ᐅ Terraced House – Adding Side Insulation to Roof Dormers

In our settlement house from the 1960s, the bathroom in the upper floor with a dormer window is going to be renovated. I have already removed the old interior cladding from the roof. I plan to install rock wool insulation between the rafters. The side walls that extend out from the roof are constructed as follows: interior wall of calcium silicate bricks 11.5 cm (4.5 inches), then approximately 5-6 cm (2-2.5 inches) of air space, followed by board cladding with copper sheeting on the exterior. I am considering filling the 5-6 cm (2-2.5 inches) air gap with suitable loose-fill insulation. However, it doesn’t seem cost-effective to hire a blower system just for this. Or should I leave the air cavity as it is? Has anyone had experience with this?
Best regards

We have a similar house and installed a drywall partition inside, using 100mm (4 inches) of bonded mineral wool insulation. Of course, with the appropriate vapor barrier and so on.