Energy saving retrofit in a heritage district: The case of the Budapest

Journal of Building Engineering 27 (2020) 100982

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Energy saving retrofit in a heritage district: The case of the Budapest �ria Suga �r a, *, Attila Talamon a, Andra �s Horkai b, Michihiro Kita c Vikto a

Hungarian Academy of Sciences, Centre for Energy Research; Szent Istv� an University, Ybl Mikl� os Faculty of Architecture and Civil Engineering, 29-33 Konkoly-Thege Mikl� os Street, Budapest, H-1121, Hungary b Szent Istv� an University, Ybl Mikl� os Faculty of Architecture and Civil Engineering, 74. Th€ ok€ oly Street, Budapest, H-1146, Hungary c Osaka University, Division of Global Architecture, Graduate School of Engineering, 2-1, Yamadaoka, Suita-city, Osaka, 565-0871, Japan

A R T I C L E I N F O

A B S T R A C T

Keywords: Energy efficiency Heritage protection Energetic retrofit Nearly zero energy Decision support

Planning an energetic retrofit for heritage buildings faces several limitations, as multiple protection guidelines narrow down the possible energy efficiency interventions. The present study introduces an energy saving but at the same time heritage respecting retrofit methodology, using the European Union directives as a baseline. The case study area in Budapest, Hungary contains traditional apartment houses, built around the turn of the 19th-20th century. Their physical and energetic state are deteriorated, resulting in demolitions, which endanger historical values. Concerning climate, according to the K€ oppen-Geiger climate classification, Budapest belongs to the humid continental climate group ‘Dfb’, and heating energy is the most significant part of households’ energy usage. After investigating the limitations of heritage protection, the authors introduce combinations of structural and engineering system upgrade scenarios, aiming to reach the European Union nearly zero-energy level. The retrofits’ effect on the buildings’ energetic characteristics were surveyed. The results were compared to find optimum solutions balancing between heritage preservation and energy efficiency. The calculation results show that reaching nearly zero-energy level is possible with heritage respecting so­ lutions. The traditional apartment houses have high energy saving potential, heating and domestic hot water energy can be reduced by 69% with certain upgrades. The methodology and results are based on numerical analyses. The paper introduces an estimation method for the energy saving potential of the above building stock, using simply accessible data. The significance of the typology-based and fast estimation is that it can be used for efficient decision support when planning a large-scale retrofit.

1. Introduction Today several energy saving measures are being taken worldwide, including the increased energy efficiency of buildings. The near-future target for the new buildings is to reach the Nearly-zero energy level. According to the European Union directives, the nearly zero-energy buildings (NZEB) should have high energy efficiency, mostly coming from renewable sources [1]. For new buildings, the Nearly-zero level is mandatory from 2021, but in the light of the ever stricter prerequisites, it can be expected to be used in the case of retrofits too. The replacement rate of old buildings to new is very slow, in Hungary only 1,7% annually

[2]. Thus the retrofit of the existing, ineffective building stock should also be considered as an energy saving measure. The historical districts and the heritage buildings constitute a special case, as several limita­ tions increase the complexity of their retrofit. The focus of the present paper is the heritage respecting energetic retrofit methodology of the traditional apartment house type of MiddleEurope, mainly built around the turn of the 19th and 20th century. Although the structural stability of these buildings is not questioned, the secondary structures are aged, resulting in unsatisfactory energetic state. In Hungary, the households use 60% out of the total energy uti­ lization of buildings. 69% of the above is used for heating, 11% for

* Corresponding author. Hungarian Academy of Sciences, Centre for Energy Research; Szent Istv� an University, Ybl Mikl� os Faculty of Architecture and Civil En­ gineering, Budapest, Hungary. E-mail addresses: [email protected] (V. Sug� ar), [email protected] (A. Talamon), [email protected] (A. Horkai), [email protected]. osaka-u.ac.jp (M. Kita). https://doi.org/10.1016/j.jobe.2019.100982 Received 3 April 2019; Received in revised form 1 July 2019; Accepted 4 October 2019 Available online 18 October 2019 2352-7102/© 2019 Elsevier Ltd. All rights reserved.

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domestic hot water which often come from the same system [3]. These ratios show that the efficiency of heating and domestic hot water pro­ duction in the residential buildings should be the significant focus of the retrofit studies. The energy used for heating and hot water production is particularly excessive in the case study area, the 7th district of Budapest, estimated to be 275 GWh non-renewable primary energy annually (calculated using Energy Performance of Building Directive ‘EPBD’ [4] methodol­ ogy, detailed below). As these downtown districts are usually the most populated parts of a given town (or in the case of Hungary, this is the most densely populated area of the country [3]), the problem affects numerous residents. The retrofit of these historical buildings requires special methodol­ ogy and technology: the conventional interventions, for example heat insulation, are seldom applicable: the ornamented façades and custommade fenestration cannot be insulated or replaced with regular solutions without damaging the heritage values. To solve the above controversial situation, the authors aim to find the consensus between energy efficiency and heritage protection, by ana­ lysing the boundaries and possibilities of the refurbishment. Structural and engineering system upgrade scenarios, their energy saving potential are surveyed to find the optimal solutions. Currently, in Hungary there is no available energy density informa­ tion system or energy saving potential map, which would make the necessary interventions foreseeable and plannable [5]. The present study also wishes to contribute to solve the hiatus. In this survey, the authors are not including the factors of cost effectiveness and social-demographic aspects of a possible retrofit, only aiming to find optimal technical solutions between energy efficiency and heritage protection.

modification is not supporting energy efficiency, rather aims to increase the green surfaces in the block. Similar methodology was used in Hungary before, with controversial results (see Section 2.2). As part of the international TABULA Episcope project [12], Csoknyai et al. [13] introduce energetic modernization scenarios for the traditional apart­ ment house type, reaching high energy saving values (61% of the total energy usage) by insulating the enveloping structures, exchanging fenestration, and applying engineering updates. Although the project declares that the historical characteristics should always be taken into account, their scenarios do not give special instructions for heritage respecting solutions. Multiple studies deal with technical solutions in detail. Iyer-Raniga and Wong [14] state that insulation of the ceiling, roof and external walls are the most effective building interventions, providing the highest energy saving, and at the same time reducing the life cycle primary energy and carbon emissions significantly. Tadeu et al. [15] survey historical buildings from the beginning of the 20th century, focusing on various insulation types and their effect. Litti et al. [16] states that simpler maintenance of the historical windows and added internal glazing provide sufficient energy saving values, at the same time the full replacement of a window does not necessary result in the highest sav­ ings. Harrestrup and Svendsen [17] investigate the heat insulation possibilities on brick historical buildings. Although heat insulation of the walls is considered as one of the most efficient supplementary up­ grades, the practice can cause damage in the original appearance. The indoor insulation (heat insulating layer on the heated side surface of the walls) can be a solution; however, it can cause major building physics problems: unwanted vapor and precipitation cause damage in the structure, and also in human health. The study is presenting a method, where moisture safety is solved by leaving a gap for proper ventilation. The above studies show that there are many aspects of heritage respecting retrofit, mainly agreeing that the three main aspects are en­ ergy saving, heritage protection and cost effectiveness. The retrofits are to consider the heritage values in order to prevent the loss of character. Built on the universal conclusions, however, the renovation guidelines should be defined based on detailed investigation, because the differ­ ences of climate and building type result in major diversities in the possible retrofit actions. The key innovation of the present paper is that complex renovation scenarios were created especially for the traditional apartment building type. These scenarios are synthesizing the results of the previous analytical and experimental studies (for example dealing with detailed analysis of wall, window etc. insulations). Concerning the methodology of creating the scenarios, the heritage protection guidelines as limiting factors were combined with the possible technological details to find optimal solutions, which comply both to the new low-energy pre­ scriptions, but at the same time help protecting the architectural char­ acter of the buildings. As a main numerical result, the energy saving potential of the sce­ narios is introduced, which can be used as simple estimation for future rehabilitation projects.

