ᐅ Request for insulation design proposal for ventilated roof underlay system needed

Hello everyone,

I have the following problem:

I need to insulate my roof from the inside. The current layers from outside to inside are: roof tiles, battens, counter battens, vapor-tight bitumen roofing membrane, wooden sheathing, 2cm (1 inch) ventilation gap (ridge vent and eave vent are present), 160mm (6.3 inches) rolled insulation between rafters with 180mm (7.1 inches) rafters. Rafter spacing is approximately 56cm (22 inches).

Now my question:

Which materials and construction method would you recommend for insulating between the rafters? WARNING: full rafter insulation is not possible due to the vapor-tight underlay with the bitumen roofing membrane (Buzi AK)!

Thanks in advance for your help!
Best regards

Why does it need to be insulated again at all? Is the attic actually being used as living space?

The house dates back to the 1980s, and the attic was originally designed for conversion into living space, which is now underway. Since the aluminum-faced glass wool insulation was installed incorrectly (for example, partially only stapled between the rafters, with various tears in the aluminum foil, etc., resulting in no continuous vapor barrier) and has deteriorated over the decades, it will be replaced before the roof structure is lined from the inside.

Therefore, my question is how to best approach the insulation setup now. (Please do not refer me to energy consultants—they have unfortunately been ineffective here. I’ve already consulted several.)

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

If there is a proper ventilation gap in place and it works correctly, a vapor barrier is not necessary. So, the aluminum foil facing is just a nice-to-have, or it was simply convenient during installation.

Or who determined that the glass wool insulation was installed incorrectly?

Caspar2020 schrieb:
If there is a properly installed and functioning ventilation gap, a vapor barrier is not necessary.

I don’t think that is correct. The dew point is within the insulation wool, which gets wet and does not dry out again.

Regarding the ventilation gap: 2 cm (1 inch) is quite tight. It is doubtful whether a continuous effective cross-section is actually available.

Knallkörper schrieb:
I don’t think that’s correct. The dew point is within the insulation, which gets damp and doesn’t dry out again.

If planned and executed properly, this is a minor issue. Using a vapor retarder can provide a greater drying potential with a ventilated roof, but it’s not always necessary. These types of roofs are less common nowadays. In my family, we have one built in the 1960s and it’s only sheathed with OSB.

Knallkörper schrieb:
Regarding ventilation: 2 cm (1 inch) is very tight. It’s doubtful that a continuous effective cross-section is maintained.

That’s why I suggested having a professional inspect the site in person. If the ventilation isn’t working properly, then the issue definitely needs to be addressed.