ᐅ OSB panels in the pitched roof

We want to insulate the entire roof with 20 cm (8 inches) of mineral wool.

We plan to build the attic floor using OSB boards.

While reading and researching online, I have come across several points that made me think more carefully...

Is there a risk of mold forming beneath the OSB boards?

My understanding is that if the insulation between the rafters is properly sealed with a vapor retarder, the attic floor project should not be a problem.

We are also considering insulating the ceiling (the upper floor ceiling under the roof).

Does that make sense?

Is there a heating and ventilation system available there?

No, neither.

Marchonisch schrieb:
My understanding is that if the insulation between the rafters is properly sealed with a vapor retarder, my plan for the attic floor should not be a problem.

Hello "Marchonisch".
Exactly as you wrote it!
The vapor retarder must be installed on the warm side, which is the underside of the ceiling facing the living space.
Otherwise, there is no reasonable explanation why wood-based panels should not be installed in a dry room (meaning: no rain ingress and the gas condensing boiler does not cause condensation on the wood-based panel surface).

Thanks for your feedback.

However, we plan to insulate the rafters under the roof tiles, but not to convert the attic into living space.

Is that a problem?

Yes, because this way you heat the attic through the radiator on the upper floor. As a result, you will probably need a higher flow temperature, which affects efficiency. So, either install a radiator upstairs as well, or insulate the ceiling of the top floor.