2. Studies and examples about retrofit of historical buildings 2.1. Retrofit studies about historical buildings Several studies deal with rehabilitation methods of historical build­ ings. Webb [6] extensively reviews the methods and problems of the energy retrofit for traditional buildings. Collecting the aspects of deci­ sion making, Ascione et al. [7] write that the energy efficiency upgrades should be a tool for maintaining the heritage buildings, and should not be seen as simple energetic retrofit. The compromise much needed for heritage protection narrows the technological choices of energy effi­ ciency. The authors define guidelines such as least invasive methods. Okutan et al. [8] also state that the need to create a framework is growing, the conflicts between conservation aims and energy reduction should be discussed and compromises reached. Their study compared multiple energy saving measures and surveyed various approaches of professional and public opinion. Apart from heritage protection guide­ lines, other problems are hindering the large-scale renovation of the buildings. Almeida and Ferreira [9] write that improving the enveloping structure is a key question, considering constraints like aesthetics and cultural heritage of buildings or neighbourhoods, which makes the consensus difficult. This is further complicated by the private ownership of flats (as in Budapest). Complex methodologies were already introduced before to deal with retrofit of historical districts. The European Union founded EFFESUS project (Energy Efficiency for EU Historic Districts’ Sustainability) [10] focuses on heritage protection. They introduce an evaluation system, in which points represent the importance of heritage value and the extent of the effect if certain changes are made. They define various heritage aspects and criteria, such as visual, physical or space related changes. Specializing in the traditional apartment house type in focus, the Hungarian Renewal of historical urban fabric [5] contains synoptic surveys about the complex problems of the stock. Ertsey et al. [11] suggest drastic intrusions in the urban fabric, such as demolitions of the inner wings of the blocks, freeing up space for recreation. This

2.2. Previous retrofits of the traditional apartment houses After World War 2 in Hungary, the central policy concerning the obsolescent historical districts was mainly demolition [18]. After the ideology-based neglect during the 1950–70’s, the renewal of historical districts again came to focus [19]. In the 1980’s, renovations started in some secluded blocks in the centre of Budapest. Demolitions, retrofit and new constructions mixed together [11] were used in case of several blocks in the 7th and 9th districts. The buildings were assessed, much of the wings inside the blocks were demolished. The remaining traditional buildings were upgraded, with new windows and other additions on firewalls, facing the newly opened parks inside the blocks [20] (Fig. 1). The plan resulted major develop­ ment in the area; however, the exchange of population and the 2

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disappearance of the original historical streetscapes are still criticized. In the case of the 7th district block, the project was stopped before the final touches of the recreational zone, and was just finished, after de­ cades of waiting [21]. The above methods only partially demolished the buildings. Unfor­ tunately, the building stock in question sustained ‘bulldozer-shaped urban regeneration’ on multiple other occasions [23]. Such large scale retrofit programs are more difficult to carry out today. The result of the privatisations in the 1990’s is that today 93% of the Budapest buildings are in private ownership [5]. Every decision on the building is based on residents’ democratic voting. Refurbishment savings are not mandatory. Partially this system is responsible for the very low number of refurbishments, and even less energetic rehabilita­ tion in the stock. The other disadvantage of the system is that without common plan or oversight, the owners can decide individually about minor renovations, resulting in unprofessional solutions. As other European examples, rehabilitations in Berlin and Vienna should be mentioned. Contrarily to Budapest, the renovation here con­ tained energy efficiency aspects. In Berlin, energetics were sometimes deemed more important than heritage protection, resulting in excessive heat insulations on the façade. The insulating boards on the outside façade often consumed and destroyed the original historical decorations and changed the original scale of the facades [5] (Fig. 2). In Vienna, the replacement of the historical houses with social flats started during the 1970s. Similarly to Budapest, it sometimes resulted in the destruction of historical values and population exchange. Later, soft rehabilitation started, encouraging the retrofit of the urban fabric itself, including details as insulation, fenestration exchange, engineering sys­ tem retrofit of the buildings. Connecting to the district heating system was highly encouraged [5].

time, D� ery [27] published an extensive collection of building data �rsony [28] and Ba �rsony et al. [29] are including construction times, Ba both descriptive books about building structures. The data were vali­ dated by field study and using the Budapest City Archives collection [30]. It is found, that the architectural style of a certain building has a close connection with its geometry and the above structural-material typology group. Architectural style thus was chosen for simple identification and classification of a certain building. This particular characteristic is convenient, no complex information is required to determine the style. This simplicity of the style-based classification is especially important, because one of the main aims of the study is that the results should serve as a decision support system for future rehabilitation plans. For further information see the authors’ pervious studies [31,36]. A short intro­ duction of the styles and other characteristics of the case study area are in Section 4. The energy efficiency aspects were defined using the currently valid EU EPDP methodology [4] and the corresponding valid Hungarian Decrees: 176/2008. (VI. 30.) Hungarian Government Decree on the certification of energy characteristics of buildings [32], 7/2006. (V. 24.) Minister Without Portfolio Decree determining the energetic charac­ teristics of buildings [33] and its 20/2014 (III.7.), amending decree of 176/2008, of Home Secretary [34]. The EU detailed report on Hungary was used to support the above [35]: In the Hungarian calculation sys­ tem, three requirement levels should be complied (levels of enveloping structure, geometry and engineering system). The levels are built on each other, ensuring that the building in question reach the desired low energy demand and utilization values. The requirements for the nearly zero-energy label are reached if the calculated values of the building comply to the limiting value stated in the Decrees, for all three levels. The basic flowchart of the calculation methodology is shown in Fig. 9. See more detail and the main equations in Appendix A. First, the present energetic state (Section 4) was investigated using the above calculation. A strong connection between energy demand, footprint area and architectural style was found, underlining the importance of architectural style as basic typology. The results show that based on the footprint area and the architectural style, the heating en­ ergy demand of a building can be closely estimated [36]. The limiting factors narrowing down the possibilities were defined next. The heritage protection guidelines and the boundaries of building geometry and urban fabric are detailed in Section 5. The possible energy efficiency interventions (Section 6) are also based on the above calculation. After combining the retrofit intervention possibilities and limiting factors, retrofit scenarios were created (Section 6). The effect of the combined structural and heating system upgrades were assessed to find an optimal solution of heritage respecting energetic retrofit (Section 7). The conclusions drawn and new suggestions for policy concerning the traditional apartment house retrofits were summarized in Section 8. Illustration of the methodology is shown on Fig. 3.

3. Methodology of study The related studies were reviewed in Section 2 above, concluding that a detailed analysis is needed for the building type in question to baseline the interventions, and to identify the optimal retrofit methods out of the multiple technical solutions. Based on [13], the surveyed building type clearly shows high potential of energy saving, their heri­ tage respecting retrofit is a pending problem worth investigating. The complex retrofit scenarios require architectural and energetic aspects. As for architectural aspects, an extensive survey of the case study area buildings was carried out. A detailed database was created building-by-building, containing geometry, function and other archi­ tectural data. The style elements, structural-material and geometry pa­ rameters of all buildings were investigated to find the common characteristics. The results include a structural-material typology, where nine categories, „Packages” were created containing the characteristic enveloping structures of the buildings, based on time of the construction (Appendix C). The typology is based on the following sources: Edvi [24] collects the construction regulations of the turn of the century Budapest. �k [25] and Pattantyús [26] write about building materials of the Ritoo

Fig. 1. Left: Block Nr. 15 after the rehabilitation. The demolished wings are marked with dashed line. The new buildings are darker shade [22] Right: Buildings from inside a District 9th block. Left: before rehabilitation as empty firewalls; Right: after rehabilitation as new facades facing a new park [11]. 3

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Fig. 2. Historical building heat insulated in Berlin. Left: the original façade; Middle: insulated façade; Right: the insulation damaging the historical decoration [4].

