Embodied energy in residential buildings-towards the nearly zero energy building: A literature review

Embodied energy in residential buildings-towards the nearly zero energy building: A literature review

Accepted Manuscript Embodied energy in residential buildings-towards the nearly zero energy building: A literature review Panagiotis Chastas, Theodoro...

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Accepted Manuscript Embodied energy in residential buildings-towards the nearly zero energy building: A literature review Panagiotis Chastas, Theodoros Theodosiou, Dimitrios Bikas PII:

S0360-1323(16)30198-6

DOI:

10.1016/j.buildenv.2016.05.040

Reference:

BAE 4514

To appear in:

Building and Environment

Received Date: 6 March 2016 Revised Date:

19 May 2016

Accepted Date: 30 May 2016

Please cite this article as: Chastas P, Theodosiou T, Bikas D, Embodied energy in residential buildingstowards the nearly zero energy building: A literature review, Building and Environment (2016), doi: 10.1016/j.buildenv.2016.05.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Embodied energy in residential buildings-Towards the nearly zero energy building: A literature review Panagiotis Chastas1*, Theodoros Theodosiou2, Dimitrios Bikas3 1

*Corresponding author: [email protected]

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Aristotle University of Thessaloniki, Dept. of Civil Engineering, Laboratory of Building Construction and Building Physics, 54124 Thessaloniki, Greece. 2 Aristotle University of Thessaloniki, Dept. of Civil Engineering, Laboratory of Building Construction and Building Physics, 54124 Thessaloniki, Greece. 3 Aristotle University of Thessaloniki, Dept. of Civil Engineering, Laboratory of Building Construction and Building Physics, 54124 Thessaloniki, Greece.

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This literature review addresses the Life Cycle Energy Analysis (LCEA) of residential buildings. As the fluctuation in the choice of functional units, boundaries of the system, life cycle inventory (LCI) methods, metrics and impact indicators complicated the potential comparability, the guidelines of Product Category Rule (PCR) 2014:02 for buildings were applied for the normalization procedure. Even though PCR provided a clear statement of the boundaries and a complete presentation of the results, uncertainty deriving from the LCI methods and the omissions in the system boundaries indicates that further standardization is needed. The sample consisted of 90 LCEA case studies of conventional, passive, low energy and nearly zero energy residential buildings (nZEB). Additional analysis identified an underestimation between case studies that use process instead of hybrid analysis, as the average value of embodied energy in hybrid analysis appears to be 3,92 times higher than in process analysis case studies. The highest value of embodied energy for a nZEB case study quantified with process analysis appears to be lower than all the input-output hybrid case studies. A revised definition, according to current trends and requirements in energy efficiency regulations, was also provided as an update of their consistency in time. Operating energy appeared to dominate in life cycle energy of residential buildings in the past. The results of this review show an increasing share of embodied energy in the transaction from conventional to passive, low energy and nZEB, despite the reduction in the total life cycle energy that could reach up to 50%. The share of embodied energy dominates, mainly in low energy and nearly zero energy buildings, with a share of 26%-57% and 74%-100% respectively. In passive buildings, the share of embodied energy varies within a range between 11% and 33% that reaches the embodied energy limits of both a conventional and a low energy building. The use of renewable energy sources (RES) in a passive house, for the production of electricity, classifies it in the range of embodied energy of a nZEB. A significant gap of 17% in the share of embodied energy, between the nearly zero and the most energy efficient building examined in the current review, is identified. This difference appears to be more important for the conventional and passive buildings, indicating the relative significance of embodied energy through time and towards the nZEB. Furthermore, if uncertainty and the underestimation of embodied energy deriving by process analysis were considered this gap could be different. The increase of embodied energy in buildings, indicates that a whole life cycle energy analysis may be needed in the methodological framework of current energy efficiency regulations. Keywords: LCΕA, residential buildings, embodied energy, nZEB, PCR, LCI 1

ACCEPTED MANUSCRIPT 1. Introduction

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The EBPD recast 2010/31/EU [1] and the regulation 244/2012 [2] initiated an effort in reducing the energy consumption and increasing the share of renewable energy sources in the building sector. Towards the goal of 2020, all European Union (EU) member states strive to meet the obligation of the complete definition and requirements of the nZEB on a national level. Even though recent reports show that the overall picture is positive, significant gaps are identified in the definition and its methodological framework [3]. Studies and research across Europe contribute a supporting role in filling the gaps and in defining the minimum requirements. Research focuses mainly on energy measures, their optimal combination and methods of cost optimization. During this process, suggestions on measures -such as the increase of insulation thickness and the increased use of renewable energy sources- aroused interest for research in the measures' effects on the embodied energy of buildings, and lead to alternative proposals and considerations of the nZEB definition [4]. This study examines the life cycle energy analysis of several case studies of residential buildings from international literature. An analysis of the embodied (EE) and operating energy (OE) in their life cycle was conducted. Several literature reviews in the past have presented the amount of embodied energy by applying a normalization procedure for the results. In order to overcome incomparability due to different functional units, methods and impact indicators, limitations in the choice of the case studies were applied. Specific attention was paid to the LCI (Life Cycle Inventory) method used in each case study. The input and the boundaries of the life cycle energy analysis were examined according to the framework provided by EN 15804:2012 [5] and EN 15978:2011 [6]. The results are normalized in MJ per 1m2 of net heated floor area and for a 50-year lifespan of the building, in line with PCR 2014:02 for buildings [7] and according to ISO 14025:2006 [8], in order to provide a common base for their analysis and interpretation. Furthermore, limitations were applied to the initial sample in order to provide homogeneity in the sample and achieve potential comparability. The goal of this study, according to the summarized and normalized results, is to provide the share of embodied energy in the life cycle energy analysis of residential buildings and its fluctuation through time and towards the nearly zero energy building. In addition issues of comparability and uncertainty that characterize the calculation, the normalization procedure, and the LCEA data are analyzed and discussed.

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2. Background and definitions 2.1.

Towards nearly zero energy buildings as a minimum requirements

Through the years, efforts for upgrading the energy efficiency of buildings have appeared in the international literature. The minimized energy demand and air tightness of a passive house and the low energy buildings have provided in the past a step forward to the energy efficiency goal and the nearly zero energy building. Towards the nZEB goal and until 2014 only a small number of the member states provided a definition and quantified its parameters. Recent reports appear to be positive for the obligations and requirements of the member states. Nevertheless, there is a significant absence in the technical systems for domestic hot water and new and passive technologies as far as the definition of energy efficiency measures are concerned [3]. The measures suggested include mainly an increase in insulation thickness, energy efficient windows, HVAC systems of high efficiency and an increased installation and 2

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2.2.

