Heat Wave: Prevention Is Better Than Air Conditioning for Buildings

Several thousand people die each year due to the heat, so the ability of buildings to protect their occupants is no longer just a matter of comfort. It is gradually becoming a public health issue. And this raises a fundamental question:

How can our buildings better protect their occupants from extreme heat?

The most obvious answer seems to be air conditioning. Its effectiveness is undeniable: when it’s 35°C outside, it quickly restores a comfortable temperature indoors. But this solution raises another question.

Should we continue to cool buildings that overheat, or should we prevent those buildings from overheating in the first place?

Because behind the debate over air conditioning lies a much broader issue: the ability of our homes, offices, schools, and public buildings to adapt to a changing climate. For decades, energy efficiency has been designed primarily with winter in mind. In the decades to come, we will likely have to place just as much emphasis on how well buildings withstand heat. In other words, the challenge is no longer just about retaining heat when temperatures drop; it’s also about keeping buildings cool when temperatures rise.

When our bodies overheat, the first thing we do is try to figure out why

Let’s imagine someone who develops a high fever. The natural first reaction is to bring their temperature down. They drink more fluids, rest, and possibly take appropriate medication. But no one considers this drop in temperature to be the definitive solution to the problem. Very quickly, another question arises:

Why does this person have a fever? Is it an infection? Inflammation? Some other condition? In other words, we aim to relieve the symptom while identifying its cause.

When it comes to buildings, we often tend to think differently. When a house reaches 30°C or higher, we naturally try to cool it down. This reaction is perfectly understandable. But it doesn’t answer a key question:

1. Why did this building reach such a high temperature?
2. Why do some homes remain relatively comfortable despite several days of intense heat, while others become practically uninhabitable?
3. Why do some schools regularly exceed comfort thresholds while others maintain more acceptable temperatures?
4. Why are some residents still sleeping at 28°C at midnight, even though the outside temperature has been dropping for several hours?

The answer doesn't depend solely on the weather. It also depends on how the building interacts with the weather. That is precisely where the topic of summer comfort begins.

Why Air Conditioning Is So Popular

Before we go any further, it’s important to be clear on one point. Air conditioning is not the enemy. It provides an effective solution to a very real problem. When a heat wave lasts for several days, it helps quickly restore more bearable living conditions. For the elderly, infants, or people with certain chronic illnesses, this ability to quickly lower indoor temperatures can even be a significant protective factor. In certain buildings, such as hospitals, nursing homes, or some healthcare facilities, its use is often essential.

Air conditioning also has a major advantage: its effectiveness is immediately apparent. Just a few minutes after it is turned on, the air feels cooler and comfort levels improve. This rapid result is a major reason for its success. With summers getting hotter, it seems natural to install more systems capable of cooling. However, this approach has a fundamental limitation.

It comes into play when the building is already hot. It therefore primarily addresses the consequences of overheating. However, a building that overheats every summer will continue to require more energy to cool. The real question then becomes:

How can we reduce this overheating before it even happens?

The challenge: cooling without preventing overheating

To understand this difference, let’s imagine a bathtub with the faucet left running. Little by little, the water begins to overflow onto the floor. There are two possible strategies. The first is to scoop out the water as it accumulates. The second is to turn off the faucet. Both actions can be helpful. But they address the problem at different levels. Scooping out the water limits the immediate consequences. Turning off the faucet addresses the root cause of the problem. Air conditioning is somewhat similar to the first approach. It removes the heat that has built up inside the building to maintain a comfortable temperature. Summer comfort, on the other hand, seeks to act further upstream. Its goal is to limit the amount of heat entering the building in order to reduce the need for cooling itself. This difference in logic is essential. The more a building is naturally able to limit its overheating, the less it depends on active systems to remain habitable.

It is precisely this preventive approach that is currently guiding much of the discussion on adapting buildings to heat waves.

How Heat Actually Enters a House

When a home becomes uncomfortable in the summer, it’s tempting to blame only the outside temperature. However, two houses on the same street, exposed to the same weather conditions, can have a temperature difference of several degrees inside. This difference is due to the way heat enters the building.

The sun first heats the roof

The roof is generally the surface most exposed to solar radiation. On a summer day, its temperature can far exceed that of the outside air. While a thermometer may read 35°C in the shade, certain roofing materials can reach over 70°C in direct sunlight. This heat does not disappear; it naturally seeks to spread toward the cooler areas below. The attic thus becomes the building’s first line of defense against extreme heat.

Windows create a greenhouse effect

Windows also play a major role. The phenomenon is similar to what happens in a car parked in the sun. Even when the outside temperature remains relatively moderate, the interior of the vehicle can become stifling within a few tens of minutes. Why? Because the sun’s rays pass through the windows, heat the interior surfaces, and are then partially trapped inside. The same phenomenon occurs in a home. Without proper sun protection, windows can be one of the main sources of heat in the summer.

Walls also store heat

The roof and windows often get the most attention, but walls also contribute to overheating. Throughout the day, facades exposed to the sun absorb some of the energy they receive. This heat is then gradually released both indoors and outdoors. In some buildings, this heat buildup explains why temperatures remain high long after sunset. We’ve all experienced this situation: by 10 p.m., the outside air has become relatively bearable again, but the walls continue to radiate the heat accumulated during the day. The building then becomes a sort of thermal battery that slowly releases the stored energy.