3.1. Case study area and its present energetic state

2016. 86% of the buildings were built before the end of the Second World War, considered traditional. The architectural styles of the stock are as follows: the traditional styles (from before WW2) are NeoClassicism, Romanticism, Historicism, Freestyle, Premodernism (1811–1942). The younger ones are Modern, Socialist Modern, Contemporary buildings (1954–2016). The identification of the buildings is relatively easy depending on style characteristics or using the year of construction (Table 1). In brief, the Neo-Classicistic buildings are usually simple, using antique elements like tympanums. The Romantic style is taking elements from the Middle Ages, with Gothic, Byzantine or Romanesque decoration. Historicism was constructing larger buildings in masses, with Renaissance or Baroque ornaments. Freestyle buildings are using most of the plot, with waving façade surface and mixed decoration (Art Nouveau, Art deco mixed with Renaissance or Baroque). In case of the above styles, the most commonly used building structures are the following: brick masonry walls (average thickness ca. 50 cm); pitched roof in which the upper closing slab is a full beam timber structure with filling (average thickness ca. 35 cm); the cellar slab is mostly Prussian vault. The windows are double-layered box-style fenestration. Premodernism is simple, with undecorated façade. Modernism is using more evolved materials, free design of façade and layout, mostly with flat roof. The walls are most commonly built with reinforced con­ crete frame and hollow brick filling (average thickness ca. 40 cm), the roofs and cellar slabs are flat reinforced concrete structures with filling, or rudimental heat insulation (average thickness ca. 30 cm). The win­ dows are joint-wing, double layered.

The major architectural heritage of Budapest is the mass of tradi­ tional apartment buildings, built around the turn of the 19th and 20th century. With their ornamented façades forming the streets, their sig­ nificance is internationally recognised as historical, cultural and city­ scape heritage. The case study area is situated in the middle of the 13 km2 area of that building type in Budapest. The area is part of the 7th district (Fig. 4), containing 475 buildings on 0.6 km2. Compared to its size, it is the most densely populated district of Budapest [37]. Information about the historical of development can be found in Strbik’s [38] and Perczel’s [39] books focusing on the district history. Although part of the area is a famous party district, the main function is still residential (88%, 386 buildings). The most characteristic building type of the area is the above mentioned traditional multi-storey apart­ ment house with courtyard (Fig. 4), but younger buildings can also be found scattered in-between. These houses were originally built for rent by investors. Standing in an unbroken row along the narrow street, they are connected to each other with firewalls on two or three sides. The street front wing is more ornamented, containing larger flats. The courtyard wings have simpler flats. The courtyard can be accessed by using the gate on the street front. Near the gate there is the main staircase. The flats on the upper stories can be entered from the hanging corridors running parallel to the walls (typical facades and layouts are in Table 1). This building type with some differences is widespread in Middle-Europe, in the past Austro-Hungarian Monarchy towns. The oldest of the surveyed stock was built in 1811, the newest in

Fig. 3. Methodology of current study. 4

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Fig. 4. Left: Map of Budapest. The area of the traditional apartment houses is marked with hatches. The black area is the case study area; Middle, left: Map of the case study area showing the urban fabric and the bordering streets; Middle, right: typical façade of a traditional apartment building; Right: typical courtyard of a traditional apartment building. Table 1 The architectural styles, their construction time and ratio in the case study area with example drawings of the street front facades and layouts. The courtyards are shown with grey.

Socialist Modernism is entirely different from the above, with its simple form and prefabricated structures. The structural solutions of the walls and slabs were mostly based on the Soviet or Danish prefabrication methods. The floor-high units are reinforced concrete sandwich panels (average thickness ca. 30 cm), the fenestration is joint-wing structures. Contemporary buildings cannot be described so simply, given the large variety of its layout and façade solution. In general, using presentday solutions in structures and materials, as well as free forming of façade with large glass surfaces might be considered as characteristics of the style. The walls are mostly reinforced concrete frames with hollow bricks, the slabs are reinforced concrete (average thickness ca. 40 cm). Both the walls and slabs are heat insulated with polystyrene or mineral wool panels. The windows are gas-filled, multilayered, highly insulating structures. The above styles and common structures had been identified using literature data from Hungarian studies. (Methodology is described in the authors’ previous paper [36].) Style classifications are based on the in­ �k’s books of Classicism, Historicism [25], and formation found in Ritoo turn of the century [42], as well as Sisa’s extensive studies of the same time’s art [40] and buildings [41]. Rados’s book was also used for style definitions [43]. The common structures were defined as explained in

Section 3 and shown in Appendix C). Today, the unique architectural and cultural values are endangered. In recent years, the demolition and modification of traditional buildings are increasing in number. Under the pretence of modernization, these projects mostly result in the loss of the original architectural character, or in more severe cases, whole buildings are demolished. On the other hand, a heritage respecting rehabilitation would be much needed for the stock. The physical condition of the buildings is often deteriorated, their poor energetic state damage their value. Pres­ ently, only 19% of the buildings are in acceptable condition, while the remaining 81% requires renovation [44]. €ppen-Geiger Concerning the climate of Budapest, according to the Ko climate classification, the city belongs to humid continental climate, warm summer subtype, group Dfb [45]. The average monthly temper­ ature, rain, humidity and solar radiation values are shown in Fig. 5. Fig. 6 and Table 2 show that the summed net heated area is the highest in case of buildings from Historicism and Freestyle, and at the same time, their average total primary energy consumption (EP [kWh/ m2a], (calculated using the EPBD conform Hungarian building energy calculation system, see details in Appendix A) is the largest here. This value is 200–300% larger than the level expected today. The summed 5

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Fig. 5. Average monthly temperature [� C], rainfall [mm], humidity [%] and daylight hours [h] of Budapest [46].

4. Limiting factors of retrofit 4.1. Heritage protection guidelines In Hungary, the monument protection system consists of multiple levels: national monument protection, local protection, conservation area protection and monument neighbourhood. The case study area is also protected in several ways: the streetscape and its scales, the organic fabric originated from the 18–19th century, multiple buildings with individual protection on national or local level. Individual protection means the protection of the forming of mass, space, the heights ratios in buildings, façade design with ornaments, fenestration form, indoor design and space relations. The demolitions are discouraged on every level, even in case of the courtyard wings. Appendix B summarizes the protection forms and their main aspects concerning the case study area. Although there is a wide range of protection forms for built heritage in Hungary, their enforcement is mostly weak. As a very common example, the individually changed windows (Fig. 8), or the destroyed ornamentation in case of an air conditioning device installment are usually not reported and penalized, as these interventions are not bound by official permission. It is also a problem that though the characteristic elements of facade are deemed protected in many documents, these el­ ements are not defined squarely [5]. Although several buildings in the area are not under monument protection, the authors are considering the full building stock as protected in the following, to maintain the historical character of the district.

Fig. 6. Connection between total primary energy consumption EP [kWh/m2a] and summed net heated area (AN [m2]) in case of each architectural style.

Table 2 Total geometry and energy values per styles. The values are calculated as introduced in Section 3 and Appendix A. Style

Neo-Classicism Romanticism Historicism Freestyle Premodernism Modernism Socialist Modernism Contemporary

Summed net heated area of the 386 buildings of the case study area

Average total primary energy consumption of the 386 buildings of the case study area per m2

AN [m2]

EP [kWh/m2a]

67 558 22 220 442 477 346 731 122 757 1926 26 027

267 276 289 259 226 153 167

101 163

102

energy usage of the traditional buildings (their total energy consump­ tion per m2 combined with the net heated area) is significantly higher than the newer styles (Fig. 7). This difference, when compared to the other styles, projecting that the renovation should start here.

Fig. 7. The summed value of the total primary energy consumption of the buildings per style, GWh/a. The summed total consumption of present state is 274,7 GWh/a. 6

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Fig. 8. Individual, unprofessional window exchange causing uneven facades and loss of character.

Fig. 9. Summary of the intervention points based on the levels of the calculation, with the analysed values.

4.2. Dense urban fabric

should be on two major interventions: architectural intervention, which should provide decreased energy demand, and engineering intervention, used to satisfy the decreased demand with efficient heating technology. The former is containing the upgrade of enveloping structures’ U value (Section 6.2.1) and geometry (Section 6.2.2), the latter is upgrade of engineering system (Section 6.3). Fig. 9 shows the three levels and their indicator comparative value to be assessed. The following values were chosen to be analysed to assess and compare the effects of the scenarios:

Using renewable energy, for example hydro or wind-power, ground source heat pump or water-based heat pump in the dense urban fabric, particularly in existing buildings are difficult or even impossible. The construction activity itself is non-accomplishable, and the required space for the above technologies are mostly not available. The biomass heat production in such an area also should be excluded, because of the transportation and storage difficulties, and the amount of dust pollution. The generally applicable solutions are solar power utilization or air source heat pump. In most cases, solar energy can be utilized, since almost all buildings have suitable roof surfaces to install collectors or photovoltaic panels [47].

