Energetic and Functional Rehabilitation of Residential Buildings in Europe: Analysis and Cataloguing of the Strategies Used

: The aim of this paper is to explore the rehabilitation strategies for multi-family dwellings on the level of function and techniques. The study employs its own methods of analysis using a sample of selected cases as a reference. Nearly 20% of EU buildings will have to be renovated by 2023, as almost 40% of the existing houses were built before 1980. The environmental impact of construction is among the highest among industrial activities due to the high consumption of resources and the generation of low-value waste at the construction and demolition stages. One way to reduce the environmental impact in this sector is to intervene in the building process, optimising the use of resources and waste generation. The principal objective of refurbishments is to reduce household energy consumption. However, the renovations in the housing sector should not be limited to energy considerations; the functional and technical aspects should also be considered. A new refurbishment model is necessary to focus on providing and improving the habitability of the housing stock and reducing the environmental impact by optimising the use of resources and waste generation. To achieve this, the renovations should be carefully monitored.


Introduction
The aim of this study is an analysis of innovations in the redevelopment of multifamily housing, driven by the need for energy-oriented rehabilitation of the existing stock, on the level of function and technique.It uses a sample of contemporary rehabilitation cases as a reference.Moreover, it introduces a systematic analysis method developed for the study of such cases and the representation of the criteria.

Obsolescence of Housing Estates
Between 17 and 22% of EU buildings will have to be renovated by 2033, i.e., up to 40 million buildings [1].Approximately 40% of the existing buildings had been built before 1980, according to the Building Performance Institute Europe.This shows the age of our residential building stock, which was built quickly, cheaply, and with minimum comfort standards.Moreover, as they had been built before the introduction of the first basic standard on thermal conditions in buildings in the eighties (Spain in 1979 with the NBE-CT-79, Germany in 1973, France in 1974, Italy in 1991, etc.) [2], the application of thermal insulation in the envelope had not been considered.
The housing stock built before the 1980s comprises residential and developmentalist buildings.They have not aged well and do not meet the expectations of modern society.Among other factors, this is due to the physical deterioration caused by inadequate conservation, the low culture of maintenance by the citizens, poor quality, and typological decadence.All this explains the obsolescence of the urban peripheries, one of the greatest challenges facing the contemporary city and public administrations, as it is a clear example of vulnerability [3].
The buildings in these residential estates are unsuitable for the intended purpose due to their high degree of obsolescence.Firstly, their current functional obsolescence, caused by a lack of building regulations, particularly affects their energy consumption and is accentuated by the social changes that have taken place since the houses were built.Secondly, their physical obsolescence, due to the poor quality of the installations and lack of maintenance, is evident.
Despite their current condition, these housing estates from the era of developmentalism have great potential to become the object of rehabilitation.Urban segregation, apparent at the time of their construction, has been compensated by the continuous urban growth, bringing the consolidated city close to these estates by occupying the previously existing voids.Some of such estates are already integrated into the adjacent urban grid, occupying favourable and valued positions relative to the city centre, with ease of transport and participation in urban life.

Environmental Impact of Building
The calculation of the ecological footprint for the entire planet, based on seven indicators, carried out in the year 2000, showed that around 164 million units have been consumed, while the biocapacity of the planet was only 125 million units, which means an overconsumption of 31% [4].The environmental impact associated with construction consists of two main elements: the effect of the operational energy (using the building) and the effect of the construction of this embodied energy.During the useful life of a building, the impact of using it accounts for 60 to 70%, and the impact of construction makes up the remaining 30-40%.The relative proportion of consumption types changes with the coming into force of successive energy efficiency regulations and the incorporation of renewable energy systems, creating a new scenario.By reducing the energy used by the building, the weight of the energy due to the construction will increase considerably, reaching the point of inverting the ratio.The only impact associated with the zero-consumption buildings will be due to the energy utilised during their construction.
The impact of construction depends on the materials used, the consumption of resources, and the generated waste.Construction and demolition waste (CDW) is the biggest waste stream in the EU by weight for over 800 million tonnes per year, i.e., around 32% of the total waste generated (as a result of excavation, execution of works, and demolition) [5].The amount of demolition waste is larger than in the construction phase, with a ratio of 8 to 1. Waste generation is a critical component in the evolution of construction technology, as it infringes the new sustainability rules [6].The Waste Framework Directive sets a target for 70% of preparation for re-use, recycling, and other material recovery for this waste stream.Closing the life cycle of materials (resources) is the only possible strategy to reduce the environmental impact and thus guarantee the fulfilment of the current requirements without compromising the needs of future generations [7].Closing the life cycle means reincorporating the waste into the production cycle (by recovery); at the same time, economic revaluation should be performed to ensure that the consumption rate is lower than the rate of recovery and replenishment of resources.
The stock of available buildings is a resource we must introduce into the cycle to take advantage of the embodied energy those buildings contain.Thus, the way to reduce the overall environmental impact of the sector is to use the existing building stock."The sustainability requirements imply changing the current dynamics of the building sector to redirect it towards rehabilitation.Such rehabilitation should be focused on maintaining and optimising the available habitability of the existing building stock by providing the fair and adequate utilities needed by the people; this should take the urban scale as a field of intervention and close the cycles of materials in all the technical processes involved", M. Poble Noguera [8].

