Application of the timber-glass upgrade module for energy refurbishment of the existing energy-inefficient multi-family buildings

Energy and Buildings 116 (2016) 362–375

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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Application of the timber-glass upgrade module for energy refurbishment of the existing energy-inefficient multi-family buildings a ˇ Tina Spegelj , Vesna Zˇ egarac Leskovar b,∗ , Miroslav Premrov b a b

Rihter d.o.o., Loke 40, 3333 Ljubno ob Savinji, Slovenia University of Maribor, Faculty of Civil Engineering, Smetanova 17, Maribor, Slovenia

a r t i c l e

i n f o

Article history: Received 29 April 2015 Received in revised form 4 January 2016 Accepted 14 January 2016 Available online 16 January 2016 Keywords: Energy-efficient design Refurbishment Residential buildings Energy saving potentials in the building sector Optimal glazing size Optimal module shape Timber-frame construction

a b s t r a c t The current research study presents two possible approaches to energy-efficient refurbishment of the existing energy-inefficient multi-family buildings. The first involves individual renovation phases, their combinations and a complex energy-efficient refurbishment of the existing multi-family buildings. The second approach analyses the effect on the multi-family building’s annual energy need for heating and cooling produced by a timber-glass upgrade module with the optimal glazing size in its south-oriented fac¸ade which is installed onto the multi-family building. For the purpose of its applicability, the study involves upgrade modules of various shapes and net floor areas, with the optimal glazing size in the south-oriented fac¸ade defined for each of the module shapes. The optimal glazing size of the module is determined by the glazing-to-wall area ratio where the sum total of the annual energy need for heating and cooling of the module is minimal. The study is based on two existing energy-inefficient multi-family buildings with different floor plans on top of which four upgrade modules are installed. The presented results show a possible positive impact the installation of an upgrade module with the optimal glazing size in south-oriented fac¸ade has on energy savings of the building. Within the energy refurbishment process the study offers a possibility of selecting various module shapes and even a different number of upgrade modules with appropriately selected parameters. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Warnings over environmental changes caused by pollutants currently appear among leading issues, with greenhouse gas emissions, particularly those of CO2 , being most frequently exposed. Fossil fuels which are responsible for a substantial part of CO2 emissions often represent the essential source of energy needed for heating the buildings. Residential buildings forming 70% of the total buildings’ surface [1] consume 63% of the total energy required to satisfy the demands of the hosing stock. A major part of the energy in residential buildings is used for heating (67%). The reason for high energy consumption lies in the age of the buildings. New buildings add annually only 1% or less to the existing stock [2], the other 99% of the buildings are the already existing ones and produce about 24% of the energy-use induced carbon emissions [3]. About 2/3 of the existing buildings are over 30 years old and

∗ Corresponding author. ˇ E-mail addresses: [email protected] (T. Spegelj), [email protected] (V. Zˇ egarac Leskovar). http://dx.doi.org/10.1016/j.enbuild.2016.01.013 0378-7788/© 2016 Elsevier B.V. All rights reserved.

approximately 40% are older than 50 years [2]. According to 2004 statistics, more than 50% of the housing stock in EU-25 was built prior to 1970, with a share of 33% dating from 1970 to 1990 period [2–4]. The latter is an important observation since most national building regulations mandating thermal properties of building envelopes were introduced after 1970 [5], which accounts for the fact that buildings built before 1970 mostly lack thermal insulation and have a high thermal transmittance value of the envelope. Until 1970, regulations on the energy efficiency of the buildings were rather loose since the energy consumption failed to be of a particular interest due to low prices of energy sources on the one hand and lack of interest in environmental issues on the other. The first oil crisis triggered the onset of changes in the domain with state regulations setting requirements on the mandatory use of thermal insulation materials in buildings. Following the first oil crises which hit Belgium in 1973, the country witnessed initial changes regarding the use of energy in buildings [6] through the onset of intensive media, conference and lecture-based advising on possibilities of saving energy in the existing buildings. After more than thirty years of informing the Belgians about better energy efficiency in buildings achieved in the case of the region of Flanders, Hens [6] conducted

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Nomenclature Symbols A a b g h n Q U V z  

net floor area (m2 ) length (m) width (m) total solar energy transmittance (%) height (m) air change rate (1/h) annual energy need (kWh/m2 a) thermal transmittance (W/m2 K) heated volume (m3 ) shading factor (%) efficiency of the heat recovery unit (%) linear thermal transmittance (W/m K)

Acronyms ADhrv additionally installed heat recovery ventilation ADlth additionally installed low temperature heating additionally installed mechanical ventilation ADmv additionally installed photovoltaic system ADPV ADsDHW additionally installed solar collectors for DHW ADsh additionally installed solar collectors for heating AGAW proportion of glazing in the south-oriented fac¸ade defined as the ratio between the total area of the south-oriented glazing and the total area of the south-oriented wall domestic hot water DHW HDD heating degree days IMPDHW improved DHW system improved heating system IMPhs MFB multi-family building PHPP passive House Planning Package REPDHW replaced DHW system REPh replaced heating system RP refurbishment phase Ubc U-value of floor slab towards unheated basement U-value of bottom plate Ubo Ud U-value of building entrance door U-value of residential units entrance doors Ued Uf U-value of external wall of residential units U-value of roof/loft ceiling Urf Usr U-value of wall separating the staircase and residential units U-value of external basement wall – submerged Usv Usz U-value of external basement wall – nonsubmerged U-value of windows Uw

a study to examine the influence of information on improving the energy efficiency in the existing buildings. The unusual finding of the study pointed to an increase in the use of energy at that time, in spite of ever stricter regulations. One of the reasons was a lack of interest and knowledge on the part of designers at the time of planning the buildings. Another reason, still encountered today, was a short-term calculation of cost-efficiency focusing solely upon the cost of construction with no consideration of the operational costs of the buildings in the future. Construction companies failed to see the necessity of stopping thermal bridges and sealing air leaks. The government naively believed in the self-awareness of investors and counted on their respecting the regulations upon their own initiative, which most strongly contributed to unfavourable results. According to the Eurostat 2011 data, the energy used for the operation of the buildings in individual member states EU-27

