Energy Renovation of Buildings the Skin of a Building 70’s Housing Developments in Barcelona Montbau’s Housing Developments Renovation

1. STATE OF THE ART. BUILDING AS A WHOLE AND THE NEIGHBORHOOD OF MONTBAU

A building is a construction whose purpose is to lodge different human activities - “a construction made of resistant materials for human habitation” (RAE) - a consumer of material resources and a great energy exchanger. From the process of its construction and subsequent use, a building will be a medium to exchange energy, which finally and in some cases is used in building deconstruction.

Construction, housing in particular, is a consumer item such as any other good (cars, computers, etc.), that can be bought, sold, possessed, inherited, and with prices that flow on markets. However, permanence of buildings over time can take years or even centuries; a fact that gives buildings a different and attractive characteristic.

Traditionally, buildings were constructed having into account varied environmental conditions such as geographical location and climate. Traditional architecture was respectful of the scope of buildings and their construction took into account such characteristics, by using transparent or opaque skins, to harness or get protection against external conditions.

In the early 20^th Century, in Europe, and especially in the US, modernity and industrialization proposed the use of new materials for construction such as steel and glass (Fig. 1), which transformed architecture and the design of building skins by reducing mass and increasing transparent surfaces. This change in materials required the use of energy sources, initially inexpensive, such as oil, which allowed a degree of comfort and habitability. Any material was then used for building skins or any constructive solution was allowed since temperature regulation was produced by energy-consuming means either for heating or cooling.

Global social and economic conditions in the mid-twentieth Century varied drastically. The growth of cities, the excessive exploitation of non-renewable natural resources, the crisis of economic patterns, among other factors, were evident in the needs and wants of large cities. In the 70’s, energy becomes expensive and the oil crisis is announced, the world revolves around the search for alternative energy resources. The United States, France, and the Soviet Union take a chance on nuclear power plants, industrialized countries without energy resources seek to decrease energy consumption in various areas: industry, transport, and housing. Regulations are driven to achieve such savings and therefore there is an impact on the way of building and living in cities.

In Spain, the first regulation on energy saving, the NBE-CT-79 standard was worded by the end of the 70’s, in response to the crisis. By then, most of the buildings constructed in the previous decades, especially in the 50’s and 60’s, that were not considered historical patrimony did not meet current minimum “sustainability” requirements and therefore the idea is to prevent the loss of energy through their skin as the materials used in their construction were not intended to such purpose.

Indoor environmental conditions depend on the heat flow established through the various enclosures comprising a building. The enclosure and the way the interiors of buildings are constructed have an impact on the indoor ‘weather’ and comfort needs.

1.1. The Skin of a Building

A building is a machine immersed in the middle of nature with its thermal, lighting, acoustic conditions, etc. These may vary over time and they do not usually match the physical needs of human beings, which are relatively constant and stable. To achieve this stability, it is necessary to act on the skin of the building. The environment is variable and it has been necessary to create envelopes, -skins protecting such variability-, leaving some indoor spaces free for various human activities to take place.

A good performance on the skin of the building, either built or renovated, will mean a decrease in energy consumption, needed to achieve desired indoor comfort. Constant indoor environmental conditions are desired. A building can correct its environment through passive systems, such as insulation, radiation, light, ventilation, etc. or even become in a producer of its environment through active systems, such as solar panels, photovoltaic cells, among others. The materials and the shape of a building are the cornerstones of a good design that results in a good construction. This study will focus on the use of insulation materials -with their ability to modify the internal environment of the building-adaptable to the shape of the building and the existing envelope (Fig. 2).

Consumption is controlled from the very design, although in areas with energy surplus, direct energy production tends to replace the right design. Peter Reyner Banham [2], defines two types of cultures that differ in their attitude towards a tree: one would build an insulating wall with the tree, and the other would burn it. Our society strongly agrees with the second standpoint, although the alarm voices of our planet are making us to be aware and change our attitude and assume our responsibilities.

That which is not achieved with these resources should be sought through external energy for its operation. Most of such energy comes from fossil resources or limited energy production plants such nuclear plants, with a cost difficult to appraise in the future, both economic and environmental (Chernobyl or Fukushim, for instance). It is important to highlight that in recent years, great progress has been made in terms of solar power plants and wind farms, mainly.

