The building sector has become a strategic area in order to accomplish the aims established by the Kyoto Protocol 20-20-20 [1]. The European Commission published Directive 2002/91/CE of the European Parliament and Council of the 16^th of December, 2002 relative to Energy Efficiency in Buildings (EPDB) [2], which was merged in Directive 2010/31/EU of 19^th May, 2010 obliging European countries to meet a number of minimum requirements for energy efficiency in buildings and their facilities.

The EPDB [3] is a legal tool that advocates for an efficient energy consumption in the building sector at European level, combining prescriptive tools (it requires Member States to establish energy efficiency requirements for new buildings and those existing which are subjected to refurbishment) and informative tools (such as the emission of energy efficiency certificates at the moment of buying, selling or leasing and requirements for heating and cooling system inspections).

In Spain, this directive has been transposed through various instruments:

• Royal Decree 314/2006 [4] approved the Technical Code on Building and its later renewal of the Basic Document Energy Savings (DB HE) on the version published on the 12^th of September, 2013 [5].
• The Rules of thermal systems in building, approved in 2007 and its later renewal through the Royal Decree 238/2013 [6].
• Royal Decree 235/2013 approved on 5^th of April, presenting the basic procedure for energy efficiency certification in buildings [7].

According to data obtained from Rehenergía project [8], there is a potential for energy savings between 12% and 40% for energy consumption reduction through intervention on façades, rooftops and windows.

Also, according to data from “Study of potential for energy savings and reduction of CO₂ emissions in dwellings of the Valencian Region” [9] on “Distribution of multifamily homes by typology in the Valencian Region”, the most common building (53%) in the Valencian Region is the isolated apartment block, which is linear and compact and is the subject of the present study.

There are various studies on energy savings obtained in different energy retrofitting experiences on the residential building stock nationwide as elaborated by WWF [10] or the RECONSOST Project [11], all of them focusing on energy refurbishment measures. The recent study “A vision-country for the Building Sector in Spain”, by Green Building Council (GBC), [12] intends to reveal that with the adequate regulatory framework, rehabilitating and updating the building stock are a feasible and economically possible action, and must stand as the activity over which the building sector should restart in Spain, currently insolvent to face the challenges on Climate Change and terribly affected by the crisis.

This first stage was aimed to collect general information about the neighbourhood and its history, social and economic aspects, as well as administrative, graphs and technical information, which may give a greater knowledge of the buildings. Accordingly, all the information available in institutions and entities such as Professional Technical Schools, Municipal Archive of Alicante, and Projects Coordination Service of the City of Alicante, Municipal Housing Board of Alicante and the Cadastral Virtual Office was consulted.

The main aim in this stage was to search for complementary data on site, mainly technical, that allowed a characterization of the initial state of the buildings’ thermal envelope and DHW, heating and cooling systems.

A series of technical visits to the buildings were scheduled for visual inspection, only including a series of tests and samples that allowed the identification of each constructive element of the building envelope and the materials of which they were constituted.

Of all of the buildings’ envelope characteristics, the most interesting were those needed to obtain the thermal transmittance U (W/m²K) of each constructive element, as well as the ones related to the air leaks.

Once all the information had been gathered, a computer program was used to be able to measure energy demand, energy consumption and CO₂ emissions. This program was also able to assess these data after applying the proposed measures for improving the buildings’ envelope. This program was CERMA (Calificación Energética Residencial Método Abreviado) in its version for retrofitting of existing buildings (it is also used for new buildings). Fredsol group, at the Polytechnic University of Valencia and the Valencia Institute of Buildings, developed this tool [15].

The CERMA application used for the energy evaluation already included the requirements established by Spanish regulation (Building Technical Code), for Alicante city. It also included the correlation between energy consumption and CO² emissions defined by the Spanish government.

At this stage, the previously obtained results on energy demand and consumption reduction as well as CO₂ emissions, by constructive elements at the buildings’ envelope were analysed individually and globally (within the entire intervention).

