Vapor barrier foil or nothing?

Hello everyone, I live in an old building from 1958, the roof is not properly insulated, only with tar paper and black chipboard panels. The attic is used as a storage room, is not heated, and cannot be used as living space. The attic floor consists of crosswise beams filled with slag concrete, which was applied on reed mats. Since the screed was crumbly and cracked, 2.5 cm high battens were screwed onto the existing beams, these were filled with Isover perlite and then covered with OSB panels. A foam element is to be laid on the OSB panel, which is then to be provided with another OSB panel as a walking protection. Can this be done like this or should I have installed a vapor barrier under the filling? Since the OSB panels are already laid and screwed... can I install a foil/vapor barrier on the OSB panel to prevent moisture and mold? Many thanks for your feedback, best regards Alex

Hello,

.. Can you do it this way or would I have had to install a vapor barrier under the insulation?

You can. Omit foam if it’s not generously vapor permeable! With a vapor retarder, it would have had to be installed on the room side in the rooms below.

.. Since the OSB panels are already laid and screwed down....can I apply a foil/vapor barrier on the OSB panel to prevent moisture and mold?

Absolutely not! It must be vapor permeable towards the attic. So do not apply any blocking coverings (e.g. linoleum or similar)!

Best regards

Thank you for your feedback. Below the storage room there is a bathroom with a bathtub that we use regularly. Since the steam rises upwards, penetrates the ceiling to cool down in the intermediate area (slag)... is it advisable to insulate the bathroom ceiling now with foil or is a waterproof paint sufficient? An answer would be nice, best regards Alex60

.... Since the steam rises upwards, penetrates the ceiling to cool down in the intermediate layer (slag)...

Correct, this causes condensation in this layer.

....So it is advisable now to insulate the bathroom ceiling with foil or is a waterproof paint sufficient?

Vapor retarders (foils) have no insulating effect, they act as barriers. The ceiling is only one component of the bathroom, i.e., if it is sealed, the moisture increases in the remaining components because a "receiver" is eliminated. Therefore, at least the AW should be included. However, the better way is always to equip the entire bathroom with a vapor retarder. In any case, avoid standing moisture. Ventilate immediately after use.
"Paint" presumably means "liquid seal" or "liquid foil," which is used as a barrier in wet rooms. Definitely an alternative.
However, it is not without problems either, because without a compensating top layer, e.g., cement plaster, you get a "stalactite cave."

regards

Hello and thank you very much for your response dear regards Alex60