick stone wool inside the space was considered.
• For the windows, replacement with a Polyvinyl chloride (PVC) frame and double-glazing glass was considered.
• For the floor, installation of thermal insulation with expanded polystyrene (EPS) 5cm thick was considered.
• For the mechanical units, replacement with high efficiency ones was considered.

Table 2 presents the thermal insulation properties of the structure after the energy upgrade, based on the nZEB EU definition and the nZEB-specified limits implemented by the Republic of Cyprus (limits presented in Table 3).

Retrofit insulation levels were designed to transform the existing building into a nearly zero-energy building. The retrofit model evolved to include a combination of passive house and green life cycle principles. The green life cycle standard was adopted to achieve a comprehensive retrofit, covering issues, such as energy consumption and embodied energy of the materials. In order for a building to be qualified as nZEB, it must comply with the administrative regulatory practices of the Republic of Cyprus, ΚΔΠ 432/2013 [37] and ΚΔΠ 366/2014 [37]. After the upgrade actions, the estimated energy consumption of the house and the energy label of the energy upgrade condition is 83 kWh/m²/y, and the CO₂ emissions are 24,47kg CO₂/m²/y, (Fig. 11).

4.1. Parametric Analysis- Life Cycle footprint

Figs. (19-22) show for each coalition the effects on the Life cycle Structure Footprint CO₂ of the three basic parameters that are related to the construction footprint. The parameters include all the materials that will be used in the construction of the existing building and for the energy and structural upgrade; thus, for each action, we calculated a bill of quantities for all actions for the CO₂ Footprint:

• Existing Structure CO₂ Footprint
• Energy Label Upgrade CO₂ Footprint
• Structural Upgrade CO₂ Footprint

The first parameter, ‘Existing Structure CO₂ Footprint’ for the building with construction time 1980, coalition A1^50-100_50-100, presents a substantial decrease in relation to the increase of the initial life expectancy of the building. This is because the total energy consumption CO₂ footprint for coalition A1^50-100_50-100 is excessively higher than the initial structure CO₂ footprint, as it is shown in Fig. (27); therefore, for each year of the building operation, the accumulated energy consumption reduces the percentage of the parameter ‘Existing structure CO₂ footprint’ compared to the total footprint of the building. As a result, the parameter of the ‘Existing Structure CO₂ Footprint’ for coalition A1^50-100_50-100, has the largest change with the increasing life expectancy. The reason is the fact that the parameter of the building's energy consumption is significantly higher in coalition A1^50-100_50-100 compared to those in coalitions B1^50-100_50-100 and C1^90-140_50-100. From Fig. (19), it can be observed that for coalitions, B1^50-100_50-100 and C1^90-140_50-100, the percentage reduction is smaller.

From Fig. (19), it can also be noted that the percentage of the reported parameter ‘Energy label upgrade footprint’ for coalition B1^50-100_50-100, is smaller than that of a coalition C1^90-140_50-100. This is because, for coalition C1, the total Life-cycle footprint CO₂ includes the parameter ‘Structural Upgrade CO₂ Footprint’. Also, Fig. (19) shows that the ‘Structural Upgrade CO₂ Footprint’ for coalition C1 is not prohibitive for the total CO₂ footprint of the building.

Correspondingly, for the parameter ‘Existing Structure CO₂ Footprint,’ the coalitions A2^50-100_50-100, A3^50-100_50-100, and A4^50-100_50-100, present similar results to those for coalition A5^50-100_50-100. Comparing Figs. (19-22), the percentage for the parameters increases as the construction year increases. The cause is the total energy consumption CO₂ footprint for the coalition A1^50-100_50-100 is larger than that for coalition A2^50-100_50-100, the one for coalition A2^50-100_50-100 is larger than that of the coalition A3^50-100_50-100 and the one for coalition A3^50-100_50-100 is larger than that of coalition A4^50-100_50-100. For coalitions B and C, the percentage reduction seems to be once more unimportant, and the percentage of the reported parameter ‘Energy label upgrade CO₂ footprint’ for coalition B is reduced more than that of coalition C. The cause is again that the total Life Cycle footprint CO₂ for coalition C includes the parameter of the ‘Structural Upgrade CO₂ Footprint’.