Rehabilitation as a Necessary Strategy
Rehabilitation means betting on environmental improvement, i.e., implies recycling of the building environment.The rehabilitation potential of the large existing housing stock, in terms of contribution to CO2 emission savings, is very high, and it has been proven to be one of the most effective strategies.It reduces the need to demolish, represents significant cost savings, and decreases material consumption and local impact.The generalisation of comprehensive renovations, reducing the weight of minor or maintenance renovations, still the most common today, is the fundamental way to improve the sustainability of the buildings.Moreover, such comprehensive rehabilitation must comply with energy efficiency requirements, achieving the lowest possible energy consumption and reducing pollution [4].However, this process cannot currently be redirected exclusively to promote energy rehabilitation but must serve as a lever for qualitative, technical, and functional rehabilitation.The main challenge to be faced is rehabilitating a specific fabric of our cities, the neighbourhoods of open-block or tower housing estates built in the 1960s and 1970s.Raising them from their current obsolescence is one of the greatest challenges facing the contemporary city.

Theoretical Framework: Evolution of Rehabilitation from the 1980s to the Present
Many residential estates were built during the first decades after the Second World War due to the shortage of housing in Europe caused by the war, migration from the countryside to the cities, and a rapid increase in population.Massive housing production units were created, following the example of the Modern Movement [9].However, these estates were built very quickly, using limited materials, resulting in final products with many functional and technical deficiencies [10].
In the 1970s, a paradigm shift took place in Europe, moving away from residential estates built on a massive scale to focus on rehabilitation.The turning point came with the oil crisis of 1973, which marked the end of the post-war period of growth and the beginning of the concern for energy saving.Among the measures taken, the International Energy Agency (IEA) [11] was created, giving rise to the first energy regulations for buildings.During the same period, the rehabilitation of the residential estates began, promoted by the strong demands of the neighbourhoods [12], for which significant resources were allocated for the repair of the houses.The first repairs consisted of mechanical rehabilitation, focusing on fixing the damage caused by deficient construction.First, the structural stability of the built complex was ensured by repairing, reinforcing, and/or stiffening the structural elements.Secondly, the health standards and habitability of the flats were guaranteed by modifying the existing plumbing, electricity, ventilation installations, etc.
Around the year 2000, the high energy consumption and CO2 emissions of these buildings became evident, and energy rehabilitation began.Work on the building envelopes was undertaken to reduce energy losses and improve the performance of the installations.
Despite these interventions, the urban morphology of these building complexes and their typologies in relation to their immediate surroundings, the growing degradation of the built stock, and the stigmatisation of the neighbourhoods [13] have provoked a process of marginalisation and rejection.It has led to the creation of an urban renewal policy with the aim of demolishing old stock and building new houses [14].In response to the French national programme committed to demolishing the 1960s and 1970s buildings, in spite of the lack of public housing, a new approach to the problem has been proposed by Lacaton, Vassal, and Druot [15].In their book "PLUS.Les grandes ensambles de logements, territories d'exception" [16], they rejected the demolition, supporting the transformation instead; they are convinced of the potential represented by this type of buildings, structurally, geographically, and spatially.They coined the phrase "never demolish, never remove or replace, always add, transform, and reuse" and put it into practice in their rehabilitation projects [17].The projects took advantage of being transformed from sustainability and, in addition, proposed functional improvements in the homes.In Germany, there is also a movement that encourages architects to design "Reduce, Reuse and Recycle" projects for existing buildings [18].
Such buildings are not normally demolished because they have reached a critical phase in terms of structural stability, but due to their obsolescence (functional, technical, and/or aesthetic) and the desire to construct new buildings with greater profitability which satisfy the current demands.We can avoid demolitions and take advantage of the existing stock, reducing the environmental impact of the extraction of raw materials and the disposal of non-recyclable waste.To achieve those goals, research and innovation activities are being promoted and funded by European institutions and programmes [19][20][21][22][23].The common objective is to improve the quality of the building envelope, reduce energy consumption, and guarantee the comfort of users.Moreover, rehabilitation should be evaluated using prefabricated modules in pilot projects in different European regions.Currently, the ENSNARE (envelope mesh and digital framework for building renovation) project is being developed.Its aim is to increase the use of renovation packages by digitalising the entire process and creating a new industrialised envelope, allowing a rapid assembly and interconnection of multifunctional and passive building components [24].The project is being developed within the Horizon 2020 programme in three pilot projects in Europe.
The movement of well-known architects in favour of rehabilitation, the proliferation of research into the process, and the promotion of industrialisation have led to a boom in renovations.Avoiding obsolescence is a pretext for introducing functional and technical improvements that solve the deficiencies.
The current study combines recent and innovative interventions for the rehabilitation of residential estates in an extensive sample of European cases.It analyses the elements of interventions (taking into account only interventions in passive and non-active elements), the techniques used and their consequences.To this end, a specific, systematic analysis system is generated to scrutinise the strategies and the technology used.It captures the data in a synthetic way, facilitating comparisons.

