Article

pubs.acs.org/est

Embodied Energy of Construction Materials: Integrating Human and

Capital Energy into an IO-Based Hybrid Model
Manish K. Dixit,*,† Charles H. Culp,‡ and Jose L. Fernandez-Solis‡
†
Sam Houston State University, Huntsville, Texas 77340, United States
‡
Texas A&M University, College Station, Texas 77843, United States
*
S Supporting Information

ABSTRACT: Buildings alone consume approximately 40% of

Downloaded by PONTIFICIA UNIV CATOLICA DEL PERU at 16:45:18:505 on June 04, 2019

the annual global energy and contribute indirectly to the

increasing concentration of atmospheric carbon. The total life
cycle energy use of a building is composed of embodied and
operating energy. Embodied energy includes all energy
required to manufacture and transport building materials,
and construct, maintain, and demolish a building. For a
systemic energy and carbon assessment of buildings, it is
from https://pubs.acs.org/doi/10.1021/es503896v.

critical to use a whole life cycle approach, which takes into

account the embodied as well as operating energy. Whereas the
calculation of a building’s operating energy is straightforward,
there is a lack of a complete embodied energy calculation
method. Although an input−output-based (IO-based) hybrid
method could provide a complete and consistent embodied energy calculation, there are unresolved issues, such as an
overdependence on price data and exclusion of the energy of human labor and capital inputs. This paper proposes a method for
calculating and integrating the energy of labor and capital input into an IO-based hybrid method. The results demonstrate that
the IO-based hybrid method can provide relatively complete results. Also, to avoid errors, the total amount of human and capital
energy should not be excluded from the calculation.

1. INTRODUCTION energy.9,13,15 The total life cycle energy use of a building is the
As a result of rising consumption of fossil fuel-based energy sum of life cycle embodied and operating energy.16
The calculation of a building’s operating energy is relatively
sources, the amount of carbon emissions in the atmosphere has
straightforward compared to embodied energy, which lacks
increased radically.1−3 Most of this emission originates from
consistent, complete, and accurate data.9,17−19 This lack of an
fossil fuel combustion in transportation, construction, manu-
established embodied energy database also impedes the
facturing, and residential and commercial operations.4−6 The application of a life cycle-based environmental evaluation to
global construction industry, on the average, consumes the building design and construction industry. Among key
approximately 40% of the world’s energy each year and challenges to establishing a complete and consistent embodied
contributes to carbon emission significantly.7−9 According to energy database is the lack of a globally accepted embodied
the USEPA,10 buildings alone contribute to nearly 40% of the energy calculation method.9 There are various embodied energy
United States’ annual carbon emissions by consuming electricity calculation methods that use process, input−output (IO), or
and natural gas in their operation. A building consumes energy in hybrid data.11−13 Although each method has advantages and
two ways: (1) through the use of construction materials, disadvantages, an IO-based hybrid analysis is currently
products, and processes during its construction, maintenance, considered the most appropriate method. However, some issues
and demolition, and (2) in its operation during the occupancy with the reliability of its results have been highlighted in the
phase.11−13 Each material or product installed in a building has literature.11−13 In the past, improvements to the IO-based hybrid
consumed some amount of energy when it was manufactured analysis enhanced its reliability, as noted by Treloar,11 Joshi,20
and delivered to end users. Similarly, each process of and Crawford.12 Despite these efforts, there remains potential for
construction, fabrication, transportation, and administration further improvement.12,13,21 Issues such as the overdependence
during the building’s life cycle also consumes energy.11,12,14,15 on product price data, sector aggregation, and the lack of an
The total energy consumed by a building over its life cycle
through the use of materials, products, and processes is called its Received: August 12, 2014
life cycle embodied energy. Once the building is complete and Revised: December 31, 2014
occupied, the total energy expended in air-conditioning, heating, Accepted: January 5, 2015
lighting, and powering building appliances is termed operating Published: January 5, 2015

