Heatwave: a comprehensive energy renovation can reduce by three the number of days of overheating in the home

Summer comfort is now emerging as one of the key challenges in energy renovation. With heat waves becoming more frequent, a home's ability to remain habitable during periods of intense heat is now just as important as its winter performance.

Heat waves are no longer exceptional. Now occurring earlier, more frequently, and more intensely, they raise questions about the ability of French housing stock to offer acceptable comfort during the summer months.

Long associated primarily with saving on winter heating costs, energy-efficient renovation is now seen as a key factor in adapting to climate change. Insulation, solar protection, and ventilation all contribute to limiting overheating in homes while reducing the need for air conditioning.

This is what the collective is highlighting. Let's renovate, coordinated by the Cler network, which is based on several research studies devoted to summer comfort in buildings.

A comprehensive renovation to permanently limit overheating

According to the group, the best results are obtained through comprehensive energy renovation combining several approaches:

• high-performance insulation
• external sun protection
• appropriate mechanical ventilation

The goal is not only to reduce energy consumption, but also to maintain more stable indoor temperatures during heat waves.

Insulation remains the first line of defense against heat

Because it limits heat loss in winter, insulation also slows down heat penetration in summer.

By reducing heat transfer through walls, roofs or floors, it helps to limit the rise in indoor temperature during the hottest periods.

According to the work presented by the Rénovons collective, high-performance insulation allows for reduce the indoor temperature by up to 5°C at the hottest time of day and divide by three the number of days of excessive heat in the home*. In very poorly insulated homes, sometimes described as "thermal kettles", where the indoor temperature can reach or even exceed that of the outside, the benefits are even greater.

Thermal simulations carried out by the research center TIPEE Examples of these discrepancies in uninsulated attics equipped with sun protection, in the south of France, illustrate.

Without renovation, they exhibit thermal discomfort corresponding to 44 days* accumulated excessive heat during the summer, with peaks that can reach 37,1 °C* inside the accommodation.

After a successful renovation and with the right practices, this discomfort is reduced to less than 15 days* accumulated over the entire summer season, while the temperature peak drops back down to 32 °C*.

Charlotte Le Droumaguet, head of public affairs at Knauf Insulation France, member of the Rénovons collective: "Energy renovation remains the first thing to do to keep your home habitable during heat waves. In order to protect the health of residents living in sweltering heat, the government must guarantee the funding of MaPrimeRénov'."

*This data comes from the work of Rénovons, based on a study conducted by the TIPEE research center. The data, expressed in degree-hours, have been translated by Rénovons experts into average degrees in the home and days of discomfort avoided during the summer.

Sun protection and usage patterns remain crucial.

The group points out, however, that insulation alone is not enough to guarantee good summer comfort.

External sun protection plays a vital role in limiting solar radiation before it passes through the windows.

Housing usage also plays a crucial role.

When the outside temperature is higher than the inside temperature, opening windows during the day allows heat to enter. Conversely, nighttime ventilation, combined with mechanical ventilation and proper insulation, helps to retain the coolness accumulated overnight more effectively.

A renovation that also reduces the need for air conditioning

Beyond thermal comfort, energy renovation also helps to limit the use of air conditioning.

According to the research center TIPEEA comprehensive energy renovation also allows for divide by three the refrigeration needs of air-conditioned accommodation.

This reduction in needs translates into lower electricity consumption, less powerful air conditioning equipment and a reduced environmental impact.

The testimony of Patrice, owner of a renovated house in Montpellier, illustrates this evolution.

Patrice, a homeowner who has thermally renovated his house in Montpellier: "In the summer, thanks to the energy renovation work, I now have the same temperature as with the three air conditioners that were previously installed in my home. We have a feeling of exceptional comfort in the house and coolness without air conditioning."

The Rénovons collective proposes six improvements to better adapt housing to extreme heat.

Beyond the technical observation, the Rénovons collective believes that adapting housing to heat waves must now become a priority objective of public policies.

The group has formulated six proposals aimed at better integrating summer comfort into public policies:

• revise the calculation method for the summer comfort indicator in the DPE
• integrate this new criterion of summer habitability into the definition of a high-performance renovation
• inform buyers and tenants about the summer comfort of the properties during sales and rentals
• to integrate summer habitability into the housing decency criteria, with an obligation for landlords to prevent overheating and guarantee satisfactory indoor air quality
• create a right allowing tenants to trigger renovations in cases of clear non-compliance with decency standards
• reinstate MaPrimeRénov' in the version and with the amounts provided for in the draft finance law for 2024

Damien Barbosa, coordinator of the Rénovons collective within the Cler network: "With all forecasts predicting more frequent and intense heat waves, it is crucial that public action regarding resilience to heat waves be proactive. Adapting to overheated homes is already a public health issue. It is therefore essential to create new rights for the most vulnerable and powerless citizens in the face of heat waves."

Beyond the energy savings achieved in winter, the work presented by the Cler network illustrates the evolving challenges of energy renovation. Faced with heatwave episodes expected to become more frequent and intense, summer comfort is gradually emerging as a new performance criterion for housing, alongside energy consumption and greenhouse gas emissions.