The climate upheaval, which is the direct consequence of our modes of production also drives a resource crisis. For instance, we can question the use of water or sand to prepare concrete given that we lack water in the summer and that extracting sand damages ecosystems. On the top of this, Portland cement has a carbon weight that is currently considerable[2].
     Steel production, which requires extracting materials from far away, results from highly energy-intensive process based on fossil fuel uses, which plays a direct role in climate change. Wood and stone are low-carbon materials, but the exploitation of these resources, from the moment they become dominant, will have to be analyzed and quantified in order to avoid unintended adverse effects.
     Apprehending all these issues requires giving oneself the means of understanding and attempting to quantify them in order to be able to make informed decisions. Indeed, although intuitively, the decision to conserve seems obvious, it is necessary to have tangible figures, and, as we’ll see, to avoid certain pitfalls. At that point, we can start talking about carbon weight, which is a novel and highly relevant indicator[3].

     “Carbon weight” is a misnomer, but it has the merit of being simple and easy to understand. What we are talking about here is the sum of all greenhouse gases weighted against carbon dioxide, something called a “CO2 equivalent” (which we’ll be abbreviating here as CO2eq).
     Every single action emits some carbon. In the fields of architecture and urban planning, each stage, from the production of a material up to the moment it is used on a project—scrap material, wastage, waste disposal, transport, and sometimes its chemical reaction (in the case of cement for instance)—causes an emission of greenhouse gases. The materials around us therefore “emit” carbon. This is what is sometimes referred to as “gray carbon” or “embodied carbon.”
   Then, once a building has been constructed, every year and for a long time after, it is used, heated, sometimes cooled down, lit, requires some material replacement, is repaired, etc. In short, during its service life, buildings also use materials and energy. And even though the carbon intensity of electricity in France is rather low, even when using renewable energy, each kilowatt-hour of energy that is used, whatever its source, emits some carbon.
      Ultimately, the building is demolished, its components are trucked off to waste disposal facilities, sorted, and stored away. Such is the building life cycle, which extends, technically speaking, “from cradle to grave”—a very revealing expression.
     In sum, there is carbon hiding everywhere in buildings. However, the two main items of use are the embodied carbon of materials and energy-related carbon. And it is the entirety of this emitted “equivalent” carbon that is bringing about an imbalance in the global climate system[4].
     It may be pointed out that though all materials emit carbon, bio-based materials—and most notably wood—are stores of carbon. Indeed, bio-based materials grow and, during that process, absorb atmospheric CO2. If we consider that the harvesting of these materials has no impact on the total quantity of CO2 that is absorbed[5] and that they are stored away for a long time, it is reasonable to assume that each kilogram of bio-based material amounts to almost the same amount of stored carbon[6].

     Carbon is like money in that it is to be approached somewhat similarly to a budget. And, just like money, it is necessary to have a few key figures a mind[7].
     To keep it simple, for each square meter that we build, we emit 1.5 t CO2eq over 50 years[8]. Out of these 1.5 t CO2eq, 50% are attributable to materials and 50% to energy. By building “low-carbon” constructions, we can almost bring this figure down by half over 50 years, to 750 kg CO2eq/m².
     The superstructure represents approximately 20% of the carbon weight of materials. The foundations (including any work on roads and networks) and the interior package amount to almost as much each. The façade accounts for a bit less, around 10%, and the technical package, 30%.
     Not rebuilding the primary structure makes up the primary gain from rehabilitation—250 kg CO2eq/m² are thus conserved in heavy rehabilitations or, in the case of light rehabilitations, part of the façade or technical installations. From an average of 800 kg CO2eq/m² for new collective housing, we can go below 400 kg CO2eq/m² with simple or low-carbon rehabilitations, or a maximum of around 600 kg CO2eq/m².
     It is safe to say, without making a hasty generalization, that the rehabilitation of a Haussmannian building, which will primarily concern doors, windows, and interiors, will have a much lower carbon weight than a building from the 1970s or 1980s which will require changing the façade entirely and involve more substantial structural remodeling. We can also posit that it is almost always possible to engage in low-carbon rehabilitation (that is, to lower the carbon impact compared with a standard rehabilitation) except when facing very specific constraints (for example, having asbestos in the façades).
     Energywise, savings are easy to achieve. The carbon emissions of a recent building can reach 750 kg CO2eq/m² in energy-related carbon over 50 years (and largely triple that amount in old buildings). In the case of a new build or a quality rehabilitation project, these emissions can be brought down to 200 kg CO2eq/m².
     When adding energy and materials together, the gap in carbon emissions between a new build and a rehabilitation project amounts to a factor of 2 to 3.
     What about the future? Imagining what construction will become: based on a modified calculation methodology (RE2020), it would be possible to achieve emissions of less than 40 kg CO2eq/m² and, with a little optimism, below 300 kg CO2eq/m². This leads us to the conclusion that, at best, zero carbon construction involves outsourcing and, at worst, is a complete misnomer.