363

varies [7]. The biggest consumers are Germany, France, Great Britain and Italy. An estimation of energy savings to be made with energyefficient refurbishment of the existing buildings needs to be preceded by the existing housing stock analysis whose basic data include the construction year, type of building and the average net floor area. Improving thermal envelope characteristics of the existing as well as of newly-built houses could lead to an annual saving of 90 Mtoe (Million Tonnes of Oil Equivalent) in EU27 by 2030 [8]. The energy consumption analysis of the existing buildings in Torino with the dwellings being classified according to their chronology and typology data proved a necessity of a joint consideration of the dimension, typology and construction as they have a joint influence on the energy efficiency of the building [9]. An analysis of the energy consumption in multi-family buildings in the Greek towns of Thessaloniki and Kalamaria shows that in three out of four cases the real annual energy used for heating surpasses that of its simulation-based calculation by several times [10]. Such discrepancy is accounted for by the use of oversized heating boilers and lack of thermostat control. The absence of the latter consequently leads to heating all residential units over the same period of time with the same amount of energy, which results in the increase in the annual energy need for heating. Most research studies on possible approaches to energyefficient refurbishment of the existing buildings deal primarily with the influence a single parameter or a set of those exert on the annual energy need for heating and cooling of the building. Studies looking for complex solutions and procedures of energyefficient refurbishment of the existing buildings are less frequent. In there article Konstantinou and Knaack [11] present an approach to energy-efficient renovation of multi-dwelling buildings and the subsequent impact on their energy efficiency. One of the solutions is seen in a complete replacement of the roof followed by installing, onto specific roof parts, an additional lightweight-construction residential unit. A research analysing the influence of individual approaches to refurbishment, such as improving the thermal envelope of the existing multi-family building, installing a ventilation device and a new heating system, points to the fact that in the climate zone of the north of China major energy savings are gained by means of fac¸ade insulation and window replacement [12]. Ten Seoul-based households located in a multi-family building dating from 1987 along with another ten from a building constructed in 2000 were included in the analysis of the dwellers’ habits exerting influence on the annual use of energy [13]. The observed habits of using energy for heating, cooling, hot water supply, cooking, lighting, ventilation and household appliances led to the following findings: there was almost no difference in the use of energy in both residential buildings; all the households demonstrated equal usage except for the energy for heating and cooling. The study of three energy-efficient refurbishment procedures involving 36 Moscowbased multi-family buildings, constructed in the period from 1966 to 1972, showed that only a complex refurbishment process of the building can reduce the energy needed for heating and hot ´ water supply by more than 65% [14]. Petrovic´ Becirovi c´ and Vasic´ [15] conducted a research of 62 public buildings situated around Serbia which were subject to the following energy-efficiency oriented procedures: window and door replacement, installing new thermal fac¸ade and roof insulation, in addition to the heating system renewal. Due to strict cost-efficient refurbishment plans the project resulted in a 47% reduction of the annual gross final energy consumption. Energy efficiency of a business three-storey building depending on the climate zone is presented by Boyano in Hernandez [16]. The buildings based in Madrid and London had a 25% and 31% lower annual energy need, respectively, than the building in Tallinn. The above presented research works mainly deal with the influence one or more parameters exert on the annual energy need

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for heating and cooling of the existing energy-inefficient multifamily building. A novelty approach of the present study goes to the analysis of the impact the already developed timber-glass upgrade module [17] has on energy-efficient refurbishment of the existing multifamily buildings. The essence of applying the upgrade module consisting of a light timber structure is seen in its installation onto the existing multi-family building whereby the “bottom plate” of the upgrade module becomes a building element between two heated rooms, i.e. between the existing building and the new module. The existing building and the upgrade module form a single thermal zone. The module’s glazing is limited solely to its southoriented fac¸ade. The content of the current paper is divided into five sections. In Section 1 the background and the objectives of the research work are presented. A brief overview of the Slovene housing stock and a set of good practice examples of multi-family building refurbishment are presented in Section 2. Section 3 considers the numerical study on two energy-inefficient multi-family buildings located in the city of Velenje, both presenting the classical Slovenian housing stock. The first part of the numerical study demonstrates the influence of individual renovation phases – basic renovation tools and their combinations on the energy efficiency of the two energy-inefficient multi-family buildings, without using an upgrade module. Such renovation phases are most often used in the energy renovation process in practice. The second part of the numerical study presents the installation of the already developed timber-glass modules consisting of a light timber structure with the optimal glazing size in the south-orientated fac¸ade [17] as an additional renovation tool providing a positive impact on the energy need of the refurbished multi-family buildings A and B, which is the goal of the presented research. Architects are usually limited in their open ideas about upgrading the existing buildings due to strict energy requirements. Two different possibilities of upgrading the same building are therefore presented in the current article in order to show that such obstacles can be successfully avoided if the optimal shape of the upgrade timber-glass module along with the corresponding glazing size is used. In Section 4, the results of the numerical study are analysed and fully discussed with final conclusions being given in Section 5. The presented conclusions are applicable to locations situated in the Cfb climate zone according to the Köppen–Geiger climate map.

2. Slovene housing stock Final energy consumption in Slovenia in 2011 was 55.16 Mtoe, as stated in EU energy and transport in figures [18]. Transport, the biggest energy consumer (40%) is followed by buildings (36%) in the second place and by industry (24%) in the third. According to the last Slovenian real estate census conducted in 2008 [19], there are 523,983 residential buildings in Slovenia with 849,138 housing units whose average net floor area is that of 167 m2 . Multi-dwelling buildings represent 11% of all residential buildings. The total net floor area of Slovenian flats is 78,793,967 m2 where one-bedroom flats with an average net floor area of 59 m2 and two-bedroom flats with an average net floor area of 79 m2 dominate. According to the 2009 data issued by the Statistical Office of the Republic of Slovenia, the largest share of the energy consumed in households was taken by heating (65.7%), followed by hot water supply (15.9%), with the remaining part (18.4%) used for lighting, cooking and operation of other electricity users. The current state of the housing stock in Slovenia is partly a consequence of frequent changes in the legislation in the past. A considerable number of legislative modifications on energy efficiency in buildings have been adopted since 1970 when the first

