Newly built apartment (KfW-55) cannot be cooled, always at least 25 °

Thank you very much. It is already good to read that turning off the ventilation during the day helps. My tenant will probably not be allowed to follow the tip with the cloths, since it is a homeowners' association and it would certainly be objected that the exterior appearance suffers.

Then she can still hang a cloth from the inside. Especially if she is not at home herself. Then it can be ugly or dark.

Then she can still hang a cloth from the inside. Especially if she is not at home herself. Then it can be ugly or dark.

True. However, I wonder why the temperature then decreases. Unfortunately, I am not familiar with this, but if the heat has passed through the aluminum shutters and the window, it is already inside the apartment. How can a cloth hung from the inside reduce the temperature?
I just want to understand and do not doubt this measure in any way. It would be really great if it is that simple. My tenant could definitely implement this tip without any problems at the two large windows in the living room.

If you want to keep it cool inside, you have to somehow insulate the interior spaces against heat influx. Insulation is created (among other things) by stationary layers of air. It is better to regulate such a stationary air layer outside, in front of the window. In this case, with the help of the roller shutters. Unfortunately, the material of today's roller shutters is no longer as insulating as wood used to be. Wood already contains stationary air layers within its structure due to the capillary structure in the wood.

If it is not or only limitedly possible to create a stationary air layer outside, then you can do it inside. A cloth casts a shadow. Linen or other rather semi-transparent fabrics do that as well. Then a quasi-stationary air layer builds up between the window and the cloth inside. It can get quite warm there and the heat is only slowly released to the interior. Theoretically, it would be even better to hang two layers of cloth with some distance in between.

I simply took cotton fabric from Ikea for 4 euros/meter. I hang it double (because of the air layers in between :D) and it creates "filtered" warm white to yellowish light in the room. At 30° or more, it's worth it for me to sit behind curtains.

At night, you can then, preferably with cross-ventilation and the ventilation system turned on again, air out the warm layers of air. And then the next warm day can come.

Thank you very much for the very clear explanation!
Do you possibly have a photo of the cotton cloth inside the window?

The topic must concern thousands of people who live in new buildings and are now complaining about excessively high temperatures.....

Yes, almost everyone is surprised how poorly a new building insulates against heat. People think that because it is new, it must be good. It is, but not in terms of temperature regulation.

I’m attaching a photo I just took. I have had white linen curtains for a week now. You can see them in the curtains in the vertical stripe all the way on the left. The regular linen curtain hangs from my ceiling and stays there all year round, mainly as decoration.

Then I mounted the proven ugly-but-cheap suction cup thing directly on the window again. A cotton cloth hangs on it, doubled in the upper third. You can clearly see the difference in the light falling through. During real heat waves, I double as much as possible. Since I have real curtains, triple. :D It’s not pitch dark in here, but pleasantly dimmed light compared to the bright sunlight. If you also lower the exterior shutters a bit during the day (light comes from above, so especially the upper part should be covered with shutters. Maybe 1/3 of the total height).

With my multiple curtain setup, I stayed below 25.5°C in all rooms all summer. You just have to make some compromises on aesthetics for that. :D Or ask the landlord if he will install expensive indoor curtains.:D

Whether the heat pump has a cooling function would have to be seen. However, I can hardly imagine it in a multi-family house. Then there would also have to be controllers in the rooms or in the apartment. That would surely have been noticed by now. Hanging rags in front of the windows - sorry, but as a neighbor, I would also complain. It looks like the slums, no offense intended. Exterior blinds would probably be an option if it weren’t so difficult to retrofit them. I see an air conditioning unit more as a solution here (if the ventilation shut-off option doesn’t help) - unfortunate because of electricity consumption. Even then, you would have to see whether it is allowed, if and how it works, and which one at all. But the southwest side is the hottest side a house has.