In order to analyse the buildings’ behaviour at a deeper level, as they consist of similar modules with 3 or 4 floors, an energy simulation from the decomposition of the buildings into 12 modules was performed Fig. (14). It presented, either by their number of floors or by their height, specific conditions on their thermal performance, so that the following aspects were taken into account as relevant: boundary conditions of the thermal envelope and height of the modules.

Fig. (14). Graphic for the identification of the different modules. (Source: Own development).

Fig. (15). Graphic for the identification of the different modules. (Source: Own development).

In order to analyse if the partial impact of each of the improvement measures is over the total, a series of possible situations were simulated. The initial status and the final status in each possible scenario were compared, analysing the improved constructive elements, both individually and globally. Table 1 summarises the different considered scenarios:

The aim was to know the partial impact of each of the improvement measures, in order to detect the relation of each improved element with its improvement rate. It should be noted that the sum of the improvements calculated partially is not equal to the improvement status calculated globally, as the simulation process is more complex and some components on the envelope affect the other components simultaneously. However, the level of error considering this is small.

The most important result was the improvement of the thermal comfort and the life quality of the users. It is worth to be noted as a high number of dwellings lacked heating and cooling systems, and this intervention does not bring up the installation of any heating or cooling systems. Therefore, taking this into account, 4 simulations were carried out per modelled module, with the number of simulations being 36.

Regarding the performance of the final solutions, U values, heating and cooling needs were determined taking into account the Spanish regulation (Building Technical Code), for Alicante city.

Façades and thermal bridges improvements presented a much higher heating demand reduction (45%) than cooling demand reduction (14%).

Windows improvements presented similar demand reduction in heating (21%) and in cooling (20%).

The rooftop improvement presented a reduction in heating demand (14%) similar to the reduction in cooling demand (12%).

The improvement level of each intervened element depends mainly on the surface it represents on the buildings’ thermal envelope, the orientation of those elements of the envelope and the compactness of the studied module. In this text, the compactness is the relationship between the volume enclosed by the thermal envelope and the amount of the surfaces of this envelope in contact with the external environment.

The reduction of CO₂ emissions by intervened element is greater at façades and thermal bridges (26%) than at rooftops (11%) and windows (17%) (Graph 2).

Graph (2). Reduction on CO2 emissions achieved by constructive element and block type (Source: own development).

The improvement by decreasing thermal transmittance coefficient in each element is shown in Graph (3), which also compares the maximum levels of U established by Spanish regulation (CTE) to show the level of performance that is reached by the intervention.

Graph (3). U values reduction compared to the initial status and the one required by legislation. (Source: own development).

Graph (4) contains information on the reduction percentages in CO₂ emissions, demand and consumption achieved with the suggested intervention. The global reduction in demand is 52%, being greater reduction in heating than in cooling demand.

As a result of the intervention, cooling is sensitively greater than heating demand. It must be taken into account that the climate zone is Alicante B4, characterised by a sever summer. On top of that, the blocks’ façades orientation to east and west is the most detrimental (with the higher radiation during summer).

The percentage in energy consumption saved is 52%. We must add as a result of the upgrading, not only the reduction in the energy bill of the users, but the improvement in their comfort and living standards, more remarkable taking into account that many dwellings lack heating and, even less, cooling systems. A notable reduction in the number of discomfort hours inside the households is achieved, obviously.

Graph (4). Reduction global percentages achieved in CO2 emission, demand and consumption (Source: own development).

It is important to point that the official energy performance evaluation programmes, require working at some theoretical set temperatures. In this regard, if these temperatures are not reached, the program considers some default systems in order to guarantee the established comfort temperatures. Therefore, the obtained results for the simulation may not be a real representation. In fact, the energy poverty situation that many families in the neighbourhood suffer stops them from paying an energy bill that would allow them to reach those comfort temperatures. Therefore, there is an important difference between theoretical and real consumption.

With the theoretical conditions for simulation, assuming heating system with gasoil and an electric cooling system with the established costs by the latest fees (which are 0.048 €/kWh for gasoil and 0.12 €/kWh for electricity), annual savings per dwelling are approximately of 170.45 € for heating and cooling altogether, in standard conditions.