      Consider a first iconic example: Tour Montparnasse, the Parisian high-rise erected in the 1970s. Now clocking in at 50 years of service life, its rehabilitation has become a necessity. The primary, logical environmental efforts concern energy use, which was divided by a factor of ten[9]. These translate into a sixfold reduction in carbon emissions as rehabilitation brings its energy-related carbon weight down from 900 kg to 150 kg CO2eq/m², which is a very good figure[10]. But, in order to achieve that performance, on the flip side, the efforts put into energy will result in poorer results in terms of the embodied carbon in materials. Quite broadly speaking, and if we’re not paying attention, between a material balance that we weigh down in order to improve the energy performance and an energy balance that effectively decreases, we can end up with a zero balance. The reverse is also true, as was well shown in the “Housing Footprint. Light and Low-Carbon Construction” exhibition curated by Philippe Rizzotti at Pavillon de l’Arsenal in 2021. It is perfectly possible to build using fewer materials and a carbon weight that is therefore lower, but then require more energy use, and ultimately end up emitting more carbon. The exercise is a subtle one indeed! Subtlety comes into play, but also beauty, because to consider the weight of carbon is to relearn how to make choices for the entirety of the building, regarding what we’re adding at the outset (the materials used) and what happens over the service life of the building (its energy consumption).
      Concerning the rehabilitation of Tour Montparnasse, the material expense amounts to 1 t CO2eq/m². This is a lot, but much less than for a comparable new build—high-rise buildings aren’t exactly low-carbon after all. Without being perfect, they have other advantages, in particular that of enabling our cities to achieve a certain density. Out of this metric ton of carbon, façades alone represent almost 300 kg CO2eq/m², which is, without a doubt, a high figure—but one that must be related to energy gains. With the building service and architectural lots (both entirely new) and the structural remodeling that such a building requires, it makes sense that it borders on the figures for new builds.
     Consider a second example—that of the renovation of a very simple Haussmannian office building—with a project that hasn’t been designed with carbon in mind. Its material balance is 800 kg CO2eq/m² and its energy balance 400 kg CO2eq/m². We can already notice that this figure is lower than the 1.5 t CO2eq/m² for a “standard” new build mentioned earlier on.
     These two examples raise questions regarding the choice that must be made between building and rehabilitation, and between standard rehabilitation and low-carbon rehabilitation.

      If we consider France’s National Low-Carbon Strategy (SNBC)[11], to the “zero net artificialization” objective, the carbon weight of new builds, to the RE2020 standards, in 2020 to build is not to build at all. This is a very radical answer, but the summer of 2022 was just as radical. That being said, given our capacity to engage in a certain level of inertia, we will continue building. It nevertheless is what is required to achieve our climate commitments, or in the terms of the SNBC: “The assumptions … lead us to consider that the volume of new builds should continuously decrease until 2050.” Clearly, stating that we cannot build means that we should engage in massive and, above all, efficient rehabilitation. The effectiveness of a rehabilitation is to be considered from a “carbon” standpoint, however, and, as SNBC points out, we must “avoid falling into the ‘impasses’ of partial renovations that could only be upgraded through more comprehensive renovations.”
     To build in 2022 is therefore to rehabilitate as much as possible by limiting construction and rehabilitating in a sufficiently intelligent way to avoid what investors would call “stranded assets,” that is, assets with no lasting value. Likewise, there is the question of the time frame of carbon emissions. Given that they must be reduced without delay, it should always be preferable to emit carbon in 20, 30, or 50 years rather than this year. Following this assumption means tending towards solutions that reduce the use of materials (which produces an impact at the time of construction) at the expense of the reduction of energy use (which will concern the building over its whole service life).
     One of the possible approaches consists in estimating the time to return on the carbon investment. Just as in finance, calculating the time to return on investment makes it possible to know at what point in time the initial investment breaks—even compared to other options. From a carbon standpoint, the idea is to determine at what point investment in materials will enable energy savings. If I use too much material and the energy gain brought about my project only materializes after 50 years, then there is a question related to whether such an intervention is indeed relevant. This, in turn, puts the full burden of focus on reducing the impact of materials.