� U, thermal transmittance value [W/m2K] for the compliance of enveloping structures � q, heat loss coefficient [W/m3K], showing the performance of the U values together with the geometry � qF net heating energy demand [kWh/m2a] � EP, total primary energy consumption [kWh/m2a] using the above values and including the heating and hot water system performance.

5. Retrofit scenarios combining possibilities and limits 5.1. Possible interventions and energetic values to be assessed According to the European Union EPBD conform Hungarian energy efficiency calculation system (See Appendix A), the focus of the retrofit

Fig. 10. Main heat insulation possibilities on enveloping structures in case of heritage respecting modernization A: Wall insulation: A1: Outside insulation of the facade, A2: Inside insulation of the façade; B: Window renovation; C: Bottom slab insulation: C1: Underside insulation of cellar vault, C2: Floor insulation, C3: Underside insulation in case of arcade; D: Roof insulation: D1: Closing slab insulation, D2: Pitched roof insulation. 7

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5.2. Architectural interventions

rooms or covering the courtyard with glass roof to decrease A/V is another option. To increase solar gains, construction of new openings on the façade should be done, which also would affect the heritage values. The present study does not deal with the upgrade of geometry in detail, because the traditional apartment buildings’ A/V ratio is commonly good, confirmed by the calculations below (Section 7.1.2).

5.2.1. Upgrade of building enveloping structures Decreasing the heat losses of a given structure enveloping the heated volume is important part of the energy retrofit. The most common form is the heat insulation of the surfaces, which, however, is controversial practice when dealing with heritage buildings. These statements are supported by both Almeida [9] and Iyer-Raniga and Wong [14]. In the case of listed monuments, regulations are highly restrictive of the normal insulation methods. Fig. 10 shows an average section of a traditional house wing to introduce the main surfaces to be insulated. Solutions for the upgrade of the protected parts considering the heritage values:

5.2.3. Retrofit scenarios for structural upgrade For the baseline of the structural scenarios, the structural typology (Appendix C) of the original structures was used. The scenarios, the structurally upgraded versions of the ‘Packages’ are created in line with the heritage protection and Nearly zero energetic aims combined. This means that the structure should comply with the prescribed U value and at the same time should be a heritage protecting technical solution on a certain level. Scenario 1 is the ‘Original structure (OR)’. This contains the data of the original structures based on the above typology. Scenario 2 is the ‘Least Invasive (LI)’ scenario. In this case, the heritage protection guidelines were fully complied and only the necessary, less visible sur­ faces were insulated. The decorated façade walls, cellar walls, arcades were left intact. The roof and cellar slabs were insulated as well as the uncovered firewalls. The windows were upgraded with the full heritage compatible solution (fitting and exchange of the glass to low-e glazing). In short, not all the surfaces are upgraded, but the insulated structures are reaching the Nearly zero U value requirement level. Scenario 3 is ‘Nearly Zero (NZ)’. Here, the aim was for every enveloping surface to reach nearly zero-energy level. Thus, the missing surfaces of the ‘Least invasive’ scenario were additionally insulated. The walls were insulated from the inside. In case of fenestration, we should differentiate between traditional and post Second World War styles from

� Outside insulation of the façade (A1): In the light of the heritage protection guidelines, the external thermal insulation of the façade can be ruled out in almost all cases. To apply traditional board insulation, it is necessary to remove the ornaments. Naturally, it is possible to replace them with plastic copies; however, these are not lasting and supported solutions. The outside insulation can only be used in the case of empty, uncovered firewalls (where no neigh­ boring building is connected to the firewall, working as an envel­ oping surface). � Inside insulation of the façade (A2): The internal thermal insulation can be an option in cases where the inner surface is not decorated. Particular attention must be paid to the appropriate material choice in order to avoid vapor and other physical problems [17]. � Fenestration renovation (B): As 40–50% of the façade is glazed sur­ face in this type, the energetic state of the fenestration is an impor­ tant question. It is avoidable to entirely exchange the traditional fenestration to new, PVC framed windows. The mass produced plastic windows are made with much thicker mullions than the traditional wooden solutions, which changes the ratios of the win­ dow (Fig. 8). There are; however, other heritage respecting solutions for modernising the original window structure fully or partially, cost effectively (stated also by Litti et al. [16] and Szalay et al. [48] dealing with historical fenestration). Some examples: fitting, Low-E glazing, replacement of inner wings with insulated wings etc.

Table 3 Summary Structural Scenarios, with their advantages and disadvantages.

Solutions not affecting the general heritage values of the buildings, thus can be utilized nearly in all cases: � Bottom slab insulation (C1, C2, C3): Most commonly, these buildings have vaulted cellars, to which bendable insulation can be installed (for example rock or glass wool), but otherwise standard technolo­ gies can be utilized (C1). In the case of flat slabs, the generally used solutions are: polystyrene or wood-wool plates. An alternative so­ lution is to renovate the structure from the heated side by installing insulation into the floor structure (C2). In this case, the wooden parquet should be ripped up, which is not an ideal solution. It is unavoidable, however, in the case where there is no cellar, and the floor layers are on the soil. C1 solution can be utilized in case of the arcade slab of the gate (C3). The value of the arcade space and ornamentation should also be considered. � Roof insulation (D1, D2): As the buildings mostly have empty pitched roof without attic rooms, the closing slab insulation (D1) is not problematic. Vapor open insulation should be chosen (for example rock or glass wool). For built-in attics, the commonly used solution for pitched roof insulation (D2) is rock or glass wool filling between and underside the rafter. 5.2.2. Upgrade of geometry To reduce energy demand, the heated volume or the enveloping surface area per heated volume (A/V) ratio should be reduced. Another possiblity is to increase solar gains indoor. Reducing the heated volume can be achieved by repositioning and grouping of heated and non-heated

Enveloping Structure

Least invasive scenario (LI)

Nearly Zero scenario (NZ)

Wall

none

Closing slab under pitched roof Pitched roof in case of attic Flat roof Fenestration

Insulation outside (D1)

Traditional apartment buildings: Insulation inside (A2) Non-traditional buildings: Insulation outside (A1)

Arcade Floor on soil Cellar slab in case of non-heated cellar Floor on soil in heated cellar Cellar wall in case of heated cellar Not-covered firewall

none none Insulation outside (C1)

Original scenario: Pro: no intervention;

Least Invasive scenario: Pro: intervention only on less-visible surfaces, or mainly not decorated surfaces; Contra: Heat-bridge problems intensify, medium energy saving

Contra: no energy saving

8

Insulation inside (D2) Insulation outside (D1) Fitting þ Low E glass (B)

Traditional apartment buildings: Fitting þ Low E glass, inner wing exchange Non-traditional buildings: full exchange (B) Insulation outside (C3) Insulation inside (C2)

none

Insulation inside (A2)

none

Insulation inside (C2)

Insulation outside (A1) Nearly Zero scenario: Pro: all surfaces are insulated, less heat-bridge problems; Contra: building physics problems may occur: vapor and low temperature of the structures. Inside decorations destroyed.

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Journal of Building Engineering 27 (2020) 100982

cooling the indoor temperature. The empty attic or the cellar can pro­ vide enough space for the system. The pipe systems with radiators should be reconstructed, the same as above. Scenario 4 contains the more common upgrade of house central gas ‘Condensation Heater (CH)’ with the same radiator system as above. Domestic hot water is produced via the same device and stored centrally.

heritage value point of view. For the former, the strict compliance of heritage protection guidelines should be applied (Section 6.2.1). In the latter case, however, because of their forming and decoration are simple, fenestration replacement is also applicable. Table 3 summarizes the measures in case of each scenario, also listing the advantages and dis­ advantages of application. A detailed example for the structural retrofit scenarios is show in Appendix D.