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use of photovoltaic panels for energy production [9-18]. The intensive use of materials, as a step to the zero energy target, indicates a significant increase in the embodied life cycle energy of buildings [19-25]. In some cases, the embodied energy of technical installations and energy production systems appear to be nearly as high as the environmental impact from the structural core of the building [26]. As the use of materials increases, studies indicate that the choice of materials appears to have an effect on the initial embodied energy and the final thermal energy demand when embodied energy is taken into account in the calculation procedure [27]. The overlooked requirements, such as embodied energy and occupants transportation, contribute more than 50% of the total energy consumption, as indicated by Stefan et al [28]. As the recurring embodied energy from the replacement of materials could represent a significant percentage of the initial embodied energy of building assemblies [29], a significant increase is identified in the recurring embodied energy, from the stages of replacement and maintenance, of net zero-energy buildings [30]. The increase in the embodied energy of buildings indicates that may be a whole life energy cycle analysis is needed. A life cycle perspective has been also suggested in the literature in order for the energy regulations to reflect the full energy use of a building's lifecycle [31]. This opinion is strengthened by other studies that identify the consideration of embodied energy and life cycle energy analysis in energy regulations as a path towards sustainability [32], by models providing a holistic approach and measure of embodied energy and life cycle energy demand of buildings beyond the energy efficiency regulations [28, 33] or even by considering the embodied energy in decision making even with a simplified life cycle approach [34].

Life cycle energy analysis of buildings

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Life cycle energy analysis is a detailed approach to estimating the total energy inputs, outputs and flows through the life cycle of a building (Fig. 1). The boundaries of the system are expanded in order to take into account both the operating energy and the embodied energy of a building. The operating energy consists of the energy for heating, cooling, ventilation, domestic hot water, lighting, appliances and auxiliary systems of the building.

Fig. 1. Boundaries of the system, inputs outputs and flows in Life Cycle Energy Analysis (LCEA) of buildings.

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Embodied energy takes into account the manufacturing of materials and their transportation to the construction site, the energy for the construction of the building, the maintenance, repair and replacement of materials and technical systems during the lifetime of the building, the energy for the demolition, the transportation of materials and their end of life management. More simplified or analytical approaches [21] can be found in literature, but in many case studies, the boundaries of the LCEA are not clearly defined [35-37]. In LCEA a range of Life Cycle Inventory (LCI) methods is used for the quantification of embodied energy. The main methods identified in the literature are process analysis and input-output analysis (I-O) and two hybrid methods based either on process or input-output data. Process analysis is based on physical quantities for the estimation of embodied energy. Even though it appears to be the oldest and most used method it is characterized by incompleteness [38, 39]. Uncertainty deriving from the limited boundaries of the system, as upstream, sideways and downstream processes outside of the boundaries are not taken into consideration resulting in the truncation error and therefore lead to an underestimation of embodied energy [39-41]. As indicated, despite the accuracy that the method provides, process analysis is only relevant to the particular system considered [42], and its 'failure' is located in its focus on detail and on limiting the boundaries of the system [43]. Input-output analysis is based on financial quantities by taking into account the price of the building material linked to an economic sector and by mapping the flows between the economic sectors and their energy intensity. Even though input-output analysis appears to be more complete than process analysis, in a system perspective, its' use is characterized as a 'black box' with a limited application when specific cases are examined [44]. Problems and limitations of I-O analysis -such as the use of economic data to model the physical flows of a system, the age of the data used, the use of national average prices, the assumption that all products of a sector consist of the same combination of inputs, the proportional relationship between the cost and the amount of products required by a sector, the sector aggregation/classification and the exclusion of capital inputs- have been indicated in the past [39, 45, 46]. In order to overcome these problems and minimize the errors from the two base LCI methods, the use of physical (process analysis data) and financial (I-O analysis data) quantities is applied through hybrid techniques. The process-based hybrid analysis focuses on reducing the truncation error of process analysis with the smaller aggregation error of inputoutput analysis, as indicated by Bullard et al. [47]. Nevertheless, the downstream and sideways truncation errors are not addressed, and this results to an increased sensitivity when a high level of detail is achieved [40, 44]. Incompleteness of the method derives from incomplete process data along with an important sensitivity related to the energy tariffs and the use of product prices for the calculation procedure [38, 48]. A hybrid input-output analysis, introduced by Treloar in 1997, is based on a disaggregation of the I-O model into direct energy paths [48]. The method improves the completeness of the system, and the integration of process data provides no further increase in upstream and sideways truncation errors [48]. Furthermore, it limits time by focusing on the most important inputs and avoids indirect effects [46, 48, 49]. The method has been further improved with the derivation of process-based hybrid values in a first step and a 'remainder' that corrects the downstream and sideways truncation error [44]. As studies indicate the need for physical and financial data in environmental assessment, an underestimation between process analysis and the other LCI methods is identified, with a significant gap in reference to the hybrid I-O analysis, that could be even more important if inputs such as capital data were included [39, 40, 44].

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ACCEPTED MANUSCRIPT 2.3.

Comparability of building life cycle energy analysis studies

In the current review, life cycle energy analysis case studies of residential buildings are examined ( Table 1 and Table 2).

Description and main characteristics Region /Country

Year

Authors

Building definition* *CS definition (updated def.)

CS 1

Germany

1997

conventional (-//-5)

Feist [36]

CS 2 CS 3 CS 4 CS 5 CS 6 CS 7 CS 8

square meters 156

low energy (conventional) passive house (passive) passive house (passive) self sufficient (nZEB) Norway

1999

Winther et al.[50]

green (conventional) current regulations (conventional) building code (conventional)

CS 9 CS 10

110

Energy Results

Primary

End Use≈ Primary

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4

Heated floor area

1

building code (low energy) Super insulated (low energy)

2

CS OE

CS EE

MJ/(m2·a)

MJ/(m2·a)

939,54 498,46

63,31 66,10

211,95 176,62

105,82 73,79

0

366,32

543,27 605,45 474,55 409,09

46,87 33,32 42,25 51,28

SC

Case study No

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Table 1. Main characteristics of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings (part 1).

