The difficulty of calculating the real carbon footprint of a building

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Climate combat with architects!In 2019, the Snohetta (Norway) and White (Sweden) agencies said that "zero carbon", even "negative" projects, by 2030.In England, eleven architects, all winners of the prestigious Stirling Prize, have undertaken to work only on low carbon certified projects.In France, fewer announcements, but low carbon buildings have been built for a few years, certified by the BBCA label, created by the association of the same name in 2016, which includes three levels of performance.The Ministry of Ecological Transition launched an appeal to manifestation of interest (friend) entitled "Mixity for low carbon construction", the deadline of which is fixed in this month of.And among the latest initiatives is the call for mixed and low carbon projects "imprints", of the Paris-La Défense developer for the rehabilitation of five business district sectors.

If the final objective - limit global warming to 1.5 ° C at the end of the century - seems derisory on the scale of human sensations, the consequences are colossal on a planetary scale and the efforts to be implemented in excessive inAll areas of economic activity.The French National Carbon (SNBC) National Strategy was drawn up in 2015, then revised in 2018-2019.It determines a roadmap to reach carbon neutrality by 2050, which implies dividing greenhouse gas emissions (GHG) by six compared to the levels of 1990.The SNBC organizes decarbonation by acting on seven major sectors: industry, energy, agriculture, forests and soils, transport, waste and building which, from 2030, will have to reduce its emissions by emissionsGes.Today, it represents 20 % of total emissions, according to the general information document published by the Ministry of Ecological Transition;25, 30 or even 40 %, according to other players, without however always specifying the sources or the effective perimeters recognized.

Apprehend the "carbon weight"

The battle of figures is at the heart of the subject."How much does your building weigh?"Questioned the architect Philippe Rizzotti, during the exhibition" The imprint of a habitat "this fall at the Arsenal pavilion (1), of which he assured the police station, taking up the question posed by Buckminster Fuller to architects at architectsAbout the mass of their works.In the same way as structural calculations, those of carbon weight are now to be taken into account during the design, a first in the history of architecture.The carbon footprint of materials, invisible value measured for the sole purpose of reducing it, or even making it disappear, is therefore added to the knowledge of architects.How to understand and manipulate it?The exhibition catalog brings together 33 buildings in the history of architecture sufficiently known to calculate their carbon footprint from tangible elements.That of the house metropolis of Jean and Henri Prouvé, very efficient, reached only 122 kg eq.CO2/M2.It is nevertheless preceded by the Eames House (93 kg eq.CO2/M2), et surtout la Walker Guest House de Paul Rudolph (60 kg éq.CO2/M2).Even adding the 80 kg eq.CO2/M2 correspondant à la mise en conformité de ces exemples aux standards actuels de confort et d'isolation, l'empreinte corrigée reste bien en deçà des seuils fixés par la nouvelle réglementation environnementale.The RE 2020 provides indeed - in a first step, by 2024 - a maximum of 640 kg eq.CO2/M2 pour la construction d'une maison individuelle, auquel il faut ajouter 160 kg éq.CO2/M2 pour la consommation d'énergie (lire le tableau ci-dessous).

It is therefore by acting as a priority on the materials and constructive systems that the total carbon footprint of a building will be reduced.

Materials at the heart of the debate

The carbon weight of a material is perfectly quantifiable.

In France, it is registered in the Inie database and available in environmental and health declaration sheets (FDES), established for five years from an analysis of the complete life cycle of the material, carried out by an independent firm respondingto a European standard.Carbon is only one of the indicators targeted by the FDES, complex documents to decipher, not transposable in the project, unless you use IT platforms establishing a correspondence between the carbon data and the material implemented.But the "truth" of the figures is misleading, and subject to many interpretation biases, underlines Damien Cuny, co -founder of the Kompoozite platform (read p.66).

Part of the FDES display only generic or default values, calculated from the most unfavorable data, for lack of information on the product or due to the uncertainties inherent in the project process."At the APD stage, due to lack of details on the nature of steels, the development of the life cycle analysis [ACV] of a metal building will be established on average values of the Inie base, whichtend to discredit the material, especially for structural steels.If the approach is laudable, it has the effect of invisiting the most efficient products and disqualifying the most virtuous, ”notes Francis Keledjian, prescription manager buildings and art structures at ArcelorMittal.

A tile placed in covering does not have the same carbon weight as a nesting tile with less overlap: with identical material, different quantities are implemented.

In addition, the carbon weight of a material does not exist in absolute terms, because it is necessary to take into account the mode of application of the product.A tile placed in covering according to local know-how has the same carbon weight as a nesting tile, whose overlap is less: with identical material, different quantities are implemented.With the introduction of ACV, the carbon footprint of a material depends on the lifespan of a product relating to the life of the building, estimated at 50 years in the RE 2020.Any material whose lifespan is lower will therefore have a carbon weight greater than its weight material before implementation.

In addition, the effective carbon weight of materials is called to lower: at the price of colossal investments, the manufacturers are trying to reduce it on the production phase, which represents a large part of the carbon emissions of the Energiesgrises.Again, the responses are specific to the sectors, even to the location of the factories."Land cooking, a 1000 ° C a untemperation, is the most consumer item, recalls Martin Piotte, Technology and Innovation Director at Terreal.Heat recovery systems - the one that comes out by the fireplace - lower the smoke temperature to 80 ° C.They are coupled with other systems to improve energy efficiency, such as biomethane use, from household waste fermentation.But this solution only applies to our nearby facilities for a waste treatment center.»Or how the carbon weight poses the local question again.

Activate all levers

The Covivio real estate operator sets up specific tools to assess the carbon footprint of its buildings."The weights of carbon and energy, at the levels of construction-renovation, exploitation and occupation, are played, for a significant part, from the first sketches of the project, by fixing the orientation ofthe building, its compactness, the presence of sun breeze, etc.."Says Jean-Eric Fournier, its director of sustainable development.Like the least polluting energy is the one that we do not consume, the least carbon material could be the one that is not implemented.Constructive lightness therefore seems to be an indicated path, but it must remain compatible with summer comfort and, in the end, its impact remains limited.Simulations comparing the implementation of different constructive solutions on the same building indicate that the wood solution improves the carbon balance down only 7 % compared to masonry solutions, due to the relative weight of the structure compared to technical lotsCVC (2).

According to Frédéric FRUSTA, founder of the environmental BET OASIIS, "the assessment is broken down according to the lots as follows: techniques, structural work, where we find the highest margins of progression, and a third for allrest.To reach low carbon, we will have to make efforts in all areas, and, in addition to new decarbonized materials, promote reuse and structure its sectors - technical floor, carpet, and even painting, false ceiling ".

The architectural expression of low carbon, if it emerges, could prefer frame structures, mixing current and old rethought techniques, re -use, to which would be added energy production devices.This portrait-robot corresponds to several buildings delivered in recent years, without knowing the new low carbon architecture (read the event P.10).This formalization is perhaps only a step in the definition of a global environmental architecture."Today, we are focused on carbon, while FDES contain many indicators - acid gas, nitrates, phosphates, nitrogen oxides, VOCs - and that they do not cover other impacts such as deforestation,'Floor artificialization… ”Note the engineer Franck Boutté (read p.67).Knowing that, according to the BBCA association, 60 % of the carbon emitted by a building is during its construction, the existing, not concerned by the RE 2020, be the real engine of decarbonation?