Slovenian regulation on thermal insulation in construction entered into force, which was nevertheless 18 years later as compared to Germany where DIN 4108 came into effect back in 1952 [20]. A historical review of legislation points to progressively stricter regulations. Until 1984, the focus was only on thermal transmittance of the building envelope with first demands, fairly minimal in comparison to the existing ones, on the energy use for heating in buildings being introduced only after that year. A substantial share, more than 78% of the multi-family residential buildings in Slovenia were built before 1984 [19]. A period extending over the next 18 years, until 2002, saw no novelties in the relevant legislation and the majority of the Slovenian multi-dwelling hosing stock (97%) had been constructed by that time. As seen in Table 1, 97% of the multifamily buildings in Slovenia prove to be energy-inefficient. In 2002, 2008 and 2010, three regulations on energy efficiency in buildings introducing progressively stricter demands were adopted. Significantly more rigorous demands regarding the thermal transmittance of the external wall came into being no sooner than in 2008, which in turn led to higher annual energy savings. The current global legislation focusing primarily on the thermal envelope of the building fails to provide the best environmental or economical solutions in the field of building construction [22]. High-quality low-energy, passive and other buildings that surpass the requirements of the legislation prove that the latter could be even stricter since their fulfillment is not impossible. Within the framework of the current Slovenian legislation, Regulations on energy efficiency in buildings [23] consider newly constructed buildings and those under reconstruction where at least 25% of the thermal envelope undergoes renovation. Nevertheless, Slovenia and other countries have already experienced such refurbishment the proof of which is demonstrated by partial and complex renovation cases of multi-family buildings constructed in the period from 1953 to 1982, Table 2. Table 2 shows savings in the energy need for heating achieved through refurbishment of individual multi-family buildings in the listed countries. Lower savings are a consequence of partial renovation of buildings, such as window replacement, thermal fac¸ade installation or roof insulation, etc. An example of a low, merely 23% annual saving is a multi-dwelling building in the Slovenian town of Sladki Vrh where the refurbishment process involved window replacement and thermal fac¸ade installation. For a substantial decrease in the energy need, a complex refurbishment process is unavoidable, which can be illustrated by a 94% saving in the case of a multi-dwelling building in the German town of Ludwigshafen which underwent the following renovation process: window replacement, thermal fac¸ade installation, roof and floor insulation, heating system replacement, installation of the heat recovery ventilation system and applying roof-mounted PV panels. Slovenian cases of energy-efficient refurbishment point to a trend of renovation per individual phases. One of the reasons for the partial refurbishment trend can be found in the establishment of the Slovenian Environmental Public Fund (Eco Fund of the Republic of Slovenia) whose purpose is to encourage the implementation of environmental protection policies and promote investments in environmental protection by granting loans with favourable interest rates, non-refundable subsidies and through other activities. Investment programmes of the Fund respect the national environmental protection scheme as well as The European Union environmental policies. According to the Slovenian Eco Fund data [25], the most common investments in 2012 included partial refurbishment of the thermal envelope in residential buildings, such as thermal insulation of the fac¸ade (11.9%) and window replacement (13.7%). Thermal insulation of the rooftop (3.4%) and installation of the heat recovery ventilation (3.1%) represented a minor part of the investments. A relatively small number of subsidy seekers applied for updating their heating systems. The listed partial energy

refurbishment of buildings does not result in major energy savings since the latter demand a complete energy refurbishment of the building.

0.28

/

0.60

0.28

3.1. The aim of the study, its basic limitations and entry data

a

In the period from 1971 to 2000 Slovenia was divided into three climatic zones. A different U value of the external wall was prescribed in each climatic zone.

1.20 0.90 0.80 1.20 0.90 0.80 1.22 0.93 0.93 1.28 1.45 1.68 1.29 1.29

1.29

1.28 1.45 1.68

1.22 0.93 0.93

1.20 0.90 0.80

/ / 70 90 100 100 100 130 170 >180

Energy need for heating (kWh/m2 a) U of the external wall (W/m2 K)a

130

100

1988–1990 1984–1987 1981–1983 1971–1977 1966–1968 Until 1965

l969–1970

1978–1980

365

3. Parametric numerical study

Year of construction

Table 1 Energy need for heating in the building and thermal transmittance of the external wall according to the year of construction, [21].

1991–1995

1996–2000

2001–2002

2003–2008

2009–2010

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The parametric numerical study is based on two possible approaches to energy-efficient refurbishment of the existing energy-inefficient residential buildings with a selection of two energy-inefficient buildings with different floor plans. The buildings’ characteristics along with the phases of their refurbishment are dealt with in Section 3.2 while the following subsection (Section 3.3) discusses the upgrade module features. The aim of the present study is to prove the influence of partial and complex energy-efficient refurbishment on the annual energy need of the selected buildings and to provide evidence of the positive impact a timber-glass upgrade module with the optimal glazing size in the south-oriented fac¸ade has on the energy efficiency of the existing energy-inefficient multi-family buildings. It is also necessary to state some important limitations of the current research. The upgrade module is constructed in a light timber-frame structural system with insulating glazing installed only in the south-oriented fac¸ade. The selected existing buildings under energy renovation are post-war multi-family buildings constructed in a massive structural system with masonry walls and concrete slabs. Climate data for Slovenia (Velenje), which is classified in the Cfb climate zone according to the Köppen–Geiger climate map, are taken into consideration. In the presented study the analysis is carried out only for the energy need for heating and cooling. The efficiency of the recuperation unit is taken into account, while the efficiency of the heating system is not considered. 3.1.1. Software Energy flows in the selected multi-family buildings and the upgrade module are computed by the Passive House Planning Package (PHPP) [26] which allows entry and variation of the parameters relevant to the present research. PHPP is a certified software programme designed for planning low-energy and passive houses. It is based on the EN ISO 13790 standard [27] as well as on other European standards. The above software programme is an instrument architects and civil engineers frequently use for planning and optimising low-energy and passive houses in addition to energyefficient refurbishment of the existing buildings, with the latter being feasible due to the EnerPhit interface. PHPP computation values concerning the annual energy need for heating and cooling of buildings show a minimal deviation from the actual, measured ones, as seen in the already conducted relevant studies [28–31]. The results of our research work are computed by using the monthly computing method whose calculations prove to be more accurate than those provided by the annual computing method. 3.1.2. Climate data The research module is based in the Slovenian town of Velenje which belongs to the Cfb climate zone according to the Köppen–Geiger climate map [32], [33]. The average annual temperature is 9.1 ◦ C with the lowest average temperature in January of −1.7 ◦ C and the highest average temperature in July being +19.9 ◦ C. The average daily temperature range in summer is recorded to be ±10.5 ◦ C [34]. On average, the HDD number for Velenje amounts to 3500 while the HDD for Slovenia ranges from 3000 to 7700 [35]. The horizontal solar radiation is 1224 kWh/m2 a. The average number of solar hours in the period from 2000 and 2008 was approximately 2036 annually [36].

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Table 2 Good practice examples of multi-family building refurbishment [24]. Country

Netherlands

Germany Sweden Denmark France Switzerland Bulgary Austria

Slovenia

Location

Construction year

Energy use prior to refurbishment (kWh/m2 a)

Measures

Energy use after refurbishment (kWh/m2 a)

Savings (%)

Haarlem Raamsdonk Hoogeveen Roermond Leidschendam Ludwigshafen Gaardsten Gyldenrisparken Lineagarden Sundevedsgade Chatelet 3 – Actis Geneva Radomir 1 Radomir 2 Radomir 3 Ried Wartberg Jesenice Ljubljana 1 Ljubljana 2 Sladki vrh Kranj Slovenske Konjice

1960 1963–69 1969 1970 1965 1960–62 1970 1965–69 1920 1880 1966 1953 1980 1980 1980 1979 1979 1961 1975 1965 1982 1963 1975–77

207 240 248 205 179 250 275 147 149 150 191 214 198 192 166 75 122 283 252 252 114 227 136