5.4. Combination of structural and engineering scenarios

5.3. Engineering interventions

All the structural and engineering scenarios were combined as per Table 4, in the following, the abbreviations below are used in the case of scenario combinations. The first part of the abbreviation is indicating the structural scenario, the second is the engineering scenario. All the scenarios and their combinations were applied to the 386 residential buildings in the case study area, and their effect on the en­ ergy efficiency values (Section 7) were calculated and compared. The results are detailed below.

5.3.1. Presently used heating and hot water systems The Hungarian Central Statistical Office [49] database was used to survey the presently used heating systems of traditional apartment houses. The Authors’ attempt for answer-sheet surveys resulted in low answer ratios on the currently used heating systems of the flats. Both the unwillingness of answering and the lack of knowledge of the residents encumbered the survey. Using the statistical database, however, the following heating systems were presumed: House-central heating with one main heater device and radiators in the flats. Two variations, an older and a newer technical solution was used for calculations to differentiate between the older and newer buildings. For domestic hot water production in the old case, electric boiler was used. In the new case, indirectly heated water tank was assumed (the house-central heated buildings are easily identifiable via their large, single chimney). Most of the traditional buildings however, are not centrally heated, but with room-by-room devices. These are mostly equipped with con­ vectors for heating and gas boiler for domestic hot water. According to the database, averagely 75% of flats in each building is still heated by convectors or even older tile stove. Averagely 25% of the flats in each building, however, has been modernized to flat central heating with more contemporary gas boilers or condensation heaters, which combine the heating and domestic hot water production. These ratios were used in the calculations as Scenario 1, or ‘Original heating system (OR)’.

6. Results and discussion The case study area was narrowed down to the residential function, which is 386 buildings, 88% of the full stock. The energetic values are calculated using the aforementioned Hungarian EPBD conform system (Appendix A). Owing to the large stock and the vast amount of input data, simplifications were done to be able to handle the database. The major simplification used in the calculation was the structural typology: the enveloping structures of each building were assumed based on their construction year (Appendix C and [36]). Also, the type of current heating system was based on statistical data. A particular uncertainty is included in the methodology of the calculation system due to the simplification methods and the estimative parts of the calculation [50]. For example: the heating and domestic hot water usage, the majority of the energy consumption is based on a ‘model user’, thus the various user habits are not included in the cal­ culations. Also, underheating of the flats resulting from energy poverty is not included either. Thus the main result of the current paper is the energy saving potential, a deduced value of multiple input data, its uncertainty is ca. 15%. The statistical normal distribution of the main input data (for example: footprint) grouped by style was investigated by KolmogorovSmirnov probe. The significance obtained during the test is around 0,00, so 95% of the sample elements are from a normal distribution. The t-probe significance was also 0,00, thus the model is estimating the actual sample values well. The R2 values of the equations provided are between 0,8–097.

5.3.2. Upgrade of heating and domestic hot water system The engineering scenarios were based on the restrictions of Section 5.2. The ‘Original heating system (OR)’ is Scenario 1. The upgraded versions are Scenario 2, 3 and 4. In Scenario 2, a new ‘District Heating (DH)’ system is assumed. It is based on the fact that presently there are undergoing constructions to expand the existing system to the inner districts. This upgrade would require the exchange of the full heating system in the traditional houses to contemporary radiators. In the cellar, a caloric centre and a heat-exchanger block is to be placed. Domestic hot water is produced by the same device and stored centrally. As the Budapest district heating system is using renewable energy and other advanced form of energy creation, thus the utilization would prove more environment friendly and modern. District heating would also reduce air pollution caused by heating. Scenario 3 is using air source ‘Heat Pump (HP)’, which absorbs heat from outside air and releases it inside the building, through hot waterfilled, low temperature radiators, and also produces domestic hot water. Its advantage is that the same system can be reversed in summer,

6.1. Effect of the structural scenarios on the energetic values 6.1.1. Change in the thermal transmittance values Fig. 11 shows the change in the U value on structural ‘Package 1’ (Appendix D) as example. The main enveloping structures and their U values in case of each scenario are shown. As mentioned above, in case of the Least Invasive (LI) structural scenario, not every surface is insu­ lated on the contrary to the Nearly Zero (NZ) scenario. The figure also shows the maximum level of requirement prescribed in the Decrees [4, 32–35].

Table 4 Summary and combinations of the upgrade scenarios, with their respective abbreviations. SCENARIO COMBINATIONS Structural/Engineering scenarios

Scenario 1: Original (OR)

Scenario 2: Least invasive (LI)

Scenario 3: Nearly Zero (NZ)

Scenario 1: Original (OR) Scenario 2: District Heating (DH) Scenario 3: Heat Pump (HP) Scenario 4: Condensation Heater (CH)

OR_OR OR_DH

LI_OR LI_DH

LI_OR LI_DH

OR_HP OR_CH

LI_HP LI_CH

LI_HP LI_CH

6.1.2. Change in the heat loss coefficient value As for the heat loss coefficient (which is affected by the U value and geometry), the value decreases moderately from the OR level to LI, and drops more significantly in NZ scenario for each style. As the geometry is not changed by the retrofit scenarios, the change is mostly caused by the decreased U value (the NZ scenario contains more extensive insulation, for example on the walls). The most significant changes are shown for Premodernism, Modernism and Socialist modernism, due to their originally lower U 9

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Journal of Building Engineering 27 (2020) 100982

Fig. 11. U values for each scenario in structural Package 1.

Fig. 12. Heat loss coefficient values in each scenario for each architectural style.

Fig. 13. Heat loss coefficient values in each scenario for each architectural style. The compliance of the limiting value of nearly zero-energy requirement level is P shown by a green line. The heat loss coefficient is surveyed together with the respective buildings’ A/V (enveloping surface to heated volume) ratio. The building is complying the requirements if the dot is under the limiting line.

values than for other styles. Contemporary buildings are not as affected, as they were originally complying more or less to the requirement levels. Although not with the highest differences, but significant change can be observed for traditional styles too (Neo-Classicism, Romanticism,

Historicism, Freestyle) (Fig. 12.). As for the heat loss coefficient, the limiting maximum value of the P regulation is dependent on the A/V (enveloping surface to heated volume) ratio. Fig. 13 diagrams show the change and the compliance in 10

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Journal of Building Engineering 27 (2020) 100982

Fig. 14. The change in the specific net heated energy demand qF [kWh/m2a] per styles.

P q and A/V in case of the three structural scenarios, sorted by archi­ tectural style. These results imply that the compactness of the buildings are suffi­ cient, and by upgrading the enveloping surfaces, the nearly zero-energy level can be met (‘NZ’ scenario). The main contrast between the LI and NZ scenarios are the insulation of walls, which causes major difference between the compliance to this limiting factor.

6.2. Effect of the scenario combinations As mentioned above, the total primary energy consumption (EP, kWh/m2a) contains the structures and geometry performance of the building, combined with the energy consumption of the engineering systems annually in primary energy. In the case of residential buildings, the consumptions of heating and domestic hot water are summed. Thus, the structural solutions are combined with the engineering solutions, their joint effect can be observed in Table 4. The abbreviations of each combination was already introduced above. To be able to decide which variation should be used for energy saving retrofit, Table 5 shows the effectiveness of each combination. The values show the percentage of the reduced EP compared to the ‘OR_OR’ original state per style. The colors indicate that energy saving is relatively high (green), moderate (orange), or smaller (red). In the case of the same engineering scenarios, the effect of the Nearly Zero (‘NZ’) solution is the most effective, resulting in smaller EP compared to the ‘OR’ and ‘LI’ scenarios.

6.1.3. Change of the specific and total net heated energy demand The change in the specific net heated energy demand qF [kWh/m2a] is introduced in Fig. 14. The largest decrease is for Modernism, where the Least Invasive (LI) scenario provided 48%, the Nearly Zero (NZ) 76% decline. The potential of energy demand cutback thus is the largest in this style. The Modernist buildings are however, few in number and net heated area (AN, m2). The demand is also reduced significantly for styles where the heated areas are the largest: Historicism (‘LI’ 26%, ‘NZ’ 66%) and Freestyle (‘LI’ 29%, ‘NZ’ 68%).