© 2015 American Chemical Society 1936 DOI: 10.1021/es503896v

Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

approach to integrate capital and labor inputs still need to be provide the flow of goods and services in monetary units among
addressed.9,12,21 various industry sectors.11,12,21 The national IO accounts are
In this paper, we propose an approach to calculate the human published periodically at a summary or detailed level.39 For
and capital energy and integrate them into an IO-based hybrid calculating embodied energy, a square matrix of direct require-
framework developed for the United States’ economy. The aim ment coefficients is used, which presents the input directly
of this paper is to enable a life cycle carbon assessment of a required to produce a unit of output for a particular industry
building by calculating the embodied energy of construction sector. The direct requirements are accompanied by indirect
materials in a complete manner (covering the energy embodied requirements.40 For example, when an industry sector “C”
in labor and capital inputs). We also analyze the results to increases its output by $1, its input providing sectors (e.g., “M”
investigate whether the energy embodied in human and capital and “E”) must increase their output proportionally. This stage of
inputs is insignificant. indirect impacts is labeled stage one. To meet the increased
output, sectors “M” and “E” exert pressure on their input
2. LITERATURE REVIEW supplying sectors to increase their output resulting in stage 2
2.1. Embodied Energy Model for a Building. The total indirect inputs. Similarly, there are stage three indirect inputs and
energy embodied in a building over its service life is composed of so on. Therefore, the impact of raising industry output by $1 can
its initial embodied energy (IEE), recurrent embodied energy be felt throughout the economy.40 This economy-wide impact
(REE), and demolition energy (DE) as shown in Figure S1 represents indirect requirements.40 A detailed description of the
(Supporting Information, SI).13,14,22 When a building is method can be found in Treloar11 and Dixit.41 One major
constructed, energy is consumed directly in processes such as advantage of an IO-based calculation is that it covers most energy
construction, transportation, and administration. At this stage, inputs to provide a relatively complete calculation.12,15,21
energy is also consumed indirectly through the use of However, its results are for an aggregated industry sector that
construction materials and pieces of building equipment installed manufactures a wide range of products. In the IO method, all
in the building. The total energy embodied in constructing and products would have the same energy intensity, which may not
delivering the building for occupancy is termed IEE.16,23,24 A be true.11,12,15
major fraction of IEE comes from building material use.25−27 There are two types of hybrid analyses: (1) process-based and
When building materials are extracted, processed, manufactured, (2) IO-based. In a process-based hybrid analysis, the basic
packaged, and delivered, they have already consumed some framework remains process-based. To improve its completeness,
energy.28−30 For instance, commonly used building materials, IO data are inserted into the framework.36,38 For instance, to
such as cement, steel, aluminum, and insulation, are highly calculate the embodied energy of a house, the actual quantities of
energy intensive.31,32 The total energy consumed in extracting, materials used are collected from the bill of quantities. These
processing, manufacturing, and delivering a building material is material quantities are then multiplied by their respective IO-
called its embodied energy.12,13,15 based embodied energy intensities to calculate the total