Uf , Ubp , Uw , ADmv , ADsh , ADsDHW , ADlth , IMPhs Uf , Urf , Uw , ADmv , IMPDHW Uf , Ubp , Urf , Ug , ADmv , REPh , REPDHW Uf , Uw , ADlth , ADhrv , IMPDHW , IMPhs Uf , Ubp , Urf , Uw , ADmv , IMPDHW , IMPhs Uf , Urf , Ubc , Uw , ADhrv , ADPV , ADlth Uf , Uw , ADsh , ADsDHW , ADhrv , IMPhs Ubp , Uf , Urf , Uw , ADhrv , ADsDHW , ADPV Uw , ADhrv , ADPV Uw , ADPV , ADhrv , ADsh , IMPhs Uw , Uf , Ubp , Urf , ADhrv Uw , Uf , Ubp , Urf , IMPhs Uf , Ubc , Urf , Uw , IMPhs , IMPDHW Uf , Ubc , Urf , Uw , IMPhs , IMPDHW Uf , Ubc , Urf , Uw , IMPhs , IMPDHW Uf , Ubp , Urf , Uw , IMPhs , IMPDHW Uf , Ubp , Urf , IMPhs , IMPDHW Ubp , Urf , Uw , IMPhs Uf , Ubp , Urf , Uw Uf , Urf , Uw Uf , Uw IMPhs IMPhs

61 120 113 103 104 15 165 69 84 86 92.5 42 107 102 90 30 47 161 92 92 89 116 80

71 50 54 50 42 94 40 53 44 43 52 80 46 47 46 60 61 43 63 63 23 49 41

3.1.3. Active technical systems A compact unit, which combines recuperation with heat recovery efficiency  = 82% and a heating pump used for space heating and for domestic hot water supply, is provided for each flat in multi-family buildings A and B in renovation phases 10 and 11. One compact unit is provided for each upgrade module. The installed compact units operate continuously. 3.1.4. Shading Overheating of the building and the upgrade module in the summer is prevented by means of external shading devices which are the most effective protection against summer solar radiation [37]. Summer shading calculation for all analysed module shapes is based on the shading factor of z = 50%. 3.1.5. Internal temperature The internal set-point temperature in the heating period is 20 ◦ C, with the internal set temperature for cooling in summer being 25 ◦ C. 3.2. Refurbishment of two multi-family buildings 3.2.1. Selected multi-family buildings In order to compare the influence of the upgrade module on the energy efficiency, two older multi-family buildings located in

the Slovenian town of Velenje have been chosen. Both buildings typically lack thermal insulation resulting in a high amount of the energy need for heating. The thermal transmittance of the envelope elements of both buildings is presented in Section 3.2.2. The selection of the two buildings exhibiting highly different geometry referring to a/b sides ratios was made for two reasons. The first reason is to clearly present the importance of using the optimal shape of the developed upgrade module. The second reason is to show that the ratio is also a vital variable parameter since it permits a selection of appropriate ground floor shapes of the upgrade module which correspond to the ground-floor shape of the existing building to undergo renovation. Multi-family building A, constructed in 1951, is a three-storey building with three separate entrances giving access to 18 residential units, as seen in Fig. 1a. The length of the south and the north sides of the building is a = 59.00 m with the glazing size in the south-oriented fac¸ade of AGAWVSA = 15.2%. The width and the height of the building are b = 9.64 m and h = 12.10 m respectively. The a/b sides ratio is 6.12. The total net floor area of the residential units is 1252.57 m2 . Underneath, there is a partially submerged non-heated basement. The building has not undergone any major renovation works except for the regular maintenance procedures. The thermal envelope has not been improved either. Multi-family building B, constructed in 1965, has a single entrance leading to the ground floor and the next four floors, Fig. 1b.

Fig. 1. Multi-family building A and multi-family building B.

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Table 3 Basic situation of multi-family buildings A (MFB A) and B (MFB B). Thermal transmittance Windows and doors (W/m2 K)

Thermal transmittance Fac¸ade (W/m2 K)

Thermal transmittance Roof-loft ceiling (W/m2 K)

Thermal transmittance Basement (W/m2 K)

Thermal transmittance Staircase (W/m2 K)

Air change rate for a pressure difference of 50 Pa (h−1 )

Ventilation heat recovery efficiency (%)

MFB A

Ud = 4.6 Uw = 5.7

Uf = 1.18

Urf = 1.38

Usr = 1.31 Ued = 2.19

n50 = 7.0

=0

MFB B

Ud = 4.6 Uw = 5.7

Uf = 1.51

Urf = 2.07

Usv = 2.45 Usz = 2.45 Ubc = 1.10 Ubp = 3.40 Usv = 3.20 Usz = 3.20 Ubc = 1.95 Ubp = 2.88

Usr = 1.86 Ued = 2.19

n50 = 7.0

=0

The glazing size of the 18.00 m (a = 18.00 m) long south-oriented fac¸ade is AGAWVSB = 34.2%. The width and the height of the building are b = 19.00 m and h = 16.62 m respectively. The a/b sides ratio is 0.95. The total net floor area of the residential units is 1138.06 m2 . Underneath, there is a partially submerged basement with parking spaces. The building has been subject only to regular maintenance works. The thermal envelope has not been improved. 3.2.2. Thermal envelope of the two multi-family buildings The thermal transmittance of each building element of both envelopes is given in Table 3. The numbers indicate poor quality of the envelopes, which is responsible for enormous annual energy losses and consequently for a considerably high amount of the energy need for heating and cooling of the two buildings. Table 3 demonstrates a lower envelope quality of the second multi-family building (MFB B) in comparison with that of the first building (MFB A), which means higher transmission losses per m2 of the envelope in MFB B. 3.2.3. Individual refurbishment phases and combinations For the purpose of analysing the impact of partial and complete refurbishment phases on the annual energy need for heating and cooling of multi-family buildings A and B, several energy-efficient renovation phases have been provided, as presented in Table 4. Refurbishment phases (RP 1–5) represent energy-efficient refurbishment of individual parts of the thermal envelope.

Refurbishment phase 1 involves replacing the existing windows with new energy-efficient ones. Applying thermal insulation to the fac¸ade is planned as phase 2, with phase 3 implying thermal insulation of the loft ceiling. Phase 4 deals with the non-heated basement ceiling insulation while phase 5 includes residentialunits door replacement along with insulating the walls facing the non-heated staircase. The phases that follow, RP 6–RP 9, are combinations of RP 1–RP 5 where theses logically complement each other. Window replacement taking place simultaneously with the fac¸ade renovation allows thermal insulation to be applied to window frames, which reduces thermal bridges caused by window installation. Installing windows once the fac¸ade has been renovated would not be a sensible order of phases due to a risk of damaging thermal insulation and the finishing coat. Additional thermal insulation of the roof follows in phase 7 with the nonheated basement ceiling insulation being foreseen as phase 8. Since hallways and staircases are partially heated, phase 9 brings thermal insulation to the walls between residential units and hallways and envisages new thermally-insulated entrance doors for residential units. The thermal transmittance values of individual building elements involved in refurbishment phases 1–5 and partial refurbishment phases 6–10 satisfy the requirements of the current legislation in Slovenia [23]. Thermal transmission of individual building elements, air-sealing and ventilation within phases 11 and 12 are identical to those of the upgrade module.