      The façade and structure are key parts of any rehabilitation project, not only in terms of carbon weight, but also, first and foremost, because the idea is to complete a project. As mentioned earlier, the environmental work amounts to reflecting on the impact of every single material used.
     The first rule is that the façade must now first be optimized for its CO2 emissions, in priority over energy-related criteria. This is a dramatic change in approach for architects and engineers, who were previously subject to thermal regulations[12], the only objective of which was to lower our energy use. The new regulatory standards, RE2020, which came out in 2021[13], goes almost entirely in the direction of carbon and imposes a maximum carbon threshold above which construction isn’t possible. This regulation doesn’t apply to rehabilitation, but its logic and the fact that a heavy rehabilitation often calls for an entirely new façade allows us to draw analogies.
     We now know that certain façade types[14] improve a building’s carbon impact. This approach should therefore dictate that of appearance and form. We also know that glazed sections are the primary weak spot of our impacts, both due to energy losses and to embodied carbon[15]. A fully glazed façade (with double or triple glazing) that contributes major energy gains can have a final weight of more than 300 kg CO2eq/m², whereas a low-carbon façade can go below under 100 kg CO2eq/m². From the perspective of material-related carbon weight, this second type of façade is the only one that can allow under 800 kg CO2eq/m² on the entirety of the building.
     For a watchful eye, it’s easy to appreciate a building’s carbon weight simply by considering its façade, especially given that the form of a building, its complexity, and its compactness, are all strictly correlated to its impact. Certain architects follow the principle of “form follows function”; we now must adhere to the precept of “form follows carbon.” Quite efficient in that regard, Haussmannian buildings are part of the way forward.
     It is no exaggeration to state that, in the case of a Haussmannian building, a carbon-oriented rehabilitation can lead to conserving 200 kg CO2eq/m² on materials and 150 kg CO2eq/m² on energy use, thus achieving, without too much trouble, a weight of 85 kg CO2eq/m², or, to put it another way, only about half as much as that of a new build.
     Among the optimizations in our example is also the fact that no additional space is being created. Instead, the existing surface is densified with the use of bio-based materials, and sobriety in the improvements. This leads to two concerns. On the one hand, users must be supported in order to make them understand the issues around a project that has smaller or shared spaces, and how they can reduce their own carbon weight. On the other hand, programming itself must be geared towards low carbon. Building parking for cars rather than bicycles, for example, is a programmatic decision that can heavily constrain a project.
     As we’ve seen, although the new façades of Tour Montparnasse are very carbon intensive, they allow for a major gain in terms of energy-related carbon weight. The particularity of this building also lies in its technical and mechanical constraints, as well as the natural ventilation system, which necessarily adds a lot to its overall carbon weight.
     Let us now evoke structure. Structural remodeling should always be carried out using bio-based materials, which are much lighter and versatile. Though these materials may require complex engineering, their use is nevertheless increasingly widespread. But their primary benefit is that emissions are shifted in time as they only appear at the end of their service life. When this time lag comes in addition to a very long service life of the building, we can then talk about carbon storage. Though the use of wood may seem an obvious choice, we can also use other types of structure, such as raw earth or stone, and only resort to concrete, ideally a “low-carbon”[16] form, only if no other material can be used in its place.
     Just as for the façade, we must review our ways of thinking. For example, concrete, and all the processes that we have been using thoughtlessly these past three years, since the last regulatory update, mustn’t be the first solution proposed, even though it appears straightaway. We are pleased to note that the RE2020 standards emphasize bio-based materials thanks to the adoption of a new calculation method referred to as the DLCA or Dynamic LCA (life cycle assessment)[17]. Based on this kind of calculation, an unoptimized structure can now achieve a lower weight of about 150 kg CO2eq/m², while a very low-carbon, optimized structure can edge on 0 kg CO2eq/m².
     That being said, there should be no self-congratulating when only the major items have been worked on. This approach must also be applied to all other budgetary items. The interior spaces and architectural lots of a rehabilitation must contribute to the carbon reduction effort. This is where reuse comes into play. As it has now become a real driver for many startups and nonprofits, designers, shops, and even insurers and sales platforms[18], there is no reason not to mainstream proposals in that field. In an office building for instance, using recovered false flooring or sanitary fixtures is very efficient. There is also a source of materials to be thought up regarding cable trays and ducts. In residential units, the reuse of sanitary fixtures and radiators is perfectly conceivable. Beyond these examples, everything remains to be reinvented from direct onsite reuse, that is, when elements are retained and reused in the rehabilitation project, to the use of materials recovered from other projects, and products that are used to manufacture a new material. We can mention the Centre Pompidou’s iconic glass “caterpillar,” which are being given a new lease of life in an office setting, or the Métisse insulating material, manufactured from recycled cotton recovered from scrap clothing as examples of this approach.
     From a carbon standpoint, we can, without being too far off-base, consider that the carbon weight of reused materials is negligible. Reuse also has the benefit of alleviating the pressure we exert on resources and waste generation—which is just as fundamental.
     Beyond reuse, frugality must guide each of our actions in the building sector. For instance, should we still be equipping office spaces both with false flooring and suspended ceilings in 2022?

