Thank you very much for your message and your questions.
I think we are now aware of our mistake and rather have the question of how we can fix the current situation and with what effort.
Let me explain our current situation:
Key data:
We have a mid-terrace house, built in 1975, 4 floors (ground floor, 1st floor, 2nd floor and the attic floor).
The roof is a gable roof with collar beams.
The attic floor is a real floor (so not just an attic) with a concrete ceiling to the 2nd floor and has a door as access and is designed as a warm roof.
One side of the roof faces north, the other south.
On the south side there is a window and photovoltaic panels on the entire roof area.
On the north side there are two windows.
Current roof structure:
- Frankfurter Pfanne roof tiles
- USB Isover Integra ZUB Light (sd value approx. 0.03 m)
- Rafters doubled up to 160 mm depth
- Insulation between rafters Isover Integra ZKF 1-032 160 mm (lying directly against the USB without ventilation/distance to the USB)
- Vapor barrier foil climate membrane Isover Vario KM Duplex UV (sd value 0.3 - 5 m)
The individual steps in detail:
1. As part of a full renovation, we first tackled the roof from the inside.
Everything removed once (KMF aluminum foil coated incl. the old diffusion-tight underlay, which on the south side had already decomposed into its components).
2. Then in August 2023 we first installed the USB from the inside per rafter field, as Isover also describes it in some videos and forums.
First, a batten was placed in the middle of the rafter field so that the USB stretched over it could form a channel in the middle to possibly lead away water without directing it onto the rafters.
Antispike was applied to the rafters and the USB was pressed firmly against the rafters with battens using Antispike and screwed.
My husband put a lot of effort into installing the USB, did a lot of research, and we also had a roofer as an advisor.
Knowing well that it does not comply with the standard. A roof renovation including a new USB from the outside was simply not in the budget.
3. In June 2024 we installed the insulation (glass wool) in the rafter fields.
4. In November 2024 we installed the vapor barrier and carefully sealed everything as accurately as possible, including all pipe penetrations, wall and floor connections, beams. We did not only use the tapes from Isover, but also DoubleFit to make everything airtight. We covered everything towards the stairwell with foil and equipped the door with a plastic dust protection door - so not 100% airtight.
However, at the roof windows we made a vertical slit in the vapor barrier each, to be able to open the windows and ventilate.
That will have been the big mistake... We were not aware that the vapor barrier must already be really finally airtight at this point and that moisture is practically sucked upwards by various physical processes. But first let's continue:
4. End of November 2024 the first layer of interior plaster was applied (the raw walls etc. were plastered thickly).
5. End of December 2024 the screed was installed on the 1st and 2nd floors (heated screed / cement screed with additives = fast screed; approx. 7-8 cm build-up height; mostly with underfloor heating pipes in between with approx. 4.5 cm pipe coverage).
6. On 06.01.2025 our heating engineer started the heating protocol and the underfloor heating on the 1st and 2nd floors was switched on, as well as one radiator in the attic floor. The ground floor is currently not heated due to the lack of a radiator.
7. On 12.01.2025 I accidentally noticed next to the window on the USB in the attic during ventilation a spot where really thick and many water drops had formed. At that time, temperatures had been around freezing for several days.
Then I looked further and found that really everywhere on the USB there were thick water droplets. I made a slit in the middle of the roof surface, pushed aside the insulation wool and looked inside, also there.
Also on the south side in one spot about 5 m away from the window, the same picture, although somewhat less thick drops.
Currently, we have NO mold or other stains or similar, but we have no idea how quickly that can change.
The insulation wool is, as far as we can see in the currently open spots, mostly only superficially wet. Water beads off the smooth upper side of the wool, but of course can also penetrate into the wool at the joints.
The cause is now clear to us; we should really have connected the vapor barrier completely airtight.
That the wool should only be brought into the house after the construction moisture was, despite a lot of research, unfortunately not clear to us.
There may now be several "problem zones":
- Moisture penetrates into wood components/other absorbent materials (at a few places one can see damp spots on the rafter/battens - but very few)
- Moisture slowly runs down to the foot purlin and accumulates there / possibly seeps into the lower floor through the concrete (but at two test spots we have not seen this so far, everything was dry)
- Moisture accumulates between USB and wool and possibly forms mold/spores (here too no visible mold/spores so far)
The question is, what can and must we do now?
Is it sufficient to free the window openings from the vapor barrier enough so that we can really ventilate properly crosswise (so far only slits and the air flows rather through the insulation layer than into the room)?
We ventilate 3-4 times a day (currently about -2°C here and very dry weather (approx. 45% humidity).
Is it good or bad that we heat upstairs in the attic floor (about 18°C, in the rest of the house currently 23°C) or is heating upstairs counterproductive?
Would it be helpful or even sensible to set up one or more drying units?
Or do we have to remove the vapor barrier and all the wool, possibly free the USB from droplets with towels or wet vacuum cleaners or similar?
Then we also ask ourselves how presumably all the wooden elements (rafters, battens etc.) gradually soak up water as well. How long and in what way would one have to dry or even wait for the summer until the moisture is also out there?
When would be the appropriate time to reinstall the insulation and vapor barrier?
Still in summer / autumn or earlier?
Would we have to carry out certain measurements (somehow measure the moisture in the components/rafters)?
Of course, we would prefer to leave everything inside and somehow get the moisture out technically with construction drying / ventilation etc., but we do not know whether that is physically possible even from the last corner if the wool lies on the USB.
If necessary, we have to take everything out.
I am attaching a few pictures so that one can better imagine the attic floor and the details.
Many thanks already and winter greetings!
Mira