ᐅ Can you mix cavity insulation materials with thermal conductivities of 0.040 and 0.035 W/m·K?

Hello,

I am about to complete my late father's project. The second floor (with a single-sided sloped roof) and the attic are to be fully finished.
About half of the roof area is already insulated with "The new Rockwool insulation wedge 040 120mm (5 inches)." Since 040-grade insulation between rafters is now hard to find, I am considering insulating the rest with 035 mineral wool. Are there any issues with that?

Roof structure as follows:

CURRENT
Roof covering
Batten
Counter batten
Roofing felt, coated with granules
20mm (0.8 inches) tongue-and-groove boarding
160mm (6.3 inches) rafters with
- 40mm (1.6 inches) ventilation gap
- 120mm (5 inches) 040 Rockwool

TARGET
As above, then
Vapor retarder with sd-value 2.3
60mm (2.4 inches) battens with
- 60mm (2.4 inches) 035 mineral wool
12.5mm (0.5 inches) drywall
Plaster

Addendum:

Or does the whole system also work without ventilation using a humidity-variable vapor retarder?

SOLL is perfectly fine. You can safely mix the thermal conductivity classes (WLGs).

If you still have leftover roofing felt and know its sd value, you can calculate whether ventilation behind it is necessary. It won’t cause any harm.