ᐅ Roof insulation between rafters: Should you choose thin or thicker insulation wool?

Sorry, I lost track with the calculations.
So far, I haven’t found the right insulation thickness at the hardware stores. But when I search online, I do find options with 80mm (and sometimes even 60mm (2.4 inches) in roll form), for example:

• Isover Metac UF 035 universal felt roof insulation
• Knauf Unifit WLS 032 glass wool insulation
• Knauf Unifit WLG 035
• Ursa GEO glass wool insulation felt, thermal insulation wool WLG 035

Would a 0.5cm (0.2 inch) air gap between the insulation wool and the breathable underlay membrane be sufficient? I will check again tomorrow regarding the breathable underlay membrane that was installed in our case. It is usually this one:

The vapor-permeable Probau roof underlay membrane is installed directly on the insulation material or wooden sheathing and protects it from moisture. Residual moisture migrating from the interior is absorbed and safely directed outwards. The membrane is three-layered, consisting of two layers of water-repellent spunbond polypropylene enclosing a special functional membrane.

What is your opinion on the "ISOVER Klimamembran Vario KM Duplex UV"? So far, I was leaning towards the "Rockwool RockTect vapor retarder Centitop." Vapor retarder / permanently diffusion-reducing.

Is the underlay vapor-permeable or not? This determines whether you need an air gap behind it and whether to use a vapor retarder or vapor barrier. Vapor retarders allow a small amount of moisture to pass through. Vapor barriers must be installed completely airtight.

The description of my roofing underlayment states the following:

The vapor-permeable Probau roofing underlay lies directly on the insulation material or the wooden decking and protects it from moisture. Residual moisture migrating from inside is absorbed and safely carried outside. The underlay consists of three layers, made up of two layers of water-repellent polypropylene spunbond fabric that encase a specialized functional membrane.

• Optimal moisture protection
• 3-layer structure
• Highly vapor permeable
• Versatile application

Both the Isover and Rockwool insulation products I mentioned act as vapor retarders, not vapor barriers. I am just wondering, since there is a big price difference between them, what exactly sets them apart?

According to the description of my roofing underlay, I could probably use insulation that matches the full rafter depth. The rafters are 85mm (3.3 inches) deep. Insulation rolls I have seen online come in 60mm (2.4 inches), 80mm (3.1 inches), and 100mm (3.9 inches). There are also insulation or acoustic mats with other thickness options, but I am not sure if these are really suitable for roof applications.

netzplan schrieb:

Both the Isover and the Rockwool that I mentioned are vapor retarders, not vapor barriers. I am just wondering, since there is a significant price difference between the two, what exactly distinguishes them?

Isover is variable. It allows less moisture to pass through in winter than in summer. This way, less moisture enters the insulation during cold outside temperatures, while still enabling drying towards the interior.

Thanks for the information. This means that in winter, less moisture from the interior moves toward the insulation, as drying out is more difficult during this season. This creates a better balance regarding the insulation’s "drying out" process. However, any moisture coming from the interior will then try to escape through other paths.

netzplan schrieb:

Thanks for the information. This means that less moisture from the interior moves toward the insulation in winter, as it dries out more slowly during this season. This creates a better balance regarding the drying of the insulation. However, any moisture potentially coming from the interior then tries to escape elsewhere.

No, not because it dries out more slowly, but because in winter, due to the temperature difference (inside 20°C (68°F), outside 0°C (32°F)), more condensation would occur within the insulation than in summer (inside 20°C (68°F), outside 20°C (68°F)). Actually, cold winter air is much drier than warm summer air since it holds less moisture. That’s why unfinished buildings were traditionally dried out over the winter. However, in occupied buildings, moisture is constantly generated through breathing, cooking, plants, and so on.