Methodology
In addition to energy consumption considerations, mainly involving the façade, the residential estates of the post-war period have other functional and technical shortcomings.Functional impairments manifest themselves on two levels.Firstly, at the urban level, due to the monofunctionality of these large complexes, designed only for habitation.Secondly, at the housing level, the homogeneity of their interior distribution and the typology is suitable for a single type of family occupying the minimum surface area and only essential rooms.The technical deficiencies have been caused by the excessive speed of construction, using the most economical way [25], and the lack of appropriate regulations at the time of building.
For these reasons, the energy rehabilitation of this particular building typology must consider functional aspects depending on the type of housing and typological diversity and eliminate monofunctionality.Moreover, interventions in the envelope have to take into account the poor construction and functional quality of the envelope, as well as its thermal quality; the systems that improve all these aspects must be considered.The introduction of functional and technical considerations in the rehabilitation of residential estates is unavoidable; these are the aspects that we want to verify in the contemporary interventions examined here (our sample of cases).
The methodology used in this research is illustrated in Figure 1, showing its different phases.Four phases can be identified: the definition of the reference sample, the study of the sample and acquisition of the data, the creation of an analysis system, and, finally, verification of the system and obtaining of results to be discussed.

Case Selection
Six fundamental criteria were considered when selecting the case studies to include in the sample to be analysed (summarised in Table 1).The first three criteria are associated with the existing buildings, and the second set of three is related to the rehabilitation carried out.First, the buildings must have been constructed in the period covered by the investigation (from 1950s to 1970s), with some exceptions for particularly interesting specific cases.The buildings, i.e., linear blocks or towers, were typical of the European constructions erected during this period.Secondly, as the objective of the study was to gather information on the rehabilitations conducted in recent years, the chosen interventions were contemporary.Some exceptions were allowed for cases of particular interest.Regarding the technology used in the renovations, the search focused on rehabilitation cases using industrialisation.The final criterion was that the renovations were not limited to improvements of the envelope and/or the accessibility of the building but that they contributed something more, went a step further; these were the cases where the need for rehabilitation was used as an opportunity to transform the building, improving its habitability.
The cases are presented in Table 2, with the most relevant data of each project.The plan shown in Figure 2 shows the location of each case in Europe, marked with an X, and an arrow that refers to the case or cases in that location (represented by an icon composed of a representative image and a reference).The cases have been found mainly in Germany, Austria, Denmark, Spain, Finland, France, The Netherlands, the UK and Switzerland.The tables that show the most relevant data necessary to form an overview of the 42 selected case studies and illustrate the general idea of the whole sample can be found in Appendix A. In the tables, you can find the bibliography for the study of each specific case and specific information about the building.

