ᐅ Vapor barrier film or nothing at all?

Hello everyone, I live in an older building from 1958. The roof is not properly insulated, only covered with tar paper and black particle boards. The attic is used as a storage space, is not heated, and cannot be used as living space. The attic floor consists of beams laid crosswise, filled with slag concrete that was applied onto reed mats. Since the screed was brittle and cracked, 2.5 cm (1 inch) high battens were screwed onto the existing beams, then filled with Isover Perlite, and finally covered with OSB panels. On top of the OSB panel, a foam element is planned to be laid, which will then be covered again with an OSB panel as a walking surface protection. Is this method acceptable, or should I have installed a vapor barrier under the filling? Since the OSB panels are already laid and screwed down, can I add a vapor barrier or foil on top of the OSB panel to prevent moisture and mold? Thank you very much for your feedback, best regards, Alex

Hello,

Alex60 schrieb:
.. Is this acceptable, or should I have installed a vapor barrier under the screed?

It is acceptable. You can skip the foam if it is not highly vapor-permeable! If using a vapor retarder, it would have to be installed on the room side, in the rooms below.

Alex60 schrieb:
.. Since the OSB boards are already laid and screwed down... can I install a foil/vapor barrier on top of the OSB boards to prevent moisture and mold?

Absolutely not! It must be vapor-permeable toward the attic/storage space. So, do not apply any vapor-retardant coverings (e.g., linoleum or similar) either!

Best regards.

Thank you for your feedback. Below the attic, there is a bathroom with a bathtub that we use regularly. Since the steam rises upwards, passes through the ceiling, and cools down in the intermediate layer (slag)... is it advisable to insulate the bathroom ceiling with a vapor barrier film now, or is a waterproof paint sufficient? I would appreciate an answer. Best regards, Alex60

Alex60 schrieb:
.... Since vapor rises upward, passes through the ceiling, and cools down in the intermediate layer (slag)...

Correct, this causes condensation in that layer.

Alex60 schrieb:
.... So is it advisable to insulate the bathroom ceiling now with a vapor barrier sheet, or is a waterproof paint sufficient?

Vapor barrier membranes (sheets) do not provide insulation; they act as a vapor retarder. The ceiling is only one component of the bathroom structure, so if it is sealed off, moisture will increase in the other remaining components since one “moisture receiver” is eliminated. Therefore, at least the external wall (EW) should be included. However, the better approach is always to apply a vapor retarder to the entire bathroom. In any case, standing moisture must be avoided. Ventilate immediately after use.
By “paint,” a liquid sealant or liquid membrane used in wet rooms is probably meant. This is indeed an alternative. However, it is not without issues, because without an equalizing top layer, for example cement plaster, you can end up with a “stalactite cave” effect.

Best regards.

Hello and thank you very much for your response. Best regards, Alex60