The definition of Areas of Rehabilitation and Urban Renovation and Regeneration (ARRU) is included in new Spanish urban plans to contribute to sustainable development by structurally intervening in the city (on Territorial Planning, 2014-07-25). This intends to support those responsible for local administration decision making by selecting the urban areas that require priority interventions and to undertake durable actions over time. Therefore, the buildings included in an ARRU would be prioritised when addressing potential refurbishment subsidies. Moreover, considering the most vulnerable areas will affect the whole city's substantial improvement.

One of the first obligations that Franco's Government had to face after the end of the Spanish Civil War was to reconstruct the country. One major part of this work was to recover urban areas, which entailed not only repairing destroyed heritage, but also constructing new housing that could accommodate a population with scarce resources.

To fulfil this purpose, immediately after the Civil War was over the first law for low-income housing was in- troduced, as was the government institution responsible for ensuring the promotion and control of building such housing all over the country, the National Housing Institute (Law, 1939-04-19) In this way, apart from updating the legislation on such interventions, governmental institutions sought to tightly control the promotion of public housing, so it was left as a private initiative on the sidelines (Go´mez, 2004).

However, the economic precariousness that the country was in during the first post-war period years did not allow the agreed quantitative targets for the building stock in successive National Housing Plans to be achieved. During the first two decades of Franco’s Government, countless laws were introduced (see Table 1) to promote building stock growth rather than regulate housing.

This objective would definitely not be met until the beginning of the 1960s, in 1963 with the last social housing legislation of Franco's Government. Governmental institutions would lose the strong interventionist character they held thereafter to behalf of the private promoter who was consolidated as a preferred agent in the building of this type of housing.

Apart from the limited implementation of successive Housing Plans, the economic shortage led to restrict the use of materials in the building, especially iron (on restriction on the use of iron in buildings, 1941-03-11), to avoid increasing the construction cost of housing buildings. Consequently for housing, builders were forced to use building systems from the beginning of the century and use alternative materials in those elements in which steel had become the most suitable option.

This meant a delay in the building technology progress made in Spain as social housing built during the first two decades of Franco's Government brings to light. After the Spanish economy opened out to the international scene and thanks to the Stabilisation Plan of the late 1950s, it managed to overcome years of shortage and, thus, the liberalisation of building stock, which would lose the tight constructive control normalisation by governmental institutions (Sambricio, 2004).

The development of the new Land Use Plan of the city of Castello´n de la Plana (Spain) needs to define ARRU. This definition was created by using 29 ad hoc selected indicators of vulnerability. They were grouped into four categories: urban (4), building (4), socio-demographic (16) and socio-economic indicators. The ARRU were those areas where all the categories concurred, so they presented multidimensional vulnerability. As a result, 17 ARRU were defined in the city ((García-Bernal et al., 2017); (Ruá et al., 2017)). The building used as a case study in this work is included in one of the ARRU. The defined ARRU are indicated in Figure 2. The selected block of buildings is included in ARRU 01. The location of the block is indicated in the top-right corner of Figure 2.

The block of buildings was built in 1959 as social housing by Ministry of Housing subsidy. The group is formed by 12 buildings, the north fac¸ade, with odd numbers 1 to 11, to Huesca Street, and opposite the Castalia football stadium. The south fac¸ade is in Mart´ınez Tena Street, numbers 2 to 12. Figure 3 shows a general view of the block of buildings. The block forms an internal courtyard and each building is formed by one ground floor and four upper floors, with two dwellings per floor, which means 10 houses per block and 120 houses in all, which account for 2339 m2.

Figure 4 shows the front side view and the side elevation plan, section and roof plan. Figure 5 represents a 3D image of the block using the Revit software, drawn after the visits made to the site.

There are four buildings in the corners of the block, while the rest are terraced buildings which present better energy performance as they are less exposed to the outer environment. The building located in the north-west corner (357º north) is the worst in energy performance terms. In order to simplify simulation, only this building was simulated. There are three types of housing, as seen in the original plans presented in Figure 6, where the selected building is indicated. Figure 7 shows the floor plan, with dwellings types I and II, included in the building selected in the block. Dwelling type III is included in the terraced buildings in the block.

In order to analyze the building’s thermal performance, some starting conditions need to be considered. Regarding external conditions, and according to Spanish Regulation for Energy Saving, CTE DB-HE1, Appendix D, the building is located in climatic zone B3 (winter severity B on a scale from A to E, from the warmest to the coldest; summer severity 3 on a scale from 1 to 4, from the least to the most severe). The thermal envelope characteristics are summarised in Table 2, where the layers of the constructive solutions are represented, together with the exposed area and the thermal transmittance data. All this, plus the facilities used to obtain domestic hot water (DHW), heating and cooling services, in this case electric appliances, will be the starting data to simulate the building to estimate its energy performance in its current state:

To evaluate the building’s energy performance in its current state, the CE3x software was used, the official software for energy certification in Spain (the open access software provided by the Ministry of Industry). Using CE3x permits energy demand to be known, energy use and carbon emissions associated with the building’s use. The software estimates the annual energy demand linked to heating and cooling in kWh/m2, and the annual CO₂ emissions due to heating, cooling and DHW in kg CO₂/m2. The sum of emissions will evaluate energy performance using a scale from A to G, with the best and the worst energy performance, respectively. This software is widely used professionally, but has also been included in research works ((Alguacil et al., 2017); (Ayala et al., 2016)), specifically to social housing (Escandón et al., 2016) to obtain simulation energy performance values.