1.2. Building Energy Performance

In order to measure energy performance, a study on the different programs validated to fit our interests has been conducted. Based on the article Contrasting the Capabilities of Building Energy Performance Simulation Programs [3], a comprehensive comparison between 20 programs was carried out to evaluate the topics they deal with and their reliability in the opinion of the experts consulted for such purpose. Such topics as “General Modelling; Building Envelope, Day Lighting and Solar; Infiltration, ventilation, (…); Renewable Energy Systems; Electrical System and Equipment; Environmental Emissions (various types of gases); Economic Evaluation; (…)” were discussed. Currently, however, topics such as gas emission (CO₂e) and economic implications have increasingly acquired more importance.

Recent works as “Establishing a Method to Perform a BPS - Building Performance Simulation” [4] deal the need to reformulate the “Building Performance Simulation” approach so that it matches more modern approaches such as BIM [5, 6].

In this paper, we have used DesignBuilder^® since it is the ‘user-friendly interface’ of Energy Plus. This latter software, along with TRNSYS, the most widely validated software by the scientific community in the field of energy simulation.

1.3. Renovation

24,000,000 houses currently exist in Spain, out of which approximately 15 million were built before the 80’s (INE, 2007). Over half of the country's houses were not built taking into account any standards to avoid excessive energy consumption, since at that time no legislation or standards existed to regulate construction processes, neither there were suggestions for their maintenance and utilization (Fig. 3).

As a strategy to improve buildings, renovation, and intervention, specifically their skin, is here proposed. Renovation is a continuous action made on an existing building in order to adapt it to current needs and conditions. Improving the skin of a building, will stabilize indoor temperature conditions and, hence, the demands for comfort.

It is necessary to renovate, reconstruct, and reuse (Il s'agit de ne jamais demolir, ne jamais retrancher ou remplacer, toujours ajouter, transformer et utilitser) [7]. Unmoderated occupation of the territory must stop. The substitution of buildings for new ones, increasingly represent higher and hardly acceptable costs in economically degraded environments. It is imperative to modify the lifespan of a building by renovating it and giving it a new life that is consistent with current times, both in terms of regulations, environment, and comfort. (See suggestions for maintenance and utilization (Fig. 4).

Technology and means to carry out renovation are available, and it is important to demonstrate the capacity and the impact said actions can have. Renovation has other facets, such as the improvement and optimization of facilities or the inclusion of active systems, such as photovoltaic cells or panels for water heating; however, this study analyzes a single aspect of renovation: how to treat a building’s skin.

1.4. Actions in our Environment

-Actions at European Level

Europe is a continent with a rich history that has been constructed throughout centuries. As such, rehabilitation and intervention of historic buildings must be respectful allowing their past features to be rescued to offer better habitability conditions for our current times. Regarding edifications, especially houses built between the 60s and 80s in the 20^th century that have not been considered as a cultural heritage or a monument, interventions can be performed on them to improve their habitability without the limitations of a property declared as cultural heritage good.

Various interventions have been performed to recover and intervene buildings and neighborhoods in different European countries such as Sweden, Italy, Austria, Germany, and France, among others. In most of them, the main criteria taken into account for intervention are energy efficiency, mobility, waste management, construction materials and vegetation, CO₂e saving levels, among others to achieve a global standard for possible interventions and results in the future. Some of the actions carried out include the need to conduct sociology studies (demography, age groups, occupations, education, economic status, etc.) so that it is possible to address management efforts and project management.

For instance, in Malmö, Sweden, the need to intervene one of the recently built neighborhoods, called Augustenborg, was considered in 1998. The neighborhood was built in the late 40s, but by the late 90s it had already started to deteriorate notoriously, mainly because of the elimination of the traditional, heavy industry dismantling and the resulting unemployment caused. The neighborhood was home to around three thousand people and needed a comprehensive intervention. The project proposed the rehabilitation and recovery of the environmental conditions, public spaces, roads, and mobility, and its inhabitants were involved through participatory workshops and specific actions to implement it and achieve a sense of belonging. Experiments were conducted with such things as an “electric pool car” inhabitants may use as long as they return it. Also, waste management procedures were implemented, so 50% of the waste was actually recycled. On the other hand, a series of actions were commenced on the buildings, such as improvements in the isolation materials of the facades, installation of thermal solar panels and photovoltaic panels, use of geothermal energy in sports facilities, and renewal of green areas and growing of vegetation on the roofs. The project is still ongoing and it has shown excellent results [8].

