Embodied energy in building materials has been
studied for the past several decades by researchers interested in
the relationship between building materials, construction processes,
and their environmental impacts.
What is embodied energy?
There are two forms of embodied energy in buildings:
· Initial embodied energy;
· Recurring embodied energy
The initial embodied energy
in buildings represents the non-renewable energy consumed in the
acquisition of raw materials, their processing, manufacturing, transportation
to site, and construction. This initial embodied energy has two
Direct energy the energy used
to transport building products to the site, and then to construct
the building; and
Indirect energy the energy
used to acquire, process, and manufacture the building materials,
including any transportation related to these activities.
The recurring embodied energy
in buildings represents the non-renewable energy consumed to maintain,
repair, restore, refurbish or replace materials, components or systems
during the life of the building.
As buildings become more energy-efficient, the ratio of embodied
energy to lifetime consumption increases. Clearly, for buildings
claiming to be "zero-energy" or "autonomous",
the energy used in construction and final disposal takes on a new
How is it measured?
Typically, embodied energy is measured as a quantity of non-renewable
energy per unit of building material, component or system. For example,
it may be expressed as megaJoules (MJ) or gigaJoules (GJ) per unit
of weight (kg or tonne) or area (square metre). The process of calculating
embodied energy is complex and involves numerous sources of data.
Refer to the
Related Resources + References page for further information
on embodied energy.
Implicit in the measure of embodied energy are the associated environmental
implications of resource depletion, greenhouse gases, environmental
degradation and reduction of biodiversity. As a rule of thumb, embodied
energy is a reasonable indicator of the overall environmental impact
of building materials, assemblies or systems. However, it must be
carefully weighed against performance and durability since these
may have a mitigating or compensatory effect on the initial environmental
impacts associated with embodied energy.
How much embodied energy is typically found
The amount of embodied energy in buildings varies considerably.
Initial embodied energy consumption depends on the nature of the
building, the materials used and the source of these materials (this
is why data for a building material in one country may differ significantly
from the same material manufactured in another country). The recurring
embodied energy is related to the durability of the building materials,
components and systems installed in the building, how well these
are maintained, and the life of the building (the longer the building
survives, the greater the expected recurring energy consumption).
Research carried out by Cole and Kernan(1)
using a model based on Canadian construction of a generic 4 620
m2 (50,000 ft2) three-storey office building with underground parking,
considered three different construction systems (wood, steel and
concrete), and yielded the following results for average total initial
embodied energy. (Note: Data were averaged for the three construction
systems as the overall differences between the building types were
|Breakdown of Initial Embodied Energy by Typical
Office Building Components Averaged Over Wood, Steel and Concrete
Structures [Cole and Kernan, 1996].
The building envelope, structure and services contribute fairly
equally and account for about three-quarters of total initial embodied
energy. The finishes, which represent only 13% of the embodied energy
initially, typically account for the highest increase in recurring
embodied energy. Embodied energy may not be significantly different
between building systems (e.g., wood versus steel versus concrete),
however, the environmental impacts associated with one material
versus another can be dramatically different.(2)
It is interesting to consider the relationship between site work
(6% of initial embodied energy) and services (24%). The reallocation
of embodied energy, and hence project budget, from conventional
services to the site management of stormwater, for example, may
have a negligible effect on initial embodied energy, but the impact
on recurring embodied energy may prove significant. Additional benefits
downstream of the building at the community infrastructure level
should also be considered. This points to one of the shortcomings
of embodied energy analysis, which typically ends at the property
line and is somewhat unwieldy in dealing with a broader context.
When recurring embodied energy in buildings is considered, yet
more interesting relationships are revealed from the work of Cole
and Kernan. First, to the credit of civil engineers, the structures
of buildings normally do not expend recurring embodied energy, lasting
the life of the building. By year 25, however, a typical office
building will see an increase of almost 57% of its initial embodied
energy due mostly to envelope, finishes and services. By year 50,
recurring embodied energy will represent about 144% of the initial
embodied energy, and it was projected that by year 100, this proportion
would rise to almost 325%. This relationship is a direct result
of what is referred to as differential
durability, where the service lives of the various materials,
components, and systems comprising the building differ dramatically.
The current preoccupation with lower first costs in buildings reveals
its disregard for sustainability when viewed from a building life
|Comparison of Initial to Recurring
Embodied Energy for Wood Structure Building Over a 100-Year
Lifespan [Cole and Kernan, 1996].
Is embodied energy a useful measure?
Embodied energy can be a very useful measure provided it is not
viewed in absolute terms. The initial embodied energy of various
materials, components and systems can vary between projects, depending
on suppliers, construction methods, site location and the seasonality
of the work (e.g., winter heating). The recurring embodied energy
is difficult to estimate over the long term since the non-renewable
energy contents of replacement materials, components or systems
are difficult to predict. For example, how energy intensive will
glass be 100 years from now? However, as buildings become more energy
efficient and the amount of operating energy decreases, embodied
energy becomes a more important consideration. There also exist
strong correlations between embodied energy and environmental impacts.
But it is widely acknowledged today that embodied energy represents
one of many measures and should not be used as the sole basis of
material, component or system selection.
1.Cole, R.J. and Kernan, P.C. (1996), Life-Cycle Energy Use
in Office Buildings, Building and Environment, Vol. 31, No.
4, pp. 307-317.
2.Comparing the Environmental Effects of Building Systems,
Wood the Renewable Resource Case Study No.4, Canadian Wood
Council, Ottawa, 1997.
|The next section
deals with Operating
Energy as a measure of sustainability.