Moisture on the vapor barrier

Hello,

I have a problem that is keeping me awake at night.
The house is a semi-detached house from 1976. In spring, I gutted the converted attic and have been working on the rebuild ever since. I insulated the slopes as follows: roof tiles on battens, 20 mm air layer, underlay membrane, 180 mm mineral insulation between the rafters, high-performance vapor barrier, 50 mm insulation below the rafters.

I had insulating material blown into the 50 mm cavity of the gable wall.

Now to the actual problem. There is a space of about 80 - 100 mm between the last rafter and the gable wall. This space is filled with sand-lime brick. It looks somewhat like steps. There I stuffed in the mineral wool as well as I could. Unfortunately, the thickness cannot be seen, probably only a few centimeters. Because now moisture is forming on the vapor barrier in these corners. A friend advised me to simply add insulation of about 100 - 150 mm from the inside on top of the vapor barrier in the corners.
I have done that. At the moment it is not so cold outside anymore, but I still have the impression that moisture is forming again on the vapor barrier underneath my corner insulation.

What else can I do? I would be very happy to receive some advice.

....Now to the actual problem. Unfortunately, you cannot see how thick the whole thing became, probably only a few cm. Because now moisture is forming in these corners on the vapor barrier. A friend advised me to simply insulate the corners from the inside with ~100 - 150 mm insulation directly onto the vapor barrier. I did that as well. Currently, it is not so cold outside anymore, but I still have the impression that moisture is forming again on the vapor barrier under my corner insulation.

The additional insulation inside the vapor barrier may be subjected to condensation moisture due to low surface temperatures and thus lose its "actual" and intended insulating effect, especially at low outdoor temperatures. The dew point lies within the insulation! This is not an unknown problem in well-insulated new buildings (fixed kitchen units without ventilation behind them). After removing the kitchen unit after 2...3 years, it sometimes becomes noticeably unpleasant. The insufficient ventilation prevents the theoretical assumption of surface temperature according to the "mold criterion".

....What else can I do? I would be very happy about advice.

The few centimeters are a thermal bridge and not sufficient! The additional thermal insulation must be protected by a vapor barrier on the inside! Existing vapor barrier in this area should be deactivated beforehand (removed, perforated).

Best regards.

Hello Euro,

thank you very much for the quick response. I had already suspected something along those lines. I am just reluctant to damage my painstakingly installed vapor barrier at the connections and have to start all over again there. Can't I leave the vapor barrier intact and lay another one over the additional insulation in the corners? Just to be safe, in case the new one is not properly sealed.

Best regards

Hello,

I had already implemented the solution that Euro suggested to me weeks ago. I installed insulation about 160 mm thick, extending from the wall to the middle of the rafter, which is 75 mm thick. Above that came a new vapor barrier. I cut open the old one. For weeks, I kept checking the moisture and felt that the corners on the rafters were colder than the rest of the surface but dry. Now, just before installing the drywall, the corners on the rafters have become damp again, even though the outdoor temperatures are not very low.

What else can I do? Or is this moisture normal?

Regards Hanseatic

...Well, shortly before the drywall installation, the corners on the rafters became damp again, even though the outside temperatures are not very low at all.

This moisture would have penetrated into the insulation without a vapor barrier and would have reduced its insulating effect! The room is part of the heated envelope and is now relatively airtight. It must be heated and ventilated just like the other rooms. Then the problem should disappear. Remote diagnoses based on verbal descriptions are always difficult. A picture or sketch can be very helpful here.

Regards

Hello Euro,

thank you very much for the quick response. Unfortunately, I am unable to upload photos here in the forum. Could I send you some by email?

Regards