My project is to improve insulation in unheated garage / workshop and I discounted multifoil due to it being very flammable.

Job is sloping rafter about 2.4 m long and only about 65 mm deep so not enough room to get "PIR" foams in and limited head room. Would now have use PIR - so much nicer to work with ... 8m x 4 m foot print building Attic trusses.

I used 2 inch mineral wool with foil on one side in 1.2 m wide 9 m rolls with intention of stapling to roof under the rafters leaving rafters and felt able to breath. Relatively cheap at £7/m2. Plus more for foil tape for joints. Ductwrap product for industry. Just feeding back what we found.

First go at it yesterday and it was a lot worse than expected to work with. Very itchy and very dusty. We had white suits, masks etc - but miserable work. 2nd layer of foil covered mineral wool would not staple with 18 mm electric stapler but it did on bench when I tried it - perhaps weight coming down was the issue. Bigger staples and stapler have been sourced for round of work or may switch to PIR sheets for some of it.

I would use PIR in future and more expensive but is easy to screw / expanding foam in place.
It is also largely fire proof but as on here what comes off in the smoke is a worry too.

Mineral wool did a good job of doing to the wall cavity bats in the cavity wall about 300 mm below the inner wall plate though. I cut slits every 600 mm about 600 long which dropped into cavity.
Foil side down in my case - we could not seal the tape very well on felt site/roof tile side and switched to foil side to room side for outer layer.