      To those that consider carbon design the ruin of architecture, it’s easy to answer instead that this is, in fact, a time of architectural renaissance. It is indeed a renaissance given that it restores the nobility of the act of building while also taking care of the planet given that it considers material realities and forces us to make thoughtful choices. This is even becoming a social issue as the world we are heading into could be designed around a carbon budget. Deciding on the attribution of kilos of carbon would then be a matter of social policy. Could we, for instance, spend a huge amount of carbon to build a hospital for which resorting to concrete is still indispensable? Certainly.
     Could we spend as much carbon simply to build housing when low-carbon solutions and rehabilitation are possible? I vote no. What about you?

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Guillaume Meunier
Guillaume Meunier, Elioth by Egis, Deputy Director.
A government-certified architect and engineer, Guillaume is a specialist in construction, low-carbon strategy and innovation, and in urban modeling and ecologies. He takes an active role in reducing the impact of the building industry in his work with architects and large contracting auhorities. Between the rehabilitation of the Tour Montparnasse and the new headquarters of the French National Forests Office (ONF), he is seeking to shift establish modes of construction towards carbon neutrality in short order.

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1. Primarily in terms of certain impacts such as the rise in sea and ocean levels.
2. To be precise, this is attributable to a component of cement, cement clinker. As an example, at equivalent characteristics, a façade made from concrete emits twice as much carbon as one made from stone or wood.
3. The same reality is often, but inaccurately, referred to as a “carbon footprint,” though footprints are, in fact, spatial indicators and should therefore use spatial units.
4. Various calculations put the use and construction of buildings at 20% to 30% of our carbon emissions, the rest being generated by transport, food, and lifestyles.
5. This is the principle behind sustainably managed forests, among other things.
6. 1 m³ of wood represents 0.9 tCO2 stored in the form of carbon (C).
7. The figures shared her proceed from the E+C—method (positive energy, reduced carbon). The arrival of RE2020 should quite substantially change these results, but we currently do not have enough feedback to date.
8. All the calculations mentioned in this study were made over a period of 50 years. This simple standard corresponds to the typical, rather than real, service life of buildings.
9. For its office spaces only, within the thermal regulation perimeter.
10. The energy-related carbon weight for the existing building isn’t that bad given that the high-rise isn’t heated with gas, thanks to the Parisian urban heat network (CPCU), which is much more environmentally friendly.
11. The French national low-carbon strategy is a road map drafted to combat climate change. It gives guidelines for bringing about a transition towards low-carbon, circular, and sustainable activity across all sectors of the economy.
12. RT2005 and, more recently, RT2021.
13. The building sector has the (curious) peculiarity of always naming its regulations based on an earlier date than when they come into force. RE2020 thus only became applicable since 2022 for certain uses and will only concern other types of uses in 2023 at the earliest.
14. Stick façades rather than curtain wall façades, with a wooden or stone frame, rather than a concrete one.
15. The carbon weight of the glazed sections of the façade is almost always higher than the carbon weight of the opaque sections.
16. This is a misnomer and must be read as forms of concrete that are less carbon intensive than the traditional CEM I cements.
17. Implemented under the RER2020 regulation, the Dynamic LCA approach takes the moment of emission into consideration—a bio-based material that only emits carbon at the end of its service life can therefore have a negative carbon weight.
18. One example is the Gallery of Reuse (La galerie du réemploi) created by Cycle Up.