Is it worth insulating rooms only partially?

Hello everyone,

I bought an older house last year, which was partially insulated. I am now considering whether to further insulate the living room, but I am not sure if it is worth it. Here is the description of the room:
- one long wall is already completely insulated
- one short wall borders the hallway and is not relevant
- the other short wall facing the garden should not be insulated at all because the wall is too large and would significantly exceed my budget.
- Now it is about the 2nd long side, which is L-shaped and half of it leads into another room, which does not need to be insulated. So only the other half is under discussion. This area mostly consists of windows (the heating is also on this side) and only a small strip would need to be insulated (about 12 sqm). The previous owner mostly left the materials for this, so the costs would only be about €400 (windowsill, roof overhang, ...).

Although the costs will be relatively low, I still only want to insulate if it pays off in the medium term, because:
- All the windows are old and will not be replaced for now
- one wall will definitely remain uninsulated.

To sum it up briefly: does insulating each individual wall save energy, or is insulation really only effective when all walls are tight and the windows are modern?

What do you think, should I insulate that one wall? Thank you very much in advance for your suggestions.

Ps: I have Alsecco 12 cm boards 035 as insulation boards.

Best regards

Guido

You don’t ask the question if you have lived in it for a winter.

Where you don’t insulate, these thermal bridges occur, I think (as a layman) that you will "feel" this wall, meaning how the cold comes in. The insulated part then ultimately is money thrown away.

My colleague bought an old house (from the 60s?) last summer. He has a panoramic window which he didn’t want to replace. Quote: "I’m not spending 3000 on a new window, I can just burn that money." After this winter, the replacement is now due. It’s very cold there, it drafts, you can feel it throughout the whole room.