92,23

USA

2001

Keoleian et al.[51]

standard (conventional) energy efficient (low energy)

228

Primary

1.277,19 415,44

126,32 145,96

CS 13 CS 14 CS 15

Sweden

2002

Thormark[52]

low energy (-//-) low energy (-//-) low energy (-//-)

120

Primary

221,76 221,76 221,76

140,66 87,76 81,58

CS 16 CS 17 CS 18

New Zealand

2004

Mithraratne et al.[37]

standard-light (low energy) standard-concrete (low energy) super insulated (low energy)

94

Primary

125,93 115,28 67,44

CS 19 CS 20 CS 21 CS 22

Australia

2006

Horne et al.[53]

common house (low energy) common house (low energy) common house (low energy) common house (low energy)

NS (≈230)

Primary

CS 23 CS 24

Spain

2006

Casals [32]

150

CS 25 CS 26

Hungary

2007

Szalay et al. [25]

≈144

CS 28

2007

Citherlet et al.[54]

Indonesia

CS 33 CS 34

Italy

CS 35

Sweden

CS 36 CS 37

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CS 31 CS 32

266

Utama et al. [55]

high rise (conventional)

2010

Blengini et al.[19]

2010

Gustavsson et al.[56]

Spain

2010

Enslic Project[57]

Canada

2011

Leckner et al.[58]

Norway

CS 45 1

2012

Dahlstrøm et al.[59]

PA

80

PA

50

PA

50

3

44,24 47,09 50,88

NS

100

125 221 140 146

57,4 41,20 53,30 44,50

PA

50

Primary

883,95 531,55

122,64 473,04

NS

30

Primary

404,28 243,72

70,92 81

PA

50

226,80

112,68

473,98 253,97

107,83 113,08

PA

NS

94,68

105,33

NS

low energy (-//-)

2009

CS 41 CS 42 CS 43

5

current regulations (conventional)

years 6

50

minergie (low energy)

CS 30

CS 44

current regulations (conventional) current regulations (low energy)

Lifespan

NS

current regulations (low energy)

Switcher land

CS 29

CS 38 CS 39 CS 40

average (conventional) efficient (low energy)

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227,45

CS 11 CS 12

LCI Method

53,17

Primary

91,34 180,40

41,67 36,77

PA

40

standard house (conventional) low energy (low energy)

192

Primary

501 134

108 157

PA

70 70

passive house (passive)

120

Primary

PA

50

PA

50

PA

40

PA

50

NS (low energy) NS (low energy) conventional (-//-3) net zero energy house with no solar systems (low energy) net zero energy house (nZEB) passive house (passive)

4.842,94 4.458,60 210

187

Primary Primary

Primary

563,28

70,08

523,73 471,87

70,08 70,08

291,43 202,56 471,73 258,65

192,12 187,52 120,51 150,51

0 0

223,41 219,51

729,75 670,38

150,33 151,80

CS OE: Operating energy originated from case study, 2CS EE: Embodied energy originated from case study , 3NS: Not clearly stated, 4CS: Case Study,

(-//-) the updated definition matches the case study definition, 6PA: Process Analysis, 7I-OHA: I-O Hybrid Analysis, 8PHA:Process Hybrid Analysis, 9I-OA: Input-Output Analysis

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ACCEPTED MANUSCRIPT Table 2. Main characteristics of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings (part 2).

CS 46 CS 47

Norway

2012

Dahlstrøm et al.[59]

Description and main characteristics Heated Energy Building definition* floor Results area square *CS definition (updated def.) meters passive house (passive) 187 Primary

CS 48

France

2012

Thiers et al.[60]

passive (passive)

CS 49

Brazil

2013

Paulsen et al.[61]

social housing (low energy)

CS 50

Italy

2013

Paleari et al.[62]

nZEB (nZEB)

4.000

NS

CS 51 CS 52 CS 53 CS 54 CS 55 CS 56

Sweden

2013

Berggren et al. [35, 63]

minergie A (low energy) minergie A (low energy) minergie A (low energy) minergie A (low energy) minergie A (low energy) minergie A (low energy)

227 440 306 314 1.087 1.056

Primary

CS 57 CS 58 CS 59 CS 60 CS 61 CS 62

Belgium

2013

Himpe,Himpe et al. [64, 65]

passive house (passive)

143,3

Primary

Belgium

2013

Stephan et al.[66]

passive (low energy)

CS 63

Spain

2013

Zabalza et al.[67]

low energy 5(-//-)

CS 64 CS 65 CS 66 CS 67 CS 68

Belgium

2013

Crawford et al.[68]

passive house (passive)

Lebanon

2014

Stephan et al.[69]

CS 69

Italy

2014

Cellura et al.[30]

CS 70 CS 71 CS 72 CS 73

Finland

2015

Takano et al.[70]

CS 74

Sweden

2015

Folkhem [71]

CS 75

Israel

2015

Huberman et al.[72]

CS 76 CS 77

Turkey

2015

Atmaca et al [73]

CS 78

Spain

2015

Oregi et al. [34]

CS 79

Lebanon

2016

Stephan et al. [74]

CS 80

India

2016

Praseeda et al.[75]

CS 82 CS 83 CS 84 CS 85 CS 86 CS 87 CS 88 CS 89 CS 90 1 5

1

CS OE

MJ/(m2·a)

2

CS EE

LCI Method

594,41 563,60

158,29 154,36

49,39

185,26

Primary

350

144

6

PA

PA 3

RI PT

Primary

48

Lifespan years

MJ/(m2·a)

132

50 80

NS

50

167,28

PA

100

115,20 176,40 136,80 122,40 122,40 158,40

PA

60

47,10 44,13 33,90 44,75 66,86 486,06

PA

60

Primary

151,56 151,56 151,56 151,56 151,56 397,61

I-OHA

100

Primary

272,58

249,14

PA

50

297

Primary

904

Primary

111,25 242,50 363,75 485,00 544,16

PA 9 I-OA 8 PHA I-OHA I-OHA

80

current regulations (low energy)

NS NS NS 366,25 964,89

near Net ZEB (nZEB)

610

Primary

137,51

391,87

PA

70

current regulations (low energy) current regulations (low energy) current regulations (low energy) current regulations (low energy)

96 316 475 1.775

Primary

366,94 289,72 256,50 186,54

46,60 31,68 29,60 23,62

PA

50

current regulations (low energy)

2.857

Primary

294,00

236,43

60

Primary

30,32

39,32

current regulations conventional) current regulations(conventional)

3.380 330

Primary

602 488

156 94

refurbishment (low energy)