In Torino, Italy, as part of the “Urban Regeneration and Development” program, an intervention project started in 2001 in suburbs that were built during the industrialization boom in the late 19^th century until the middle 60s in the 20^th century. The economic crisis of the late 20^th century caused important changes in those suburbs, and intervention was planned to include urbanism, renovation, infrastructure, socio-economic changes, participation of inhabitants, sociology analysis, participatory workshops, and culture activities. Arguata, another neighborhood, built in the 1920s, was intervened in terms of public spaces, roads, infrastructure, and edifications, already deteriorated due to abandonment, through efficiency measures such as insulation of roofs, reduction of thermal bridges, new low-emissivity windows, and solar protection. Likewise, a natural gas co-generation plant was built to heat the buildings and produce electric energy. Actions reduced CO₂ emissions (1997) in 52% per year [9].

Another example is Winarskyhof in Viena. Between 2005 and 2008, rehabilitation of a set of buildings constructed in 1924 started. 127 homes (7,200 m²) were intervened. New low-emissivity windows and doors were installed; facades and balconies were insulated; water and electricity grids were renewed; lifts were installed, and yards and parks were redesigned. 76 new apartments were built to replace the mansards, which also allowed the project to be funded [10].

In France, architects Fréderic Druot, Anne Lacaton & Jean Phillippe Vassal, proposed action strategies in the “Banlieue”, bad reputation suburbs of large French cities. The French State wanted to demolish them to substitute them with small two-storey houses. The proposal of the architects was to revalue these neighborhoods and reclassify them based on the idea of projecting the houses to the external areas so that day light and sun energy was harnessed, increase the surfaces and create large terraces with greenhouses to control energy. By reinterpreting the large buildings in terms of height and their enormous potential, the concentration of utilities and the optimization of the territory occupancy was going to be allowed. The project proposed to potentiate community spaces and the way the buildings relate to bathroom areas, community lounges, restaurants, playfields for children, among other characteristics. It was a constructive, typological, and global program of intervention on residential buildings [7].

This study especially approaches the importance of rehabilitating buildings, house developments, and neighborhoods, and the advantages of their costs over time versus the consequences of demolishing and build again. The rehabilitation of a building in the Boulevard du Bois le Petre clearly showed the advantages and feasibility of conducting such type of interventions over the simple act of demolishing and build again.

1.5. Estate and Local Actions

Interest in rehabilitation has increased in Spain as a consequence of the crisis of the construction industry. However, many working groups have rigorously and continually been working for a long while now on documenting the topics dealt with in this document.

In Zaragoza, the Mayorship Housing Rehabilitation Service is conducting a rigorous work by acting on several levels not only in terms of buildings and environment, but also in terms of social issues- as most of the inhabitants of the area are low-income population. This working group attempts to act in terms of urbanization and planning, improving the deficit of community spaces, facilities, etc. First, they study the buildings and, according to results, act on such topics as accessibility, roofs, damp, insulation, window and door frames, climatization, etc [11].

In Madrid, a working group of Universidad Politécnica, led by Margarita de Luxán, conducted a study paid by “Empresa Municipal de la Vivienda” in order to study the buildings of the center of the city, their physical characteristics: shading, ventilation systems, and possible enhancements. A very interesting document addressed to architects and builders in that specific area of the city is the result of such study [12].

Margarita de Luxán and Gloria Gómez intervened a building of apartments in San Cristóbal de los Ángeles, Madrid, by reoriented the windows of the building so that sunlight could easily and efficiently be collected by eliminating the roof and the façade and creating a ventilation system (heating and cooling the air), and the interior spaces were improved. The building now satisfies the habitability comfort need of its inhabitants [13].

In Cataluña, ADIGSA has intervened various buildings in the neighborhood of Verdum, and has also intervened buildings in Trinidad, Barcelona, and in the cities of Terrassa and Planoles. The company has improved accessibility, window and door frames, insulation, and many other features. Interventions commence with a diagnosis of the building status as well as the areas to be intervened by simulating energy performance possible savings [14].

2. HOUSING DEVELOPMENTS IN BARCELONA

After the postwar period, economic emergence affected the growth of the city and its population. Housing developments built in Barcelona during the 60’s and 70’s were an answer to the need to accommodate a growing population who came to the city looking for work and were hired as cheap labor.

In 1929, the Housing Board was created and its aim was to eliminate shantytowns and commence housing construction for its inhabitants. The first barracks were built by the new residents who arrived in the city and were retained as labor for the construction of the 1929 Barcelona International Exposition, built mostly in the mountain of Montjuïc. Then, inexpensive houses were built to accommodate this population [15].