Summary of the Obtained Sample
Firstly, the sample was made as heterogeneous as possible, and the cases met the six requirements set out above.The tables below show the characteristics of the sample obtained after an exhaustive search for current rehabilitation projects (employing the six criteria).First, the factors associated with the existing building were taken into account.Table 3 shows that the cases were built in the studied post-war period, with more than half built in the 1960s during the housing construction boom.The buildings erected were usually linear blocks (81%), the type most commonly used throughout the study period.The height was pB + 4 (ground floor + 4 floors above) the maximum construction height permitted without a lift.This building type is characteristic of the decade to which most studied cases belong (1960s), before the introduction of the tower typology (22%).Cases involving terraced houses have been included due to the interest in the technology used in the research development.Finally, a large majority of the cases, 62%, are located in Central Europe (Germany, Austria, The Netherland, and Switzerland).More than half of the cases are in Germany, 26% are in Southern Europe (Spain and France), and 12% are in Northern Europe (Denmark, Finland, and the UK), as shown in Table 4.  Secondly, the criteria for the rehabilitation type were also followed.Table 5 shows that, in almost 75% of the cases, the intervention was carried out in the last 10 years.The numbers of cases using the industrialised and the conventional systems are almost equal (Table 6); cases using industrialisation in the construction process were actively sought."Industrialised" construction refers to a construction system using the components manufactured in the factory; once assembled, they are transported to the construction site, where they are placed in their final position.It is a dry construction with mechanical joints.In contrast, conventional construction is executed in situ after transporting the different materials to the site, where they are manipulated and assembled.It is a wet construction with chemical seals.
Finally, it seemed interesting to compare the types of tenure in the different cases.The vast majority, almost 70% of the cases, are of public tenure.This type of tenure facilitates renovations and is the main type of tenure for this type of building in European countries, unlike Spain.Source: Own data.

Interventions Identified in the Sample
Various interventions identified in the case studies have different objectives and affect different areas of the building.They can be divided into 4 sections: the envelope, the built volume, the housing space, and the community space.The four sections are interrelated, as interventions in one section may affect elements included in another.Thus, for example, changes in the volume of the building will cause an obligatory modification in its envelope and possibly in the configuration or number of dwellings.The chosen technology plays a fundamental role as it has been one of the requirements in the selection of cases.However, the technology used is not a strategy in itself but a way of carrying out the interventions, which is why it is not included as a section.Once the strategies are identified, the technology will have to be studied within each intervention.
The interventions in the 42 cases of the sample are represented in a tree-type scheme that hierarchises and classifies them according to the criteria of the authors.In the graph (Figure 3), the four sections are hierarchised and differentiated.The different strategies are covered by each section, called first-level interventions, and the objective of each one is given (functional objective, energy objective, and/or both).Achieving the functional objective will improve the use of the building, and the energy objective will improve its energy performance.Interventions in which specific attention is required are highlighted.The analysis of the used construction systems, conventional or industrialised, acquires importance in the envelope interventions, the increase in volume, and the renovation of the house, represented in the graph by highlighting these interventions with a dotted line and *technology.
A set of profile cards was put together (Figure 4) to summarise all the strategies; they were catalogued, described, and labelled using icons.The aim was to represent each rehabilitation project in a synthetic and visual way to facilitate comparing different projects.The card is made up of four identifiable elements.The case is identified using the reference, name, and icon of the project.The information for the rehabilitation project is presented in two different parts.To the left are four sections, into which the interventions are divided, indicating the interventions carried out.To the right of each section, the general or specific strategies identified are represented, highlighting those carried out in each case.
The strategies are represented by icons.These icons (as well as the square accompanying each intervention) are highlighted in blue when the strategies are included in the rehabilitation project.Various colours are used to pinpoint the different construction systems.Thus, the conventional systems are marked in blue, while the icon for industrialised systems will be highlighted in green.Below, there are the icons (Figure 5) and a couple of examples of project profile cards from the sample to show how they work (Figure 6).