9.484

Primary

69,6

22,92

PA

50

904

Primary

138,65

660,20

I-OHA

50

PA

50

297 4.458

SC

75,16

104,40 176,40 140,40 93,60 133,20 176,40

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CS 81

Year

current regulations (low energy)

current regulations (low energy) conventional (low energy)

EP

4

Region /Country

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Case study No

78

220,00

24,80

55

NS

150,00

82,40

65

40,00

24,60

28

51,00

36,00

42

62,00

36,00

152

50,00

49,00

152

50,00

48,60

202

33,00

57,80

150

40,00

55,20

256

10,00

40,80

152

55,40

53,80

7

PA 3

50

PA

50

(-//-) the updated definition matches the case study definition, 6PA: Process Analysis, 7I-OHA: I-O Hybrid Analysis, 8PHA:Process Hybrid Analysis, 9I-OA: Input-Output Analysis

6

50

NS

CS OE: Operating energy originated from case study, 2CS EE: Embodied energy originated from case study , 3NS: Not clearly stated, 4CS: Case Study,

The sample consists of 90 case studies of residential buildings around the world with a time horizon that varies between 1997 and 2016. The comparability of results has been a significant issue as great variation in the functional units, the lifespan of the building, the LCI methods, the life cycle stages considered and the year of the study was located (Fig.2). In life cycle energy analysis, as in life cycle assessment, the main core is structured by the boundaries of the system and the assumptions that are made for the calculation procedure.

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Fig.2. Issues of comparability in Life Cycle Energy Analysis (LCEA) of buildings.

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When an LCA is conducted, the common framework provided by ISO 14040:2006 [76] and ISO 14044:2006 [77] is followed. In the majority of the LCEA studies in international literature, the operating energy is added in the boundaries of the system, according to EN 15804:2012 and EN 15978:2011, in order to provide a complete analysis of the full life energy cycle of a building. As stated in the past, life cycle energy analysis is an approach that takes into account all energy inputs to a building in its life cycle [21]. The operating energy consists of the energy for the heating, cooling, ventilation, hot water, lighting, appliances and auxiliary systems of the building. In many case studies, the energy use for the lighting and the household appliances of the building is not taken into account in order to simplify the calculation processes and to limit the boundaries of the system [30, 37, 62, 64, 65]. These limitations can be significant as the difference in the share of embodied energy in the LCEA could decrease from about 33%, when only space heating is taken into account, to 25% when the energy for household electricity, hot water, and ventilation are included [56]. When the energy for the lighting of the building is excluded from LCEA, this can lead to an increase of 20% in the share of embodied energy in its life cycle [57]. A significant variation is identified in the functional unit of the study, which usually consists of the area that is used for the calculations and the lifespan of the building. The area used in the literature varies between the total gross area, the net floor heated area, the total built-up area or 1m2 of them. The significant difference is identified between the heated and the total area of a building as the second one takes into account building elements that compose the non-heated areas and other faculties of a building, which increase the embodied energy [62] and may lead to great differences in the results [30]. The lifespan of the building varies between 30 and 100 years, with a most common used lifespan of 50 years observed both in the current and previous literature reviews [20, 21, 78]. The definition of the buildings' lifespan appears to be of great importance [79], as an increase from 25 to 75 years could lead to a decrease from 14% up to 29% in the total embodied energy despite the increase in the recurring embodied energy [80]. In addition, the total operating energy 7

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increases with a final reduction in the total share of embodied energy in the life cycle of the building. The year that a study is conducted is of significant importance for the buildings' energy efficiency definition. A residential building characterized in a study as low energy in the year 1999 [50] appears to have 70% more primary energy consumption than a similar one from a case study conducted in 2010 [19]. The construction methods are changing through time along with the energy efficiency regulations. These factors affect the final result and the potential future comparability of LCEA studies. Nevertheless, as time cannot be normalized, the definition of a building could be updated according to the current national and international trends and regulations. Another important issue is the use of final or primary energy as a metric in LCEA. Studies have pointed out that the energy demand could be doubled when the primary energy is taken into account instead of final energy [30]. Nevertheless, in LCA a common approach is the use of primary energy for the calculation procedure, as conversion factors are proportional to CO2 and a more suitable metric for the environmental impact of energy [81]. The lack of national data in LCA for the calculation of embodied energy leads to the use of international databases such as Ecoinvent. An approach for limiting the uncertainty is to normalize the average energy data with the electricity mix of the country where the study is conducted [54, 59, 65], as the electricity mix and primary energy factors could have a significant effect in both embodied and operational energy results [82]. In this way, the different sources of energy, used in the electricity mix, are taken into account in the calculations. Primary energy appears to be a more suitable metric in LCEA, as it embodies the different sources of energy, the losses and the potential impact of its use. The boundaries of an LCA can also have an effect on the final environmental impact of the buildings' life cycle. When a lifecycle process or a stage is excluded from the boundaries of the study it affects the environmental burden and the share of embodied and operating energy. In some case studies the end of life is excluded from the boundaries of the LCEA [37, 50, 66]. This limitation may lead to a decrease (landfill disposal scenarios) or increase (reuse or recycling) of the share of embodied energy in the life cycle energy analysis of a building. The effect on the life cycle is considered to be minor (1-2%) [71], although the recycling or reuse potential could reach about 30% in the life cycle energy of a low energy building [19] or could lead to a reduction in the embodied energy of about 40% when a nearly zero-energy building is examined [52]. Along with the boundaries of the system, the LCI method used for the quantification of embodied energy appears to be of significant importance in the calculated results and the normalization procedure. As indicated by Lenzen et al. the truncation error of process analysis appears to be about 50% [39]. In a passive house examined in previous research with process, input-output, process-based hybrid and I-O-based hybrid analysis the share of embodied energy in the total life cycle energy was 13,1%, 28,5%, 42,8% and 56,9% respectively, identifying a significant underestimation of embodied energy by all LCI methods compared to I-O-based hybrid analysis [68]. An average gap of 64% is also identified by Crawford between process analysis and I-O-based hybrid analysis in building case studies [40]. In addition, a significant difference is estimated in the recurring embodied energy of building assemblies calculated with different LCI methods indicating increased values for the I-Obased hybrid analysis [29]. As the gap in the results among case studies examined with different LCI methods cannot be normalized in the current review, additional analysis should be addressed in order to examine the uncertainty and comparability of the results.

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ACCEPTED MANUSCRIPT 2.4.