In 1941, a tool, whose operator was the Association for Home Labor -the main statewide-level promoter linked to the vertical union- was created. In Barcelona, together with the Municipal Patronage, the first actions inspired by the housing development proposals submitted by the dictatorship of 1929 began. Garden neighborhoods with a rural character, located in rural districts, started to be built and featured single-family houses larger than the ones built in previous years.

The first housing developments that were built, such as El Polvorín, in Montjuïc in 1953 (Fig. 5 Aerial view of the buildings of El Polvorín) have completely been renovated. Others, such as Torre Llobeta located at Paseo Maragall, follow the urban design proposed in 1950, where five blocks of the Eixample are built. The church supplied, in some cases, the deficiencies of the State and to celebrate the 35^th Eucharistic Congress, it commences the construction of Viviendas del Congreso Eucarístico (Houses of the Eucharistic Congress) with around 2,700 units built over ten years, between 1952 and 1962 (Fig. 6). Aerial view of Viviendas del Congreso Eucarístico).

In 1953, the “Regional Plan” is worded as an ideological and urbanistic instrument for Barcelona and its area of influence. This plan, in the context of a permissive management, led to speculative processes that ended up over populating housing developments and the city in general. In 1957, the Social Emergency Plan was worded which most of the housing developments were created and foreseen, including Southwest Besòs, Montbau, Guineueta, Badalona, Baró de Viver, Bon Pastor, St. Martín, Cornellà and, Bellvitge [16] (Fig. 7 Situation of polygons in the Barcelona area).

Large housing developments create small cities -within the city- with thousands of houses and inhabitants. In 1968, Bellvitge housing development created a city with 13,000 houses and between 30,000 to 40,000 inhabitants (Fig. 8 Bellvitge). In view of the need of construction projects, the private sector decided to become a key player in the construction of houses, both in lands made available by the town hall an in private-owned lands. Construction to house increasing population in house developments opens the door to larger populations and speculation for profit [17]. (Fig. 9 Southwest Besòs).

Housing developments, for better or worse, over time became one of Barcelona’s key growth drivers. Allowing expansion to strong demographic pressures, housing developments started to grow in a disorderly manner and had late responses to a number of estimates and calculations that were never sufficient.

Therefore, in the past twenty years, policies towards the construction of a large number of houses -without considering urban furnishing and public space- as well as the deficient and poor-quality construction of houses generated a sort of unfinished city [16]. The arrival of democracy began to balance these weaknesses by fostering a heavy investment in public space and utilities and by renovating buildings.

3.1. Block A - Duplex Buildings (Common Hallway Distribution)

Block A was one of the first buildings ever to be built in Montbau. It is a Ground-Floor Building+10, with only one staircase and an elevator; hallways lead to each one of the apartments. There are two types of apartments: 20 two-bedroom apartments and 30 three-bedroom apartments (Figs. 13 and 14). The building located towards the Northeast - Southwest, with the facade of the living rooms facing the afternoon sun. Such orientation is favorable in winter for heating. Sun can be handled using blinds, curtains, etc. in summer to avoid overheating (Fig. 15). The other duplex building, Block N, is located in the same orientation.

3.2. Linear Block E-F (Apartments per Floor)

Block E-F is the typical type of apartment building in the area. These blocks are shallow, where apartments face both facades, allowing good illumination in all the rooms and a good cross-ventilation design for summer time (Fig. 16). Their orientation is Southeast - Northwest. The Southeaster facade receives most of the sun hours and the rear part receives just a few sun hours and only during summer time (Fig. 17).

3.3. Tower Q (4 Apartments per Story)

Block Q, with a tower design, holds 9 towers, each with 15 floors and 4 apartments per floor (Fig. 18). This block has two different types of space: A and B. A-type apartments face South. B-type apartments face East and West. In general, all apartments have good ventilation and illumination in all rooms (Fig. 19).

3.4. UNI14 - (Single-Family Attached Houses)

There are two types of houses in core Montbau. This study focuses on a unit of 14 UNI14 attached houses that include workshops as a complement to the shopping mall (Figs. 20 and 21). This housing development is organized as the linear blocks: Southeast - Northwest. The building is located on a slope of land. Access to houses is located on the main road, and following the topography, a staircase connects, at a lower level, the adjoining workshop floor. The houses are in pairs and distributions have minor differences (Fig. 22).

5. EVALUATION OF TYPICAL BUILDING IN MONTBAU

Building skins is the envelope that allows specific conditions to occur and isolates living spaces from the external environment. Its design and construction must provide solutions for comfort with a minimum energy consumption, through the contributions provided by constructive solutions, adapted to weather and environmental conditions. The skin takes on a greater role in situations of lack or reduction of energy resources [19, 20]. The skin is a physical barrier that should help produce balanced climatically buildings. Skin conditions will impact the mechanical environmental needs -low or null- that allow regulating a low energy consumption during the lifespan of the building and ensure an adequate internal thermal comfort [21].

