It’s eleven at night. After a 41 degree day, we opened all the windows in the house wide. Logic suggests that this way fresh air can get in, but in reality, we are trying to cool several tons of brick, concrete and plaster. Something impossible… Or not? The reality is that behind this daily habit lies one of the most effective cooling strategies known to building engineers. Is called night flushingor night ventilationand consists of use night air to cool not only the environment of a home, but also its walls, ceilings and floors. Its structure.
The idea may seem simple, but numerous studies show that, applied correctly, it can reduce the indoor temperature by several degrees and significantly reduce the need to use air conditioning, especially in buildings with good thermal inertia. And when we talk “significantly”, one study points out up to 6º less while another mentions a decrease in cooling demand of up to 94%.
What is the science behind this? When a home has been exposed to the sun for hours, the problem is not only that the air inside is hot. The walls, ceiling, floor, furniture and even books store part of that heat as if they were a thermal battery.. Even though the outside air cools down at dusk, these materials continue to slowly release the energy accumulated throughout the day.
That is why many houses remain suffocating even well into the early hours of the morning. Opening a window for a few minutes hardly changes that situation. What really needs to be cooled is the structure of the building itself and that is precisely the principle of night flushing. When the outside temperature falls below the inside temperature, windows located on opposite facades or on different levels of the home are strategically opened to generate a continuous flow of air.
This flow extracts the heat accumulated on the interior surfaces for several hours. At dawn, the house not only contains cooler air. It has also reduced the temperature of its walls and ceilings. And since those materials will take hours to reheat, the interior will remain more pleasant for much of the next day. In a way, it is like introducing a huge invisible block of ice inside the house. It does not directly cool the air, but rather absorbs heat as it accumulates again. The key is in the temperature difference ypFor it to be effective, there needs to be a sufficient difference between the outside and inside temperatures. Studies show that the greater this contrast and the more hours the home is ventilated, the greater the amount of heat that can be eliminated.
That is why this technique works especially well in areas where the days are very hot, but the nights cool down relatively quickly. Instead, During the so-called tropical nights (when the thermometer barely drops below 25 °C) its effectiveness decreases considerablysince the outside air can hardly extract heat from the building. There is another decisive factor: thermal mass. Buildings made of heavy materials, such as concrete, brick or stone, act as enormous heat accumulators: during the day they absorb part of the solar energy and slowly release it.
It may seem inconvenient, but during the night it becomes an advantage. If these materials manage to cool down through night ventilation, they will begin the next day acting as a cold reservoir. Very light constructions, on the other hand, hardly store heat… but they also do not store coolness. He night flushing Can be combined with fans to increase air movement. Although these do not cool the home by themselves, they do promote the exchange of heat between hot surfaces and the night air, accelerating the process.
So… what determines when to open the windows? It’s not the time, but the heat balance: the outside air must be cold enough to start extracting heat from the walls and furniture. And the aforementioned studies point out three main criteria. The first is the simplest: open when the outside temperature falls below the inside. From that moment on, the heat flow changes direction and the building begins to cool. The second adds a margin of safety. Some simulations They consider it more efficient to wait until the outside air is between 2 and 3 °C colder than the inside. This avoids introducing air that barely cools and guarantees that the building really begins to “discharge” the accumulated heat.
The last of them incorporates the weather forecast. If it is known that the night will be long and cool, it may be advisable to start ventilation relatively early, even if the temperature difference is still small, because the cooling will accumulate over more hours. On the other hand, if the night is barely going to cool down, opening too early may provide little benefit. The curious thing is that Most of us do just the opposite: we open the windows when we get home, around seven or eight in the afternoon, thinking that this is the moment when the heat begins to subside. However, in summer the exterior walls continue to radiate a large amount of energy and the air can remain very hot.
In most Spanish cities, during a heat wave, the really interesting moment usually comes between ten at night and one in the morning, when the outside temperature begins to drop clearly below the temperature of the home. In inland areas, where the nights cool down more quickly, this moment can come earlier; On the Mediterranean coast, where the temperature drops very slowly it can be delayed until well into the early morning. The best sign to open the windows is not that it has gotten dark, but that the outside air is clearly cooler than the inside of the home. If it is also two or three degrees colder, the building will begin to lose heat much more efficiently.