4.2. Parametric Analysis- Life Cycle Cost

Figs. (23-26), show for each coalition the effects on the Life Cycle Structure Cost of the three basic parameters related to the construction cost. The parameters include all the materials that will be used in the construction of the existing building and for the energy and structural upgrade, thus for each action, we calculated a bill of quantities for all actions for the cost:

• Existing Structure Cost
• Energy Upgrade Cost
• Structural Upgrade Cost

From Fig. (23), the percentage of the parameter ‘Structural Upgrade Cost’ for the scenario C1⁹⁰₅₀, represents 6.34% of the total cost of the building and is lower than the percentage of the parameter ‘Energy Upgrade Cost’, which is 11.56%, while the percentage cost of the parameter ‘Existing Structure Cost’ for the same scenario is 8.57%. Although identical energy upgrades have been made to coalitions B and C, the percentage cost of the parameter ‘Energy Upgrade Cost’ for scenario B1⁵⁰₅₀ is greater than that for the scenario C1⁹⁰₅₀ since the percentage cost of the structural upgrade is absent from the former. Also, it seems that the percentage of the cost of the parameter ‘Energy Label Upgrade’ for the scenario B1⁹⁵₉₅ tends to be identical with the percentage of the scenario C1¹³⁵₉₅. Also, it seems that the percentage of the cost of the parameter ‘Existing Structure Cost’ for the scenario B1⁹⁵₉₅ tends to be identical with the percentage of the scenario C1¹³⁵₉₅. This is because the decreasing percentage of the parameter ‘Existing Structure Cost’ of the life expectancy for coalition B1 is higher than that for coalition C1. Also, for coalition A1, it is observed that the percentage of the parameter ‘Existing Structure Cost’ decreases rapidly as the life expectancy increases. This is also confirmed in Fig. (27), where it is shown that the percentage of energy consumption increases as the life expectancy increases. Figs. (23-26) show intersections between coalitions B and C. As the time of construction for the existing building is newer, the intersections are getting earlier for the parameters ‘Energy Upgrade Cost’ and for the parameter ‘Existing Structure Cost’. In Fig. (23), the identical scenarios are B1⁹⁵₉₅ with C1¹³⁵₉₅. In Fig. (24), the scenarios B2⁷⁰₇₀ with C2¹¹⁰₇₀ and in Fig. (25) the scenarios B3⁵⁰₅₀ with C3⁹⁰₅₀. In Fig. (26), there is an intersection between coalitions A4 and B4, only for the parameter ‘Existing Structure Cost’, specifically the identical scenarios are A4⁶³₆₃ with B4⁶³₆₃. Also, only in Fig. (23) there is a section for the coalition A and C for the parameter ‘Existing Structure Cost’ with scenarios A1⁵⁵₅₅ with C1⁹⁵₅₅. On the other hand, in Fig. (23) there is an intersection between the parameters ‘Structural Upgrade Cost’ and ‘Existing Structure Cost’ for scenarios A1⁷⁷₇₇ with C1¹⁷⁷₇₇C in Fig. (24) for scenarios A2⁶⁵₆₅ with C2¹⁰⁵₆₅, and in Fig. (25) for scenarios A3⁵⁵₅₅ with C3⁹⁵₅₅. As the time of construction for the existing building is newer, the intersections are getting earlier for the parameters ‘Structural Upgrade Cost’ and ‘Existing Structure Cost’. It is important to note that the intersection of coalitions reflects the age of the initial life expectancy of the structure. Beyond the specific age, the values will show which coalition offers the optimum results to the figure parameter.