Table 4 Individual energy-efficient refurbishment phases for multi-family buildings A and B. RP

Thermal transmittance Windows and doors (W/m2 K)

1

Ud = 1.6 Uw = 1.3

7 8 9 10 11 12

Thermal transmittance Roof-loft ceiling (W/m2 K)

Thermal transmittance Basement (W/m2 K)

Thermal transmittance Staircase (W/m2 K)

Uf = 0.280

2 3 4 5 6

Thermal transmittance Fac¸ade (W/m2 K)

Urf = 0.20 Ubc = 0.35 Usr = 0.70 Ued = 1.60 Ud = 1.6 Uw = 1.3 Ud = 1.6 Uw = 1.3 Ud = 1.6 Uw = 1.3 Ud = 1.6 Uw = 1.3 Ud = 1.6 Uw = 1.3 Ud = 1.0 Uw = 0.7 Ud = 0.9 Uw = 0.7

Uf = 0.280 Uf = 0.280

Urf = 0.20

Uf = 0.280

Urf = 0.20

Ubc = 0.35

Uf = 0.280

Urf = 0.20

Ubc = 0.35

Uf = 0.280

Urf = 0.20

Ubc = 0.35

Uf = 0.165

Urf = 0.165

Ubc = 0.165

Uf = 0.100

Urf = 0.100

Ubc = 0.10

Usr = 0.70 Ued = 1.60 Usr = 0.70 Ued = 1.60 Usr = 0.165 Ued = 1.00 Usr = 0.10 Ued = 1.00

Air change rate for a pressure difference of 50 Pa (h−1 )

Ventilation heat recovery efficiency (%)

n50 = 5.0

=0

n50 = 6.5 n50 = 6.5 n50 = 6.0 n50 = 7.0

=0 =0 =0 =0

n50 = 4.5

=0

n50 = 4.0

=0

n50 = 3.0

=0

n50 = 3.0

=0

n50 = 2.0

 = 82

n50 = 0.6

 = 82

n50 = 0.6

 = 82

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Fig. 2. Installation of the upgrade module on the existing multi-family building and the energy flows of the system.

3.3. Timber-glass upgrade module An upgrade module is installed onto the existing multi-family building after the completion of energy-efficient refurbishment phases 10, 11 and 12, as schematically presented in Fig. 2. The upgrade module is a one-storey building unit. The building materials of the upgrade module are solid or glued-laminated timber and insulating glass, combined into the light timber-frame construction system. The advantage of such construction system is its low weight compared to other massive construction systems. Low weight of the timber-frame construction allows its installation on the existing massive multi-family buildings. Old buildings with a massive masonry construction system often have over-dimensioned load bearing elements. According to preliminary static calculation, additional weight can be installed on top of the existing buildings in the majority of cases. A good practice example is seen in upgrading ˇ a hotel in Cateˇ z, Slovenia [38], where a timber upgrade unit was installed on the existing massive-construction-system building. A further advantage is seen in the insulation of the existing roof with a light upgrade module. Pre-fabrication of the construction elements (walls, roof) allows for easy and time-efficient installation of the upgrade modules on the existing buildings. In the process of installing the upgrade module on the existing building the bottom plate of the module replaces the roof of the existing energy-inefficient building and becomes a building element between two heated rooms having the same indoor temperature, which means negligible, i.e. zero computation transmission losses through the module’s bottom plate. The thermal transmittance values of the module envelopes are given in Table 5. Sections 3.3.1–3.3.3 describe further upgrade modules’ properties, in addition to those presented in Section 3.1. 3.3.1. Shape of the module and its orientation The chosen module’s floor plan is rectangular, with sides a and b whose length is subject to variation, as schematically shown in Fig. 3. In our analysis, side a with the glazing installed always faces south, with side b being perpendicular to side a. The shape of the

module is defined by the sides a/b aspect ratio within a range of ratios from 0.18 ≤ a/b ≤ 8.0. The net floor areas are Anet1 = 200 m2 , Anet2 = 400 m2 and Anet3 = 600 m2 . The net heated volumes are Vnet1 = 500 m3 , Vnet2 = 1000 m3 and Vnet3 = 1500 m3 . 3.3.2. Glazing in the south-oriented fac¸ade Triple glazing used with a glass configuration of 4E-12-4-12-E4 and krypton-filled interspaces has a thermal transmittance coefficient of Ug = 0.51 W/m2 K. The total solar energy transmittance is g = 52%. A PVC spacer between the glass panes has the value of  = 0.033 W/m K. The thermal transmittance of the whole window is Uw = 0.68 W/m2 K. Solar gains in the module will largely depend on the proportion of glazing in the south-oriented fac¸ade (AGAW), i.e. on the ratio between the glazing size and the size of the south fac¸ade. The optimal glazing size (AGAWopt ) in the south fac¸ade equals the proportion of glazing in the south fac¸ade at which the annual energy need for heating and cooling in the selected size of the module is minimal. Ambient lighting is not the subject of the current study. The north, east and west oriented fac¸ades have no glazing installed. 3.3.3. Construction of the module Basic vertical load bearing elements in a timber-frame construction system are timber-panel walls composed of load bearing timber frames and sheathing boards. Thermal insulation is inserted between the timber studs and girders. The sheathing board on the internal side of the wall is usually a wood-based or a fibre-plaster board. Additional thermal insulation is inserted on the external side (fac¸ade side) of the wall. The total thickness of thermal insulation depends on the required thermal transmittance of the wall. The numerical analysis of the module is conducted for a structure with two different thermal transmittance values. The thermal transmittance of the external walls (TF-1) and the roof in the first module case is U1 = 0.100 W/m2 K, with that of the second module case (TF-2) being higher, U2 = 0.165 W/m2 K. Since the module is

Table 5 Characteristics of the upgrade modules. Module

a/b sides ratio

Net floor area (m2 )

Windows (W/m2 K)

Fac¸ade (W/m2 K)

Roof-ceiling (W/m2 K)

Air-sealing (h−1 )

Ventilation (%)

AGAWopt (%)

A1.1 A1.2 A2.1 A2.2 B1.1 B1.2 B2.1 B2.2

4.50 4.50 2.42 2.42 0.98 0.98 0.72 0.72

400 400 2 × 200 2 × 200 200 200 200 200

Uw = 0.68 Uw = 0.68 Uw = 0.68 Uw = 0.68 Uw = 0.68 Uw = 0.68 Uw = 0.68 Uw = 0.68

Uf = 0.165 Uf = 0.100 Uf = 0.165 Uf = 0.100 Uf = 0.165 Uf = 0.100 Uf = 0.165 Uf = 0.100

Urf = 0.165 Urf = 0.100 Urf = 0.165 Urf = 0.100 Urf = 0.165 Urf = 0.100 Urf = 0.165 Urf = 0.100

n50 = 0.6 n50 = 0.6 n50 = 0.6 n50 = 0.6 n50 = 0.6 n50 = 0.6 n50 = 0.6 n50 = 0.6

 = 82  = 82  = 82  = 82  = 82  = 82  = 82  = 82

43 41 45 42 68 64 80 75

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Fig. 3. Altering the upgrade module shape: (a) a/b = 0.18, (b) a/b = 2.00, (c) a/b = 8.00 and the glazing orientation.