The construction of buildings in Spain, did not take into account these considerations until the energy crisis in the 70’s, -a crisis that affected all the industrialized countries, especially in Europe, with limited sources of energy- leading, among other measures, to a considerable increase in energy prices. Given the crisis and the economic costs of maintaining the minimum comfort conditions, it was necessary to reconsider the construction system, forcing the emergence of regulations that governed and regulated such type of envelope - NBE-CT-79 Standard-.

Buildings constructed between the 50’s and 70’s, -subject to analysis in this study- despite their acceptable conditions, do not meet the minimum comfort standards. Currently, these apartments consume large amounts of energy to achieve acceptable internal comfort conditions [22].

This study analyzes the current consumption of the 4 types of buildings in the neighborhood of Montbau. This study will allow the analysis of the current characteristics of the skin of several buildings, as well as knowing the energy demand of air conditioning, C0₂e emissions and their economic cost to guarantee inhabitants the acceptable level of thermal comfort. The analysis, limits its scope to the evaluation of consumption, costs, and CO₂e emissions, needed to achieve the thermal comfort required to warm the different spaces, without refrigerating them. Weather, location -the neighborhood of Montbau in Barcelona, and the arrangement of buildings- their current position in the territory through cross-ventilation, allow air currents that generate a regular acceptable temperature in summer time [23].

Simultaneously and complementarily, the study includes an analysis of the behavior of buildings in other climatic zones of Spain -Madrid and Girona- to assess the impact suffered in different climatic conditions.

For this particular case, DesingBuilder [24] was used. This tool allows controlling temperature, lighting, and energy analysis of buildings in any climatic situation, as well as assessing comfort and CO₂e emissions, among others. The tool offers the possibility of analyzing both existing buildings and future buildings during their any of the stages of the design process.

The software allowed to model buildings type A, E-F, Q, and UNI14. Modeling basically consists in geometrically defining the building, characterizing its constructive elements, defining its occupancy rate, its air conditioning system and the external environment and climate. The software provides a series of results that will allow the analysis and the evaluation of the building behavior and thus estimate the various possibilities for future building envelope improvements.

All buildings have high energy consumption, produce high CO₂e emissions, and are considerably expensive in terms of internal comfort. However, the buildings with the highest consumption rate are the single-family UNI14 type, followed by duplex apartments -Block A-, linear blocks -Block E-F- and finally apartment towers -Block Q (Figs. 23-25). The most compact buildings -E-F and Q- consume less. The type of housing affects consumption; the more compact a building is, the less energy it losses is. This is so because there is less skin in contact with the outside, a key situation to achieve comfortable spaces with a minimum consumption.

Current global study of the neighborhood proves the use of significant amounts of energy with its corresponding CO₂e rates and a considerable economic cost. It is assumed, based on the studies, that most houses present discomfort.

8.1. Block A Efficiency

Block A is a duplex building. In all the three time points established -0, 25, and 50 years-, the reduction of emissions is evident along with the economic costs over years 25 and 50 (Table 3 Block A Efficiency. CO₂e emissions and Economic Cost Reductions after Renovating the Skin of Block A Buildings, years 0-25-50).

The table shows the evolution -resulting from the difference between the building without any renovation and the various renovation proposals- CO₂e emissions, with the use of envelopes of various thickness for the years proposed above, and the speculative energy price increase. Year 0 is identical for all the constructive proposals and economic scenarios, because this is the year in which the investment has to be made to renew the facade.

The mid-term results year 25 using constructive solutions T1 (overlying) and T2 (ventilated facade) with wooden structures and 7 to 15 cm insulation thickness, show reduction values for CO₂e emissions, being a little more elevated if 15 cm insulation thickness is used. In scenarios with differences of 3 and 6% in the cost of the stored energy, deficit seems to occur. In the case of an alleged increase in the cost of energy by 10%, the graph shows, in year 25, a benefit obtained from the investment made. (Table 3) Block A Efficiency. CO₂e emissions and Economic Cost Reductions after Renovating the Skin of Block A Buildings, years 0-25-50).

For year 50, it is evident that the use of solutions T1 or T2, with wooden structures and 15 cm insulation thickness, would achieve a better relation between reducing CO₂e emissions and recovering investment. In the three alleged economic scenarios, the recovery of investment is shown in the medium term.