Table 5 The average percentage of EP compared to the ‘OR_OR’ variation per styles. Bold variations were used for optimization (Section 7.3)

11

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Journal of Building Engineering 27 (2020) 100982

Table 6 The average EP for each style in kWh/m2a with each scenario combination.

Fig. 15. The total energy saving potential all the buildings summed together, with each scenario variation in GWh compared to the total consumption of OR_OR variation.

Comparing the engineering scenarios, that variations containing ‘CH’ condensation heater for engineering solutions are less effective. Here, EP is relatively larger than the other engineering solutions (‘HP’ or ‘DH’) with the same structural scenario. The District Heating ‘DH’ and the Heat Pump ‘HP’ scenarios, however, show the same effect in reducing EP. The most effective combinations are ‘NZ_DH’ and ‘NZ_HP’. Although the ‘LI_DH’ and ‘LI_HP’ are less effective, the difference is not significant. The average EP for each style in kWh/m2a with each scenario com­ bination is shown in Table 6. If the net heated areas (AN, m2) of each building are considered, the total primary consumption of each building can be calculated. Fig. 15 shows the total energy saving potential of each scenario variation in GWh/a. The values show the amount of energy which can be saved if the given scenario variation is applied to all the buildings in the case study area (compared to the total primary energy consumption of OR_OR scenario variation).

applied more strictly than the post-WW2 styles (Modernism, Socialist Modernism, Contemporary). In the light of the above, for traditional buildings, the ‘LI’ Least Invasive Structural scenario should be applied. Their façades are more elaborate, the forming is more complex. For newer buildings, the ‘NZ’ Nearly Zero Structural scenario is recommendable. Their historical values are not high priority, also their façade ornaments and forming elements can be upgraded with insulation easily, without changing the character. From an engineering system point of view, the above tables and di­ agrams show that District Heating (‘DH’) and Heat Pump (‘HP’) sce­ narios are the most efficient (with almost the same energy saving potential). As the construction of the district heating system in the case study area is only a future plan, the authors decided to choose the ‘HP’ scenario as the best option. Combining the above, for traditional styles the ‘LI_HP’, for post-WW2 styles the ‘NZ_HP’ combination were chosen as optimal from heritage protection and energy saving point of view (see bold cells in Table 5). By calculating the EP total primary energy consumption before and after the theoretic retrofit, the energy saving potential of each style and the full district can be assessed. Fig. 16 illustrates the results. It can be concluded, that the building stock of Historicism and Freestyle have the largest potential if the summed primary energy saving of the styles are analysed. On the individual building level, the Modernist and Social Modernist buildings can be upgraded with the

6.3. Optimized energy saving potential In Section 7.2, the effects of scenario variations were observed, assuming that for every building the same upgrade is applied. The traditional buildings, however, should be dealt differently than the postWW2 ones. For traditional styles (Neo-Classicism, Romanticism, His­ toricism, Freestyle, Premodern styles), heritage protection should be 12

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Journal of Building Engineering 27 (2020) 100982

Fig. 16. The summed total primary energy consumption before and after renovation.

highest energy saving ratio compared to their original state. If the average energy saving potentials are compared to each other, the buildings of Freestyle, Historicism and Romanticism stand out with the highest saved energy per building, and per net heated m2. The en­ ergy usage, and energy saving potential on district, style, individual building and m2 level are shown in Appendix E. If using the above scenario variations for upgrade, totally 188 GWh

heating and domestic hot water production energy can be saved annu­ ally, reducing the original consumption by 69%. 6.4. Change in energetic classification and complying the nearly zero energy level As last step of the energetic calculations, the buildings are classified

Fig. 17. Decision support: estimation of energy saving potential [kWh/a] using architectural style and footprint [m2] as input. 13

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Journal of Building Engineering 27 (2020) 100982

into groups based on their difference to the Nearly zero requirement level (in %), prescribed in the aforementioned decrees. After the pro­ posed retrofit, 85% of the upgraded buildings are complying the Nearly zero requirements compared to the original 4,7%, which shows the effectiveness of the above described scenario variations.

the buildings, which are considered historical values. In case of newer buildings, built after the Second World War, the Near Zero ‘NZ’ and ‘HP’ scenarios combined can be applied to reach the maximum level of en­ ergy saving. The results show, that with certain upgrades, high energy savings can be reached even in case of historical buildings. Concerning the statistical relevance, there is a particular uncertainty of the values, due to the simplifications of the input data and the esti­ mative parts of the calculation system. The total primary energy usage values’ uncertainty is ca. 15%. The energy saving potential values are comparing the original and upgraded energy usage values, thus their uncertainty is lower. Analysing the energy saving potential gives the following results. If considering the energy saving ratio of the styles, Premodernist, Modernist and Socialist Modernist buildings show the highest percent­ age compared to their original consumption, their quantity is, however, low in the case study area. On a full district level, the buildings of Historicism and Freestyle have the largest energy saving potentials due to their large quantities, but they also exceed with their high average energy saving potential per building. By using the above scenarios, 69% of the currently used heating and domestic hot water production energy can be saved on average, which amounts to 188 GWh annually. Considering the energetic classifications, before retrofit, only 4,7% was complying the Nearly Zero requirements. After upgrade, 85% of the case study area reached this high efficiency level. As a main result, diagrams and equations were introduced for the two optimal retrofit scenario combinations (the Least Invasive structural scenario with heat pump and the Nearly Zero scenario with heat pump, mentioned above), helping the estimation of energy saving potential of a building, using the architectural style and footprint as input data. The significance of the above is that the energy saving potential of a building from the stock can be estimated using only simply accessible data. The estimation thus can be carried out even by non-professionals in a fast and efficient way. As a future step, the economic factors of the retrofit scenarios should be surveyed. By adding the crucial data of payback time and costefficiency, the usability of the above suggestions as decision support will increase further. It can be concluded that even without destroying the heritage values, historical buildings can be renovated to reach the highly energy-efficient level of Nearly Zero requirements. With careful planning and combi­ nation of heritage protection guidelines and energy efficiency measures, optimal solutions can be reached between energy saving and heritage protection. The average energy consumption values per heated m2 can help the typology based energy usage estimation in the case of a similar building stock. To support the above, the valid regulations should be expanded to deal separately with the heritage buildings, by allowing certain ex­ emptions from the compliance of structures. For example, the required U value for the façade walls should not be complied for heritage buildings. Using the above special permit, the heritage values could be protected more effectively, but at the same time the Nearly Zero level certificate could be awarded for these buildings, increasing their viability and sustainability.

6.5. Decision support: estimation of energy saving based on simple data of a building In the preliminary steps of decision making, especially when a largescale project is planned, simple solutions for estimations are highly recommended. Fig. 17 introduces an estimation diagram which can be used to approximately calculate the energy saving of a building from the stock (applying LI_HP and NZ_HP scenarios), using the footprint and the architectural style. The footprint (ground floor area) is easily accessible data, as open-access satellite applications can be used for measuring (for example Google Earth Pro [51]). To define the architectural style too, a field survey or satellite image is mostly enough (Table 1 helps the identification). Both input data can be accessed even by non-professionals, thus the diagram can be used by residents and housing associations too, to assess their energy saving potential. 7. Summary and conclusions Buildings consume significant amount of energy. Increasing their efficiency is an important focus in the light of the ever stricter energy saving guidelines. The newly constructed buildings of the near future should already comply to the nearly zero-energy requirement level prescribed by the European Union. Their quantity is, however, insig­ nificant compared to the existing, older stock. Upgrading the existing buildings is essential to reach the national energetic aims. The energetic refurbishment of the historical districts and protected buildings is a complex question. Generally applied insulation methods are rarely applicable due to the heritage protection guidelines. The dense fabric also limits the possibilities. The case study area in Budapest contains traditional apartment houses built around the turn of the 19th and 20th century. Their physical and energetic state are deteriorated, the heating energy used by the residents during winter is excessive. The historical values of the area is in danger due to the demolitions and modifications of the buildings, under the pretence of modernization. The present paper introduces a new methodology to renovate the traditional apartment building type of Middle-Europe. The methodology includes the results of previous studies concerning technical solutions for insulation, and combines them with the limiting factor of heritage protection guidelines. This complex approach helps to decrease the energy consumption, but at the same time protects the architectural value. The results of calculations help to estimate the energy saving potential of the buildings, thus can serve as decision support for future rehabilitations. By analysing the possibilities and limitations, two scenarios for up­ grade were created. These scenarios are considering both heritage pro­ tection and energy efficiency aspects. Two groups of scenarios were considered: structural upgrade and engineering system upgrades. Each and every scenario and their combinations were applied to all 386 res­ idential buildings in the area. Their effect on the buildings’ energetic indicator values were analysed and compared. After assessing the effectiveness of each scenario on energy saving, the optimal variation of structural and engineering upgrades was cho­ sen. It is concluded that in case of traditional buildings, the Least Invasive ‘LI’ structural scenario combined with Heat Pump ‘HP’ engi­ neering scenario is sufficient for energy efficiency aims. At the same time in this structural scenario, the insulations are used on a minimal, non-intrusive level, preserving the forming and elaborate decoration of