Featured Interventions
In the set of identified, analysed, and structured interventions, certain strategies are of great interest to the research.Strategies that concern the building envelope are unavoidable due to its impact on the climatic conditions of the houses.Moreover, some strategies try to tackle functional obsolescence from the point of view of current society and its way of life.Therefore, the interventions highlighted are the envelope, increase in volume, and type of housing, which are among the most common in the sample.Approximately 30% of these used industrialised systems, which were not employed in modifications of the flats.
The state of the existing envelope affects the decision to renovate or construct a new envelope.In this particular study, the façade rehabilitation and the construction of a new envelope is considered in cases of total or partial demolition of the existing façade.Thus, rehabilitating the existing envelope could mean replacing it with a new one located in the same place.However, constructing a new envelope as a direct consequence of the modification in the built volume is not considered at this point but is included in the interventions in the built volume.The rehabilitation of the envelope includes rehabilitation of the façade and the roof.Nevertheless, the information on the rehabilitation of the roof provided by the architects is scarce, and in cases where it is specified, the strategies do not provide any noteworthy innovation.Consequently, the most noteworthy strategy highlighted in the intervention of the envelope is the rehabilitation of the existing façade using industrialised systems.
The variations in the built volume include new construction and increasing and decreasing the surface area.However, while the increase in surface area is carried out in almost 90% of the cases in the sample, the new construction and decrease in surface area is found in approximately 15% (Figure 7).Therefore, the main strategy seems to be increasing the surface area.This strategy affects other aspects of the building, such as constructing a new envelope for the increased space and typological changes in the house to which a new space is added.It is worthwhile to study the technology used for the construction, the type of spaces it generates, and its effect on or relationship with the existing housing typology.Therefore, such intervention is of interest from the point of view of function and technique.The housing rehabilitation can include changes in the number of dwellings, their renovation, and modification of their type.The interest of the renovation lies in the use of industrialisation in its process; however, none of the cases in the sample make use of such technology, or they do not specify it (Figure 8).Therefore, the highlighted strategies will be the modification of the number of dwellings produced by the division of existing buildings, which implies the modification of the existing typology and the introduction of typological variety in the building.

Significant Strategies
The interventions highlighted in the previous section were subjected to a more exhaustive analysis in cases that used them as part of their rehabilitation projects.This allowed us to follow their functional and technological development and pinpoint any predominant lines of action.

Rehabilitation of the Façades Using Industrialised Systems
The rehabilitation of the façades of multi-family housing buildings is a primary objective on the agendas of institutions to comply with the currently established energy efficiency requirements.This type of renovation is a common practice to improve the thermal performance of the building; it is mostly carried out using conventional techniques, ETICS (External Thermal Insulation Composite System) and/or ventilated cladding.The analysis focused on the rehabilitation of the façade using industrialised construction systems.The process can be carried out by replacing or superimposing the existing façade.Table 7 shows the ten cases that rehabilitated the façade using industrialised systems; these were the object of study in terms of the technology they used, the characteristics of the façade of the original building, and the fastening system.The materials, function, and state of the existing façade set the course of action in a rehabilitation project.When the façade has a structural function (i.e., it is load-bearing), it cannot be demolished.Then, the rehabilitation is carried out by superimposing the prefabricated panels on the existing façade.In such cases, the cladding, insulation, and exist-ing carpentry of the façade are removed, and the load-bearing function element is preserved.Then, thin insulation and wooden battens are superimposed.This allows the irregularities to be smoothed out; new panels can then be fixed to form the enclosure.In some cases, the openings can be enlarged without affecting the load-bearing function of the façade.However, if the façade has no structural function and only needs to meet enclosure requirements, then it is rehabilitated through superimposition and/or replacement, depending on the materials it comprises.First, the superimposition is performed on the façades composed of concrete sandwich panels (Table 7).The first layer of concrete and the insulation were removed, and a prefabricated panel was superimposed on the inner layer of the concrete sandwich.In both cases, the connectors had badly deteriorated (which is why this decision was made).Second, in the substitution of non-load-bearing façades made of lightweight sandwich panels, e.g., PVC, the existing façade is completely demolished, and a new enclosure is built; this strategy allows the configuration of the façade to be changed.Consequently, the rehabilitation strategy will depend on the type of façade of each specific building, its materials, function, state of deterioration, and the need or desire to modify the existing openings (Figure 9).The predominant technology used in façade rehabilitation is the industrialisation of large wooden panels, as shown in 10 of the 11 cases using industrialised construction systems (Table 4).This technology stands out mainly because of the lightness of the materials, which avoids the need to reinforce the structure and for the speed of assembly.The prefabricated wood panels are based on a light framing system.It consists of a structure made up of wooden uprights between which the thermal insulation is placed, and on top of these, an OSB board is fixed to stiffen the panel.However, there are some differences in the orientation of the panels, their dimensions, and the degree of prefabrication and the final cladding of the façade.These differences are shown in Table 8.The orientation of the panel in its final position can be horizontal, in which case the height is always equal to the height of a floor of the existing building (and with variable length), or vertical, with the height equal to the total height of the building and the width (or length) is variable.The orientation of the horizontal panels makes it possible to cover the entire front of the house with a single panel, facilitating sequential rehabilitation of dwellings (Figure 10), and does not limit the size of the windows of the houses, a fact that becomes important in cases where the renovation is carried out by replacing the façade.The dimensions of the panels, regardless of their orientation, are 8, 10, or 12 m by 3 m (Table 8), due to the fact that the dimension of the panels is limited by transportation.The degree of prefabrication of the panels determines the work needed to be carried out in the workshop (to be completed on-site).There are three types of prefabrication levels: low, medium, and high.The lowest level of prefabrication only includes the assembly in the workshop of the wood panel and isolation.The medium level also incorporates carpentry and glass at a low prefabrication level.Finally, the high level of prefabrication supplies all the elements in the panel, including the carpentry with its solar protection (if any) and the façade cladding, reducing the work to be completed on-site to the installation of the panels in their final position and the sealing of the joints.Logic suggests that the appropriate way to reduce the duration and amount of on-site work is to use the highest possible level of prefabrication; so, once the panels are installed, the house is watertight and can be inhabited again very quickly if the tenants have had to leave their homes.However, this is not always the case in the examples included in the studied sample.The prefabricated panels used in rehabilitating the façade can be joined to the existing building, either ported or self-supporting.The ported panels are mechanically fixed to the load-bearing element of the building and the self-supporting panels have their own foundations, a new construction made expressly for this purpose.They are mechanically attached to the existing building to prevent tipping over and to transmit horizontal loads, allowing their movement relative to the existing building.

