Insulation of the attic of an old country house

If it is bricked, there are small gaps in the masonry.

If it is brick-faced, there are small gaps in the masonry.

Please read the first post again. It is about INTERNAL insulation and the question of how the whole thing should be executed as a ventilated construction.

When it comes to the space directly under the roof that is visible in the photos, you have the brick/clinker wall right in front of you.

Judging by the photos, these are red bricks with white plaster on them.

Then you could see the structure as a double-shell masonry.

Brick -> Air layer -> Insulation -> Barrier -> Dry construction

Of course, I can't say if that is how it is intended there.

How spacious are the rooms? How much area is lost inside from something like that?

If it is about the space directly under the roof that can be seen in the photos, then you have the brick/clinker wall right in front of you. Judging by the photos, these are red bricks with white plaster on them. Then one could see the structure as a double-shell masonry. Brick -> air layer -> insulation -> barrier -> drywall I cannot say for sure if it is intended like that there

Yes, that is correct so far. The bricks are plastered from the inside. I can post photos again tomorrow and will also check the clinker on the gaps.

How spacious are the rooms? How much area is lost inside in such a case?

At the roof, I lose about 30cm towards the inside, at the wall about 20cm.

when I see the pictures of the roof, simply stuffing insulation in between is not enough. First, the roof is in poor condition (you can partly see through it, semi-circular beams...) and second, the achievable insulation thickness is too low and third, something windproof should be placed under the roof panels. Depending on what kind of rooms are to be created and how much money is available, planning can be done. I would not do wall insulation at all unless the wall is really very thin. It is not financially worthwhile at all and the risk of building damage is very high. At most a wood fiber board directly on the plaster.