I have a 1.5-story house from the 1930s. It has raised knee walls so the outer wall goes up one meter on the top floor and then slanted roof. Today, there are substantial knee walls on both sides. I am now going to completely renovate the attic and tear out everything (all interior wall materials, everything in the ceiling all the way up to the outer roof/ridge board). The plan is, among other things, to get a bit more floor space and use the old knee walls for smart solutions like built-in wardrobes/the foot of the children's beds, etc. I will insulate with something other than stone wool/fiberglass, it will be eco-fiber or Vital or similar and no plastic. Now to my question. I understand air gaps in the ceiling and so on, but what do you do with the outer walls where there used to be knee walls? I am going to insulate these walls (about 1m high) but should I put the insulation directly against the inside of the outer wall or should I leave an air gap (like a mini knee wall of 5 cm) before I frame a new (inner) wall that is insulated? Air gap feels right, and if so, I had thought to connect the air gap in the wall up to the air gap in the ceiling.
Exactly right! Air gap also in the wall. If you have an opening at the eaves where the roof gap and wall gap meet, then you're following the textbook as you should. Between the wall gap and wall insulation, you can use asfaboard - black side out. There is surely someone here in the forum who is more knowledgeable and knows alternatives to asfaboard.