ᐅ Insulation of the roof with insulation wool installed between the rafters

Hello!
We are currently in the process of insulating our roof.
Yesterday, we partially removed the old insulation from the inside to get a better understanding of the structure.
The house is about 50 years old. The roof assembly consists of:
1. Roof tiles
2. Battens
3. Overlapping horizontal bitumen membrane
4. Currently, insulation wool directly attached to the bitumen membrane, without any ventilation gap
5. A vapor barrier foil that was not airtight

While removing the insulation wool, I noticed that the bitumen membranes are damp on the inside (this is called "not vapor-permeable" in technical terms, I believe). The insulation wool is also somewhat damp on the outer side (which is understandable). However, the rafters are still dry.

My question is: in a construction like this, do we need to install a ventilation gap, meaning a space between the bitumen membrane and the insulation wool?

I am planning to use 14 cm (5.5 inches) thick insulation with a thermal resistance of about 0.032 W/m·K. The rafters are also approximately 14 cm (5.5 inches) deep. On top of this, I was thinking of adding battens of about 3 cm (1.2 inches) in thickness to create a ventilation space.

Would this be a suitable approach?

Of course, an airtight vapor barrier foil will be installed on the inside, hopefully more effective than the current one.

Thank you for your advice.

Best regards,
Robert

Basically correctly understood: the bitumen membrane acts as an absolute vapor barrier, preventing indoor humidity from escaping outward through the construction—apart from accidental leaks in the loosely overlapped areas. In principle, it should be removed because it is located on the wrong side of the roof—on the outside. Building assemblies and materials need to become progressively more vapor-permeable from the inside to the outside (decreasing sd-values). You can remove the bitumen membrane by cutting it out, but then you lose the secondary waterproofing layer, such as an underlay or roofing membrane, and you lose the ventilation layer beneath the roofing tiles in the form of counter battens.

If you keep the bitumen membrane, you must install a ventilation layer underneath it. This is possible as a workaround, but you must ensure that properly sized ventilation openings to the outside air can be installed at the eaves and ridge. Assessing this and developing a flawless renovation concept can only be done by a specialist on site.

For all insulation measures, you must comply with the requirements of the energy-saving regulations. With 14cm (5.5 inches) minus 3cm (1.2 inches) = 11cm (4.3 inches) insulation thickness, you will not meet the standards and will need to add additional insulation below the rafters.

Just out of curiosity, since I have something similar coming up.

Who is the right professional for this? Various companies offer this kind of service.

-Roofers
-Drywall contractors / interior builders
-General construction service companies
-Renovation companies
etc...

An independent planner: architect/structural engineer or a specialist in carpentry/roofing crafts. The planner should not be the contractor and should also oversee the execution to ensure accuracy and quality. Craftsmen provide "free" quotes but are not planners and end up inspecting their own work: this usually does not work.

@wpic,
thanks for the information.
The moisture on the inside of the bitumen membrane is moisture from indoors, right? Not from the outside.
Regards,
Robert

Basically, it is usually from the interior side, especially if the attic is used as living space or if, due to significant leaks, indoor humidity from the upper floor escapes into the attic. However, it can also be localized rain or wind-driven snow, often occurring with damaged bitumen membrane.