ᐅ Insulation between rafters in addition to an insulated roof for ceiling heating

Hello everyone,

The question might be a bit unusual, but here it is:

In the attic, we have a ceiling heating system with the following setup:

- Exterior insulation above rafters, 160mm (6.3 inches) thick with a U-value of 0.14
- Rafters (currently still an empty cavity)
- OSB boards
- Ceiling heating
- Drywall panels (still to be installed)

I still have some external thermal insulation composite system (ETICS) leftover from the house renovation, made of mineral wool and polystyrene foam.

From your point of view, is there any reason not to simply fill the empty spaces between the rafters with this loose insulation material, fitting it precisely? Otherwise, the ETICS materials would have to be disposed of.

I expect this might provide a further—albeit small—improvement in thermal insulation and help the ceiling heating direct more energy towards the interior surface.
Or could this cause problems with mold or moisture?

See attached pictures.

Thanks in advance

OSB panels are vapor-tight if the joints are sealed, so no damage is to be expected.

OSB boards are vapor-tight when the joints are sealed, so no damage is to be expected. However, the heated cavity provides additional security.