ᐅ Which insulation material is recommended?

Hello,
I want to insulate a small extension to some extent. That means where there are cavities in or in front of the masonry, I want to seal them.
Overall, it will be about 500 liters (13 cubic feet) in volume over approximately 10m² (108 square feet) that I want to fill.
I am thinking of using either foam insulation or wood wool for filling.
What would you recommend and why?
It should not be excessively expensive, easy to work with, and not harmful to health.
Best regards,
Frank

Maybe packed hemp fiber could also be suitable?!?

Of course, it should also provide good insulation. That is the main task. ;-)

"In" and "in front of" are different applications. Maybe you could take a picture so we can better understand what it’s about.

WilderSueden schrieb:

Maybe you could share a photo so we can get an idea of what it's about

Here are some pictures.
You can see the holes in the wall.
And you can also see how uneven the (natural stone) wall is. Unfortunately, the previous owner painted it with tar paint, so no other paint will stick to it. That’s why I thought I’d install OSB boards in front of it, with insulation material between the boards and the wall.