Increasing Surface Area
In almost 90% of the cases in the sample, the surface area has been increased in the renovation process.Analysing such cases can help us in scrutinising the ways in which industrialised construction systems are employed.It can also reveal how these spaces were incorporated into the existing building; for this reason, the analysis of the integration of outdoor spaces was omitted.The increase in surface area can be achieved by enclosing an existing space or by a new construction to add a new space to the built volume.
The enclosure of the existing space is very common in this building typology due to the small surface of the dwelling and is often carried out by the tenants themselves.This allows them to expand the useful area by, e.g., incorporating a balcony whose dimensions are insufficient for any particular separate function.The objective of a renovation is to incorporate these spaces to increase the interior area of the house and/or galleries or to homogenise the enclosure of such spaces already performed by some tenants according to their convenience.Moreover, on many occasions, this is accompanied by incorporating a new outdoor space for private use in each home to ensure that all the dwellings in the building can enjoy this type of space.The gallery space type involves the enclosure of the existing outdoor space by glazing around its perimeter.In this way, the space created can function as an outdoor space, gallery (greenhouse), or interior space; its use might change with the seasons of the year.The modification of its function will depend solely and exclusively on the tenants of the property.The gallery is not accompanied by a new outdoor space for the house since the gallery itself can function as such.The existing façade does not need modification since it already has an exit to the balcony; however, it can be improved by incorporating thermal insulation or increasing the glazed surface (Figure 11).The increased interior areas have a greater impact on the existing building.The new space is incorporated and involves a new façade and the modification of the existing façade.This modification, carried out to unify the spaces, might range from the elimination of the existing carpentry to the total demolition of the façade.However, the increase in the surface area is usually not very large, and the newly generated space is functionally integrated into the adjacent area, generally the kitchen, dining room, and/or living room.
The increasing surface area by the generation of a new construction involve to consider three aspects: What happens to the existing façade when an adjoining gallery-type or interior space is built?How does the increase in surface area affect the typology and/or number of dwellings?What technology is used in the construction of the additions?
What happens to the existing façade when a new space is annexed?The strategies differ depending on the type of space being added (interior or gallery) since the existing façade will acquire different functions.If the adjacent newly constructed area is an interior space, the existing façade becomes an interior partition without having to meet the requirements of the enclosure.The conservation or demolition of this element will depend on the function of the incorporated space and, above all, on its dimensions.Two types of solutions were implemented among the case studies: total demolition when it did not have a load-bearing function, conserving only the load-bearing element (if any); conservation without modifications, given that its demolition involves a technical and economic effort (Figure 12).When the newly built area is a gallery space, the existing façade must fulfil some additional requirements as it can function as an exterior, intermediate, or interior space.Thus, it is reasonable that it should meet the requirements of the most restrictive function, which is the envelope.Moreover, it must facilitate access to the new space that did not exist before, so the size of the opening has to be enlarged.When the galleries are incorporated into the built volume, the aim is to increase the glass surface of the existing façade.However, it is necessary to consider the climate and the orientation of the façade to choose the best solution in each particular case.The incorporation of new interior spaces means an increase in the surface area of existing flats and/or the addition of new dwellings to the building.The increase in interior surface area can be carried out in two ways.It can be performed by adding new dwellings without modifying the existing ones or by incorporating newly constructed space into the existing dwellings, thus expanding their surface area.On the one hand, adding new dwellings in vertical, attics, or horizontal increases involves a rise in the number of homes and the ability to take advantage of the existing passages and facilities in the building.This strategy entails an economic benefit.On the other hand, there are several possible strategies for enlarging existing homes by adding the newly created spaces, depending on the dimensions of the space to be incorporated and the adjacent rooms.If the surface area is large enough, then new rooms can be added to the existing typology (such as an additional bedroom or study).However, if the surface area is insufficient to add new separate rooms to the dwelling, then it can be incorporated into the existing spaces.Thus, one can choose to modify the space distribution in the house or maintain the same distribution with larger spaces.Generally, such modifications are used to update and expand the kitchens and bathrooms, which tend to be in the worst state.Finally, some cases use the increase in surface area to access the flats through these new spaces, changing the position of the vertical circulation included in the built volume to be able to add the elevator, ceding this space to the existing homes.
Among the studied cases, 65% resort to industrialisation to construct the elements to be added to the built volume (Table 9) in the structure or the enclosure.The study focused on the use of industrialised construction systems of different types, as is considered to be the innovation introduced in the rehabilitation works.In all cases, the structure generated by the increase in height was supported by the existing structure of the building and increased its load.In contrast, in the spaces created by horizontal expansion, in all the cases, the structures were self-supporting, constructed independently of the existing volumes but connected to them to avoid tipping over.Therefore, it stands to reason that there is a clear difference between the systems used for horizontal and vertical space expansions.An increase in height must be carried out using light construction systems, avoiding overloading the existing structure.Thus, the systems based on light materials such as wood are the most commonly employed.The predominant construction systems in use are prefabricated panels with light timber framing.Such lightweight construction systems combine structural and enclosure functions; however, they can only be used for a limited number of floors.In addition, as these are linear load-bearing elements, the existing structure is not overloaded at specific points; the loads can be distributed evenly.The level of prefabrication used in the studied cases was low, the panels are assembled on-site without joinery or cladding, only to supply a load-bearing function and to generate space; then, the work is completed in situ in a conventional way.Other construction systems use steel, e.g., prefabricated panels with steel mullions for the enclosure or three-dimensional modules with tubular steel structures.In these cases, the modules are highly prefabricated so that, once on site, only the joints are treated, and the cladding is added.
Extension in the horizontal plane is not limited by weight, which is reflected in the predominant construction systems used in such cases (Figure 13).A lightweight construction system can be replaced by a heavy one, such as prefabricated reinforced concrete elements, used in the structure.Prefabricated reinforced concrete elements have several advantages: the cost, the well-developed industry that ensures good finish, good fire resistance, structural efficiency, and the possibility of using recycled aggregates.Two exceptions must be mentioned: a case employing three-dimensional modules and another with a composite slab with steel profile structure, chosen due to the transport efficiency.The type of generated space, the gallery, is key to understanding the lack of industrialisation in the enclosure, while it is used in the structure.The enclosure of the space was totally translucent (glass or polycarbonate) to capture the sunlight in winter; specific carpentry was used to achieve it and, at the same time, to open up the space completely.Carpentry is not considered an industrialised element, even though it arrives at the construction site ready for assembly.

