ᐅ Moisture on the vapor retarder layer

Hello,

I have a problem that is keeping me awake at night.
The house is a semi-detached from 1976. In spring, I gutted the converted attic and have been working on the rebuild since then. I insulated the sloped roof as follows: roof tiles on battens, 20 mm (0.8 inches) air gap, underlay membrane, 180 mm (7 inches) mineral wool insulation between the rafters, high-performance vapor barrier, 50 mm (2 inches) insulation below the rafters.

I had insulation blown into the 50 mm (2 inches) cavity in the gable wall.

Now to the main problem. Between the last rafter and the gable wall there is about an 80–100 mm (3–4 inches) gap. This is filled with sand-lime bricks laid in a stepped pattern. I stuffed mineral wool in there as best as I could. Unfortunately, I can’t see how thick the insulation ended up being, probably only a few centimeters. However, moisture is now forming in these corners on the vapor barrier. A friend advised me to add about 100–150 mm (4–6 inches) of insulation from the inside directly on the vapor barrier in the corners.
I did this as well. Currently, it’s not very cold outside, yet I have the impression that moisture is still forming under my corner insulation on the vapor barrier.

What else can I do? I would really appreciate any advice.

Hanseatic schrieb:
....Now to the actual problem. Unfortunately, it is not possible to see how thick the whole thing became, probably only a few centimeters. Because now moisture is forming in these corners on the vapor barrier. A friend advised me to simply add about 100 - 150 mm (4 - 6 inches) of insulation from the inside directly onto the vapor barrier in the corners. I did that as well. 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.

The additional insulation inside the vapor barrier may be exposed to condensation moisture due to low surface temperatures (OFT) and thus lose its "actual" and intended insulation effect, especially at low outdoor temperatures. The dew point is within the insulation!
This is a known problem in well-insulated new buildings (fixed kitchen units without rear ventilation). If the kitchen unit is removed after 2 to 3 years, the situation can become noticeably unpleasant. Insufficient rear ventilation prevents the theoretical assumption of surface temperature according to the "mold criterion."

Hanseatic schrieb:
....What else can I do? I would really appreciate some advice.

A few centimeters are a thermal bridge and not sufficient! The additional thermal insulation must be protected by an internal vapor barrier! Any existing vapor barrier in this area should be deactivated first (removed or perforated).

Best regards.

Hello Euro,

thank you very much for the quick response. I had already suspected something along these lines. I am just reluctant to damage the vapor barrier I carefully installed at the connections and then have to start over there. Is it possible to leave the vapor barrier intact and install an additional one over the extra insulation in the corners? As a precaution, in case the new one is not properly sealed.

Best regards

Hello,

A few weeks ago, I implemented the solution suggested to me by Euro. I installed insulation about 160 mm (6.3 inches) thick, extending from the wall to the center of the rafters, which are 75 mm (3 inches) thick. Above that, I applied a new vapor barrier membrane. I cut open the old one.

Over the weeks, I regularly checked for moisture and found the corners on the rafters to be colder than the rest of the surface but still dry.

Now, shortly before installing the drywall, the corners on the rafters have become damp again, even though the outside temperatures have not been particularly low.

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

Regards, Hanseatic

Hanseatic schrieb:
...Well, just before the drywall installation, the corners on the rafters became damp again, even though the outside temperatures were not very low.

This moisture would have penetrated into the insulation and reduced its thermal performance if there had been no vapor barrier! The space is part of the heated building envelope and is now relatively airtight. It needs to be heated and ventilated like the other rooms. Then the problem should disappear. Remote diagnoses based on verbal descriptions are always difficult. A photo or sketch can be very helpful here.

Kind regards,

Hello Euro,

thank you very much for the quick reply. Unfortunately, I’m unable to upload photos to the forum here. Could I send them to you by email?

Best regards