Constructive solutions T1 (overlying) and T2 (ventilated facade) using aluminum structures, show a high initial investment in CO₂e, which will be stabilized over time, but will always be higher than the results showed by the use of wooden structures. The economic cost in this case will be very similar to the cost of using wooden structures and its amortization will produce similar values between them.

A summary of the above analysis would consist in relating the building skin renovation investment and reducing CO₂e emissions over time -in this case, year 50-. Therefore, it is possible to evaluate the results obtained and relate them to the angle occurring when point zero is joint to the studied point and the abscissa axis. The widest angle indicates the optimal renovation choice and the narrowest angle indicates the most deficient choice. If both angles are known, then the rest of the angles will give us guidance on the most feasible proposal.

For Block A (Table 4 Block A, Efficiency between Renovation Investment and Reduction of CO₂e Emissions in Year 50, and Figs. (35 and 36)), graphs show that the widest angle occurs using wooden structures with 7cm insulation thickness in either of the constructive proposals. The narrowest angle is produced using aluminum structures with 40 cm insulation thickness in solutions T1 (overlying) and T2 (ventilated facade).

The most efficient point between investment and reduction of CO₂e emissions is proposal T2, using wooden structures and 7 cm insulation thickness to reduce emissions by 1,235,423 kg CO₂e with an investment of €395,198. Angles in proposal T2 (ventilated façade), with 7, 10 and 15 cm insulation thickness are very close, as shown in the graphs (Fig. 37), in which 15 cm insulation thickness is to produce a greater reduction of CO₂e emissions, but with a slightly higher economic cost.

8.2. Block E-F Efficiency

Block E-F is a linear block with 4 staircases and two apartments per floor. Year 0 is when renovation investment occurs and investment shows deficit the renovation process will produce more emissions than the ones emitted in an entire year. However, after that in the years to come, the reduction of emissions will be evident as well as the economic cost (Table 5 Block E-F Efficiency. Reduction of CO₂e emissions and economic costs after Renovating the skin of a Block E-F building, Years 0-25-50).

The table above shows the evolution of CO₂e emissions, with the various insulation thicknesses for the years studied and the three assessed energy increases. Year 0 is identical for all the constructive proposals and scenarios, since it is the year in which investment is made to renovate the facade.

In year 25, using wooden structures in both solutions T1 (overlying) and T2 (ventilated facade) and 7 -15 cm insulation thickness, CO₂e emissions reductions are very close to each other, with the highest change given by constructive solution T1 if 7 cm insulation thickness is used. The use of aluminum in both proposals, T1 (overlying) and T2 (ventilated facade), show a wider gap between both proposals when using wooden structures, and even some values are negative. From the economic point of view, the constructive proposals will be in deficit, saved on the assumption of an annual increase of 10% in the accumulated price of energy.

For year 50, the table shows that by using wooden structures for solutions T1 (overlying) and T2 (ventilated facade) and 15 cm insulation thickness, a better relationship between the reduction in CO₂e emissions and return on investment would be achieved, followed, at a short distance by the same type of solution and aluminum structures with an insulation thickness of 10 cm (Table 5 Block E-F Efficiency. Reduction of CO₂e emissions and economic costs after renovating the skin of a Block E-F building, Years 0-25-50.

Constructive solutions T1 (overlying) and T2 (ventilated facade) using aluminum structures show a high initial investment in CO₂e emissions, which is gradually balanced over time, but will always be higher than the results provided after using wooden structures.

As in case A, we relate the investment in the renovation of the building skin and the reduction of CO₂e emissions over time -50 years.

In Block E-F (Table 6 Block E-F, Efficiency between investment in renovation € and CO₂e emissions reduction for year 50 and Figs. 38 and 39) the graphs show that the widest angle occurs when using wooden structures and 7 cm insulation thickness in any of the constructive proposals. The narrowest angle occurs when using aluminum structures with 40 cm insulation thickness in solutions T1 (overlying) and T2 (ventilated facade).

The most efficient point between investment and reduction of CO₂e emissions will be proposal T1 using wooden structures and 7 cm insulation thickness to reduce CO₂e emissions by 911.097 kg with an investment of 444,375€. The angles of proposal T1 and proposal T2 with 7, 10 and 15 cm grades of are very close, as shown in the graphs (Figs. 38 and 39). Proposal T2 would get the maximum reduction of CO₂e emissions with 941,270 kg but with a significantly higher cost of 941,270 €.

