Come on, I bring my modest stone to your building:
1) you said:
In fact, I only plan to stay for a maximum of 5 years in this house, because I risk being transferred ... So I want to do the minimum of work for a maximum result in insulation ...
OK, but don't forget that if you take out invoices, photos, certificates, you can count on added value. If your house costs 50 euros / month cheaper to heat, it is worth at least 12000 euros more and the successor will also be ready to pay 50 euros more / month.
2) concerning the 1, take measurements (= regular index readings) now and write them down to be able to prove the result (even if only to you).
3) I insulated the attic from my house (I am a tenant) entirely in recycled materials! (I work in construction). I even insulated a wall with household appliance packaging polystyrenes, cut and glued on top of each other. It cost me the screws and plates and a jar of ad hoc glue.
4) I know I'm going to make more than one scream again, but I'm in favor of the aluminum vapor barrier. So, even if you don't change your attic insulation, remove the insulation, place an aluminum foil or aluminized mylar and replace. (if at least your ceiling is a thin ceiling) Objectively, the benefit is limited, but the impression is more comfortable.
5) and of course, if not already done, the best performance / price: programmable thermostat. I have one wireless: in the morning it is in the kitchen, and in the evening in the living room.