developed for the purposes of installation onto the existing building the floor-plate is not a part of its structure. 3.3.4. Optimal upgrade modules Different floor plans of the existing multi-family buildings demand a variation of the upgrade module floor plans. The optimal glazing size in the south-oriented fac¸ade defined for different upgrade module shapes with different areas Anet1 –Anet3 and with the thermal transmittance values U1 and U2 , numerically developed and presented in a research study [17], is shown in Fig. 4. The optimal glazing size in the south fac¸ade of the module with the shape a/b < 0.50 and the area Anet1 = 200 m2 exceeds 100% in both construction systems, Fig. 4. In the module with the area Anet2 = 400 m2 the optimal glazing size exceeds 100% for the shape a/b < 0.72 with U1 = 0.100 W/m2 K and for the shape a/b < 0.98 with U2 = 0.165 W/m2 K. The glazing size furthermore exceeds 100% for the shape a/b < 0.98 with U1 = 0.100 W/m2 K and for the shape a/b < 1.28 with U2 = 0.165 W/m2 K in the module with the area Anet3 = 600 m2 . The reason lies in small glazing sizes in the southoriented fac¸ade and the subsequently low solar gains in the heating period. The prevailing part of the energy need is required for heating. Extending the length of side a results in the enlargement of the glazing size and the consequent decrease in the module’s energy need for heating in winter on the one hand and an increase in the energy need for ventilation in summer, on the other. Fig. 4 provides values defining the optimal glazing size in the south-oriented fac¸ade for the module with the shapes where the glazing size does not exceed 100%. The energy need for heating and cooling (Qh + QC ) of the modules with the optimal glazing size in the south-oriented fac¸ade is shown in Fig. 5. The annual energy need for heating and cooling (Fig. X) of the modules with U2 = 0.100 W/m2 K is approximately 50% lower than that of the modules with U1 = 0.165 W/m2 K, at the three net floor areas (Anet1 –Anet3 ). The lowest annual energy need Qh + Qc = 5.27 kWh/m2 a is observed in the module with U2 = 0.100 W/m2 K at Anet3 = 600 m2 and represents 25% of the maximum annual energy need Qh + Qc = 21.48 kWh/m2 a of the module with U1 = 0.165 W/m2 K at Anet1 = 200 m2 . 3.3.5. Characteristics of the selected upgrade modules On account of a very longitudinal shape (a/b = 6.12) of the existing multi-family building A (MFB A) two optional upgrade solutions will undergo separate analysis and comparison. Step one consists of placing an upgrade module with the net floor area of A1 = 400 m2 as a single piece (A1) on the existing construction (Fig. 6a). The corresponding optimal glazing size (AGAWopt ) will be determined according to the given equations in Fig. 4, separately

for two different envelope U-values, those of U1 = 0.165 W/m2 K (A1.1) and U2 = 0.100 W/m2 K (A1.2). In the following step, two upgrade modules (A2) having the net floor area of 2A2 = 2 × 200 m2 will be installed on the existing old building, with (A2.1) belonging to a low-energy and (A2.2) to a passive energy upgrade solution. Since an almost square floor-plan of the existing multi-family building B (MFB B) proves to be completely different from that of MFB A only the solutions with the upgrade module in a single piece will be analysed in this case. Two different selected a/b values of the modules (B1 and B2) with the net floor area of 200 m2 will be analysed with the envelope types in a low-energy and passive standard, Fig. 6b. It is important to point out that the selected upgrade module shapes determined according to Fig. 4 allow for their installation on the existing multi-family buildings A and B whose different floor plans assure various design solutions. Fig. 6a shows the shape of multi-family building A with upgrade modules A1 and A2, while Fig. 6b features the shape of multi-family building B with upgrade modules B1 and B2. The optimal glazing size in the south-oriented fac¸ade (AGAWopt ) for both buildings is taken from the developed functions presented in Fig. 4. The chosen AGAWopt values and the characteristics of the selected upgrade modules are listed in Table 5. The selection of different module shapes was made with a purpose of analysing options where the existing roof is covered either with a single large module (cases A1, B1, B2) or with two smaller modules (case A2), all of whose “b” side length corresponds to the width of the existing building. It is evident from Table 5 that the difference in the optimal proportion of glazing in modules A1 and A2 is minimal (≤4%), while the difference in AGAWopt in modules B1 and B2 proves to be higher (≤16%). The shape of module B2 exhibits quite a small south oriented wall surface in comparison to the perpendicular fac¸ade and its AGAWopt is consequently higher in order to reach the appropriate glazing area. Table 6 illustrates all of the considered refurbishment phases (RP) of multi-family buildings A and B combined with the upgrade modules. As seen in Table 6, individual combinations of the refurbished multi-family buildings (RP 10 to 12) and the upgrade modules generate new renovation phases, RP 13 to 20. 4. Analysis of the parametric numerical study results Individual approaches to energy-efficient refurbishment of the existing energy-inefficient buildings are discussed separately in the following subsection. Section 4.1 treats the first approach to energy-efficient refurbishment of multi-family buildings A and B,

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Fig. 4. AGAWopt depending on the module shape (sides ratio a/b) and area (A) for the values of U1 = 0.100 W/m2 K (a) and U2 = 0.165 W/m2 K (b) [17].

24

Anet1 = 200 m2 U1 = 0.100 W/m2K

22 20

Anet2 = 400 m2 U1 = 0.100 W/m2K

Qh + Qc [kWh/m2a]

18 16

Anet3 = 600 m2 U1 = 0.100 W/m2K

14 12

Anet1 = 200 m2 U2 = 0.165 W/m2K

10 8

Anet2 = 400 m2 U2 = 0.165 W/m2K

6 4

Anet3 = 600 m2 U2 = 0.165 W/m2K

2 0

0

1

2

3

4 a/b

5

6

7

8

Fig. 5. Energy need for heating and cooling (Qh + Qc ) of the modules with the areas Anet1 = 200 m2 , Anet2 = 400 m2 and Anet3 = 400 m2 and thermal transmittance of the thermal envelope U1 = 0.165 W/m2 K and U2 = 0.100 W/m2 K.

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Table 6 Multi-family buildings A and B after energy-efficient refurbishment phases (RP) 10 to 12 combined with upgrade modules A1 and A2 and upgrade modules B1 and B2.

Fig. 6. Installation of modules A1 and A2 on multi-family building A (a) and installation of modules B1 and B2 on multi-family building B (b).

with renovation phases 1 to 12 (Table 4). The results of the second approach to energy-efficient refurbishment of multi-family buildings A and B, with renovation phases 13 to 20 (Table 6), are the subject of Section 4.2. 4.1. Refurbishment of two existing multi-family buildings without the upgrade module Fig. 7 shows the sum total of the annual energy need for heating and cooling (Qh + Qc ) of the existing multi-family buildings A and B prior to refurbishment (RP 0), the (Qh + Qc ) for refurbishment of individual building elements (RP 1–RP 5) along with that of refurbishment combinations (RP 6–RP 12).

