I live in the Netherlands. My apartment has two floors. The upper floor is basically a greenhouse, so 40C is absolutely normal if I close the windows. During the summer I always keep at least one window open, which cools down the house to around 30-35C. This sucks during summer but during winter it's a blessing because I can have 30C on a sunny day even if it's 0C outside. The only drawback is that the windows are very old so in winter that heat escapes immediately once the sun is gone. HOA has been planning window replacement for years already.
The lower floor has windows facing north so it's always much cooler there. I've never seen it reach more than 30C, which was in the middle of a heatwave, usually it keeps 25C during summer.
> During the summer I always keep at least one window open, which cools down the house to around 30-35C.
Yeah this is exactly my point. If your house is so poorly insulated that your internal temperatures go above the ambient temperature, you can open a window.
Chances are that ambient are doesn't provide enough cooling. Maybe you can open only window, or all windows on the same side of your home, meaning there's no draft. Maybe you're in an urban environment, with a micro climate even hotter than outside the city. Maybe both. Heat will enter from sunlight and from people inside, and will exit only very slowly.
There's a world of difference between a house in the countryside, with maybe even some trees providing shade, and an apartment in a densely populated area with very low opportunities for thermal control.
The lower floor has windows facing north so it's always much cooler there. I've never seen it reach more than 30C, which was in the middle of a heatwave, usually it keeps 25C during summer.