KFW55 Roof Insulation - Is Attic Ventilation Necessary?

Hello,

I am currently having a KFW55 house built and would like to convert the attic afterwards. In this process, I need to insulate the roof. The rafter width is 24cm. In theory, I should use insulation with WLG035 and 24cm thickness. However, many have told me that I should leave 2-4 cm of space between the insulation and the diffusion membrane. Unfortunately, I haven't really found anything about this on the internet. Only old posts. Should/must space be left or is it not necessary?

Regards
Skloter

Yes, a little ventilation is sensible. Then simply build counter battens, perpendicular to the rafters, a 4cm installation layer which you insulate. Then you reach your 24cm and reduce the thermal bridges.

What kind of underlay membrane is this? If it is diffusion-open, no ventilation gap needs to be left. We have fully utilized these exact 24cm and left no air behind. The underlay membrane is a DELTA Max Plus from Dörken. SD value 0.15m, so completely open.