Types of Housing
This section discusses the strategies that lead to modifying the typology of existing dwellings and increasing the typological variety in the built complexes.The union, division, and modification of the houses modify the existing types.The difference between the three strategies is shown in Figure 14: the division and union of dwellings would change the number of dwellings after the renovation; the modification changes the typologies of the dwellings but does not modify their number; the implementation of any of these strategies will depend on the objective of the rehabilitation project and the surface area of the existing dwellings.The analysis focused on the cases employing the housing union strategies to modify the usable surface area per bedroom and/or to introduce typological variation in the building.The surface area of existing flats is decisive in this type of strategy.The area is expressed in m 2 /bedroom as it allows the square meters of the homes to be equalised, regardless of their typology.However, it is necessary to note that the types with a single bedroom tend to have a larger surface area per bedroom than the multiple-bedroom apartments.Joining the dwellings to increase their usable surface area is the most common strategy among the cases in the sample.The apartments in the buildings constructed in the studied period were rather small, and their rooms were often inadequately sized.The objectives of this strategy are to increase the surface area of the dwellings and introduce new typologies in the building.This will reduce the number of dwellings unless the existing surface area is increased by incorporating new flats created by a new integrated building.The unions can be carried vertically or on the same floor by merging the adjoining flats; the latter method is more commonly used.Only two cases merged the spaces vertically compared to ten joining the flats on the same floor.Table 10 shows the changes in housing typologies and their surfaces achieved by joining the dwellings.First, the change in the With the exception of two cases out of the fifteen that carried out the union and/or division of the dwellings, the interventions were used to incorporate new typologies and, therefore, increase the typological variety (when the original building only contained flats of one type).However, one of the two exceptions introduced typological variety by adding new apartments.Only a single case did not increase the typological variety in the building.After the renovation, six cases maintained the single typology in the building complex.Of these six cases, three are located in Spain; here, the tenure type is private, which makes it difficult to implement this type of strategy in rehabilitation projects.