Once the building is occupied, it is maintained, refurbished, embodied energy of the house.11,12 In an IO-based hybrid
and some of its components are replaced. All such activities analysis, the process data of direct energy use are collected and
involve the use of construction materials and processes.33 The inserted into an IO-based framework.11,12,21 The process data
sum of all energy expended in maintenance, repair, refurbish- replace comparable IO-data in the framework.11 The basic
ment, and replacement activities is REE.7,19,23 The amount of assumption is that including more process data results in a more
REE depends on material and equipment selection and the reliable model. The replacement of IO data with process data can
service life of a building.25,34 For instance, selecting a durable be accomplished in many ways. For instance, if energy use data
product with a low maintenance requirement would lower the are available for all industry sectors, they can be incorporated
value of REE.16,23 Because REE is time dependent, the longer the directly into the IO model.42 If direct energy data are available
service life of a component, the larger the value of life cycle REE only for a few sectors, then incorporating them into the IO model
for a building.16,26 When the building reaches the end of its may cause unwanted indirect impacts.11 Treloar11 proposed a
service life, it is demolished and demolition waste is sorted and method for integrating energy use data into the economic model.
hauled for recycling, reuse, or disposal. The total energy This method involves the identification and extraction of direct
consumed at this end-of-life stage is known as DE.14,22,27 For a energy paths from the IO model in order to integrate the study-
comprehensive carbon assessment of buildings, it is critical that a specific process data to avoid any unwanted indirect effects.11
net embodied energy value is calculated by taking into account According to Treloar,11 the incompleteness or error in typical
any recovery through reuse and recycle, particularly at the end- embodied energy calculation and analysis is approximately 20%
of-life phase.35 and, thus, no method provides accurate results. However, an IO-
2.2. Embodied Energy Calculation Methods. There are based hybrid analysis is considered more complete and
three commonly used methods for calculating embodied energy: consistent than other existing methods.8
(1) process-based, (2) IO-based, and (3) hybrid analyses. Each of 2.3. Embodied Energy Calculation: Major Issues.
the three methods has limitations based upon the type and 2.3.1. Incompleteness and Inconsistency. Among the key
availability of data it uses. Because the three methods also differ in parameters causing incompleteness and inconsistency are system
the extent to which their system boundaries cover the energy boundary definition and data quality.13,19 A system boundary
inputs, their results are not comparable.9,19 A process-based demarcates the energy inputs that are included in an embodied
embodied energy calculation is a bottom-up approach, which energy calculation.14,43−45 In most studies, the system boundary
starts with the building material as a final product and goes is defined subjectively since there are no system boundary
backward (upstream) to include as many direct and indirect definition guidelines.14,46 The results of such studies are not
energy inputs as possible.11−13,36−38 Most direct energy use data comparable and, therefore, cannot be used.17,18 Data quality is
comes from the material manufacturers. An IO-based embodied another parameter responsible for the incompleteness and
energy calculation utilizes the national IO accounts, which inconsistency of embodied energy values.14 Studies often use
1937 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