Current standards on the life cycle assessment of buildings

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In 2014 PCR 2014:02 v.01 was published to provide guidance for the Environmental Product Declaration (EPD) of buildings [7]. This report defines the methodological framework, the calculation procedure, the quality of data and other significant issues that are necessary in order to conduct an EPD certification for a building according to ISO 14025:2006. It is based on the framework of ISO 14040:2006, ISO 14044:2006, EN 15978:2011 and EN 15804:2012. The main core is structured on the fundamental principles and guidelines for the life cycle assessment and sustainability of construction works. PCR 2014:02 defines a 50-year lifespan of a building and the net heated floor area (Atemp) as a basis for the calculation procedure. It follows a cradle to grave approach that takes into consideration all the upstream (A1-A3), core (A4-A5) and the downstream processes (B1-B7 & C1-C4) that could be located in a life cycle energy analysis (Fig. 3), as they are also defined but in a more simplified form in previous literature reviews [20, 21, 78]. The final output is suggested to be given in certain impact categories and indicators providing the possibility for future comparability between products-buildings. Additional information such as operating water use and alternative optional scenarios for the end of life is also provided. The final output, as the first EPD for a whole building was published in 2015 [71], appears to be a very descriptive and accurate analysis and presentation of the procedure, the boundaries and the results of the building assessment. Many of the impact categories could be chosen in order to analyse and characterise the environmental impact of the building with the further potential comparability with other studies conducted on the same principles.

Fig. 3. Processes and stages of the building assessment according to PCR 2014:02, as reference from EN 15804:2012 [5] (PCR 2014:02,p.10 [7]).

One set of impact indicators provided is the use of renewable and non-renewable primary energy through the buildings' life cycle. As a result, the consideration of these guidelines could be a very useful way of conducting LCEA and presenting its output, as it can also be found in international literature when the principles of EN 15978:2011 and EN 15804:2012 are followed [19, 67, 70].

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Defining operational energy efficiency levels

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Through the years, adjustments and recasts on the energy efficiency regulations have provided the current limits and trends of operational energy efficiency levels. A conventional building is a building constructed according to specific regulations of a country and during a specific period of time. As the energy efficiency level of a conventional building is defined by the occasional regulations and standards a specific value cannot be applied. A low energy building is a building designed with special features and measures in order to reach a level of energy efficiency lower than required by energy efficiency regulations of a country. Feist in 1997 defined a low energy house as a building with an annual heat requirement below 70 kWh/(m2·a), with a consumption at least 50% lower than required by the German energy regulations [36]. In 2007, Sartori and Hestnes applied a total operating primary energy requirement of 202 kWh/(m2·a) as a limit between low energy and conventional buildings [22]. The Passive House Institute has recently defined the limits and requirements of energy efficiency for a low energy building when compliance with the passive house criteria cannot be accomplished (Table 3) [83].

Parameters Heating demand Cooling and dehumidification demand Pressurization test result n50

Units kWh/(m2·a) kWh/(m2·a) 1/h

Renewable primary energy demand Renewable energy generation

kWh/(m2·a) kWh/(m2·a)

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Table 3. Certification criteria for the Passive and Low Energy House (Passive House Institute [83]).

Criteria of Passive House ≤ 15 ≤ 15 + dehumidification contribution ≤ 0,6 Classic Plus Premium ≤ 60 45 30 ≤ 60 120

Criteria of Low Energy House ≤ 30 ≤ Passive House Requirement + 15 ≤ 1 ≤ ≤

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A passive house as defined by Feist is a building in which the heat requirement is so low that a separate heating system is not necessary and there is no loss of comfort [36]. According to the Passive House Institute, passive houses are characterized by an especially high level of thermal comfort with minimum energy consumption, followed by specific requirements for their certification (Table 3) as defined in 2015 and substituting the limit of 120 kWh/(m2·a) for the non-renewable primary energy demand. A nearly zero-energy building (nZEB) is a building with high energy performance and a low amount of energy consumption covered by renewable energy sources [1]. The operational energy efficiency levels of a nZEB are defined at a national level by the member states of the European Union. Latest reports show that the primary energy consumption for heating and cooling of residential buildings varies between 30-95 kWh/(m2·a) with the most common range of 45-50 kWh/(m2·a) [84].

3. Analyzing existing life cycle energy analysis studies on buildings 3.1.

Selection of existing studies

In the current review, LCEA case studies of residential buildings were examined. Geographical limitations were not applied and the sample consisted of available case studies of residential buildings from Europe, USA, Australia, Canada, New Zealand, Lebanon, Turkey, India, and Brazil. International literature provides a more extensive sample of cases studies than the one examined in the current review. Many case studies provided the needed information in CO2 emissions and other environmental impact indicators or were 10

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Analysis and normalization of data

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characterized by a lack of required data for the current normalization procedure. In studies with a variety of buildings examined, the selection was focused on buildings with the lowest energy intensity as this is the scope of this study. The specific criteria applied for the selection of the case studies were: • the results are provided in a primary energy form, with specific information for the total amount of embodied and operating energy, • the boundaries of the system, the lifespan and the area of the building are clearly stated, • the LCI method used clearly stated. The final sample consists of 90 case studies of residential buildings. Case studies from literature that did not fulfil the requirements mentioned above were excluded. Nevertheless, some case studies with lack of available information were retained in the sample in order to expand the geographical boundaries, the building typology and to fulfil the requirements and scope of the current review.

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Table 1 and Table 2). The case studies cover a 20-year time horizon (Fig. 4). The building lifespan ranges between 30 and 100 years with a most common value of 50 years (Fig. 5). In order to provide the potential comparability between their results, the primary energy was selected as a metric and was a significant criterion for the choice of a case study for the current review. Nevertheless, all the issues described in section 2.3 had to be overcome in order to provide a complete picture.

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6

4

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0 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Publication year

Fig. 4. Distribution in time of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings.

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Fig. 5. Building lifespan, in years, of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings.

