The gray energy of construction, the hidden face of the sector!

The gray energy of construction and the “gray CO2” of components, the hidden face of the building and construction industry.

The orientations of European policy on the gray energy of construction.

The new data on climate change obliges us to significantly reduce our fossil energy consumption, especially in the construction sector, which represents 40% of the total energy used in the European Union and almost a quarter of greenhouse gas (GHG) emissions.

The European Union has issued a directive 19 May 2010 to improve the energy performance of buildings, which at the time was already hailed as a big step. This directive provides for the generalization of "passive" or energy-producing buildings on the 2020 horizon, with an obligation for public buildings to implement this policy from 2018. This text stipulates that in a quarter of a century, at least 25% of new buildings are neutral or positive on the energy balance of their consumption.


But should the energy balance of a construction stop at the simple notion of energy expenditure of use?

This new energy performance threshold requested by the public authorities leads us, de facto, to pursue a coherent logic of integration of all the energies present in a project in order to improve its environmental performance. However, if the notion of consumption of use is put forward (heating, domestic hot water, lighting, ventilation, air conditioning), the consideration of gray energy in the implementation of buildings and on-board equipment was , to date, rather understated.


It is now unavoidable that the policy of very low consumption decided for future thermal regulations highlights the importance of the gray energy factor.
As Alain Maugard (President of Qualibat) points out in one of his columns on the Xpair website:

"In the past for a home that used 80 Kg eq CO2 / m2.an, we needed 5 years of operation to cover the gray energy used in construction. We must now realize that a building low consumption that will require only 5Kg eq CO2 / m2.an for its uses will put 70 80 years to compensate this post »

gray energy of construction: Comparison of gray energy and use for 4 types of buildings
Comparison of embodied energy and use for 4 types of buildings

To set an order of magnitude, we know that the amount of gray carbon required for the construction of a building is of the order of 300 to 500 Kg eq CO2 / m2 for a single house, from 300 to 600 Kg eq CO2 / m2 for a small collective and from 500 to 800 Kg CO2 / m2 for offices.


But what is the gray energy of construction ?

The ICEB (Institute for the Eco Responsible Design of Buildings - annex 1) proposes a definition integrating two types of energies:

  • non-renewable gray energy called process energy (energy inputs needed in the implementation process during the life cycle),
  • Renewable gray energy over the entire life cycle of non-working life.


This concept is close to the standard EN 15 978 which has the only difference to integrate, in addition, the post maintenance. To schematize, the gray energy is the energy "lost": it is the energy debt, while the energy material (also called "feedstock") is rather the energy stock mobilized temporarily. The material energy can be recovered at the end of its life either by recycling or by energy recovery.

Below, an example of a comparison between a steel frame and a laminated timber frame from FDES (environmental and sanitary declaration sheets) on 100 ans:

gray energy of construction: Energy comparison of a metal and wood frame
Energy comparison of a metal and wood frame

This comparison shows us that, although the total primary energy of the steel structure is more interesting, we note however that its balance sheet process of energy "lost" is very unfavorable with regard to the wood structure that allows us to do an economy of 46% gray energy within the meaning of the ICEB definition.

What are the options to reduce the share of gray energy ?

The main areas of improvement can be seen in several stages:

  • at the scale of architectural programming by implementing functional diversity, choosing a suitable site, optimizing the architecture through compactness and simplicity;
  • at the building level, we will have to focus on implementing equivalent energy-efficient materials relying on the lowest possible LCA (life cycle analysis), item by item;
  • at the site level, we need to apply the same analytical approach to save water and fuel and give priority to nearby resources, waste management, sustainable staff travel, etc.
  • at the stage of exploitation, we can reduce embodied energy through various maintenance, upkeep, equipment renewal, sobriety, information on best practices for users, etc.
  • manage the end of life of the project in the most optimal way possible, thinking of the reuse on site or in a nearby sector, recycling, energy recovery.

A new vision of the energetic construction is starting!

The 18 February 2016, the association for the development of low carbon BBCA, the scientific and technical center of the building CSTB and its certifying body CERTIVEA presented the first label Low Carbon.

Jean Christophe Visier, Energy Director at CSTB, explains that:


"This label is based on scientific methods (LCA for example) that led to the development of the BBCA repository
(see annexes 2 and 3) which relies on the collective work of the sector and the State (in particular HQE performance) ".


Mrs. Ségolène Royal also welcomed the establishment of this label which contributes, according to her words, "to the realization of exemplary buildings that will participate in the implementation of the objectives of the energy transition for green growth and the pursuit of commitments the Paris Agreements on Climate adopted at COP 21 ".


Article written by Philippe Lefèvre, Certified European Passive House consultant

Article published on following a request for publication on the site, feel free to submit your own publications and we will study them!

References and Annexes:

The ICEB website
Low Carbon Building BBCA
BBCA repository in .pdf

Go further: technical discussions on embodied energy in construction

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