data that are either nonrepresentative or secondary. A detailed approach to desegregate industry sectors in order to calculate
discussion can be found in Dixit et al.13 product-specific energy intensities.
2.3.2. Lack of a Standardized Comprehensive Calculation 2.3.6. Human and Capital Inputs. Another major issue with
Method. The second big challenge to creating a consistent and the IO-based hybrid calculation is its inability to account for
complete embodied energy database is the lack of a globally inputs such as human labor and capital investment.14,50 Current
accepted calculation method that provides specific and complete embodied energy methods fail to include the human and capital
results.9,19,47 A method’s accuracy relates to the specificity of the energy component of total embodied energy.50−54 Studies
calculation. If a calculation approach provides embodied energy (Langston and Langston;52 Pulselli et al.55) have emphasized a
values specific to the product under study, the calculation is need to incorporate human energy into embodied energy
considered more accurate than one that provides aggregated analysis. However, this is often not accomplished due to the lack
results. Completeness is governed by the extent to which most of a clear human energy calculation method.56 There needs to be
energy inputs are covered by the system boundary.14 Some a system to apportion the household consumption that is
energy inputs, such as energy of labor, capital equipment, and counted toward human labor.50,57 In 1978, Costanza58 quantified
services, if not included in the calculation, could cause serious the energy embodied in labor and government services by
incompleteness.12,14 For instance, the choice between process- making the household sector endogenous to the conventional IO
based and IO-based methods depends on how well a method model. However, Cleveland50 argued that Costanza58 allocated
provides complete and study-specific results. entire household income to employment, which may not be
2.3.3. Process Data Integration. The reliability of the IO- accurate. Cleveland50 identified three energy components that
based embodied energy results can be improved by integrating should be accounted for when quantifying human energy: (1) the
process data of energy use. Studies such as Treloar11 and calorific value of food consumed by workers, (2) the embodied
Crawford12 improved the IO-based hybrid method incremen- energy of food, and (3) fuel purchased by workers. To apportion
tally. Treloar11 suggested systematically extracting IO data from human energy toward employment, Cleveland50 suggested
an IO model and replacing them with process data of actual comparing an earner’s expenditure to that of a nonearner.
energy use. Treloar11 also highlighted the problem of double Each industry sector of an economy requires considerable
counting of energy sources in an IO-based method and investment in capital goods in addition to labor. Capital goods
recommended keeping all energy and nonenergy inputs to include all building and nonbuilding structures such as
energy providing sectors at zero, using primary energy factors administration buildings, warehouses, workshops, electrical
(PEFs) instead. A PEF represents the total primary energy yards, loading and unloading areas, etc.59 The construction of
consumed and lost in delivering end-use energy such as these structures is energy intensive and this energy use should be
electricity. The PEF should be calculated by accounting for all treated as capital energy.14,60 Industry sectors also make
direct and indirect energy use.48,49 In Treloar,11 it is assumed that purchases of a range of equipment, software, and automobiles,
the process data of direct energy use are calculated in a complete which also consume energy during their manufacturing or
manner; incompleteness may leave gaps in the calculation. Also, construction, delivery, and installation.12,50,58 Because the
it is assumed that PEF values incorporate all direct and indirect detailed IO accounts are not published annually, some capital
energy use of energy source generation and distribution. If the
expenditures that occur in a year other than the year of the
values of PEFs are obtained from a secondary source, they need
detailed IO accounts may not show up in the IO table and may
to be used with caution. Crawford12 identified an issue with
remain excluded from the calculation.11,12,15
Treloar’s approach and proposed that, instead of extracting a
direct energy path, the total energy path should be removed.
However, when the total energy paths are removed and replaced 3. RESEARCH GOAL AND OBJECTIVES
with process data of direct energy use, there may remain some This paper aims at improving the completeness of an IO-based
incompleteness. Acquaye21 analyzed the proposed approaches hybrid method. The main goal of this study is to propose a
and opted for direct energy path extraction. human and capital energy calculation approach, integrate it into
2.3.4. Use of Price Data in IO-Based Analysis. Most IO-based an improved IO-based hybrid method, and calculate embodied
methods involve the use of price data to convert direct and energy of commonly used construction materials. Additionally,
indirect energy flows from monetary to energy units. If energy we calculate embodied energy intensities in energy units/$
prices are inflated or deflated, a serious error in the calculation output, thereby avoiding the use of energy prices. We also
may occur.11 Furthermore, the energy intensities obtained from calculate embodied energy values using the traditional IO
IO-based methods are in energy units/monetary unit (e.g., method to compare results. We also quantify the contribution of
MBtu/$) and, therefore, require product prices to convert them human and capital energy to the total embodied energy. The
to energy units/unit of mass or volume.12,15 This excessive research goal is accomplished by achieving the following
dependence on price data can be problematic, particularly if objectives:
reliable sector-specific prices are not available.21 The improve-
ments suggested by Trealor11 and Crawford12 still involve the • Construct an IO model for the United States’ economy
use of prices multiple times which could introduce large errors • Gather and refine the data of actual energy use by various
even if the prices are slightly off the actual values.21 industry sectors and integrate them into the IO model to
2.3.5. Sector Aggregation. Because an IO framework is used develop an IO-based hybrid model
in an IO-based hybrid method, the embodied energy intensities • Calculate human and capital energy values and insert them
are obtained for an aggregated sector producing a wide range of
products including the material under study. Such results for an into the IO-based hybrid model
aggregated sector lack specificity.9,11 In addition, manufacturing • Calculate and compare embodied energy using IO and IO-
technology differs across products and using aggregated based hybrid methods with and without human and capital
intensities may not be accurate. Joshi20 recommended an energy
1938 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

