Roof between-rafter insulation: Thin or thicker insulation wool?

Right, sorry, I have forgotten how to calculate.
So far, I haven’t found anything in the hardware stores regarding insulation thickness. However, when I google, I find 80mm (and sometimes even 60mm in rolls), for example:

[*]Isover Metac UF 035 Universal felt roof insulation
[*]Knauf Klemmfilz UNIFIT WLS 032 glass wool
[*]Knauf Klemmfilz Unifit WLG 035
[*]Ursa GEO insulation felt glass wool thermal insulation wool wlg 035

Would 0.5cm of air between the insulation wool and the underlay be sufficient? I will check again tomorrow regarding the underlay that was used here. Normally, it is this one:

The diffusion-open Probau roof underlay lies directly on the insulation material or the wooden sheathing and protects it from moisture. The residual moisture migrating from the inside is absorbed and safely carried outwards. The underlay is constructed in 3 layers, consisting of two layers of water-repellent polypropylene spunbond, embedding a special functional membrane.

What is the opinion on "ISOVER-Klimamembran-Vario-KM-Duplex-UV"? So far, I would have tended towards the "Rockwool RockTect vapor barrier Centitop." Vapor barrier / permanently diffusion-inhibiting.

Is the [Unterspannbahn] vapor-permeable or not? This determines whether you need a ventilation gap and a vapor retarder or vapor barrier. Retarders allow small amounts of moisture to pass through. Barriers must be installed completely airtight.

In the description of my underlay membrane it says the following:

The diffusion-open Probau roof underlay membrane lies directly on the insulation material or the wooden sheathing and protects it from moisture. The residual moisture migrating from the inside is absorbed and safely carried outwards. The underlay membrane is built in 3 layers, consisting of two layers of water-repellent polypropylene spunbond, which embed a special functional membrane.

[*]Optimal moisture protection
[*]3-layer construction
[*]Highly diffusion-capable
[*]Versatile application

Both the Isover and the Rockwool that I mentioned are both brakes and not barriers. I just wonder, since both have a large price difference, what exactly distinguishes them?

According to the description of my underlay membrane, I could probably use insulation with the full rafter width. 85mm rafters (insulation rolls are available in 60mm, 80mm, and 100mm, which I have seen online. There are also insulation/acoustic mats with further graduations, but I do not know if they are really suitable for the roof.)

Both the Isover as well as the Rockwool that I mentioned are both vapour retarders and not vapour barriers. I just wonder, since both have a large price difference, what exactly distinguishes them?

Isover is variable. It lets less water through in winter than in summer. This is supposed to allow less water to enter the insulation at cold outside temperatures while still enabling drying to the inside.

Thank you for the information. This means that less moisture comes from the inside towards the insulation in winter, as it is harder to dry out in winter. Thus, a better balance regarding "drying out" has been created here in terms of insulation. However, the moisture (which may come from the inside) then tries to escape elsewhere.

Thank you for the info. That means less moisture comes from the inside toward the insulation in winter because it dries out more slowly in winter. Thus, a better balance regarding "drying out" is created here, concerning the insulation. However, the moisture (which possibly comes from the inside) then tries to escape elsewhere.

No, not because it dries more slowly, but because in winter, due to the temperature gradient (inside 20, outside 0), more condensation would occur in the insulation than in summer (inside 20, outside 20). Actually, cold winter air is much drier than warm summer air because it can hold less water. That is why shell constructions were dried over the winter in the past. However, in inhabited buildings, moisture constantly accumulates due to breathing, cooking, plants...