8.3. Block Q Efficiency

Block Q is a ground floor tower with 14 floors and four apartments per floor. CO₂e emissions in year 25 will be reduced only by using wooden structures in both constructive proposals T1 (overlying) and T2 (ventilated facade). In year 50, the two constructive solutions will provide positive results, although the proposals using aluminum structures are at a considerable distance from the rest (Table 7 Block Q Efficiency. CO₂e Emissions and Economic Cost Reductions after Renovating the skin of a Block Q Building, Years 0-25-50).

The table shows that in the medium term -year 25- when investment is yet to be returned, for all the constructive solutions and insulation thickness, and with a difference in the accumulated annual energy costs in the 3%, 6%, and 10% proposed scenarios. This is so, because Q-type buildings, due to their constructive features -compact buildings with a balanced ratio of solid parts and openings- show heating costs of 9,935€ without being intervened. In case of an intervention using any of the constructive solutions, either wooden or aluminum structures, with 15 cm insulation thickness, the cost will be 4,678€ per year. Therefore, economic investment will require a longer time for repayment.

For year 50, it is evident that the use of solutions T1 (overlying) and T2 (ventilated facade) with wooden structures and 10-15 cm insulation thickness, would show a better relationship between reducing CO₂e emissions and obtaining a return on investment between 6% and 10% of the differences of annual cumulative energy costs.

As in previous cases, the investment in the renovation of the building skin and the reduction of CO₂e emissions is related over time -50 years.

For Block Q (Table 8 Block Q, Efficiency between Renovation Investment and Reduction of CO₂e Emissions in Year 50 and Figs. (39 and 40), the graphs show that the widest angle occurs using wooden structures, and 7cm insulation thickness in either of the constructive proposals. The narrowest angle occurs using aluminum structures and 40 cm insulation thickness in solutions T1 (overlying) and T2 (ventilated facade). For solution T1, emission reduction is minimal.

The most efficient point between investment and CO₂e emission reduction will be proposal T1-7 and T2-7 with similar investment and reduction values -T1/7 428.668€ - 701.00 kg CO₂e and T2/7 433.808€ - 701.357 kg CO₂e. The angles of the values between 7 and 15 cm insulation thickness using wooden structures are really close. The proposal that reduces CO₂e emissions the most is proposal T2, with 15 cm insulation thickness and a slightly higher cost.

8.4. Block UNI14 Efficiency

A UNI14 building contains 14 single-family attached houses, each one with three levels, which originally had a level that was used as a workshop or production area, intended to render community services for the inhabitants of Montbau.

CO₂e emissions show negative results in the graph for years 25 and 50, using any of the constructive solutions, T1 (overlying) and T2 (ventilated facade), and aluminum structures and insulation of any thickness. Same thing happens if wooden structures and insulation of any thickness are used for year 25, with constructive solution T2. For year 25, making improvements in year 50, solution T1 will show positive results if wooden structures and 7-15 cm insulation thickness are used, a proposal that would prove a greater beneficial impact in the medium and long terms. (Table 9 Block UNI14. Efficiency. CO₂e Emissions and Economic Costs Reductions after Renovating the skin of a Block UNI14 Building, Years 0-25-50).

Negative results are caused by the high amount of envelope single-family houses have -the roof in this case-, which would require a higher initial investment in relation to the volume and inner surface of the building.

For proposals T1 (overlying) and T2 (ventilated facade) T2, using aluminum structures and insulation of any thickness, there is a deficit in the initial cost of CO₂e emissions investment made for the renovation of the envelope. The economic costs will be negative in the medium term -year 25- in any of the scenarios in which the cost of energy is increased and using any of the proposed solutions. The numbers will be positive for year 50 in increase scenarios of 6% and 10% and constructive solutions T1 (overlying) and T2 (ventilated facade) using aluminum and wooden structures and insulation of any thickness.

It is evident, for year 50, that by using solutions T1 (overlying) or T2 (ventilated facade), with wooden structures and 10 cm insulation thickness, a better relationship between CO₂e emissions reductions and return on investment would be achieved.

In the case of single-family houses, lower emissions and costs reductions will occur if compared to the rest of the buildings studied. The greatest amount of skin surface linked to a higher investment in renovation would produce more accurate results.

As in the previous cases, we relate investment in the renovation of a building skin and the reduction of CO₂e emissions over time -50 years-.

