Vapor barrier and new ceiling on top of the old one?

[Dea_34]

We have a house from 1960 with a mono-pitched roof, without an attic or other space.

Upon switching from roof tiles to double-lock standing seam metal, it was discovered that the roof decking was completely ruined, due to the lack of a vapor barrier in the inner ceiling (warm and moist air had risen through the inner ceiling, through the insulation, and condensed when it hit the cold roof decking, damaging it from the inside). In connection with this, we added extra insulation to the roof (now 250mm), fixed an extra high ventilation gap, and installed new roof decking. To prevent the same issue from occurring again, we are now supplementing with an internal vapor barrier.

The current inner ceiling is made of fine-sawn paneling, which is in good condition. The inspector (whom we called when we found the damaged roof decking) and the company that did the outer roof considered that we could place the vapor barrier on the existing paneling, use thin battens (17 mm), and install new paneling (the cost saving of not tearing down the paneling is significant, and some parts cannot be removed due to the construction).

The reasoning was that the current inner ceiling paneling will be outside the vapor barrier, but since it will always remain warm, no moisture will condense on it, thus no problems will arise.

By chance, I discussed this with someone at work, who believed that the current inner ceiling paneling should be removed to prevent reverse moisture migration in the summer from causing moisture issues, as the moisture would be trapped at the vapor barrier.

I would appreciate some opinions on what we should do since the workers who will handle the areas with the highest ceiling height (4.2m) will start in 1.5 weeks..... and I have also taken the opportunity to run plenty of cables in the ceiling for Ethernet and TV distribution in the house.

/Dea

 

[Lago]

As I see it, there will be no problem with having the moisture barrier on the existing panel, it's important that it remains undamaged and completely intact; it is only an advantage to have it as close to the warm side as possible. Reverse moisture migration can occur in houses that are not used year-round, where it is better to install a vapor retarder instead.