Summary of the morning REX session n°1 — fire: protecting against solar radiation, September 16, 2025

• Current news reminder

Heatwaves are increasing in frequency and becoming more deadly: Santé Publique France recorded +280 excess deaths during the September 2025 heatwave, mainly among people over 75. The question arises of finding a balance between mitigation (reducing carbon emissions in the long term) and adaptation (taking immediate action to protect residents).

• Program workstreams

The program has started, with several workstreams now underway:

• • Operational support: launch of discussions and proposal of an upstream specifications document for early October
  • An RE2020 calculator integrating the evolution of the DH threshold in the 2050 weather scenario and the tightening of the threshold with ISB, location, and ICU: V0 expected during November (with CSTB)

Practical guides (fact sheets)

• • The first program insight: Heat pumps and heat waves – What are the limits? What solutions?
  • A publication on shading devices, with system ratings to facilitate and systematize their implementation (no-regret actions).

"Bioclimatism is common sense: prevent heat from entering, allow air to circulate, and use movement to improve comfort."
Three principles guide this approach:

• Reduce heat gains: through exterior shading devices, insulation of opaque walls, optimized window management, and limiting internal gains (equipment, domestic hot water).
• Thermally discharge: by promoting natural night ventilation, preferably cross-ventilation, when the outside temperature drops.
• Improve comfort through air movement: via fans, cross-ventilation, and more broadly through architecture that encourages airflow.

Some lessons:

• A window insulates six times less than a wall: managing glazed surfaces is therefore a critical issue.
• East- and west-facing openings are the hardest to protect, since they receive low-angle sunlight in the morning and evening.
• Even north-facing openings can be subject to gains from diffuse solar radiation.
• The solar factor (g, gtot, Sw) remains the reference indicator for measuring and comparing performance.

Bioclimatism does not eliminate heat, but it creates conditions to better manage it, relying on design before resorting to systems.

Effectiveness

• Exterior shading devices are the most efficient: they stop between 93 and 97% of solar flux before it enters the living space.
• Interior shading devices (blinds, curtains) are much less efficient, allowing between 20 and 50% of the heat to pass through, since it has already come in through the glazing before being trapped inside.

Selection criteria
The choice of a shading system cannot be based solely on its thermal effectiveness. Other important criteria must also be considered:

• Ensuring sufficient natural light
• Preserving views to the outside
• Security (especially for ground floor and first floor apartments)
• Ease of use (motorization, automation)
• Durability and maintenance of the device

Sector professionals are exploring new solutions to combine efficiency and user comfort:

• Hybrid roller shutters with adjustable slats (shutter mode / louver mode)
• Solar-powered motors that avoid complex wiring
• Connected systems integrated with home automation (automatic closure in case of high heat, wind detection for awnings)
• Perforated shading devices enabling night-time ventilation while preventing intrusions
• Integrated insect screens allowing increased night-time ventilation while keeping harmful insects out

The challenge with shading devices is twofold: ensuring measurable thermal effectiveness and gaining residents’ acceptance, so these systems are truly used daily.

Focus on the mosquito issue

The question of insect screens has emerged in discussions as a frequently overlooked but essential point: to ventilate at night, windows need to be opened. Yet in many areas, especially in densely populated urban regions or in the south (and increasingly in the north), residents face the problem of mosquitoes.

Some striking figures:

• In 2024-2025, France experienced several local outbreaks of dengue and chikungunya.
• The tiger mosquito is spreading: its expansion rate increased from 6 km/year in 2006 to around 20 km/year in 2024.
• More frequent heatwaves, longer summers, and increasingly erratic rainfall are expanding its habitat: swales, retention basins, infiltration basins, drainage trenches, rain gardens...

And what if insect screens became a widespread, no-regret measure, an automatic adaptation reflex? They can be:

• Integrated with roller shutters: close the shutter and the insect screen drops down.
• Fixed: discreet grilles on windows, terraces, skylights.
• Mobile: roll-up, sliding, magnetic. Easy to install and remove.
• Innovative: treated fabrics, ultrafine meshes that let air through—but not mosquitoes!

In new construction and renovation: insect screens should be planned just like insulation—naturally, as an automatic feature. The same goes for schools, hospitals, and social housing; installation should be systematic.

Albédo, cool roof : comment ça marche ? Par Emilien Paron (CSTB)

Le cool roof consiste à appliquer en toiture des revêtements très réfléchissants et à forte émissivité. Leur efficacité est mesurée par le SRI (Solar Reflectance Index).

