Air tightness of the wooden beam ceiling for the attic conversion.

Hello, we want to convert the attic in our bungalow house. However, I need your advice on this.

The wooden beam ceiling (240mm) is already insulated with rock wool (220mm). Below it is equipped with a vapor barrier to prevent moisture from rising from below, or it is airtight.

Now we want to convert the attic above, but I have two questions.

1) At the exterior masonry, the wooden beam rests on it, but it is not airtight. How do I execute this now since I can no longer reach from above? (see sketch) Is it sufficient if I insulate the rafters of the wooden beam ceiling with the remaining 20mm and install a vapor retarder above?

2) How is the further structure arranged: wooden beam ceiling, vapor retarder, OSB board (I already have), dry screed? Is there then an insulation strip below the OSB board for impact sound insulation? Is this laid floating or screwed?

I would be grateful for tips.

Regards, olli

So, I am not an expert. But in my opinion, a vapor barrier should physically always protect the connection to the coldest spot beforehand to prevent condensation from forming there. A second vapor barrier/vapor retarder on the floor is not sensible, especially since the converted attic will get an access at the latest, which will also render the vapor retarder UNDER the ceiling absurd. Warm air absorbs moisture – then rises again and meets a cool surface: Therefore, according to the rules of the physical properties of heat and moisture, the foil belongs under the roof (if not already present). Much more interesting than the question of the floor construction is therefore: Is the roof well prepared for the conversion?

Hello, thank you for the response. The roof will still be insulated and then fitted with a vapor barrier, I know how that is done. However, it is not airtight in the area I have marked on the drawing. Here I have no idea how this area must be properly treated.