window insulation

Okay thanks so far.. doesn’t sound very good though or rather quite expensive :(

link removed by moderation; construction expert

which ones would be the most sensible?
Unfortunately, I’m not the wealthiest, so blinds will probably have to do..

Structural measures? Are you a tenant or an owner? If tenant, I wouldn't do anything yourself... At most, fabric draft stoppers in front of the apartment door and on the windowsill. Beyond that, you can check whether your landlord has insulated the top floor ceiling and towards the basement, and how old your heating system is. The same applies if you are an owner. Check the measures.

And I repeat myself: It depends on the overall situation. Please answer the questions that were asked. Then we might be able to help you. A picture often says more than a thousand words.

I assume that no questions will be answered by the OP because he was more interested in getting rid of his link :P

Regards