Graph (5). Scale of energy classes at buildings, both for the original and the final status, in KgCO2/m2 year. (Source: own development).

• Initial consumption: 3.7 €/m² dwelling per year
• Final consumption: 1.776 € /m² dwelling per year
• Savings: 1.924 € /m² dwelling per year

The global savings in CO₂ emissions is of 47%. In addition to the improvement in energy parameters, there is an improvement in the scale of energy qualification against energy qualification for new construction block buildings (for existing buildings, at the time when the study was carried out, the scale was not published, it includes two more letters below E). The achieved qualification is a D, which is equal to 13.4 kgCO₂/m²year, (Graph 5).

Assuming an average saving of 13.8 kgCO₂/m² dwelling per year, the saving in emissions for a standard dwelling is 994 kgCO₂ per year. This quantity, for the total 324 dwellings would result in an annual total saving of 322 TCO₂.

From the results obtained by the application of the improvement measures to standard envelopes, it can be concluded that even increasing the total thickness of the insulation layer to 80mm at the same time in all the elements of the envelope, (the original project established 40mm), it is impossible to reduce the emissions with the purpose of modifying the original energy evaluation, graded with a D.

If, in addition to increase 40 mm the thermal insulation in roof and walls, compared with the original design (80 mm in total, with a conductivity of 0.04W/m2K), the glass (3.3 W/m2K) and the frames (3.3 W/m2K) are improved simultaneously, and a mix system of DHW and biomass heating systems are added, the qualification obtained would be an A and would be a high energy efficiency building.

Acting on the DHW and heating systems separately a qualification of C was obtained but only using biomass as fuel. However, if both systems are combined, a B score can be achieved if biomass is still used. Thus, a mix system of DWH and heating systems, with a high performance seasonal boiler (95%) will also obtain an improvement on the energy qualification, reaching a C level by using natural gas as fuel or an air-water heating pump.

To obtain a qualification of A, an important effort must be done, not only on the envelope but also on the systems, with the use of renewable energies due to the low emissions produced by this type of fuel.

Although the residential unit’s trend is very similar, a slightly better performance can be observed on the 4 stories units compared to the 3 stories units by square meter (kgCO₂/m² a year). It is due to a higher compactness of the first one. Certainly, the global emissions (kgCO₂ a year) of the 4 stories units are greater than the 3 stories blocks, because they have a larger number of houses, although the emissions ratio per square meter is lower.

If we relate the improvement percentage on thermal transmittance reduction, with the improvement percentage on emissions reduction and the price of m2 on roof, façades and windows, we acknowledge that windows are more expensive (252 €/m2) although they do not take into account the maximum emissions reduction, which is given by the façades with a cost of 42 €/m². Roofs improvements are the cheapest (28€/m²), but they are the least contributing to emissions reduction. If this analysis is realised with total prices per element, we can verify how the façades are precisely the most relevant element regarding emissions reduction, but with the most expensive cost, approximately 760,000 € (a 50% of the total cost of the intervention on the envelope). In fact, rooftops improvements represent 177,000 € (a 12% of the total price of the intervention), much cheaper than the façades improvements, although they are the ones with less effect on emissions reduction. It must not be forgotten that, due to the shape of the blocks, the total façade area adds up to 18,000 m², against 6.300 m² of the rooftops, almost 3 times more.

If the total cost of the intervention of each element (rooftops, windows and façades) is reflected on the dwellings’ m², the following costs are given:

− Rooftops: 7.59 € /m²dwelling

− Windows: 25.36 € /m²dwelling

− Façades: 32.52 € /m²dwelling

− Total intervention: 65.47€ /m²dwelling

Despite the long investment-return periods, the results revealed that the houses which were out of the neighbourhood real estate market before the improvement, entered again in it after the refurbishment, showing a slight increase in the number of sales and purchases. Another effect that should be highlighted is the increase in property value that the intervention has generated. The comparison of the façade picture before and after the project clearly reveals the indirect effect (Figs. 14 and 15).

Thus, a deeper economic study considering these aspects, not just the technical solutions, may reduce the investment-return periods.