Considerations/Conclusions
An intervention to improve thermal performance acts as a driving force for rehabilitation.However, renovations should not be limited to energy aspects but should also embrace the functional and architectural aspects.The building will remain in a state of obsolescence if only energy efficiency is improved.These functional aspects, as shown by the case studies, are considered a part of the rehabilitation intervention to modify the useful surface area of the dwellings and incorporate typological variety.Thus, their functional obsolescence is resolved, adapting to the current way of life.
Technological improvements in renovation mean increasing the level of industrialisation of construction systems to shorten construction times and, consequently, reduce inconvenience to tenants.Such systems make more efficient use of available resources, reduce waste generation, and help close the cycle of materials and components to reduce the environmental impact of the building.The rehabilitations of the studied sample prove that this is possible.A European collaborative project, E2ReBuild, is involved in creating an industrial platform for energy rehabilitation by introducing industrialised renovation methods and advanced renovation processes.
The analysis system generated here was very useful for the study of the sample, synthesising all the strategies using our own icons and symbols.This system could be used for the analysis of future cases and/or for decision making in rehabilitation projects.Moreover, as an open system, it is suitable for including new strategies necessary for other projects.

Figure 2 .
Figure 2. Location of case studies in Europe.

Figure 3 .
Figure 3. Interventions identified in the case studies.

Figure 4 .
Figure 4. Model of a standard profile sheet.

Figure 6 .
Figure 6.Example of cards used to represent the results.

Figure 9 .
Figure 9. Examples of envelopes rehabilitated using industrialised systems.

Figure 10 .
Figure 10.Examples of panel types used and their level of industrialisation.

Figure 11 .
Figure 11.Examples of increasing surface area using an existing outdoor space.

Figure 12 .
Figure 12.Examples of increasing surface area by new construction.

Figure 13 .
Figure 13.Examples of industrialised technology used to increase the surface area.

Figure 14 .
Figure 14.Examples of flat typologies and joining strategies.

Table 1 .
Criteria for the selection of case studies.

Table 2 .
List of cases in the sample.
Source: Own data, obtained from the study cases.

Table 3 .
Classification of the cases according to the decade of construction.
Source: Own data.

Table 4 .
Distribution of the number of cases by country.
Source: Own data.

Table 5 .
Classification according to the five-year period in which the rehabilitation intervention was carried out.
Source: Own data.

Table 6 .
Type of construction system used in the intervention.

Table 7 .
Summary of the cases rehabilitating the façades using industrialised systems.

Table 8 .
Characteristics of wood panels used in renovation.
Source: Own data.

Table 9 .
Technologies used to increase height.
Source: Own data.