4.3. Parametric Analysis- Energy consumption footprint and cost

From Fig. (27), the scenario C1⁹⁰₅₀, has the lowest percentage of 64.45% for the parameter ‘Energy consumption cost’ and scenario B1⁵⁰₅₀ follows with a percentage of 64.57%. At the other end of the range scenario B1¹⁰⁰₁₀₀ has a percentage of 55.11%, and the scenario C1¹⁴⁰₁₀₀ follows with a percentage of 66.27%. Considering the parameter ‘Energy consumption footprint’, for the scenario A1⁵⁰₅₀ it is 96,94%, and for the scenario A1¹⁰⁰₁₀₀ it is 98.20% of the Life cycle Structure CO₂ footprint. This shows that the energy consumption footprint for coalition A1 is ample to reduce; furthermore, the percentage of the parameter of the initial structure footprint to the total footprint of the building, as the life expectancy of the structure is increasing. Also, from Fig. (27), the percentage for the parameter ‘Energy consumption cost’, has a noticeable increase in coalition C1^90-140_50-100, compared to those in coalitions A1^50-100_50-100 and B1^50-100_50-100. For coalitions B1^50-100_50-100 and C1^90-140_50-100, the energy label upgrade has the effect of reducing the total energy consumption cost, but for coalition C1^90-140_50-100 the extension of the final life expectancy is the reason for the energy consumption cost increase for the total life of the building. In Figs. (27-30), it can be observed that for all coalitions the percentage for the parameter ‘Energy consumption cost’ is decreased as the construction age of the existing building is reduced. Also, as the time of construction is newer, the technical energy characteristics based on the time of construction (age) of the building are different, as shown in Table 4. This is the reason that for the parameter ‘Energy consumption footprint’, the interval between coalitions B and A and coalitions C and A is getting bigger. Likewise, for the parameter ‘Energy consumption cost’, the percentage of the consumption cost, as the time of construction is newer, is reduced for all coalitions. Specifically, for the parameter ‘Energy consumption cost’, the interval between coalitions B and A is getting bigger compared to that of coalition C and A. ‘It is important to note that the intersection of coalition B and C reflects the age of the initial life expectancy of the structure. More specifically, in Fig. (27), the intersection of scenarios B1⁵⁰₅₀ and C1⁹⁰₅₀ shows that coalition B offers optimum results with regards to the parameter Life cycle energy Consumption Cost.’

Summarizing the above results, it can be concluded that from the three coalitions that can be applied to the existing building, coalition C appears to be the most efficient coalition. For coalition C, the scenario with the lowest initial life expectancy C1⁹⁰₅₀ has the following results.

• Life Cycle CO₂ Footprint 12.47 ton/year, which is 45.25% less than the scenario A1⁵⁰₅₀ and 35.13% less than scenario B1⁵⁰₅₀.
• Life Cycle total cost €4345 per year, which is 39.43% less than the scenario A1⁵⁰₅₀ and 37.70% less than scenario B1⁵⁰₅₀.
• Life Cycle Energy Consumption Cost € 2,801 per year, which is 49.76% less than scenario A1⁵⁰₅₀ and 38.80% less than scenario B1⁵⁰₅₀.
• Seismic Upgrade total cost (€) is €24,778, which is 6.4% of the Life Cycle Structure Total Cost (€).
• Scenarios C1⁹⁰₅₀ and B1⁵⁰₅₀ have Energy Consumption 83,46 kWh/m²/year, which is 91.70% less than scenario A1⁵⁰₅₀.

In scenario C1⁹⁰₅₀ the earthquake resistance upgrade CO₂ Footprint is 29.26 (tons), which is 2.6% of the Life Cycle Structure Total CO₂ Footprint, and the Energy Upgrade CO₂ Footprint is 11.77 (tons), which is 1.22% of the Life Cycle Structure Total CO₂ Footprint. Table 5 shows which coalition has the Maximum and the Minimum of the main parameters of the life cycle footprint of the building, which has been examined in the study.