4. RESEARCH METHODS employment (according to BLS63). Finally, the value of TEE per
4.1. Hybrid IO Model. Although numerous approaches exist hour per employee was calculated. The total number of
for integrating process data into an IO model, we used the employees was sourced from the USCB,66,68,69 USBLS,70
approach proposed by Carter et al.42 In this approach, the energy USDA (NASS71), and USDOC.72 A more detailed explanation
inputs in monetary units are replaced with the process data of of calculation can be referred from Dixit.40
actual energy use in energy units. This way, the calculated values 4.3.1. Apportioning Personal Consumption. The trans-
of direct and total requirement coefficients are obtained in portation energy data for 2002 were sourced from the USDOT.73
energy unit/$, omitting the need to use energy prices. We used Not all personal transportation is used for work-related purposes.
the 2002 U.S. Benchmark Accounts for constructing the IO The Federal Highway Administration (USDOT) published data
model. A detailed description of the model development can be on work-related trips for the years 1983, 1990, 1995, 2001, and
found in Section 1 of the SI. We used MBtu/lb as the functional 2009 (Santos et al.74), which showed the percentage of work-
unit to make the calculation and analysis simpler. However, these related trips consistently between 22 and 30%. On the basis of
values can be translated into the units of volume using these data, we apportioned 27% of the total personal trips toward
appropriate material densities. Because of varying material employment. Similarly, all consumer expenditure cannot be
density, the functional properties of each material per unit of assigned to the industry of employment. The USBLS provided
mass would be different. Consequently, the material require- the consumer expenditure for single and more than 2 consumers
ments in the unit of volume would also vary. The calculated under the category of no earner and one earner (BLS75).
values can be converted from English units (kBtu/lb) to SI units Excluding the food and utilities expenses, the difference between
(MJ/kg) by multiplying with 2.326. the earner and nonearner for single and multiple consumers was
4.2. Primary Energy Factor (PEF) Calculation. To avoid in the range of 26−35%. An average of 30% of the total personal
the double counting of energy inputs, we adopted the approach consumption expenditure (excluding food, utilities, and trans-
proposed by Treloar11 and calculated the primary energy factors portation) was allocated to the industry of employment as
(PEFs) for all five energy-providing sectors: (1) oil and gas suggested by Cleveland.50 The personal consumption expendi-
extraction, (2) natural gas distribution, (3) coal mining, (4), ture data were sourced from the USBEA’s 2002 Benchmark
petroleum refineries, and (5) electric power generation, Input-Output Accounts. To account for the energy embodied in
transmission, and distribution. A detailed description of PEF personal consumption items, the 2002 Benchmark Input-Output
calculation is available in Section 2 of the SI. A more detailed direct and total requirements tables were used.76 The residential
explanation of PEF calculation methods can also be referred from energy consumption data for electricity, natural gas, and
Dixit et al.16 petroleum were sourced from the 2002 Annual Energy Review
4.3. Human Energy Calculation. We used the approach to account for the household energy use.
suggested by FAO61 to calculate the total energy expenditure 4.4. Capital Energy Calculation. Although the USBEA
(TEE) of an average human body. Three types of data were publishes capital flow data as supporting information to the
sourced to quantify TEE of an average working employee in the benchmark accounts, the latest data were from 1997, and hence,
United States. First, the average range of weight and age by could not be utilized for quantifying capital inputs in 2002. Data
gender was gathered from BLS62,63 and Borrud et al.64 to relating to capital expenditures of various industries were
calculate the basal metabolic rate (BMR) values. Next, an hourly collected from diverse sources such as the USDOC,59 USCB,77
activity schedule of a typical day of an employee was developed. USBEA,78,79 USDA,80 and USCB.81 IO-based hybrid energy
The number of total employees was sourced from USBEA65 and intensities were used to calculate average energy intensities of
USCB.66 Finally, the physical activity ratio (PAR) values of the capital products. The energy intensities were averaged on the
activities of employee were quantified to calculate an average basis of relative share of capital inputs in the total net change in
physical activity level (PAL). The activity hours and PAR values the fixed assets’ value at the end of 2002. For instance, capital
were based on FAO61 and the American Time Use Survey Data inputs under the structures category included a wide range of
for 2003 from the United States Bureau of Labor Statistics residential (e.g., single and multifamily) and nonresidential
(USBLS).67 The TEE was calculated as buildings (e.g., healthcare, commercial, and manufacturing
buildings) belonging to different industry sectors, and energy
TEE = BMR × PAL intensities of those were averaged using their % share in total
The value of BMR is dependent on the age and body weight of capital inputs in buildings. Likewise, the equipment category
a person and was calculated as follows: 61 included a variety of equipment such as computers, printing and
Males, age 18−30 years metalwork machinery, and electrical and mechanical equipment,
etc., which were used to calculate an average energy intensity.
BMR = 0.063 × Wtbd + 2.896 Tables S1 and S2 in the SI show the breakdown of the structure
Males, age 30−60 years and equipment categories with their average energy intensities
and percent share in the total capital expenditure.
BMR = 0.048 × Wtbd + 3.653
Females, age 18−30 years 5. FINDINGS
BMR = 0.062 × Wtbd + 2.036 5.1. Human and Capital Energy. The total energy
embodied in household expenditure of an average employee in
Females, age 30−60 years the United States in 2002 was calculated as 45.7 MBtu (excluding
BMR = 0.034 × Wtbd + 3.538 work-related personal transport and food). The total energy
consumed by an average employee was found to be 78.8 MBtu.
where Wtbd is the body weight of a worker in kg. The TEE values This energy use included the energy of food (4%), consumables
were calculated for both male and female employees separately (31%), residential energy use (27%), and all work-related
and were then averaged using their percent share in the total personal transportation (38%). On the basis of these results, the
1939 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

