Insulate the roof or the floor ceiling? Or both?

Hello everyone from Bavaria,

I am about to insulate our attic. The reason for this is the heating costs and also the heat in summer. The attic is not going to be converted.

My problem is that the roof is quite new. So a new roof with external insulation under the tiles does not matter. Basically, rafters, boards, roofing felt, and then tiles.

The rafters are only 12cm. Doubling is possible but not desired.

The top floor ceiling consists of a board layer and a cavity of 18cm. Here you can see very old glass wool that was either only half installed or has collapsed. I have already found a professional company for disposal.

What do you think? I would replace the old glass wool on the top floor ceiling with wood fiber insulation 180mm or 200mm and lay a vapor barrier over the beams beforehand. Above that, I would like to lay OSB boards, which would then be diffusion-tight.

For the rafters, I would install Pavatex boards from the inside (with a batten distance to the boards).

What would be the sensible solution?

Simplified from bottom to top. [Dampfsperre] (so tight that no moisture can get from the upper floor into the insulation), insulation, OSB boards

Or alternatively, simply renew the old insulation and add mineral wool on top again. However, then the floor is no longer walkable. You also don’t need a barrier then.

first of all, thanks for the reply.

So, laying OSB boards on top is no problem as long as there is a vapor barrier to the lower floor. Is there anything to consider? Does the manufacturer matter?

A big question mark for me is also the building material I should use. I like wood fiber insulation because it cannot settle and is natural. Although the insulation value is worse.

I don't want mineral wool. It should be walkable and serve as storage space. OSB boards for that reason, because I don't like the rest. I was also told that I don't need a vapor barrier as long as you lay boards again that allow air to pass through.