ᐅ Upper floor ceiling – additional insulation? What makes sense?

Hello everyone,

I bought a house built in 1991 and have been living in it for several months now.

The house has one and a half floors, and the ceiling of the upper floor (about 40 sqm (430 sq ft)) leading to the attic is currently only partially covered (right half of the house) with chipboards (approximately 24 mm (1 inch) thick). Between the rafters, there is Isover Rollisol-SB (WL-B1/040) insulation with a thickness of 120 mm (5 inches). Below this insulation, there is an aluminum vapor barrier (or vapor retarder?). This is not properly stapled to the rafters everywhere, and in those areas, there are black spots (mold?) on the chipboards.
I noticed this when I removed one of the boards to install insulation on the rest of the floor.
The goal is to make the entire attic walkable to store items like Christmas decorations, children's clothing, suitcases, etc.

My plan is to remove the insulation, install new foil (Isover Vario Duplex) from above to cover the rafters, reinstall the insulation without the aluminum "foil," and then put OSB boards or chipboards back on top.

Now I am wondering if I should double the thickness of the rafters from above and add extra insulation (for example, 120 mm (5 inches) Isover Integra ZKF 1-035) on top.

Does this make sense—adding the additional insulation—and is it worth it?
Is it even necessary to install a new vapor retarder everywhere?
The costs for everything are reasonable (several hundred euros), but the work itself is quite daunting :-(

I appreciate any advice, ideas, and recommendations.

Best regards,
Patrick

Patrick, for walk-in attics, it’s better not to use OSB boards for the flooring, but rather solid timber boards. This creates a much more breathable surface, not so sealed, which helps prevent mold. Just nail board to board. Slight gaps between the boards are intentional. Karsten

That may be so, but with the properly installed new vapor retarder, no moisture should get through – right?

A brake slows down moisture but does not stop it completely.
And what if it doesn’t pass through? Where should it go then? Should it stay in the insulation and cause mold?

Do not use a vapor retarder; instead, install a vapor barrier from below and a vapor-permeable membrane above the insulation that allows moisture to escape upwards through the OSB boards, in case moisture does get into the insulation.

saar2and schrieb:
venting upwards through the OSB panels

However, OSB panels are not vapor permeable, hence the comment from @Nordlys