Proposal for insulation construction in a ventilated under-roof needed

Hello everyone,

I have the following problem:

I have to insulate my roof from the inside. The existing structure from outside to inside is: roof tiles, battens, counter battens, vapor-tight bitumen roofing membrane, wooden sheathing, 2cm ventilation gap (ventilation ridge and ventilation at the foot purlin present), 160mm roll insulation between rafters with 180mm rafters. Rafter spacing approx. 56cm.

Now my question:

which materials and structure would you recommend for a between-rafter insulation? ATTENTION: full-rafter insulation not possible due to the vapor-tight underlayment with a bitumen roofing membrane (Buzi AK)!

Thanks already for your help!
Best regards

Why does it have to be insulated again at all? Is the attic even inhabited?

The house is from the 80s and that attic was intended for conversion to living space during construction and now the conversion is taking place. Since the aluminum-coated glass wool was installed incorrectly (e.g., only partially stapled between the rafters and various tears in the aluminum foil, etc. => no continuous vapor barrier anymore) and has deteriorated over the decades, it will now be replaced before the roof structure is lined from the inside.

Therefore, my question is how I should best handle the insulation setup now. (Please do not refer me to an energy consultant => unfortunately, they are incompetent in our area. I have already been through several.)

Well, I would have a roofer take a look at it.

If there is a flawless ventilation present and functioning, no vapor barrier is necessary. In other words, the aluminum foil coating is just a nice-to-have. Or rather, it was nice during the installation.

Or who found that the glass wool was installed incorrectly?

If there is a flawless ventilation and it works, no vapor barrier is necessary.

I don't think that is true. The dew point is in the wool, it gets wet and doesn't dry again.

On the subject of ventilation: 2 cm is already very tight. Whether a continuous effective cross-section is present there can be doubted.

I don't think that's true. The dew point is in the wool, which gets wet and doesn't dry again.

If properly planned and executed, that's a minor problem. With a vapor retarder, you can achieve a higher drying reserve in a ventilated roof. But it's not necessary. Nowadays, such roofs are built less often. In our family, we have one, and it's only clad with OSB. Built in the 60s.

Regarding ventilation: 2 cm is very tight there. Whether there is a continuous effective cross-section can be doubted.

That's why the note whether someone from the trade has already had a look at it on site. If the ventilation doesn't work, you really have to get to grips with the whole thing.