Module/RP Building A

A1.1

A1.2

2 × A2.1

2 × A2.2

RP 10 RP 11 RP 12 Module/RP Building B RP 10 RP 11 RP 12

RP 13 RP 17

RP 15

RP 14 RP 18

RP 16

B1.1 RP 13 RP 17

RP 19 B1.2 RP 15

B2.1 RP 14 RP 18

RP 19

RP 20 B2.2 RP 16 RP 20

Evidently, the sum total of the annual energy need for heating and cooling (Qh + Qc ) of the existing multi-family buildings A and B was highest before the process of refurbishment (RP 0). In phases 1 to 5, involving renovation of individual building elements, most of the annual energy need for heating and cooling (Qh + Qc ) of both multi-family buildings (A and B) is saved on account of the improved thermal insulation of the fac¸ade (RP 2), i.e. of the largest surface where the transmission losses occur. In the case of multi-family building A, phase 2 leads to 21.6% annual energy savings as compared to 22.9%, relevant to the savings in building B. The lowest decrease in the annual energy need for heating and cooling (Qh + Qc ) of both multi-family buildings is recorded in phase 5 which comprises thermal insulation of the residential units walls facing the non-heated staircase and fitting new energy-efficient entrance doors to residential units. Building A thus saves 2.6% of the annual energy need, with building B attaining 5.5%. A significant decrease in the annual energy need is observed in phases 6 to 10 covering a combination of renovation processes. The lowest point is reached through phases 9 to 10 which involve complex energy-efficient refurbishment of the two existing multi-family buildings, performed in compliance with the current Slovenian legislation. Owing to phase 9, which does not comprise heat recovery ventilation, the annual energy saving of building A is 63.9% with that of building B amounting to 67.8%. Installation of the heat recovery ventilation device (RP 10) results in additional 7.5% annual energy savings in both multi-family buildings. Further thermal envelope improvement (buildings A and B) in refurbishment phases 11 and 12 leads to even more savings due to lower transmission losses occurring through the building envelope. In phases 0 to 8 comprising refurbishment of individual building elements and a combination of individual renovation procedures, the annual energy need for heating and cooling (Qh + Qc ) of

Fig. 7. Annual energy need (Qh + Qc ) for the basic situation and for refurbishment phases 1–12 of multi-family buildings A and B.

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Fig. 8. Annual energy need (Qh + Qc ) for refurbishment phases 10 and 13–16 performed on multi-family buildings A and B.

multi-family building A proves to be lower than that of multi-family building B. On the contrary, in phases 9 to 12 within which multi-family buildings A and B undergo a complex energy-efficient refurbishment, the annual energy need of building A surpasses that of building B. 4.2. Timber-glass upgrade module’s influence on the energy efficiency of multi-family buildings A and B The influence of the timber-glass upgrade module with the optimal glazing size in the south-oriented fac¸ade on the annual energy need of multi-family buildings A and B is shown in Figs. 8–10. Installation of the timber-glass upgrade module onto the existing multi-family building results in the enlarged surface of the thermal envelope in addition to the increase of the net floor area and the volume of the building. Fig. 8 shows the annual energy need for heating (Qh ) and cooling (Qc ) using the upgrade module for refurbishment phase 10 performed on multi-family buildings A and B. Combinations RP 13 to RP 16 with different positions of the upgrade modules presented in Fig. 6 are taken from Table 6. Fig. 8 gives evidence of a positive impact the upgrade module has on the annual energy need (Qh + Qc ) of multi-family buildings A

and B. The annual energy need in refurbishment phases 13 to 16 is lower than in phase 10. The influence of upgrade modules A1 and A2 on the annual energy need of multi-family building A is bigger than that of modules B1 and B2 in the case of multi-family building B. Installation of upgrade module A1.1 (RP 13) on multi-family building A brings a 29.2% decrease in the annual energy need, as compared to phase 10 while the savings based on upgrading with module A1.2 (RP 15) rise by 32.3%. Savings in the annual energy need in refurbishment phase 14, where upgrading involves two modules A2.1, prove to be even higher than in phases 13. Installation of upgrade module A2.1 (RP 14) results in a 28.8% decrease in the annual energy need with that of upgrading with module A2.2 attaining 31.5%. Annual energy savings of multi-family building B show no significant discrepancy between refurbishment phases 13 and 14 on the one hand and phases 15 and 16 on the other. In phases 13 and 15 the annual savings amount to 18.8%, as compared to phase 10, with those relevant to phases 14 and 16 being 20.5%. Multi-family building B with an almost rectangular floor plan allows installation of two different upgrade modules, B1 and B2, with a similar shape (Fig. 6 and Table 6). The annual energy need of modules A and B is consequently similar too and such is their influence on the energy efficiency of building B.

Fig. 9. Annual energy need (Qh + Qc ) for renovation phases 11 and 17 to 18 of multi-family buildings A and B.

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Fig. 10. Annual energy need (Qh + Qc ) for renovation phases 12 and 19–20 of multi-family buildings A and B.

Fig. 9 displays the annual energy need of multi-family buildings A and B refurbished within renovation phase 11 and the upgrade module, which encompasses improvement of the thermal envelope transmittance to U = 0.165 W/m2 K. Combinations RP 17 and RP 18 with different positions of the upgrade modules are taken from Table 6. The annual energy need (Qh + Qc ) in phases 17 and 18, which involve installation of the upgrade modules onto multi-family buildings A and B, previously refurbished through phase 11, is lower than that in refurbishment phase 11. The impact of upgrade modules A1 and A2 on the annual energy need of multi-family building A, already refurbished in phase 11, is more powerful than that of modules B1 and B2 installed on multi-family building B which underwent phase 11 beforehand. Refurbishment phases 17 and 18 performed on multi-family building A reduce the annual energy need by 32.2% and 30.5% respectively, with reference to the energy need in phase 11. It is evident that installation of two A2 upgrade modules instead of a single A1 upgrade module has a lower impact on the reduction of the energy need for building A although the difference in the total energy need is not of a significant nature. A 19.4% annual energy saving in the case of multi-family building B is identical in both phases (17 and 18), which means that the chosen shape of the upgrade module in this case is of absolutely no importance. Fig. 10 displays the influence of refurbishment phases 19 and 20 exerted on the annual energy need (Qh + Qc ) of the upgrade modules installed on multi-family buildings A and B previously renovated through phase 12, Table 6.