Table 1. Embodied Energy of Materials under Study

IO-based hybrid + human and capital
conventional IO-based method IO-based hybrid method energy
embodied energy (A) (B − A)/A embodied energy (B) (C − B)/B
study material (kBtu/lb) (%) (kBtu/lb) (%) embodied energy (C) (kBtu/lb)
carpet (3/8 in. thick), level loop 235.25 −3 228.21 6 242.10
wood lumber 2.19 11 2.42 11 2.70
hardwood plywood & veneer 11.54 21 13.95 9 15.19
softwood plywood & veneer 3.01 21 3.64 9 3.97
paints & coatings 28.99 −21 22.82 6 24.12
adhesives 56.16 −61 21.64 6 23.00
plastic pipes & fittings 42.23 11 46.86 4 48.74
polystyrene foam insulation 104.84 0 104.70 5 110.12
bricks 2.07 −24 1.57 6 1.66
clay wall & floor tiles (1/4 in. thick) 18.99 −24 14.38 6 15.20
vitrified clay sewer pipes 8.39 −24 6.36 6 6.72
flat glass 10.60 −3 10.29 3 10.62
cement 1.91 64 3.13 3 3.23
concrete 0.46 20 0.54 7 0.58
gypsum, bldg. products 9.05 12 10.12 3 10.38
lime 1.67 12 1.87 3 1.92
stone 1.31 −7 1.22 17 1.43
mineral wool insulation 11.83 1 11.90 6 12.60
virgin steel 10.41 −3 10.11 3 10.39
primary aluminum 29.19 172 79.30 1 80.17
copper 18.76 31 24.67 4 25.77

Table 2. Share of Each Energy Source in Total Embodied Energy

% of various energy sources in total embodied energy
study material oil and gas coal electricity natural gas petroleum human energy capital energy
carpet (3/8 in. thick), level loop 1.60 4.21 36.35 23.11 29.00 3.55 2.18
wood lumber 1.35 0.81 27.33 10.46 49.76 5.85 4.44
hardwood plywood & veneer 1.20 1.12 32.94 14.01 42.58 5.13 3.01
softwood plywood & veneer 1.20 1.12 32.94 14.01 42.58 5.13 3.01
paints & coatings 2.76 4.45 23.87 21.67 41.88 2.75 2.62
adhesives 2.53 4.66 26.02 21.49 39.37 3.13 2.80
plastic pipes & fittings 3.27 2.12 23.26 20.15 47.34 1.84 2.02
polystyrene foam insulation 2.56 3.07 24.34 24.61 40.48 2.62 2.31
bricks 0.31 2.14 23.20 51.91 17.02 2.96 2.46
clay wall & floor tiles (1/4 in. thick) 0.31 2.14 23.20 51.91 17.02 2.96 2.46
vitrified clay sewer pipes 0.31 2.14 23.20 51.91 17.02 2.96 2.46
glass 0.28 1.18 28.36 55.97 11.10 1.47 1.65
cement 0.22 38.30 28.16 6.03 24.37 0.71 2.21
concrete 0.41 21.05 24.28 14.61 32.98 3.12 3.54
gypsum, bldg. products 0.30 22.49 18.50 26.56 29.65 1.21 1.28
lime 0.30 22.49 18.50 26.56 29.65 1.21 1.28
stone 0.72 3.76 33.64 17.11 30.27 9.27 5.23
mineral wool insulation 0.52 3.82 39.01 35.55 15.54 2.45 3.10
virgin steel 0.18 26.43 38.07 24.70 7.91 1.52 1.19
primary aluminum 3.54 0.48 64.69 9.17 21.04 0.58 0.50
copper 0.13 6.29 51.09 26.96 11.24 2.19 2.09

total energy embodied in human inputs was equal to sectors. Tables S3 and S4 in the SI list the top 9 sectors with
approximately 11 quadrillion Btu. higher human and capital energy use.
The total energy embodied in capital inputs such as structures, 5.2. Embodied Energy of Construction Materials. Table
equipment, and software incurred during 2002 was calculated as 1 lists the calculated values of embodied energy of 21 commonly
11.5 quadrillion Btu. The share of capital structures and used construction materials. As shown in Table 1, the embodied
equipment/software in total capital energy was approximately energy values of adhesives, cement, aluminum, and copper
46% and 54%, respectively. Interestingly, all primary energy calculated by the conventional IO-based method were quite
supplying sectors had more capital energy under the category of different from those quantified using an IO-based hybrid
structures or buildings, whereas the equipment and software approach. Surprisingly, the IO-based hybrid embodied energy
category dominated the capital energy use of secondary energy of aluminum and cement was over 2.5 and 1.5 times their IO-
1940 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

