Insulation of the top floor ceiling, wooden beam ceiling

Good morning everyone,

my wife and I have bought an old house built in 1935. Among other things, the top floor ceiling, which is a wooden beam ceiling, is to be insulated. Old wooden floorboards are laid out in the attic. The attic is only to be used as storage space and possibly to temporarily set up a Carrera / model railway, but no permanent use and therefore nothing heated.
Below it there is a bathroom and a bedroom.
A few days ago, we had craftsmen here who said that for insulation it would be sufficient to lay a vapor barrier on the floorboards and mineral wool on top. Won't I have a mold problem below the vapor barrier (wooden floorboards and beams)?
Unfortunately, I cannot say more about the structure of the beam ceiling at the moment, as we still need to open it up.

Best regards
Heiko

Hi Heiko,

with proper execution (properly installed vapor barrier, sealed at all rising parts,....) there should be no problems.
The dew point will then be somewhere in the insulation layer, where condensation might occur. But since no water gets there due to the vapor barrier, you won't have any problems.
If you are interested in the topic, play around a bit with the U-value calculator (Google), I think it helps to understand the concept well.

Regards

Thank you for your quick response. But that also means that I do not have to use mineral wool, but can take PUR boards over the vapor barrier and accordingly need less height. On the exterior walls, we want to use something diffusion-open, since the old building should manage without penetrations for a ventilation system (exceptions are the bathroom and the exhaust hood in the kitchen).