All the costs described are total costs including labour and materials indirectly.

It is impossible to modify the original energy evaluation, graded with a D, only with isolated intervention on the envelope, even increasing to double the total thickness of the insulation layer from the first project.

If apart from acting on the envelope, according to the technical project, an intervention on the systems would have been implemented, the analysis of the results obtained from the application of standard measures concluded that, in general, the emissions are reduced but not in a sufficient manner to achieve a C energy qualification. Only with the use of biomass for DHW or heating, an energy efficiency qualification increase can be obtained.

A qualification C can be achieved combining interventions on roofs, walls and windows.

Combined interventions on the thermal envelope and installing systems using biomass as fuel, a B score can be achieved.

In any case, the improvement of the systems with high efficiency equipment produces a substantial improvement on the qualification, scoring a maximum of C, and more if it is combined with improvements on the envelope.

It is only possible to achieve a qualification of A, improving the thermal systems with an important economic effort, through the installation of renewable energies.

Compactness is defined as the relation between the total air treated volume of the building if compared with the envelope surface area in contact with the exterior environment. The energy behaviour between units is sensibly different depending if B’ (4 stories) has a compactness level higher than the other B unit (3 stories), because with a similar construction quality (equal values of U on the envelope) the energy demand is reduced by square meter (kWh/m² a year). This occurs because the area of thermal transmission is smaller on the building with greater compactness. Therefore, greater compactness improves the energy efficiency if calculated based on surface and not in global units (kWh a year). It is evident that, globally, the 4 stories unit has greater emission and consumptions as it has a higher number of housing units.

Because of the conditions of the buildings, occupied by low income citizens, the results focused, mainly, on studying how the demand can be reduced. In this sense, the savings obtained for the heating demand are close to 72%, while the reduction of air conditioning demand is estimated at 52%. This causes that on the improved-final result, the demand for air conditioning is higher than the heating demand.

It should be highlighted that these results are produced by the shape of the building on the microclimate created on its emplacement. Therefore, if solar contribution is analysed, it can be concluded that the studied building has a linear form with façades to East and West orientation. As a result, the solar radiation on the eastern and western façade is much higher in summer than in winter season, while the northern façade receives low direct radiation and mainly in summer time. Therefore, on the East and West façades, the use of solar protection systems should be emphasized, such as awnings or something similar, as the great benefits for reducing solar radiation in summer are demonstrated. The improved rooftop also results as being very beneficial in avoiding this overheating situation.

The buildings with a higher compactness have a slightly better performance. However, the global emissions of these constrictions are greater, because they have more houses, and the emissions ratio per square meter is lower.

The most relevant result is the improvement of the thermal comfort and the life quality of the users, especially if the high percentage of housing units is considered that do not have any heating or air conditioning systems along with the absence of such systems on the refurbishment intervention. In consequence, the investment-return periods, if there is no real energy consumption, reach 20 years, making unviable, initially, this type of intervention.

The energy retrofit of the envelope with external thermal insulation revealed that houses that were out of the neighbourhood real estate market entered again in it after the refurbishment, showing a slight increase in the number of sales and purchases.

Thus, a deeper economic study considering these aspects, and not just the technical solutions, may reduce the investment-return periods.

This paper reveals the importance of energy refurbishment in residential buildings, especially in social housing, and how important it is to work for improving the processes, the financial mechanism or launching awareness campaign for the users.

In addition to achieving tangible results in emissions reduction, energy refurbishment permits a transformation of the construction sector, highly affected by the economic crisis, attending the necessities of groups in risk of suffering energy poverty as well as creating green jobs.

In addition, this paper aims to point out the important exemplary role that governments should have in the regeneration of vulnerable neighbourhoods of our cities. These goals are not only focused on reducing CO₂ emissions, but also on improving the quality of life of people. It has been demonstrated that without large financial investments, prioritizing what is important and durable, an interesting refurbishment project can be undertaken.

This experience demonstrates the social character that has motivated the intervention in the buildings at this neighbourhood, beyond the regulation. The benefits generated by the energy retrofit are worth noting in terms of improving the urban scene and rising property values.