Heat Wave: Building Design Determines the Indoor Climate
14.08.2026
Figure: Artyom Korshunov/Unsplash
A new heat wave is sweeping across Germany, pushing not only people but also many buildings to their limits once again. While some indoor spaces remain pleasantly cool despite the intense heat outside, others reach unbearable temperatures as early as the morning. The building design often determines why this is the case.
According to official measurements, summers have been getting warmer since the 1980s, and the number of hot days is on the rise. This summer, temperatures in classrooms, hospitals, and long-term care facilities are increasingly exceeding 30 °C. In Baden-Württemberg, an educational center even had to close temporarily in June because children and staff were experiencing circulatory problems and nausea. A recent analysis by the German Weather Service described the June heat wave as unprecedented in terms of duration, intensity, and extent—and warns that such extreme events could increase significantly with global warming of two degrees.
This brings one question more sharply into focus: How can we design and build buildings so that they don’t overheat in the first place during the summer?
Buildings Can Slow Down Heat
Many buildings today react more quickly and intensely to heat partly because design decisions were made under different climatic conditions. Large window areas without external sunshades, especially on south- and west-facing sides, cause rooms to heat up faster. Lightweight structures with low thermal mass can barely store heat.
At the same time, the use of air conditioning is on the rise. While they can provide short-term cooling, they also increase energy consumption and release the extracted heat into the environment. In densely built-up neighborhoods, this can further exacerbate the so-called heat island effect.
Building physics shows that a building’s thermal mass—that is, the ability of its components to absorb, store, and release heat over time—is crucial for maintaining stable indoor temperatures.
Solid construction methods using masonry or concrete act as a natural temperature buffer and can significantly mitigate heat spikes. In combination with other factors, such as the possibility of nighttime and cross-ventilation, summer heat protection can thus be demonstrably improved. Studies by the Federal Environment Agency, the Federal Institute for Research on Building, Urban Affairs, and Spatial Planning (BBSR), and the University of Natural Resources and Life Sciences, Vienna confirm this effect.
In addition, thermal mass can be utilized in a targeted manner: Through building component activation in combination with heat pumps (ground-source, groundwater), solid building components can be temperature-regulated using water-carrying pipes. This stabilizes indoor temperatures with minimal energy consumption.
Buildings Without Energy-Intensive Cooling
Various projects in German-speaking countries demonstrate that the principle of thermal mass works. These include buildings based on the “2226” concept, where the name refers to the target indoor temperatures of between 22 and 26 degrees Celsius. Examples include an office building in Lustenau, Austria, and the residential complex “House (Almost) Without Heating” in Ingolstadt. Both maintain stable indoor temperatures between 22 and 26 °C—entirely without conventional heating, ventilation, or cooling systems.
Another approach is evident in the “RESI” office building in Bamberg, also a low-tech building without conventional heating and cooling systems. Building simulations were used to account for shading, daylight, and future climate impacts as early as the planning phase. By using reused clinker bricks and recycled concrete, the building also achieves a recycling rate of 75 percent.
Climate adaptation starts with construction methods
“Current temperatures make it clear just how important it is to build structures that don’t let heat through immediately. This isn’t a minor detail—it’s genuine protection,” says Thomas Zawalski, CEO of Solid Unit. “When it stays hot for days on end, you can feel the difference between buildings that absorb heat and release it slowly, and those that heat up immediately. This has nothing to do with comfort; it’s about the very practical experience of how well a building supports people during extreme weather. The construction method determines the indoor climate. That’s exactly why we need to talk about how we’ll build in the future—and which qualities really matter.”
CONTACT
Solid Unit
Kristine Hebenstreit
+49 621 423 01 42