• Avantages : réduction de l’échauffement en toiture, amélioration du confort d’été, limitation de l’îlot de chaleur urbain, gain de productivité pour les panneaux photovoltaïques
• Technologies : peintures réfléchissantes (économiques, mais durabilité limitée), membranes blanches synthétiques, bitume clair, SEL clairs
• Limites : efficacité surtout sur bâtiments peu isolés et/ou climatisés ; entretien régulier nécessaire (nettoyage) ; risque d’augmenter les besoins de chauffage en hiver

Limitations et points de vigilance : 

• Équilibre été/hiver : les atouts de l'été peuvent devenir des défauts l'hiver (hausse du chauffage car moins d'apport solaire gratuits) : il faut raisonner sur l'année
• Effet qui disparait si la toiture du bâtiment est bien isolée
• Pertinent plutôt dans les zones sud : H2d, H3 où le gain hiver/été peut être favorable
• Entretien : l'encrassement peut réduire rapidement l'efficacité et nettoyage (type karcher) peut dégrader le revêtement et l'étanchéité
• Possible incompatibilité chimique entre certaines peintures et échanchéités

Emilien Paron : « Le cool roof, c’est un revêtement “froid” (réflectance élevée + émissivité forte) qui refroidit la toiture et soulage la clim — ou l’inconfort d’été. Mais ce n’est pas une baguette magique : efficace surtout quand le contexte s’y prête (zones très ensoleillées, toitures peu ou pas isolées et occupant une part importante du bilan thermique, pentes et étanchéités compatibles...). Sinon, on risque d’annuler en hiver ce qu’on gagne l’été ! »

 

Les toitures réfléchissantes de CDC Habitat, par Elodie Espeout

Les enseignements

• Les peintures réfléchissantes (type CoolRoof) se distinguent par leur rapport coût / efficacité : rapides à appliquer, elles apportent un bénéfice immédiat pour les logements situés aux derniers étages
• Les membranes présentent une durabilité intéressante, mais nécessitent une instrumentation fine pour bien en mesurer les effets
• Les habitants ont exprimé un ressenti positif immédiat dès l’application des revêtements réfléchissants

Limits and conditions for success

• The effectiveness depends on the initial condition of the building: the less insulated the roof, the greater the benefit.
• Regular maintenance (cleaning about every 2 years) is necessary to preserve the reflective properties.
• Safety during maintenance must be planned ahead: roof access, slope, waterproofing.

For CDC Habitat, cool roofing is emerging as an operational solution that is both fast, cost-effective, and suitable for social housing, capable of addressing urgent needs during heatwaves.
A key point to note: it would be useful to test these coatings over a longer period (a full year) and place sensors inside the apartments.

Elodie Espeout: "Reflective roofs are among the simple and quick solutions we are studying—in response to emergencies—to help people 'live better' during heatwaves in our homes and to avoid the installation of individual air conditioning units."

The MIMESIS project is a complex building, bringing together laboratories, offices, and collaborative spaces. The goal is to create a high-level scientific facility while ensuring optimal comfort in a hot Mediterranean climate.

Solutions implemented:

• Bioclimatic facades that reduce solar gain by 70% compared to a classic facade, inspired by the ferocactus
• Organisation of spaces to maximize diffuse light, limiting reliance on artificial lighting while reducing overheating
• Outdoor spaces (terraces, walkways, balconies) designed as living areas, not just technical add-ons
• Reversible design: the building can accommodate other uses over time (housing, offices, amenities)

 

Campus BAOBAB – Dakar (completed)

The BAOBAB campus is a site located in West Africa, characterized by intense heat, high humidity, and the need to create an urban and educational landmark. The objective of the campus is to host 3,000 students in a sustainable, iconic, and comfortable building without heavily relying on air conditioning.

Implemented solutions:

• Design concept inspired by the palaver tree, the traditional African communal space for dialogue and exchange, reinterpreted as an architecture fostering collective, shaded interaction
• Vertical campus: a dense and compact urban form optimized for city integration, articulated around large naturally ventilated and solar-protected voids and circulation spaces
• Terracotta façades: locally sourced material with high thermal inertia, ensuring passive thermal regulation and minimizing risks of overheating
• Optimized orientation and bioclimatic strategies achieving up to 60% reduction in cooling demand and reliance on artificial lighting
• Didactic architecture: the building becomes an explicit pedagogical tool, exemplifying sustainable design practices and positioning architecture as an agent of ecological transition

Teachings

• Summer comfort can be integrated from the design stage and should not be reduced to a mere technical add-on
• Climatic constraints become drivers of creation: crenellated façades, deep balconies, shaded outdoor spaces
• Architecture can contribute to the long-term reduction of energy consumption without sacrificing the project’s aesthetics or identity

These feedbacks demonstrate that a project conceived holistically can go far beyond a defensive logic (protecting from the sun) and move toward a positive approach (creating comfort and value).

Luca Bertacchi: "We want to transform the climatic constraint into an architectural opportunity. Summer comfort should not be an add-on, but a signature."