6th IPCC report: solutions for decarbonizing buildings?

The part of the IPCC’s 6th report from the 3rd Working Group (GW III), which looks at methods for mitigating climate change, was released in its final version in 2023. We’ve summarized the main points in the link above, and now we’ll take a closer look at the subject of buildings. What, according to the IPCC, is its responsibility in greenhouse gas (GHG) emissions, and what are the avenues for reducing them? There are a lot of elements here, so let’s concentrate on the most operational.

Data and major trends

Buildings accounted for 12 GtCO2eq in 2019, or 21 u total. 57% (6.8 GtCO2eq) was due to external generation of electricity and heat, 24% (2.9 GtCO2eq) was produced on site and 18% (2.2 GtCO2eq ) was due to the production of cement and steel required for their construction. Overall demand accounted for 128 EJ in 2019, 43 of which was electricity. Its CO2 emissions increased by 50 between 1990 and 2019. (p.955)

There are several trends for residential buildings:

  • increasing demand for cooling. The number of cooling units is 4 billion, and could rise to 14 billion by 2050.
  • Rising demand for electricity. Between 1990 and 2019, it rose by 161 EJ to 43 EJ, or more than 18 e of global demand. This increase is generated by rising incomes, income distribution and the “S-curve of ownership rates” (see Wolfram et al. 2012; Gertler et al. 2016). One important trend is the use of electricity for thermal purposes, such as boilers or cooking, not least due to the spread of heat pumps.
  • The digitalization of energy demand, particularly in construction, with the development of modeling (“Building Information Modelling/Management (BIM)”), 3D printing, robots, drones, 3D scans, sensors and connected objects.

Different aspects of the building sector

The IPCC distinguishes between two aspects of the building sector: what relates to construction itself (components and construction methods) and the services that make them more comfortable, practical, efficient and safe (e.g. heating and ventilation). (p.961)

The sector is said to face a number of barriers to decarbonization, including

  • The fact that many investments are the responsibility of the owner, but often benefit the tenant. For example, in Germany, having a good energy performance rating does not seem to influence rent (inf. supp, p.9SM-8)
  • Investment in decarbonizing buildings was estimated at $164 billion in 2020, which would not be enough (p.956)

The study is divided into three sections: Sufficiency, Efficiency and Renewability (SER).

  • Sufficiency is the equivalent of sobriety: lower overall demand. This would mean, for example, reducing the demand for m² per person, designing buildings that are dense, compact and bioclimatic (?), promoting the circular use of materials, adapting housing to changing household needs, optimizing the use of buildings through lifestyle changes, etc. This type of measure could reduce the sector’s emissions by 17% by 2050. What’s more, these changes would be profitable for those implementing them. (p.955) In-depth technologies include all measures that do not require energy to operate:
    • better insulation materials,
    • trombe walls, plant walls, PCM systems, AAC walls, double-skin walls,
    • cooling roofs,roof ponds and green roofs. (p.979)
  • Efficiency refers to the ability to meet demand with as little energy as possible. Efficiency measures (EJ/m²) are the continuous improvement of technologies. They could reduce emissions by 30% by 2050. Advanced technologies include measures that improve energy intensity:
    • Thermally activated building systems
    • heat pumps, organic rankine cycles, heat recovery systems, fuel cells
    • adiabatic/Evaporative condensers, intelligent ventilation
    • Thermal energy storage
    • Liquid pressure amplification“,“Chilled-ceiling“, “Desiccant cooling“, “Ejector cooling” and“Variable refrigerant flow” (p.979)
  • Renewable” measures aim to reduce the environmental impact of demand (MtCO2/EJ). They would reduce emissions by 43 ‘by 2050. These include :
    • Geothermal energy or geothermal heat pumps
    • photovoltaic or thermalsolar energy
    • biomass energy (p.979)

Further on (p.1006), the following categories are mentioned:

  • building design and performance
  • changes in construction methods and circular economy
  • envelope improvement
  • heating, ventilation and air conditioning (HVAC)
  • Appliance efficiency
  • Change in building materials
  • “Demand side management (= demand reduction)
  • Renewable energy production

No tech” (NT) solutions

Non-technological” measures would be “key” to low-carbon buildings. (p.983-) For example, simple good practices such as placing refrigerators away from radiators and ovens. It can be more extensive, such as minimizing the need for heating by wearing warmer clothes. There are two types of management:

  • Passive management refers to adjustments in human behavior, such as adjusting clothing or organizing activities in rooms to minimize energy use.
  • Active management concerns the human control of building energy systems, where efficient lighting and cooking practices can significantly reduce energy consumption. This can be as simple as putting the lid on the pan when cooking or turning off unused lights. Adjusting the temperature in winter and summer can result in energy savings of 5 to 25%. Deferring usage can reduce the need for electricity by 10-20% during peak periods. Smart home applications could facilitate these practices.

Technologies with multiple positive effects

The recommended actions would generate numerous additional benefits (p.1000 – 1005).

They would help reduce energy poverty. In 2018, 790 million people had no access to electricity, and 2.8 billion relied on polluting fuels in inefficient tools, generating indoor pollution. an estimated 3.8 million deaths were caused by these practices in 2016. Improving these household appliances, using liquid fuels such as ethanol and biogas, and electrifying their use would enable us to effectively combat this scourge. (p.1000)

Even in Europe, 44.5 million people couldn’t keep their homes warm in 2016, and 16.3 had disproportionately high energy bills. In the United States, 25 million households had to give up food or medicine to pay their energy bills. In short, fuel poverty is a global problem. It’s a problem not only in terms of comfort (which can be very stressful), but also in terms of health: cold and damp in winter increase several types of illness. The recommended measures would help reduce this precarious situation.

