Combating heat has never been easy. It is no coincidence that in hot, humid climates traditional dwellings tend to be lightweight, easy to ventilate, and shielded from the sun. In hot, arid regions the response was the opposite: thick earthen or stone walls that soak up thermal energy by day and release it as night falls. In the desert cities of Persia, builders also erected windcatchers that caught high breezes and funneled them down into living quarters, sometimes drawing them across a pool of water first. Across the Mediterranean we find narrow alleys and shaded courtyards, occasionally with a fountain bringing humidity to the air. These were practical strategies to cool the living environment and make summer tolerable. Alongside them came the siesta during peak afternoon heat, the instinct to seek shade, and the routine of drawing the blinds. Where architecture fell short, culture stepped in.
Cold is another matter entirely; everything is simpler. You bundle up, burn fuel, and warm up quickly. And if walking around wrapped in a blanket is impractical, you wrap one around the building instead: that is thermal insulation. Now imagine for a moment being able to do the exact opposite: a blanket that absorbs your body heat and cools you down instantly, or a 30-euro device that cools an entire room just as effectively as a small space heater warms it. If the world were growing colder, we would have inexpensive solutions ready to distribute across homes in warmer regions. Yet the opposite is happening.
Climate change brings protracted heatwaves to regions where cold weather has always prevailed, and where buildings were constructed to let heat in easily and keep it trapped. The climate is shifting, and many buildings must adapt as part of a new era of urban resilience.
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When we think of Bilbao, in northern Spain, we usually call to mind a persistent drizzle, an umbrella just in case, and mild summers. Heat rarely crosses our minds because the challenge has always been guarding against cold and damp.
Times have changed, however. In Torre Bolueta, an 88-meter-tall residential tower built to high energy-efficiency standards, residents can find themselves uncomfortably warm as early as May. Completed in 2018, it was engineered to deliver indoor comfort with minimal energy consumption.
Climate change brings protracted heatwaves to regions where cold weather has always prevailed, and where buildings were constructed to let heat in easily and keep it trapped.
Thermal insulation slows the penetration of outdoor temperatures, but it also makes it harder to shed the heat that has already entered. The problem arises when solar radiation entering through glazing is left unmanaged: the home simply cannot cool down. It behaves like a thermos flask: hot liquids stay hot, and cold liquids stay cold. If heat enters through windows and has no escape route, it remains trapped indoors.
Bolueta is compelling precisely because of its location in Bilbao. In this part of the country, many buildings were conceived primarily to retain warmth. Now we are beginning to realize that they must also learn how to release it.
A car parked in the sun can reach unbearable internal temperatures even on an afternoon that does not feel especially warm. Solar radiation penetrates the glass and heats every surface inside. From that point on, removing that trapped thermal energy becomes far more difficult.
Something similar occurs in residential buildings. Windows admit solar radiation, and once walls, floors, and furniture absorb that energy, the building envelope's thermal insulation impedes heat dissipation.
The design follows the historical logic of cold climates: orienting living spaces to the sun and capturing every possible unit of solar gain throughout the winter.
The issue is that in summer the sun continues to enter through those same windows, only now that thermal gain is entirely unwelcome.
Historically, part of the heat accumulated during the day could be flushed out at night. Residents opened windows when the air cooled down and closed them again in the morning. It is a straightforward strategy, but for it to work, outdoor temperatures must drop below indoor levels for several hours, typically overnight. That cooling window is shrinking.
A study published this year in Communications Earth & Environment analyzed satellite data to track land surface temperature shifts across more than 1,400 cities over the past two decades. Some of the fastest temperature increases are emerging precisely in cities with historically cold climates.
Some of the fastest changes are emerging in cities with historically cold climates. [...] Tallinn, Warsaw, and Oslo rarely had to consider heat protection in the past.
Tallinn, Warsaw, and Oslo are cities that historically gave little thought to keeping heat out. Furthermore, urban centers themselves are slow to cool at night. Asphalt and concrete, unshaded by tree canopy, absorb solar radiation throughout the day and release it slowly at night. This is the urban heat island effect, and it can keep city centers several degrees warmer than surrounding rural areas.
The warmer the night, the less time remains to purge heat, open windows, and flush thermal loads out of the building before the next day begins.
Across much of the Mediterranean, coping with heat is a cultural practice. Window blinds are drawn before sunlight strikes the glass. The home is aired out at night and sealed shut during peak afternoon heat. Seeking shade is second nature. What is obvious in Alicante, however, is far less customary in Oslo.
This distinction matters critically in high-performance architecture. The more efficient a building is, the more essential it becomes to understand its operational dynamics. The case of Bilbao might suggest that the issue lies in over-insulating the envelope, but insulation itself is not the enemy. In fact, it can be our most valuable asset. When outdoor temperatures reach 35°C or 37°C, the goal is to delay the transfer of that heat through the building envelope for as long as possible.
Homes built to similarly rigorous efficiency standards in much warmer climates perform well through the summer. The key difference is that they incorporate external solar shading in front of the glazing, enable cross-ventilation, and typically include mechanical air conditioning for peak temperature spikes.
The central challenge does not lie in the buildings we will construct from now on, but in the infrastructure already in place. The housing blocks of Oslo, Montreal, Berlin, and Bilbao will still be occupied 30 years from now, even as the climate conditions for which they were originally engineered continue to shift.
Interventions across the existing building stock will be indispensable, and they will not follow a uniform blueprint. In some cases, installing exterior solar shading will suffice. In others, upgrades will require improving natural ventilation, retrofitting roofs, or adding vegetation.
A significant share of urban planning will have to take place beyond the building envelope itself: planting shade trees, unsealing paved ground, deploying low-heat-absorbing materials...
A significant share of urban planning will also have to take place beyond the building envelope itself: planting shade trees, unsealing paved ground, integrating water features, deploying low-heat-absorbing materials, and establishing district cooling networks where feasible.
Addressing these risks does not require a single, massive overhaul. Adapting the existing building stock will unfold through a mosaic of varied measures—many of them incremental—distributed over years across individual buildings, streets, and entire districts to reduce their climate vulnerability.
For a long time, energy retrofits focused primarily on curbing winter heat loss to improve thermal comfort. That remains necessary, but buildings must now also be shielded from incoming summer heat and equipped to reject thermal energy when required.
There is an added complexity: we may retrofit a building once with the expectation that the intervention will endure for decades, whereas the climate will continue to evolve throughout that lifespan. That is why we may need to rethink buildings not just for the conditions we know today, but with enough margin for those they will have to face tomorrow. The building will endure, and people will expect to live comfortably within it.
Sources:
- Communications Earth & Environment (Nature) — Two decades of urban heat intensification and exposure across 1400 cities (mayo de 2026): https://www.nature.com/articles/s43247-026-03665-y
- Estudio de la Torre Bolueta — licitación de VISESA “Propuesta de soluciones al sobrecalentamiento de las viviendas de la Torre de Bolueta (Bilbao)”, adjudicada en agosto de 2020: https://www.euskadi.eus/web01-tramite/es/contenidos/anuncio_contratacion/expjaso32546/es_doc/es_arch_expjaso32546.html
Jordi Martí is a building engineer and science communicator specializing in architecture, construction, sustainability, and technology. He brings 20 years of sector experience, has taught at EPSEB-UPC, and spent over a decade as head of content at the Agenda de la Construcción Sostenible.