ᐅ Should the “attic space” in a barn be ventilated or should insulation be applied to the wall?

I am currently building a tack room for my girlfriend to store all the riding equipment. Since there will also be a small toilet inside, I want to insulate the tack room as a precaution.

You can imagine it as a small freestanding shed inside a fairly large barn. The room is located directly in the corner, so two sides are in direct contact with the barn’s masonry. The barn walls are made of quite solid lightweight expanded clay aggregate blocks.

I could either place mineral wool insulation between the beams and directly against the barn wall, then cover it with OSB boards, or install an additional layer of OSB boards between the masonry and the insulation to hold the insulation back and allow for ventilation. I have already left a few centimeters (inches) of air gap, so the beams are not in direct contact with the wall.

What would you recommend?

Thanks a lot for any advice!

Best regards,

Michael