For Block UNI14 (Table 10 Block UNI14. Efficiency Between Renovation Investment and CO₂e Emission Reduction in Year 50 and Figs. 41 and 42), the graphs show that the widest angle point, therefore the most efficient, occurs when using wooden structures with 7 cm insulation thickness for proposal T1 (overlying) with a cost of 163,814€ and CO₂ emissions reduction of 187,113 kg. The graphs (Figs. 41 and 42) show negative angles indicating a higher initial investment of CO₂e emissions for materials used in the construction, than the benefits obtained in such period.

Proposal T1 (overlying) produces a greater reduction in CO₂e emissions within the whole set with a minimum investment difference if compared to the most efficient one which is 4,716€.

8.5. Efficient Renovation of Montbau

At this point, it is time to analyze the overall performance of the neighborhood of Montbau and its buildings. We start with the selection of the most efficient solutions and extrapolate the remaining buildings per square meter, studying their status in the medium term -year 25- and normal term year 50.

Depending on the point of optimization, the constructive solution is selected, as well as the materials for the structures and insulation thickness in order to establish optimal reduction rates for all the neighborhood.

The analysis of efficiency has led to propose more effective solutions for the conditions established in this studio. For type A duplex buildings, the most efficient constructive solution is T2 (ventilated facade), using wood materials and 15 cm insulation thickness. The same constructive proposal is useful for Linear Blocks E-F and Q Towers. Finally, for Block UNI14 or single-family houses, the most effective constructive solution would be solution T1 (Overlying) using wooden materials and 10 cm insulation thickness.

The results for the four types of buildings will be extrapolated to the remaining buildings in Montbau, to get an overall picture of what jointly acting would be like in this residential neighborhood.

We extract the values of heating energy consumption in kWh and CO₂e emissions to choose the evolution in the cost of energy from 6%, as it is an intermediate situation between cheap energy, 3%, and a significant increase of 10%. (See Table 11 Montbau - like buildings with no renovation, year 25, and Table 12. Montbau-like buildings with no renovation, year 50)

The values per square meter are extended to other buildings in the neighborhood, by setting two tables with the values of buildings that are not renovated for the years 25 and 50 (Tables 13 and 14 Summary of non-renovated buildings in the neighborhood of Montbau) and two additional tables with the values of buildings renovated for the years 25 and 50 (Tables 15 and 16 Summary of buildings renovated with optimal proposals in the neighborhood of Montbau).

In year 25, consumption would be 179,776,348 kWh, with CO₂e emissions released into the atmosphere equivalent to 49,498,919 kg and an energy cost, with 6% price increases, of 28,150,418€ in case 1,959 houses and apartments within an area of 160,406 m² were not renovated. If we do the exercise for year 50 -non-renovated houses and apartments- the total energy consumption would be 335,603,543 kWh, with CO₂e emissions released into the atmosphere equivalent to 97,093,345 kg, and a cumulative energy cost of 146,925,283€. Accumulated consumption values and emissions are doubled over time and economic costs are fivefold.

By Renovating with the optimal values estimated in this chapter -which include the necessary renovation energy, gas emissions released into the atmosphere, and their economic cost in year 2012- the following results are obtained for year 25: energy consumption equal to 106,051,437 kWh, 31,531,808 kg of CO₂e emissions, and a price increased energy cost of 6% equal to 41,993,197€. The same variables for the year 50 are 157,999,235 kWh in energy consumption, 46,743,719 kg of CO₂e emissions, and a cost of 82,410,581€ in energy. Between one and the other date, the increase in energy consumption and CO₂e emissions is approximately 49%, and costs almost doubled between the two dates.

The results summary table (Table 17 Overall summary of renovated and non-renovated buildings scenario in the neighborhood of Montbau) shows that in the first 25 years, with the proposed improvements to buildings, reduction in energy consumption is 41% and 36% in CO₂e emissions. The cost of energy shows a deficit of 49% caused by the initial investment in the renovation of facades. The economic value may vary, as already mentioned, depending on energy prices. On the other hand, the energy consumption reduction values and reduced emissions of greenhouse gases far exceed purely economic values.

When comparing the scenario in year 50, for non-renovated and renovated buildings, there is a decrease in energy consumption of 53%, CO₂e emission reductions of 52%, and a decrease in energy costs of 45%.

Renovating the envelope of buildings using an optimal solution between investment costs and results can lead to reductions in consumption, emissions, and economic costs over 50% of consumption in non-renovated buildings. Usage values are considerably reduced, but initial investment values impact the overall results. Overall results confirm the need to act on the envelopes of the buildings and the benefits in the medium-term -year 25- and normal-term -year 50-.