Improved air quality and thermal comfort would also be associated with higher productivity in commercial buildings. (1p.004)

Better thermal insulation could also improve soundproofing, for example with the installation of double-glazing, which would have considerable positive effects: in the UK, these benefits could amount to £400 in 2030. (p.1003)

Additional information: the heart of the document

The additionalinformation” document added to this chapter is particularly interesting. It details the various solutions envisaged.

  • Passive wall strategies
    • Improved insulation materials, to protect the home from heat and cold. Conventional materials are derived from petrochemical substances, but new, more durable materials are also more expensive. In addition, care must be taken not to break the “insulation barrier” at the risk of creating a thermal bridge. Studies show that energy savings can range from 28% to 64%.
    • Trombe” walls are systems that use a plate in front of the wall to retain heat. This reduces the need for heating and reduces humidity in damp areas. They are inexpensive to install, but can also retain heat in hot weather. The figures quoted, ~20-40%, are based on simulations only.
    • Plant walls / facades reduce heat loss and heat gain, while also being aesthetically pleasing. The gains of the studies cited range from 12 to 58.9%. Their disadvantages are that they are difficult to maintain, require a lot of water and can harbour undesirable insects.
    • PCM (Phase Change Materials) wall systems involve storing excess energy within the wall itself. This solution has major challenges (low thermal conductivity, flammability, low thermal and chemical stability) and studies appear to be inconclusive (0-29%).
    • Autoclaved aerated concrete (AAC) walls are aerated cement walls with numerous closed micro-bubbles that reinforce the insulating properties of the cement. Their disadvantage is that they are more expensive, less resistant and consume more energy to design. The only study referenced mentions an insulation gain of 7n.
    • Double-skin walls consist of two walls with a void in the middle. They are more expensive, and can increase heat when it’s hot. Studies put their efficiency at between 8% and 51%.
  • Passive strategies for roofs
    • cool roofs” are roofs with reflective paint, reflecting the sun’s rays. They are of interest in hot zones, but not in cold climates, as their cooling effect operates independently of heat.
    • Roof ponds consist of a small pool on the roof that can be covered automatically. They’re a great way to combat both heat and cold. However, they increase weight, pose a risk of leakage and are only an option for one- or two-storey flat-roofed homes. Savings can be in the order of 30%.
    • Green roofs, like green walls, have many advantages, but the same problem: maintenance. Gains are small but significant (7-16%).
  • Efficiency-enhancing technologies (mixed)
    • Thermally Activated Building Systems (TABS) are systems that use pipes in the walls to generate heat or cold. They reduce energy requirements, but have a high level of inertia. Energy savings are estimated at between 15% and 24%.
    • Heat pumps can reduce energy consumption by up to 60%.
    • Organic Rankine cycle machines produce electricity from heat thanks to a “Rankine thermodynamic cycle” using an organic fluid. This technology requires a lot of space and high investment. Its efficiency would be 41n hot period and 63n cold period.
    • Intelligent ventilation
    • There are three heat storage system technologies available: sensible, latent and thermochemical. However, they would be particularly expensive.
  • Efficiency-enhancing technologies (heating)
    • Heat recovery system (exhaust air heat pump?)
    • Fuel cells convert hydrogen into electricity and heat. [I didn’t understand their calculation, especially given the price and low commercial availability of hydrogen]
  • Efficiency-enhancing technologies (cooling)
    • Adiabatic or evaporative condensers are effective cooling systems in hot, dry climates.
    • Direct cooling by evaporation
    • Indirect cooling by evaporation
    • “Liquid pressure amplification
    • “Ground-coupled
    • Chilled ceiling
    • Desiccant cooling is an air-conditioning technology that uses chemicals to remove moisture from the air. It is an environmentally-friendly, energy-efficient alternative to traditional air-conditioning systems based on the vapour-compression refrigeration cycle.
    • Ejector cooling
    • “L “Variable refrigerant flow
  • Renewability-enhancing technologies
    • Geothermal energy or geothermal heat pumps. [I don’t understand why they mention it here, since they’ve already mentioned heat pumps previously and the operation of a geothermal heat pump is not fundamentally different from that of an aerothermal heat pump]
    • Photovoltaic solar energy
    • Solar thermal [rq: these are much more interesting, rising to 75.8 in winter. I’m a little sceptical, given that this technology has very little mass market appeal.)
    • Biomass energy has the advantage of being cheap, abundant and available in less developed countries. Its problem is the release of pollutants and the amount of land it takes up.


I have omitted several passages from the report, which I felt were not very useful/operational:

  • The report discusses several reports / trends (p.963-974),
  • The passage on energy / carbon “embodied” in materials (p.975-978), which I didn’t understand. A priori, the answer can be found in this article: https://www.researchgate.net/publication/344049062_Advances_Toward_a_Net-Zero_Global_Building_Sector
  • The discussion on methods to induce change (p.1007-1016)
Type of luminaireCode in drawingLumens per watt [lm W-1] Colour temperature [KColor temperature [K]Life [ h]Energy consumption [W]
Incandescent candleCnL14.02700100025
Fluorescent TL8FluT880.03000-65002000030-40
Fluorescent compactCfL66.02700-65001000020
LED GLSLeD100.02700-50004500010
LED spotlightLeD Pin83.82700-6500450008
Fluorescent T5FluT581.82700-65005000022
LED DT8LeDT8111.02700-65005000015
IPCC, 6th report, WGIII, Table 9.1 | Types of domestic lighting devices and their characteristics, p.980