Declaration of competing interest None. Acknowledgement �n We would like to thank the architect master students of Szent Istva �s Faculty of Architecture and Civil Engineering for University, Ybl Miklo their help in the field survey.

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Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Appendix Appendix A Currently valid EU EPDP methodology Based on European Union regulation 2010/31 EU, the Hungarian Government Decree 176/2008. (VI. 30.) (2018.10.11)] on the certification of energetic characteristics of buildings [32] implemented, with the 7/2006. (V. 24.) Minister Without Portfolio Decree determining the energetic characteristics of buildings [33], and its amending decree of Home Secretary number 20/2014. (III.7.) [34]. (the calculation system includes the EN ISO 13790/2008 standard methodology). The requirements of each level are satisfied, if the calculated values of the building are complying to the limiting value stated in the Decrees above. For new buildings, the Nearly-zero level is mandatory from 2021, but in the light of the ever stricter requirements, it can be expected to be extended to the renovation of the existing buildings as well. To reach the Nearly zero building classification, all the three levels should be fulfilled. The three levels of requirements are the following: 1.) Compliance of structures (U, thermal transmittance value [W/m2K]); This requirement aims for the sufficient heat insulation capability of the structures enveloping the heated volume. A maximum value is defined in Ref. [29]. The value is affected by material, layering and position of the structure. 1 U¼ P ¼ R

Rsi þ

1 � � P di λi

þ Rse

where: U: thermal transmittance [W/m2K] R: thermal resistance [m2K/W] R si: thermal resistance of the indoor surface [m2K/W] R se: thermal resistance of the outdoor surface [m2K/W] d: thickness of structural layer [m] λ: thermal conductivity of the structural layer [W/mK] The U value should be modified (Umod) if the structure is not connecting with outside air (ξ-value): In case of cellar: ξ ¼ 0,5, in case of closing slab-roof: ξ ¼ 0,9. Further modification is needed when considering heat bridges (χ -value): For example: in case of a wall with strong heat bridges: χ ¼ 0,4 (see full table in [33)] Thus: Umod ¼ U*ξ*ð1 þ χ Þ 2.) Compliance of geometry (q, heat loss coefficient [W/m3K]), The second level of requirement is using data from the first step also combining it with geometry of the building (areas and volumes). The aim of this limit is, to have adequately low heat losses, which is why the limit encourages compact buildings. In this level, the calculated values are only dependent on architectural data: the building geometry itself is considered, calculating the heat losses caused by the enveloping surface to heated volume ratio, including the solar gains through fenestration, excluding the engineering systems. The value is represented by the heat loss coefficient. � � X 1 X Qsd q¼ A * Umod þ l*Ψ V 72 where: q: heat loss coefficient [W/m3K] V: heated volume [m3] A: surface of enveloping structures [m2] Umod: modified thermal transmittance[W/m2K] l: perimeter of structure in connection with soil (floor) [m] Ψ : linear heat transmission [none] Qsd: Direct radiation gain for heating season

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Another important energy indicator values calculated here are: specific net heating energy demand qF [kWh/m2a], and total net heating energy demand QF [kWh/a]. QF ¼ H * V * ðq þ 0; 35 * nÞ * σ

ZF * AN *qb

where: QF: total net heating energy demand [kWh/a] H: the thousandth of the annual heating degree days (constant [33]) V: heated volume [m3] q: heat loss coefficient [W/m3K] n: average heat exchange rate (combination of indoor ventilation possibilities and protection from outside wind, from table in Ref. [33]) σ : correction factor of periodic run (depends on function, [33]) ZF: length of the heated season (constant [33]) AN: net heated area of the building [m2] qb: Inner heat load (table in Ref. [33]) qF ¼

QF AN

where: qF: specific net heating energy demand [kWh/m2a] QF: total net heating energy demand [kWh/a] AN: net heated area of the building [m2] 3.) Compliance of engineering systems (EP, Total primary energy consumption [kWh/m2a]) The third level contains the energy consumption of the engineering systems annually in primary energy. The value shows the total energy usage of all the engineering systems, containing their efficiency on common primary energy value. In case of residential buildings, the heating energy and the domestic hot water energy consumption should be summed, as they are the predominant form of energy usage. EP ¼ EF þ EHMV where: EP: total primary energy consumption [kWh/m2a] EF: primary energy demand of heating [kWh/m2a] EHMW: primary energy demand of domestic hot water [kWh/m2a] � X � � Ck * αk * ef þ EF;Sz þ EF;T þ qk;v *ev EF ¼ qF þ qf ;h þ qf ; v þ qf ;t * where: EF: primary energy demand of heating [kWh/m2a] qF: specific net heating energy demand [kWh/m2a] qf,h: losses caused by heat demand and efficiency fitting inaccuracy [kWh/m2a] (Table in Ref. [33]) qf,v: losses of heat distribution [kWh/m2a] (Table in Ref. [33]) qf,v: losses of heat storage [kWh/m2a] (Table in Ref. [33]) Ck: heater performance coefficient ] (Table in Ref. [33]) αk: energy ratio covered by the heater (1, if only 1 heater device ef: heater primary energy conversion factor (1 in case of natural gas) EF,Sz: support energy demand of circulator [kWh/m2a] (Table in Ref. [33]) EF,T: support energy demand of storage [kWh/m2a] (Table in Ref. [33]) qk,v: support energy demand [kWh/m2a] (Table in Ref. [33]) ev: electricity primary energy conversion factor � qHMV;v qHMV;t � X EHMW ¼ qHMV 1 þ þ * ðCk * αk * eHMV Þ þ ðEc þ EK Þ*ev 100 100 where: EHMV: primary energy demand of hot water [kWh/m2a] qHMV: net energy loss of hot water supply [kWh/m2a] qHMV,v: hot water distribution losses [kWh/m2a] (Table in Ref. [33]) qHMV,t: losses of hot water storage [kWh/m2a] (Table in Ref. [33]) Ck: heater performance coefficient ] (Table in Ref. [33]) 16

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V. Sug� ar et al.

αk: energy ratio covered by the heater (1, if only 1 heater device

eHMV: primary energy conversion factor used for hot water (1 in case of natural gas) Ec: support energy demand of circulator [kWh/m2a] (Table in Ref. [33]) EK: support energy demand of storage [kWh/m2a] (Table in Ref. [33]) ev: electricity primary energy conversion factor Appendix B Monument protection types and main aspects in the case study area. PROTECTION TYPE

LOCAL PROTECTION

UNESCO World Heritage

Area of historical monuments

Office of Cultural Heritage Protection rehabilitation guide*

Level of protection

National level by law

Local government level by regulation

National level by law

Law or regulation

2001. LXIV. act law; 39/2015. (III. 11.) regulation

Local regulation of Erzs� ebetv� aros 9/2008. (IV.25.)

