Which insulation material is suitable?

Hello,
I want to slightly insulate a small extension etwas isolieren. That means where there are Hohlräume on / in / in front of the masonry, I want to abdichten.
In total, this will possibly be 500 liters volume over about 10m² that I want to fill.
I am thinking of Insulation foam or wood wool to fill it.
What would you recommend and why?
It should not be excessively expensive, should be easy to work with, and not harmful to health.
Regards,
Frank

Maybe [Stopfhanf] is also suitable?!?

It should of course also insulate well. That is the main task. ;-)

Im and vor are different applications. Maybe you could take a picture so that we can imagine what it is about

Maybe you could take a picture so we can imagine what it's about

Here come pictures.
You can see the holes in the wall.
And you can see how uneven the (natural stone) wall is. Unfortunately, the previous owner painted it with tar paint, so no other paint will surely stick. That's why I thought I'd put OSB boards in front and insulation material between the board and the wall.