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The results were normalized according to the principles and guidelines provided by PCR 2014:02 for buildings, and summarized into the following: • Functional unit. The life cycle energy results were normalized for a building lifespan of 50 years and for the net heated floor area of the building (Atemp). The useful floor area and the inhabitant-habitable floor area are assumed to be the same with the net floor heated area of the building. • Energy metrics. The results, in primary energy, are recalculated and given in MJ of primary energy per square meter of net heated floor area and for 50 years, and in MJ of primary energy, for the total net heated floor area of the building and for 50 years. Even though in case studies CS6-CS10 the results are given in end use [50], they are not excluded from the sample, as it is justified that due to the electricity mix of Norway the difference between the two metrics appears to be very small. • Lifecycle processes and stages. In each case study the life cycle processes and stages that are taken into account were identified and assigned to the upstream, core and downstream processes [7], in order to provide a comprehensive analysis. For the process of replacement of materials, building components and technical systems the definition given in the case studies varies between replacement, maintenance and refurbishment. As the authors appear to describe the downstream processes of maintenance and replacement [6, 7], these processes (B2 and B4) are selected in all case studies. The final outcome of these adjustments is presented in Table 4 and Table 5. • Building definition. Revising the definition of a building, according to current regulations and trends, appeared to be a difficult issue to be handled. The Passive House Institute recently revised the standards for the passive houses and provided the minimum requirements for the certification of a building as low energy (Table 12

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3). As the necessary parameters-criteria were not available in the examined case studies, a simplified approach to the definition of buildings was considered. The Passive House Institute provided the potential use of evidence for the previous primary energy demand limit, as a transitional phase between the old and new standards, for the passive house classic [83]. The passive house requirements for the heating demand, cooling demand and pressurization test are considered as defined in Table 3, retaining the primary energy demand (non-renewable) below 120 kWh/(m2·a) as a limit for the passive house classic. As the limit values for the low energy buildings vary in national level, a standard international limit has not been specified yet. The annual heat requirement below 70 kWh/(m2·a) for a low energy building as defined by Feist in 1997 [36] was followed and analysed further by Sartori and Hestnes in 2007, with a total operating primary energy requirement of 202 kWh/(m2·a) [22], as a reference to Feist's results and calculations. The primary energy demand limit in the current study for the low energy building is defined at 120 kWh/(m2·a), as evidence for the PHI Low Energy Building Standard can be provided in a transitional phase by proving compliance with the requirement for (PE) of QP ≤ 120 kWh/(m2·a) [83]. This value appears to be significantly lower than previous studies and literature reviews. Nevertheless, this could be justified by the increasing requirements of the energy efficiency regulations through time. The nZEB or Net ZEB characterization, as the limits and definitions vary in national level, was defined by the energy balance of the building as provided in the case studies and the national reference limit values [84]. The case studies that did not match any of the criteria mentioned above are defined as conventional. • Operating energy. By providing limitations in the processes of the operating stage of the building, many case studies were excluded and the sample was significantly reduced. As a result, in a first step, it was decided to retain the initial sample and justify any extreme values or uncertainties in the results. Secondly, limitations according to the life cycle stages and processes included in the studies were applied, and the reduced sample was examined and analysed by the same point of view. After the normalization process, the primary energy is calculated in two main categories as a simplified and more compact point of view that is also followed in other studies [35, 53, 56]. The first category takes into account the operating energy required for the use of the building by its occupants and consists of all the energy related to space heating, ventilation, cooling, domestic hot water, lighting and auxiliary systems. The second category consists of all the other lifecycle processes, as there is a direct or indirect connection with the embodied energy (manufacturing, transportation, maintenance, repair, replacement, refurbishment, demolition, disposal and reuse or recycling). The transportation of the buildings' occupants is not considered in the calculation of embodied energy. The embodied energy, is finally calculated as the difference between total life cycle energy and the operating energy:  =  −  ,

(Eq. 1)

where:  is the total embodied energy (MJ of primary energy),  is the total life cycle energy (MJ of primary energy) and  is the total operating energy (MJ of primary energy) through the life cycle of the building. The results are finally calculated as the share, in a percentage form, of the total embodied and total operating energy of the building through its entire life cycle.

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Table 4. Lifecycle processes and stages of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings (part 1).

Raw material supply A1

A/A

2

NS

Product stage (A1-3)

Construction process stage (A4-5)

Transport Manufacturing Transport

Benefits and loads beyond the system boundary

Downstream Processes Use stage (B1-7)

Constructioninstallation process

Use

Maintenance

Repair

Replacement

Refurbishment

A2

A3

A4

A5

B1

B2

B3

B4

B5

NS

NS

NS

NS

NS

NS

NS

NS

NS

Operational Energy Use*

Operational Deconstruction/ Transport Water Use Demolition

B6

1

End Of Life Stage (C1-4)

H,VC,DHW,L&A

Waste Processing

Disposal

C4

B7

C1

C2

C3

NS

NS

NS

NS

NS

NS

NS

NS

Reuse, recovery,recycling potential D NS

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CS 1 CS 2 CS 3 CS 4 CS 5 CS 6 CS 7 CS 8 CS 9 CS 10 CS 11 CS 12 CS 13 CS 14 CS 15 CS 16 CS 17 CS 18 CS 19 CS 20 CS 21 CS 22

Core Processes

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Upstream Processes

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NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

NS

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NS

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CS 23 CS 24 CS 25 CS 26 CS 27 CS 28 CS 29 CS 30 CS 31 CS 32 CS 33 CS 34 CS 35 CS 36 CS 37 CS 38 CS 39 CS 40 CS 41 CS 42 CS 43 CS 44 CS 45

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H,VC,DHW,L&A H,VC,DHW,L&A H,VC,DHW,L&A H

1

NS

H,VC

NS

NS

H,VC,DHW,L&A

NS

NS

NS

NS

NS

NS

H

NS

NS

NS

NS

NS

NS

H,DHW,L&A H,VC,DHW,L&A H,VC,DHW,L&A VC,L&A H,VC,DHW,L&A H,VC,DHW,L&A

H,VC,DHW,L&A H,VC,DHW,L&A

H,VC,DHW,L&A 2

NS:Not clearly stated in the case study

:Yes

:No

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H: Space heating, VC:Ventilation-Cooling, DHW:Domestic Hot Water, L&A:Lighting and Auxiliary systems

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Table 5. Lifecycle processes and stages of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings (part 2).