International level existing legislation does not impose any powers of authority or competence. 2011. LXXVII Act the World Heritage

What is covered

Individual buildings

Individual buildings

Part of the case study area

Main points

Changes should not affect or endanger the ‘set of values’ (mass, space relations, ratios, symbolic content, façade design, etc.). A protected building cannot be demolished under any circumstances.

The building cannot be demolished. No intervention can result in total or partial destruction, deterioration, transformation or partial or complete alteration of its architectural character.

Part of the case study area To protect the integrity and authenticity of outstanding universal values of the World Heritage Site. ‘ … One of the most authentic sites … the preservation of historical settlement structures and buildings of the protection zone. ‘

* is not considered as protection from, or mandatory itself, however the detailed, building-by-building guideline was created especially for this area to highlight the significance of values of the area. The authors deemed the study especially important from heritage protection point of view. Full area and individual buildings A unique historical quarter of its kind represents a special value for the country, its capital and its district The scale, the size of the existing buildings to be retained, their height, the special arrangements for their construction, the parcel structure and the space structure of the design area to be retained.

þ þ 0

þ þ þ

þ þ þ

0 -

þ -

þ þ

0 -

0 þ -

þ -

WHAT IS PROTECTED? (- -no, 0 -neutral; þ -yes) plot structure mass, space þ þ street view, þ þ street side façade inner courtyard þ structure, þ materials ornamentation þ þ mandatory? þ þ enforced? þ þ/-

7/2005. (III. 1.) Regulation

The parts of the settlement placed under such a protection have characteristic structure, fabric, connection to landscape, the buildings and spaces in-between as a system have historical importance and thus therefore worth historic protection.

Appendix C Summary table about the structural typology Packages. The Packages are containing the type and details of the main enveloping structures, with their U values. A certain building can be classified based on the year of construction. See the supporting methodology in Ref. [6]. Package

Package 1 (1800–1840) Package 2 (1841–1850) Package 3 (1851–1860) Package 4 (1861–1892) Package 5 (1893–1918) Package 6 (1919–1930) Package 7 (1931–1941)

Enveloping structure External wall

Closing slab

Cellar slab and arcade slab

Fenestration

Brick-stone U ¼ 0.98 W/m2K

Covered beam U ¼ 0.83 W/m2K Full timber U ¼ 0.62 W/m2K

Brick barrel vault U ¼ 0.41 W/m2K

Plank-type U ¼ 2.28 W/m2K

Brick U ¼ 1.2 W/m2K Brick U ¼ 1.66 W/m2K Hollow brick wall with concrete frame U ¼ 1.34 W/m2K

Prussian vault U ¼ 0.56 W/m2K Steel with filling U ¼ 0.64 W/m2K

Reinforced concrete U ¼ 2.13 W/m2K Reinforced concrete U ¼ 2.18 W/m2K

Reinforced concrete with filling U ¼ 2.32 W/m2K

Box-type U ¼ 2.28 W/m2K

Joint wing U ¼ 2.5 W/m2K (continued on next page)

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(continued ) Package

Package 8 (1955–1980) Package 9 (1981–2016)

Enveloping structure External wall

Closing slab

Cellar slab and arcade slab

Block with reinforced concrete frame U ¼ 1.4 W/m2K Reinforced concrete with burnt clay U ¼ 0.41 W/m2K

Advanced reinforced concrete U ¼ 2.36 W/m2K

Advanced reinforced concrete with filling U ¼ 2.32 W/m2K

Fenestration

Contemporary reinforced concrete U ¼ 0.58 W/m2K

Contemporary reinforced concrete with filling U ¼ 0.39 W/m2K

Contemporary one-layer PVC or wood U ¼ 1.5 W/m2K

Appendix D Example of the structural Scenarios and upgrades on a Package introduced in Appendix 3. The original state (‘OR’) and the two upgrade scenarios (‘LI’, ‘NZ’) are introduced on the structural typologies’ ‘Package 1’. This’ Package’ contains the characteristic enveloping structures for the traditional apartment buildings built between 1800-1840. Enveloping structure

Structure type

‘OR’ Original state Layers

A

External Wall

Stone brick mix

Not covered, empty firewall

Stone brick mix

Cellar Wall (in case of heated cellar)

Stone brick mix

B

Window

plank framed (prebox style)

C

Cellar upper slab (in case of none heated cellar)

brick vault

Floor on soil

standard wooden parquet

Arcade

D

Closing upper slab

Attic

Brick vault

Covered beam

standard pitched roof

λ [W/ mK]

Thickness [cm]

‘OR’ U [W/ m2K]

‘LI’ Upgrade

‘LI’ U [W/ m2K]

‘NZ‘ Upgrade

‘NZ’ U [W/ m2K]

Requirement level U [W/ m2K]

Whitewash sand 0,81 1,5 mortar Stone brick 0,65 79 mixed (average of limestone and brick) Whitewash sand 0,81 1,5 mortar Whitewash sand 0,81 1,5 mortar Stone brick 0,65 79 mixed (usually limestone and brick) Stone brick 0,65 79 mixed (usually limestone and brick) Standard plank framed window structure with two wing layers opening outside and inside. Wooden frame.

0,98

none

0,98

Indoor insulation (A2) 20 cm (e.g. glass foam boards)

0,24

0,24

1,00

Outdoor insulation (A1) (e.g. mineral wool) 20 cm

0,16

Same as LI

0,16

0,24

0,86

none

0,86

0,30

0,30

2,28

Fitting and Low E glass replacement

1,45

1,14

1,15

Wooden plank (Oak) Wooden plank (Oak) Filling Brick Wooden plank (Oak) Wooden plank (Oak) Filling Wooden plank (Oak) Wooden plank (Oak) Filling Brick Whitewash sand mortar Filling Wooden plank (Oak) Timber (oak)/air Wooden plank (Oak) Reed 3 layers Whitewash sand mortar Wooden plank (Oak) Insulation stone wool Timber (oak)/air Sheetrock

0,41

0,21

0,21

0,26

2,5

Underside insulation (C1) (e.g. mineral wool) 5 cm þ mortar 1,5 cm none

Indoor insulation (A2) 10 cm (e.g. glass foam boards) Fitting, Low E glass for outside layer, new wing for inside layer Same as LI

2,5

In-layer insulation (C2) (e.g. mineral wool) 15 cm

0,23

0,30

0,81

none

0,81

Underside insulation (C3) (e. g. mineral wool) 20 cm þ mortar 1,5 cm

0,17

0,17

0,83

Outdoor insulation (D1) (e.g. mineral wool) 20 cm

0,16

Same as LI

0,16

0,17

0,81

Indoor insulation (D2) (e.g. mineral wool) 30 cm

0,15

Same as LI

0,15

0,17

0,22

2,5

0,22

2,5

0,58 0,78 0,22

30 30 2,5

0,22

2,5

0,58 0,22

10 2,5

0,22

2,5

0,58 0,78 0,81

30 30 1,5

0,58 0,22

10 2,5

R¼ 0,22

0,14 2,5

0,06 0,81

3 1,5

0,22

2,5

0,048

5

R ¼ 0,14 0,4 2

18

V. Sug� ar et al.

Journal of Building Engineering 27 (2020) 100982

Appendix E The energy saving potential based on style, individual building and m2 level. Style

Quantity of buildings

Original energy consumption for heating and domestic hot water (summed) [MWh/a]

Upgraded energy consumption for heating and domestic hot water (summed) [MWh/a]

Difference: The total energy saving potential of the case study area (summed) [MWh/a]

Difference (total) [%]

Average energy saving potential per individual buildings [MWh/a/building]

Average energy saving potential per m2 [kWh/ m2a]

Neo-Classicism Romanticism Historicism Freestyle Premodernism Modernism Socialist Modernism Contemporary Total

33 8 176 91 46 1 7

17 282 5695 123 282 87 169 27 142 295 4238

5319 1787 38 639 26 829 7351 69 1055

11 962 3908 84 643 60 340 19 791 226 3183

69,2 68,6 68,7 69,2 72,9 76,6 75,1

359 484 476 657 428 226 454

182,4 188,1 195,9 177,4 163,9 117,6 125,7

24 386

9560 274 662

3939 84 988

5621 189 674

58,8

234

61,2

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20