OSB boards (oriented strand board) are coarse chipboards and are made from long wood chips, which are pressed into boards with adhesives. There are different quality classes (1-4) for different areas of application. OSB/3 and OSB/4 are particularly suitable as vapour barrier levels, as they have the necessary static properties and thus serve not only as a vapour barrier and airtight level, but also as a stiffening of the roof truss. The panels are glued airtight to the joints with tongue and groove or by means of suitable adhesive tapes. When expanding the roof space, the drywall panels can be attached directly to the OSB boards or sanded boards can be used to remain visible as inner roof planking. Compared to foil-like vapour barrier layers, damage to the airtight layer is very unlikely with OSB boards.
Source: bauwion