The input data in the software result in the energy performance presented in Figure 12. According to the simulation, the building emits 29.5 kg CO₂/m2 due to heating, cooling and DHW, with 15.98, 5.09 and 8.41 kg CO₂/m2, re- spectively. Altogether, the building obtains an E energy label, where DHW presents the worst energy performance, with F, followed by heating with E and a medium D for cooling.

The next stage consists in analysing some refurbishment solutions to improve the building’s energy performance and to select the optimal one. To do so, four perspectives are observed: technical, environmental, economic and heritage. The technical perspective analyses the advantages and disadvantages of actually implementing the con- structive solution. The environmental perspective is based mainly on the resulting transmittance of the refurbished solution, together with the reduction of thermal bridges. The economic perspective examines the viability of refur- bishment. Finally, the building’s heritage value is considered by looking at the aesthetic variation of the building’s external envelope as it can be representative of the building type built at a particular socio-economic period of time, as explained in Section 3.2.

Regarding the thermal envelope, the refurbishment aimed to add insulation layers that did not exist in the original building's state. Insulation material is generally the most cost-effective solution when buildings are refurbished (Hamdy et al., 2011)Rua´ & Lo´pez-Mesa, 2012). This should be done on fac¸ades together with im-provements to carpentries, with double glazing solutions, and also in roofs. Adding insulation on fac¸ades and roofs could be done basically in two ways: on the outer or the inner layer.

Table 3 shows some constructive solutions (Huedo et al., 2016), along with their advantages, disadvantages and features by bearing in mind the technical, environmental, economic and heritage perspectives. The selected solutions are shaded in grey in Table 3 and represented in Figure 13 For fac¸ades, inner insulation (FI) was ruled out because of the dimensions of rooms. It would mean reducing the areas of rooms as they could not fulfil the minimum required dimensions according to new standards. Therefore, the outer solution with the External Insulation System was adopted in this case because it was more convenient than the ventilated fac¸ade,

For roofs, the inner solution proved more convenience when looking at advantages and disadvantages. The same solution involving mineral wool and finishing plaster board could be applied to roof 1(SRI) and roof 2 (FRI).

Theoretically, other measures can be proposed. For example, solar energy to cover a high percentage of DHW demand could have been proposed. However, this would need a further structural analysis to check that the structure would support the new load added by fitting solar energy. The presented proposal is intended mainly to be realistic and economically viable by considering the building's socio-economic features.

The selected construction solutions, together with improved facilities, by going from electrical new boilers to condensed natural gas ones, shows a relevant improvement in energy performance and reduced annual carbon emissions from 29.5 to 15.3 kg CO₂/m2. Table 4 shows the heating and cooling demands, together with emissions, due to different facilities and the savings made after refurbishment.

To analyze the economic viability of refurbishment, the guidelines of Commission Delegated Regulation (EU) No. 244/2012, of 16 January 2012, which supplements Directive 2010/31/EU of the European Parliament and the Council on the energy performance of buildings by establishing a comparative methodology framework to calculate cost-optimal levels of minimum energy performance requirements for buildings and building elements, was used. According to this methodology, energy performance measures are considered an investment and the Net Present Value is used. This indicator is appropriate for long-term investments. Buildings are products of a long service life that require maintenance, replacing certain elements, etc. Therefore, the global cost, referred to as starting year τ_0, was calculated using the following equation (1) over calculation period , for a set of measures j during year i:

Table 5 presents details for each term of Equation 1 and the estimated values using the macroeconomic perspective by considering the carbon cost.

The graphic represented inFigure 8 shows the accumulated NPV for two discount rates, r1% and r4%. NPV is C53,184.12 for 1% and is C4,736.1 for 4%. With the starting hypothesis of Table 5, we can see that the investment starts to be positive from year 20 and 28 for the 1% and the 4% discount rate, respectively. This is not a surprising result if we consider the private costs and only the carbon price a social cost.

Some subsidies can be considered in the calculations, if we bear in mind that the building is included in an ARRU, as mentioned in Section 3.1. This would result in a more optimistic scenario with a shorter payback, as calculated in previous studies (Alguacil et al., 2017). For example, estimating a hypothetical subsidy of C5,000.00/dwelling, the investment would be returned in less time, as seen in Figure 14.