Advice on thinning the roof and stripping out walls

[Soffpotatisen]

We are going to space out the ceiling and frame out the exterior walls in a house from 1947. The walls will be framed out 45mm to accommodate electrical wiring and possibly PEX tubing for the radiator loop. 45mm insulation will be installed along with a vapor barrier. Then OSB and drywall. The ceiling will be spaced out and aligned if no alternative exists.

Conditions:
Plank walls with vertical planks, reasonably straight walls so probably no need to level. But there are local peaks and valleys as usual with plank walls. The ceiling needs to be aligned, likely differing by 5cm or more between the highest and lowest points. Ceiling joists are spaced at 80cm centers.

Questions:

1. How should I frame out the walls? Since there are local peaks and valleys, it seems unwise to screw the studs vertically into the wall. I suspect the peaks and valleys would also carry through into the drywall, even if minimally. I see two options:
   • Option 1. Stud is screwed horizontally into the ceiling joists and into the floor, then vertical studs are screwed between ceiling and floor. In this way, a new and straight wall is built, somewhat like constructing a new interior wall.
   • Option 2. Studs are screwed horizontally at 60cm centers across the wall, in this way the new wall is based only on the peaks in the plank wall. Screws are placed where each peak is on the plank wall and masonite or similar is used to shim if I need to screw into a valley. Vertical blocking is screwed at 60cm centers to provide support for sheet joints.
2. Is 22x70mm spacing adequate if the ceiling joists are spaced at 80cm, or do I need to use 28x70?
3. Should I space out the ceiling first and then frame up the exterior walls? If so, how do I best attach the horizontal wall stud at the top? One option I can see is to attach it to the plank wall with possible shimming if needed to match the horizontal stud at the floor.
4. Is there anything negative about screwing a horizontal wall stud into the floor, e.g., if in the future we need to replace the floor? The floor currently is tongue-and-groove pine directly on the floor joists.
5. Are there alternatives to spacing out the ceiling to maintain ceiling height? For instance, double layers of 6mm drywall with staggered joints. I understand that unevenness will show, but I have a hard time visualizing how uneven it will be and how much would be visible.

 

[Leif i Skåne]

If the house from 1047 I would have added more insulation, for example, 2x70 and placed the diff barrier 1/3 in from the warm side.

 

[Soffpotatisen]

If the house is from 1047, I would have added more insulation, for example, 2x70 and got the vapor barrier 1/3 from the warm side.

Thanks for the input. The house is insulated from the outside about 70mm. So 45mm from the inside is enough for us and there shouldn't be a problem with where the vapor barrier is then.