Which insulation materials are most effective against heat waves?

With increasingly hot summers and more frequent heat waves, France is facing a major climate emergency that directly impacts how we design and renovate buildings. Faced with this new reality, the construction industry can no longer focus solely on winter comfort. Summer comfort is becoming a top priority, particularly to ensure tolerable living conditions without systematically resorting to air conditioning. But which insulation materials are most effective against heat waves?

SUMMER COMFORT

1.1 How can summer comfort be defined?

Summer comfort is not limited to an ideal temperature in the home. It is a complex balance between various physical, physiological, and subjective parameters. From a scientific point of view, it is based on thermal comfort (laws of thermodynamics), physiological comfort (the body's ability to regulate its temperature), and the feeling of well-being, which is unique to each individual.

According to the AICB white paper, summer comfort depends in particular on the ambient temperature, air speed, humidity, building orientation, wall thickness and, above all, the materials used in the building envelope.

1.2 How is it measured?

Since the RE2020 regulation came into force, summer comfort has been assessed using theDH (Degree-Hours of Discomfort) indicator. This indicator accumulates every hour during which the indoor temperature exceeds certain thresholds (26°C at night, 28°C during the day). Below 350 °C.h, comfort is considered to be assured. Between 350 and 1,250 °C.h, we enter a risk zone. Above this, the building is non-compliant and deemed uncomfortable.

1.3 What factors influence comfort in summer?

Several physical properties of materials influence their effectiveness in hot weather:

• Thermal inertia: the ability of a material to store heat and release it slowly.
• Phase shift: the time it takes for a heat wave to pass through a wall, thereby delaying the heat inside.
• Amortization: ability to mitigate the intensity of heat.
• Effusivity: the ability to absorb heat without overheating.
• Diffusivity: thespeed at which a material transmits heat—the lower it is, the better.
• Hygroscopic properties: ability to regulate humidity and therefore thermal sensation.

INSULATING MATERIALS

2.1 Conventional insulation materials

Traditional insulation materials such asglass wool,rock wool, andpolystyreneare widely used because they are inexpensive and perform well in winter. But what about summer?

• Glass wool: low thermal conductivity (λ ≈ 0.032–0.040 W/m.K), but low thermal capacity (≈ 1,030 J/kg.K) and low density (15–50 kg/m³). It therefore heats up quickly and offers little thermal inertia.
• Rock wool: better density (up to 150 kg/m³), but its ability to phase shift heat remains limited.
• Expanded polystyrene (EPS): very good insulation in winter (λ ≈ 0.038 W/m.K), but poor performance in summer with low effusivity and virtually no phase shift.

According to studies by the CSTB and ADEME, these materials are effective in winter but show their limitations in summer. They havehigh diffusivity (up to 10.5 x10^-7 m²/s), which means that heat passes through them quickly, significantly reducing summer comfort.

In summary, conventional insulation materials provide good thermal resistance, butdo not offersignificant phase shift or hygroscopic regulation, two essential elements in combating heat.

2.2 Bio-based insulation materials

Bio-based materials are characterized byhigher density,good thermal capacity, and above all by theirhygrothermal and moisture-regulating properties. The main ones are:

• Wood fiber: phase shift of up to14 hours, damping of95%. It combines low conductivity with excellent thermal capacity (up to 2,100 J/kg.K) and a density of 50–270 kg/m³.
• cellulose insulation :11-hourphase shift and96%damping in unused attic space. In addition, its low diffusivity slows down heat transfer.
• Hemp (panels or concrete): high-performance material with highdampingpropertiesand natural hygroscopicitythat regulates ambient humidity. Hemp concrete has an exceptional phase shift of24 hours, one of the best on the market.
• Straw: very good thermal inertia, especially in bales. Significant phase shift, very low VOC emissions, ideal for wood-frame walls or partitions.
• Recycled cotton or mixed fibers (linen, jute): phase shift of7.6 to 11.8 hours, damping of 82 to 88%, very good ecological alternative.

A comparison between bio-based and conventional insulation materials shows a clear advantage in terms of effusivity and phase shift:

• Bio-based effusivity:63 to 93 J.K⁻¹.m⁻².s⁻½
• Petroleum-based effusivity:26 to 37 J.K⁻¹.m⁻².s⁻½

THE REAL IMPACT OF INSULATION ON HEAT WAVES

It's not enough to choose the right material:using it in the right placeis just as important. Here are some things to consider:

• Wall density: walls in France are already thick, so they are less sensitive to the comfort provided by insulation in summer. However,unused attic spaceand roof slopes, which are highly exposed to the sun, are critical areas.
• The thermal inertiaof interior walls, particularly those made of hempcrete, acts as a thermal buffer and delays overheating.
• Phase shift and dampingare particularly useful in roofing. cellulose insulation , for example, offerstwicethephase shiftof mineral wool in summer.
• The hygroscopic propertiesof certain materials such as hemp or straw help maintain a comfortable relative humidity of between40 and 60%, which improves thermal comfort.

"During heat waves, cellulose insulation keeps your home much cooler. When it's 95°F outside, it's only 75°F inside. It feels like you have air conditioning!" (Mr. and Mrs. B.)