Laminate on OSB boards over concrete

Good day everyone,

I am in the process of laying a floor covering in the attic (above is still a loft with insulated pitched roof, below is an inhabited floor with bathroom and several bedrooms). For this, I first laid 24 mm high wooden slats on the concrete floor and screwed them down. OSB boards are now to be placed on these slats. Laminate is to go on top. The house is 18 years old.

Now I have been made aware that this is not a good idea because the OSB boards act as a vapor barrier and moisture could accumulate under the OSB boards over time. The keyword here was ventilation. The advice came from a trained carpenter.

Do I have to lay a vapor retarder, if yes directly on the concrete floor? Do I have to seal anywhere else?

If I understand correctly, moisture would settle if there are greater temperature differences. What would a correct construction look like in this case?

Thanks in advance!

Hello questioner. According to your hints, the wood-based panels (OSB panels) rest on battens. Just make sure there are sufficient edge gaps so that the structure is ventilated and no "Staunässe" (the term is very exaggerated in connection with the described construction method) can form. OSB panels actually form a vapor retarder, but not a vapor barrier. However, since no excess moisture from the manufacture will certainly escape from the old concrete ceiling (which is very unlikely!), you do not need a foil layer between the concrete ceiling and the wooden supports either. Sure, it can’t hurt, but it is not really sensible either. The ventilation of the battens and thus the wood-based panel layer is important, nothing else! Regards: KlaRa

Hello questioner. According to your notes, the wood-based panels (OSB boards) rest on battens. Just make sure there are sufficient edge gaps so that the structure is ventilated from behind and no "trapped moisture" (the term is somewhat exaggerated in connection with the described construction method) can form. OSB boards do indeed form a vapor retarder, but not a vapor barrier. However, since no excess moisture from the old concrete ceiling manufacturing will escape (this is very unlikely!), you do not need a foil layer between the concrete ceiling and the battens. Sure, it can’t hurt, but it also isn’t really necessary. The ventilation of the battens and thus the wood-based panel layer is important, nothing else! Regards, KlaRa

Thank you very much for your assessment. I have taken edge gaps into account.