Figure 1. IO-Based hybrid embodied energy comparison.

based values, respectively. There was minimal difference in IO- than the fossil fuel-based energy sources. Carpet was the most
based hybrid and IO-based embodied energy values of human (8.6 kBtu/lb) and capital energy (5.3 kBtu/lb) intensive
polystyrene foam, mineral wool, and concrete. The inclusion of material. Human and capital energy contributed to over 9% and
human and capital energy inputs in the embodied energy 5% of the total embodied energy of stone, respectively.
calculation increased the IO-based hybrid embodied energy Figure 3 illustrates the percentage of direct and indirect energy
values by 1−17%. Among the most human and capital energy in the total energy of construction materials. Manufacturing
intensive construction materials were stone, wood, and plywood construction materials such as carpets, paints and coatings,
products. The least human and capital energy use was for adhesives, plastic products, concrete, and copper consumed
aluminum production. Among the top three energy-intensive more indirect than direct energy (72−92% indirect energy).
materials were carpet, polystyrene foam, and aluminum. Among Other materials, such as bricks, clay tiles, clay pipes, glass,
the materials with low embodied energy were concrete, stone, cement, lime, and gypsum, used relatively lower indirect energy
brick, and lime. (11−27%). The embodied energy of all wood-based con-
Table 2 provides the relative share of each energy source in the struction materials included approximately 46−51% of indirect
total embodied energy of construction materials. A clear energy.
dominance of electricity, natural gas, and petroleum was seen
when the total embodied energy was broken down by energy 6. DISCUSSION
source. Among the most electricity-consuming materials were The energy embodied in labor and capital inputs was quantified
carpet (88 kBtu/lb) and aluminum (52 kBtu/lb). Industry using approaches suggested by published studies. However, there
sectors producing products such as plastic fittings and pipes and were a number of assumptions made when apportioning
polystyrene insulation use petroleum products as feedstock personal consumption expenditure toward employment and
materials. These sectors were among the most petroleum- disaggregating capital expenditure in the categories of structures,
consuming sectors. Surprisingly, carpet remained the most fossil- equipment, and software. More reliable data are needed to
fuel consuming material. Energy intensities broken down by reliably compute human and capital inputs. Overall, human
energy sources are illustrated in Figures 1 and 2, which show the energy contributed to approximately 1−9% of the total
results of IO-based hybrid and conventional IO-based methods, embodied energy of the studied construction materials. Although
respectively. Interestingly, the IO-based electricity intensity of this percentage seems insignificant and confirms the assertion
aluminum was approximately 1/12th of its IO-based hybrid that human energy could be negligible, for a building with
value. Human and capital energy was found to be much smaller excessive use of stone and wood-based materials, the total human
1941 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

Figure 2. Conventional IO-based embodied energy comparison.

Figure 3. Share of direct and indirect component in total embodied energy.

energy embodied could be much higher. Similarly, capital energy capital energy could contribute from 1 to 14% of total embodied
was calculated as 1−5%, which was insignificant compared to energy. Excluding the energy of labor and capital investment
other fossil fuel-based energy sources. Collectively, human and from embodied energy calculation could cause incompleteness in
1942 DOI: 10.1021/es503896v
Environ. Sci. Technol. 2015, 49, 1936−1945
Environmental Science & Technology Article