The occupants generate their own heat

We often forget that our daily activities also generate heat. Appliances, computers, televisions, lighting, and cooking all contribute to rising indoor temperatures. Taken individually, these sources of heat may seem modest. But when combined with an already high outdoor temperature, they further reduce comfort. A house doesn’t heat up simply because it’s hot outside. It heats up because multiple heat sources accumulate simultaneously.

Summer comfort isn't just about a single factor

In the face of extreme heat, it’s natural to look for simple benchmarks to compare the performance of buildings and materials. Thermal resistance (R), lambda, phase shift, and thermal damping all provide useful information about how a wall behaves. These indicators help us better understand how heat flows through materials and how a building responds to temperature fluctuations. Thermal phase shift, for example, refers to the time it takes for a heat wave to pass through a wall. Thermal damping, on the other hand, measures the wall’s ability to reduce the intensity of a heat peak before it reaches the interior of the building. These phenomena are very real and play a role in a building’s thermal performance. However, it is important to remember that no single indicator can predict a building’s summer comfort on its own. In reality, numerous parameters interact simultaneously: the building’s orientation, glazed area, solar shading, nighttime ventilation, wall thermal mass, the immediate environment, and even the occupants’ habits.

What Really Determines Summer Comfort

If there is no single indicator that can predict summer comfort, what are the most important factors?

1. Sun Protection

The best way to keep the heat out is often to prevent it from entering the building in the first place. Shutters, adjustable sunshades, exterior blinds, or roof overhangs can block a significant portion of the sun’s rays before they pass through the windows. This makes a fundamental difference. Once the sun’s rays have entered the building, it is much more difficult to dissipate the accumulated heat.

2. Nighttime Ventilation

When the outdoor temperature drops at night, it becomes possible to release some of the heat that has built up inside the building. This simple strategy remains one of the most effective ways to improve summer comfort—provided, of course, that nighttime temperatures actually allow for cooling.

3. The quality of the envelope

The roof, walls, windows and doors, and insulation all contribute to a building’s ability to withstand temperature fluctuations. The more cohesive and efficient the building envelope is, the more stable the indoor temperatures remain.

4. The building's inertia

Heavy materials can absorb some of the temperature fluctuations and limit temperature variations. This thermal storage capacity can contribute to comfort when properly combined with appropriate ventilation.

5. The occupants' activities

Daily habits also play a major role. Closing the shutters during the hottest hours of the day, limiting certain electrical uses, or ventilating the home at night can have a significant impact.

The Five Most Effective Ways to Prevent Overheating

Adapting Buildings Without Exacerbating the Climate Crisis

There is another question worth asking.

How can we adapt buildings to extreme heat without increasing greenhouse gas emissions?

This is because the heat waves currently affecting France are part of a broader context of climate change. The solutions chosen to improve summer comfort must therefore also be analyzed in terms of their environmental impact. Not all materials have the same carbon footprint. The manufacture of certain insulation materials requires highly energy-intensive industrial processes.

For example, the production of mineral wool involves melting raw materials at extremely high temperatures. These processes require significant amounts of energy. Conversely, other solutions rely more heavily on recycled or renewable materials. The goal is not to systematically pit one material against another, but to remind us that a building designed to withstand heat waves must also take into account its overall environmental impact.

What role does cellulose insulation play cellulose insulation

As part of this comprehensive approach, cellulose insulation one of the solutions used to improve the thermal performance of building envelopes. Made primarily from recycled paper, it is part of an effort to make use of existing materials. Its production through “simple mechanical pulping” requires neither water nor heat, making the manufacturing process very energy-efficient. It also acts as a carbon sink, since recycled paper—derived from wood—continues to store biogenic carbon throughout its entire lifespan. As with any material, its performance also depends on the quality of its installation and its integration into a coherent strategy for summer comfort.

Prevention or Treatment: Two Complementary Approaches

FAQ: Air Conditioning, Heat Waves, and Summer Comfort

Is air conditioning essential during heat waves?

In certain situations, yes. Healthcare facilities, housing for vulnerable individuals, or certain buildings that are highly exposed may require a cooling system. However, air conditioning is primarily effective when heat is already present inside the building. It does not replace strategies aimed at limiting heat gain at the source.

Why do some houses stay cool longer?

Summer comfort depends on many factors: the building’s orientation, the amount of glass, sun protection, nighttime ventilation, the quality of the building envelope, and the occupants’ habits. As a result, two homes located on the same street may have very different indoor temperatures even under identical weather conditions.

Does insulation also help keep out heat?

Yes. Insulation isn’t just for retaining heat in the winter. It also helps limit heat transfer in the summer and contributes to stable indoor temperatures. However, it must be combined with other measures, such as sun protection and ventilation.

What is summer comfort?

Summer comfort refers to a building’s ability to maintain an acceptable indoor temperature during hot periods without relying excessively on a cooling system. It depends on a combination of factors related to the building’s design, its environment, and its equipment.

Is the thermal lag sufficient to ensure a comfortable summer?

No. Phase shift is a real phenomenon that measures the time it takes for a heat wave to pass through a wall. But summer comfort also depends on the glazing, sun protection, ventilation, the building’s thermal mass, and the occupants’ habits.

Will buildings need to be better equipped to handle the heat in the coming years?

Heat waves are becoming more frequent and more intense. Adapting buildings is therefore an increasingly important challenge for maintaining comfort, reducing cooling needs, and protecting occupants during periods of extreme heat.

Why is there more and more talk about building resilience?

Resilience refers to a building’s ability to continue functioning despite more challenging weather conditions. During heat waves, a resilient building is able to limit overheating and protect its occupants even when outdoor temperatures become exceptionally high.

Conclusion