Raw material supply

A/A

A1

Core Processes Construction process stage (A4-5)

Transport Manufacturing Transport A2

A3

A4

Benefits and loads beyond the system boundary

Downstream Processes Use stage (B1-7)

Constructioninstallation process

Use

Maintenance

Repair

Replacement

Refurbishment

A5

B1

B2

B3

B4

B5

Operational Energy Use*

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CS 46 CS 47 CS 48 CS 49

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Upstream Processes Product stage (A1-3)

B6 H,VC,DHW,L&A

End Of Life Stage (C1-4)

Operational Deconstruction/ Transport Water Use Demolition B7

C1

NS

NS

C2

Waste Processing

Disposal

Reuse, recovery,recycling potential

C3

C4

D

CS 50 CS 51 CS 52 CS 53 CS 54 CS 55 CS 56 CS 57 CS 58 CS 59 CS 60 CS 61 CS 62 CS 63 CS 64 CS 65 CS 66 CS 67 CS 68 CS 69 CS 70 CS 71 CS 72 CS 73 CS 74 CS 75 CS 76 CS 77 CS 78 CS 79 CS 80 CS 81 CS 82 CS 83 CS 84 CS 85 CS 86 CS 87 CS 88 CS 89 CS 90

NS

NS

NS

NS

NS

NS

NS

NS

NS

H,VC,DHW H,VC,DHW,L&A

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H,VC,DHW,L&A L&A 2

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NS NS

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NS:Not clearly stated in the case study

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:No

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H,VC,DHW,L&A H,VC,DHW,L&A H,VC,DHW,L&A

H,VC,DHW,L&A H,VC H,VC,DHW,L&A H H,VC,DHW,L&A VC,L&A

H: Space heating, VC:Ventilation-Cooling, DHW:Domestic Hot Water, L&A:Lighting and Auxiliary systems

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100% 90%

CS 48, CS 79, CS 89

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CS 87

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As indicated in the results the share of embodied energy in the total life cycle energy of the examined buildings ranges between 5% and 100% (Fig. 6 & Fig. 7). The share of embodied energy in conventional buildings ranges between 5% and 36%. In low energy buildings this percentage varies between 10% and 83%. Although there is a decreased energy consumption through their life cycle, in some case studies the significant share of operating energy could be justified from the effect of the recycling potential (CS70-CS73, [70]), the building methods and materials (CS20 [53], CS80 [75] ) and a simplified approach to the total exclusion of the end of life processes (CS9, [50]). In case studies CS 87 and CS 89 [75] the low operating energy contribution is justified by the limited processes of the use phase (space conditioning and lighting) along with the building methods and materials used for the construction of the building. The extreme low contribution of operating energy in CS 79 [74] occurs from several energy measures applied and the use of renewable energy sources. Apart from these case studies, low energy buildings appear to have a share of embodied energy between 23% and 58%.

conventional passive low energy

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Fig. 6. Total share of embodied (EE%) and operating energy (OE%) in the Life Cycle Energy Analysis (LCEA) of the 90 case studies of residential buildings (step 1).

In the passive case studies the share of embodied energy ranges between 11% and 57%, except for case study CS48 (79%) [60]. The extremely low percentage of operating energy for the CS48 is justified by the use of RES for electricity production in a passive house along with the use of a district heating system, as previews studies indicate a reduction in the total primary energy when district heating is used in a passive house [31, 85]. This house could be certified as a Passive House premium or plus, according to the new passive house standards, with its share of embodied energy reaching the range of the nZEB case studies. The nZEB share of embodied energy ranges between 69 and 100% for a nearly zero balance. The LCI method used for the calculation of embodied energy appears to be important to the final results as a significant difference in the energy intensity between case studies that use process and hybrid analysis is identified (Fig. 8) (the case studies that do not state clearly the LCI method used are not considered in this comparison).

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Fig. 7. Normalized results, for the share of operating (OE%) and embodied energy (EE%) in the total life cycle energy, for a 50-year lifespan, of the 90 Life Cycle Energy Analysis (LCEA) case studies.

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Fig. 8. Operating (OE), embodied (EE) and life cycle energy (LCE), in GJ/m2 of primary energy and for a 50-year lifespan, of the 90 Life Cycle Energy Analysis (LCEA) case studies of residential buildings. .

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The embodied energy in process analysis case studies ranges between 1 and 12 GJ/m2 for the 50-year lifespan used in the normalization procedure, except for values between 18 and 20 GJ/m2 of embodied energy for the nZEBs examined (Fig. 8). For the hybrid analysis case studies embodied energy ranges between 18 and 33 GJ/m2. The average value of embodied energy in hybrid analysis appears to be 3,92 times higher than in process analysis case studies. The highest value of embodied energy for a nZEB case study quantified with process analysis appears to be lower than all the I-O hybrid case studies. Furthermore, the lowest value of 18 GJ/m2 of hybrid analysis case studies appears to be 1,50 higher than the highest of process analysis, excluding the nZEBs. As indicated, an uncertainty is identified in the potential comparability of the case studies and in the final share of embodied energy to the total life cycle energy of a building, deriving from the LCI method. Moreover, limitations are applied in order to increase the comparability between the case studies and the accuracy of the current analysis: • Operating energy. As energy for lighting and auxiliary systems appears to have an important contribution to the LCEA results (2.3) and the majority of the case studies calculated the contribution of these processes, it was decided to exclude the studies that did not take them into account for their calculations. • Energy for construction and demolition of the building. The energy for the construction of a building (A5) contributes with a very small percentage (0,2-1%) in the total lifecycle energy [57, 71, 86]. The share of energy for the demolition of a building (C1) ranges between 0,1% and 1% in the total life cycle energy [21, 22, 57, 71, 86-88]. The cut-off rules of PCR 2014:02, according to EN 15804:2012, indicate that a percentage limit of 1% for insufficient data of a unit process or 5% for a module may be excluded from the calculations. Due to their minor contribution and the validity of excluding them from the analysis, no distinctions are applied to the case studies that either take into account these processes or not in their calculations. • Module D. The significance of the reuse, recovery or recycling potential could be important in the LCEA results of a building (2.3). Nevertheless, as this module is defined as an optional step in PCR 2014:02 and EN 15804:2012 and in certain case studies the results for the modules C3-C4 and D were not given separately, no limitations are applied in this step. • LCEA processes and stages. A final limitation is applied for the case studies that did not include the transportation processes (A2 and C2) and the waste processing and disposal (C3 and C4). The analyzed case studies and literature show that the transportation contribution ranges between 0,1% and 7% [19, 52, 56-59, 65, 71, 89] and the waste processing and disposal contribution ranges between 0,1% and 3,5% [52, 56-58, 71, 90] in the life cycle energy of the building. As the contribution range of these processes in the majority of the case studies exceeds the limits of the cut-off rules, in addition to the exclusion of A5 and C1 processes, the case studies that did not take them into account in their analysis are excluded. As a normalization procedure could not be performed for the LCI methods used, no limitations were applied. The final reduced sample consists of 39 case studies of residential buildings (Fig. 9). The sample after the limitations applied consists only of case studies calculated with process analysis, even though this was not an intention or a part of this procedure. This outcome does not limit the truncation error and the potential underestimation of embodied energy occurring by process analysis but provides a homogeneity in the sample. 19

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The share of embodied energy in the life cycle of conventional buildings ranges between 6%-20%, for the passive ones between 11% and 33%, for the low energy buildings between 26% and 57% and for the nearly zero energy buildings between 74% and 100% (Fig. 9). Although an overlapping is located between conventional and passive, and between low energy and passive buildings (Fig. 9), a significant gap of 17% is identified as the difference between the minimum share of embodied energy of an nZEB and the maximum share of embodied energy of a low energy building. This gap appears to increase compared with the passive and conventional buildings respectively. It must be stated that Fig. 9 does not indicate that a low energy building is more energy efficient than a passive one. The operating energy of low energy buildings appears to be higher than passive buildings but a lower relative contribution of operating energy in their total life cycle energy and in a percentage form place them closer to the vertical axis and the nZEB area of the graph.

