New insulation for attic (between / under rafters)

Hello,

I would like to have my attic apartment re-insulated. I have already removed the cladding and found 10cm of aluminum-coated PU panels.
I now have the possibility to add 15cm of insulation between the rafters and then another 5cm of insulation below the rafters when installing the battens for the drywall cladding.
Can you give a general recommendation on whether this makes sense and, if so, which insulation you would advise? Rather 0.32 or is 0.35 sufficient?
In the end, about 115 sqm of roof would need to be re-insulated – it is ultimately a cost issue. Does this even make sense or is it not worthwhile?
I am grateful for any tips.

Here are some additional details about the construction of the house:

- Foundation: waterproof concrete basin
- Basement exterior walls: B25, interior: perforated brick 12
- Exterior walls: unipor, 49cm, except for the west side where a garage with 24cm walls is connected; here 36cm was used.
- Load-bearing partition walls: perforated brick 12
- Non-load-bearing partition walls: perforated brick 12
- Ceilings: B25
- Floor: screed and insulation
- Roof structure: purlin roof truss
- Roof: tile covering, 34 degrees pitch, underneath 10cm aluminum-coated PU

Best regards André

Hello André,

just cautiously asking upfront: Do you own the entire house or are you the owner of the apartment according to [WEG]?

If the latter is the case: Have you obtained the approval for the structural modification of the building from the owners' meeting?

Regards,

Dirk

hi,
sorry, am the owner of the house :)
best regards

Anyone have a tip? After reading a bit, I would now choose 14cm 0.32, below that a diffusion-open vapor barrier (possibly Isover Vario Klimamembran), then the battens for the drywall (without insulation), then the drywall. I was advised not to insulate the battens anymore, to leave an intermediate space to "breathe." What do you think about that?

Before the vapor barrier, seen from the room side, a maximum of 20% of the insulation value of the insulation behind the vapor barrier (usually between-rafter insulation) should be installed to prevent condensation. With 14cm of 032 between-rafter insulation, this would be about max 3cm of 032 below-rafter insulation. I actually find below-rafter insulation only really useful where it is important to utilize every cm for insulation. If this is not the case for you, I would rather double the rafters, add correspondingly more between-rafter insulation, and omit below-rafter insulation. Doubling means less work for the same insulation effect than adding an additional insulation layer without doubling. In addition, you then have a free installation space.