the range of 1−17% at a material level (see Table 1). This approach to disaggregate an industry sector, which can be utilized
incompleteness could be more at a building level depending to compute more product-specific energy intensities.
upon the type and quantity of materials used. Whereas this study The results of this study indicate that even if a process-based
focuses on the United States, the results could be much different analysis is carried out covering one upstream stage, a significant
if calculated for countries such as India and China with more error may occur due to the incompleteness of a truncated system
labor-intensive economies. boundary. Although the incompleteness inherent in a process-
The total embodied energy calculation results of the IO-based based analysis can be improved, covering a system boundary
hybrid method were quite different from the conventional IO- similar to an IO analysis seems impractical. On the other hand,
based values, particularly for industry sectors with higher the results of an IO-based hybrid analysis can be made more
electricity use. One main reason for this may be the use of PEF study-specific by using the process of sector disaggregation
(4.12 times for electricity) for converting secondary energy to (Joshi20). However, the results should be interpreted and used
primary energy, which also takes into account transmission and with caution, as there are issues with IO analyses that still need to
distribution losses. Also, the calculated values of PEFs for coal, be addressed. For instance, using unreliable product price data
natural gas, and refined petroleum included energy use and losses contributes to the uncertainty of the results of IO-based
that may not show up in annual IO accounts, as no monetary methods. In addition, the assumptions of proportionality and
transaction occurs for such onsite energy use or loss. The homogeneity inherent in an IO table adds to this uncertainty. IO
embodied energy results demonstrated that carpet was the most data are conventionally published late and using such old data
energy-intensive material, particularly in the fossil fuel category. without proper adjustment may yield misleading results. To
This was due to the heavy use of synthetic materials in its avoid the use of price data, the total monetary output of a
production, such as nylon, polyester, PVC, and adhesives having commodity in physical units, if available, can be used to convert
a higher embodied energy. As discussed in the earlier section, embodied energy intensities to embodied energy (per unit of
materials such as carpets, polystyrene, plastic pipes and fittings, mass or volume). The total energy embodied in a commodity can
paints and coatings, and adhesives showed a higher petroleum be divided by the total output in physical units to calculate
embodied energy. In addition, the process of sector disag-
product use (10−70 kBtu/lb) probably due to their use of it as
gregation can help address some of the issues relating to the
feedstock material. Such high feedstock energy use was visible in
proportionality and homogeneity assumptions.
a higher percentage of indirect energy (72−92%) of these
The life cycle embodied energy of a building and its
materials as seen in Figure 3. Materials that require relatively less
constituent materials can be a comprehensive indicator of the
energy-intensive ingredients such as clay, sand, and limestone, building’s life cycle carbon contribution. Mostly, embodied
showed relatively lower percentage of indirect energy use (11− energy has been calculated as an aggregated energy value without
21%). For instance, clay products that are made of clay, a knowing the individual contribution of energy sources such as
relatively less energy-intensive material, held a lower indirect coal, natural gas, oil, and electricity. In such a case, determining
energy (16%). Similarly, glass and cement showed 21% and 11% carbon impacts becomes difficult, since an average carbon
of indirect energy, respectively, due to the use of ingredients with emission coefficient is used. However, if embodied energy
less embodied energy such as silica and limestone. The embodied intensities are computed for each energy source, a more detailed
energy results for cement and concrete were interesting. Because carbon emission evaluation can be performed. We calculated
cement, a material with higher embodied energy, is one of the aggregated as well as disaggregated embodied energy values that
main constituent materials of concrete, the indirect energy of showed the composition of energy sources. Embodied energy
concrete was approximately 87%. However, the indirect energy intensities shown in terms of various energy sources also help
represented less than 11% of the total embodied energy of distinguish materials that are more coal, oil, or electricity
cement. Furthermore, the IO-based embodied energy of intensive. The manufacturing process of such materials then
materials such as cement and aluminum, which mainly included could be improved to reduce the use of carbon-intensive energy
coal and electricity usage, increased significantly when calculated sources. In this paper, we demonstrated that the embodied
with IO-based hybrid method probably due to the use of PEF for energy of construction materials can be quantified in a complete
electricity. In addition, most aluminum production plants manner by including capital and human energy and by avoiding
installed near hydropower plants buy electricity at a considerably the excessive use of energy prices.
low price. The same may be true with most cement plants, which
may be buying coal at a cheaper price. The IO-based embodied
energy of adhesives was significantly higher than IO-based hybrid
■
*
ASSOCIATED CONTENT
S Supporting Information
values probably due to a larger consumption of secondary energy Additional text and tables as mentioned in the text. This material
sources such as petroleum products. Because the problem of is available free of charge via the Internet at http://pubs.acs.org.
double counting is more pronounced in the case of secondary
fuels, the IO-based embodied energy value for adhesives is
significantly higher than the IO-based value.
■ AUTHOR INFORMATION
Corresponding Author
As shown in Table 2, some construction materials such as *E-mail: mandix72@hotmail.com.
hardwood and softwood plywood and lumber share the same Notes
embodied energy intensity because they come from the same The authors declare no competing financial interest.

■
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1945 DOI: 10.1021/es503896v

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