40%

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60%

70%

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The results also indicate a difference in the share of embodied energy between the case studies of the final sample in contrast to the case studies with hybrid analysis of the initial sample. For hybrid analysis case studies the share of embodied energy for a conventional building is 36%, for the passive house the range is between 43% (process-based hybrid analysis) and 57% (I-O hybrid analysis) and for a low energy building with RES is 83%. A significant increase is identified in the share of embodied energy between process analysis case studies and hybrid case studies. Nevertheless, the number of case studies using hybrid analysis represent a small percentage of the initial sample and an extended comparison cannot be applied as a nZEB calculated with hybrid analysis is not available in the initial sample examined by the current review. Finally, the results of the 39 case studies were recalculated in kWh/(m2·a) and compared with 40 case studies from a literature review published in 2007 [22] (Fig. 10). The majority of the studies of the current review are located after 2007 with the ones before this year being also a part of the sample of the previous review. The current and the previous review appear to have a similar linear relation between LCE and OE, with a transposition of the results distribution to the left, indicating a decrease for the current review. Life cycle energy appears to be higher, between 35 and 1.175 kWh/(m2·a), in the previous review while in the current 20

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Comparability appears to be a significant issue when an LCEA review is conducted. In many case studies the limits, boundaries and assumptions were not clearly stated. The fluctuation in the choices of functional units, boundaries of the system, LCI methods, metrics and impact indicators complicated the potential comparability, an issue also indicated in previous LCEA reviews [20, 22, 91, 92]. Differentiation between case studies is inevitable as the available data, limits and assumptions are characterized by time, personal judgment, geographical variations and lack of a standardization procedure is identified. A common and clear presentation of the boundaries and the results is needed. Standards and guidelines such as EN 15978, EN 15804 and the recent development and use of PCR 2014:02 for buildings appear to be suitable for the presentation and interpretation of a life cycle energy analysis. Nevertheless, as uncertainty appears to be of significant importance in LCA and LCEA, a clear statement and presentation is just a first step, and issues that have been addressed in the past such as standardization of the requirements [32, 91] and a need for an embodied energy protocol [92] should be considered. Uncertainty in the quantification of embodied energy deriving from the LCI method used in the case studies examined appears to be important. The highest value of embodied energy 21

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for a nZEB case study quantified with process analysis appears to be lower than all the I-O hybrid case studies. The average value of embodied energy in hybrid analysis appears to be 3,92 times higher than in process analysis case studies, with previous studies indicating that hybrid analysis could provide between 4,36, 4,94 and 3,78 higher values than process analysis [68, 69, 93]. Even though in the current review an effort was made to increase comparability between case studies, by addressing issues of uncertainty, the uncertainty deriving from the methodological framework, the calculation procedures, the LCI methods, the truncation error occurring from process analysis, the omission in the boundaries of the system and even uncertainty from the current normalization procedure with the potential error by extracting values from graphs, when accurate values were not available, are issues that should not be ignored. In the current review, 90 LCEA case studies of residential buildings were examined. Operating energy appeared to dominate in life cycle energy analysis of residential buildings in the past. Through time as the energy efficiency regulations conform towards the nZEB, the embodied energy leads with a dominating role in LCEA of residential buildings. The LCEA case studies examined in the current review indicate that in a transaction from a conventional to passive, low energy and nearly zero energy building the share of embodied energy increases, even though a reduction in the total life cycle energy even up to 50% is identified. The share of embodied energy dominates, mainly in low energy and nearly zero energy buildings, with a share of 26%-57% and 74%-100% respectively. In the conventional LCEA case studies examined the share of embodied energy is between 6% and 20%. Ramesh et al. in 2010 reviewed conventional residential buildings, resulting in a similar share of embodied energy that ranges between 10%-20% [21]. Sartori et al. in 2007 indicated that for low energy buildings the share embodied energy in the total life cycle energy ranges between 9% and 46% and for conventional between 2% and 38% [22]. A previous review by Karimpur et al. in 2015, that takes no account nearly zero energy buildings, results in a range of the share of embodied energy between 5,1%-42,4% [20]. These ranges appear to be close to the findings of this study if only conventional and low energy buildings are taken into account providing a range in the share of embodied energy between 6% and 57%. In the passive buildings examined, the share of embodied energy varies between 11%-33% and reaches the limits of both a conventional and a low energy building, indicating the influence of building methods, choice of materials, heating systems and LCI methods. This value appears to be significantly lower than the one presented by Crawford et al. with 56,9% of the total life cycle energy [68]. This is justified by the use of input-output hybrid analysis in contrast with the use of process analysis in the reduced final sample of case studies in the current review. As indicated in the same study by Crawford et al, if process analysis was used embodied energy would contribute with 13,1%, a value that reaches the lower limit resulting from the current study. The use of renewable energy sources in a passive house or a low energy building, for the production of electricity, classifies it in the range of embodied energy of a nearly zero energy building. A significant gap of 17% in the share of embodied energy, between the nearly zero and the most energy efficient building examined in the current review, is identified. This difference appears to be more significant for the conventional and passive buildings, indicating the relative significance of embodied energy through time and towards the nZEB. If uncertainty and the underestimation of embodied energy occurring by process analysis were considered this gap could be different, indicating that future research is needed with an extended analysis and a number of hybrid based case studies from international literature if available. As the share of embodied energy in buildings increases, a whole life energy cycle

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17% gap in the share of embodied energy between nZEBs and low energy buildings Significant uncertainty from LCI methods indicated in LCEA of buildings PCR 2014:02 for buildings appear to be suitable for the presentation of LCEA Further standardization is needed in LCEA of buildings

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