ᐅ KfW55 Roof Insulation – Is Ventilation in the Attic Space Necessary?

Hello,

I am currently having a KFW55 house built and plan to convert the attic afterwards. In this process, I need to insulate the roof. The rafter width is 24cm (9.5 inches). In theory, I would need insulation with a thermal conductivity of WLG035 and a thickness of 24cm (9.5 inches).

However, many people have told me that I should leave a 2 to 4 cm (0.8 to 1.6 inches) gap between the insulation and the vapor barrier.

Unfortunately, I couldn’t find much information about this online—only old forum posts.

Is it necessary or recommended to leave such a gap, or is it not required?

Regards,
Skloter

Yes, a bit of ventilation is sensible.
Simply install counter battens across the rafters, creating a 4cm (1.5 inches) service cavity that you insulate. This way, you reach your 24cm (9.5 inches) thickness and reduce thermal bridges.

What kind of underlay membrane is this?
If it is vapor permeable, there is no need to leave ventilation space behind it.

We have fully utilized these 24cm (9.5 inches) without leaving any air gap.
The underlay membrane is a DELTA Max Plus from Dörken. It has an Sd value of 0.15m (0.5 feet), meaning it is completely vapor open.