As expected, according to Figs. 9 and 10, phases 19 and 20 both result in identical, 21.4% annual energy savings of multi-family building B, with reference to phase 12. Phase 19 decreases the annual energy need of multi-family building A by 34.1%, with reference to phase 12, while the annual savings of refurbishment phase 20 (building A) attain 33.5%. A difference in the energy need between modules A1 and A2, in this case, proves to be smaller than that achieved through renovation with a low-energy module (Fig. 9) and can be practically neglected. A comparison of all the discussed renovation phases shows that a major part of energy savings through renovation phases RP 1 to RP 9 arise from the reduction of transmission losses through the improved thermal envelope of the existing multi-family buildings A and B. Only a smaller part of energy savings are due to improved air-sealing. Renovation phases RP 10 to RP 12 lead to additional energy savings appearing as a result of the installed heat recovery ventilation. In renovation phases RP 13 to RP 20, comprising installation of the timber-glass upgrade module, there is evidence of an essential decrease in transmission losses through the existing roof. In addition, the optimal south-oriented glazing size of the module allows for high solar gains. The costs of different renovation phases of the multi-family buildings depend on the building’s properties, such as window area, compactness ratio, fac¸ade type, roof type, etc., and on the building’s location. The costs of individual renovation phases for the current case study are presented in Fig. 11. The costs of different renovation phases for multi-family buildings A and B, presented in Fig. 11, include only those of the

Fig. 11. Costa of different renovation phases (RP 1–RP 20) of multi-family buildings A and B.

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embedded material and work. As expected, the lowest costs are seen in renovation phases 1–5 where only one building component is improved. The costs of renovation phases 6–9 involving combinations of individual energy improvements of the building components are higher. The highest costs appear with renovation phases 10–12 where all the components are improved and the system for heating and ventilation is installed. In renovation phases 13–20 comprising the installation of upgrade modules on multifamily buildings A and B, the net floor area increases by 200 m2 (RP 13, RP 15, RP 17, RP 19) and 400 m2 (RP 14, RP 16, RP 18, RP 20). With both multi-family buildings, A and B, the highest costs arise in renovation phase 20 and the lowest in renovation phase 13. The difference in individual renovation phases costs between multi-family building A and multi-family building B is minimal. The production price of the upgrade module with the thermal transmittance of the external wall U2 = 0.165 W/m2 K is around 1050.00 D /m2 , while the price of the upgrade module with the thermal transmittance of the external wall U1 = 0.100 W/m2 K is slightly higher, amounting to approximately 1100.00 D /m2 . According to the Report on the Slovenian real estate market for 2014 [39], the average 2014 market price of the existing flats in Slovenia was set at 1490.00 D /m2 . Market prices of newly built flats surpass those of the existing flats by more than two times. The market price of the flat-unit in the upgrade module is higher than the production costs of a new module. By selling the flat-units in upgrade modules a part of costs for energy refurbishment of the existing multi-family building can be covered.

5. Conclusion and recommendations The aim of the study was to present that an important additional decrease in the annual energy need resulting from energy-efficient refurbishment of the multi-family buildings is achieved by installing a timber-glass upgrade module with the optimal glazing size in the south-oriented fac¸ade onto the existing multi-family building. Different floor plans of the existing multi-family buildings led to developing a variety of module shapes (a/b) along with the optimal proportion of glazing in the south-oriented fac¸ade (Fig. 4), which allows for installation of a single module or more modules having the same or a different shape onto the existing multi-family building with a given floor plan. A positive effect of the upgrade module on the annual energy need of the multi-family building is seen in Figs. 8–10. Application of the upgrade modules to multifamily buildings A and B leads to an 80% to 92% range of annual energy (Qh + Qc ) savings, with reference to the basic situation of the multi-family buildings (RP 0). Selecting modules B1 and B2 with a similar shape (a/b) showed that the impact of similarly shaped modules exerted on the energyefficient refurbishment of the multi-family building is the same. With respect to energy-efficient renovation of a multi-family building whose floor plan resembles that of multi-family building B, with the sides aspect ratio of a/b = 0.95, a reasonable choice is to choose the upgrade module with a poorer thermal envelope (U2 = 0.165 W/m2 K). Installation of the upgrade module with better thermal features (U1 = 0.100 W/m2 K) means only a minimal annual energy saving, Fig. 7. Hence, in the case of a multi-family building with the “B” floor plan with almost a square shape, it is sensible to apply energy-efficient renovation by means of installing a single upgrade module with a poorer thermal envelope (U2 = 0.165 W/m2 K). Renovation of multi-family building A was realised by applying two upgrade modules of different shapes, A1 and A2. The floor plan of multi-family building A allows installation of a single larger A1 module or that of two smaller upgrade modules A2 with a more

dynamic overall appearance of the upgrade structure. The coverage area of the multi-family building’s roof is identical in both cases. The obtained numerical results demonstrate that the difference in the annual energy need following the refurbishment by installing a single upgrade module A1 or two upgrade modules A2, having the same floor size but a different proportion of glazing in the south fac¸ade (AGAWopt ) along with a different surface area of the thermal envelope, is minimal and can be neglected. This is an important fact which enables architects to opt for more attractive shape solutions by means of structural upgrading of the existing old buildings or simply by following the ground floor shape of the existing building. Application of the upgrade module with better thermal features (U1 = 0.100 W/m2 K) to multi-family building A brings only a 3% higher reduction of the annual energy need (Fig. 8), in comparison to the results given by installing the module with a poorer thermal envelope (U2 = 0.165 W/m2 K). The given conclusions may present general guidelines to be applied in practice – choosing the upgrade module shape and area according to the shape of the existing building along with selecting the optimal AGAW of the upgrade module, taken from the already developed relation in Fig. 4. The results of the presented study show a positive impact of the upgrade module on energy savings of the two selected multifamily buildings with different floor plans and a different number of storeys. Applying one or more upgrade modules on an optionally selected multi-family building will lead to a different range of savings which depend on the shape and number of upgrade modules as well as on the existing multi-family building selected for renovation. Besides a beneficial influence on energy savings, the application of upgrade modules on the existing buildings proves to have positive economic and sustainability-based impacts. With installation of an upgrade module on the existing multi-family building new additional net floor areas are obtained and could be offered to the real estate market. The costs of the upgrade module are in this manner covered by selling of the new areas. Moreover, upgrading of the existing buildings is particularly suitable for urban areas where it is often the only possible way of increasing the density of residential surfaces. The developed functional dependence between the optimal glazing size in the south-oriented fac¸ade (AGAWopt ) and the module shape (a/b) enables civil engineers and architects to make a time-saving selection of the shape and the number of modules to suit upgrading of the existing multi-family building with a given floor plan. The currently developed upgrade module has its glazing installed only in the south fac¸ade, which is the most optimal choice due to solar gains in the winter even though the selected glazing position requires efficient shading in summer time in order to prevent overheating. The given conclusions can be generally implemented in locations situated in the Cfb climate zone according to the Köppen–Geiger climate map. With a view to enhancing the applicability of the upgrade module, we aim at focusing our further studies onto the optimal glazing sizes in other fac¸ade orientations.

Acknowledgements Operation part is financed by the European Union, European Social Fund. Operation is implemented in the framework of the Operational Programme for Human Resources Development for the Period 2007–2013, Priority axis 1: Promoting entrepreneurship and adaptability, Main type of activity 1.1. Experts and researchers for competitive enterprises.

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