Attic insulation - What thickness, vapor barrier foil?

Hello dear forum, since our plans have changed and we have purchased an existing property, we are now thinking about the right choice of insulation for our attic. The attic will not be developed, so the ceiling between the floors is to be insulated. Since a suitcase or similar items are still to be placed there occasionally, the insulation should remain walkable. What have you chosen, what thickness, which vapor barrier foil, etc. Thank you for your tips. Best regards Bauer2018

What is the current structure like?

If something reasonably vapor-retardant has already been installed, you can simply lay wood fiberboards with a compressed surface on top.

Otherwise, remove the old insulation and then install new insulation with a vapor barrier underneath.
Work with soft material in the corners and slopes and insulate the chimney wall at least 60cm high.
Make sure to seal and insulate the weak point at the stairs.

It is a concrete ceiling without any additives, i.e. still completely bare. The insulation only begins now with our measures.

You don’t need a vapor barrier with concrete. I simply used a mix of EPS boards for the walkable area and wool for the areas where you can’t reach anyway. Lay the boards in two layers, offset and crosswise. Then put OSB boards on top and done. It is much cheaper than ready-made products with a walkable surface and I think the result is better too.

You don't need a vapor barrier with concrete. I simply used a mix of EPS boards for the walkable area and wool for the area you don't reach anyway. Lay the boards in two layers, staggered crosswise. Then put OSB boards on top and done. It's significantly cheaper than ready-made products with a walkable surface and I think the result is better too.

That's exactly how I want it too. Only my site manager wants to use screed. (Instead of OSB boards like you)

Exactly, with a concrete ceiling you can do without the vapor barrier foil. However, from my experience with loosely laid EPS panels, they can shift over time with frequent foot traffic and thus create thermal bridges, especially if flexible material is used around the edges. Additionally, the floor must be even without waves or cables because EPS panels do not adapt at all. Otherwise, an air layer forms in hollow spots, which makes the entire insulation pointless. Other walkable insulation panels (wood fiber, mineral wool) are significantly more tolerant in this regard. EPS is of course the cheapest solution.

I would recommend wood fiber panels with a compressed surface in the walkable area. These can be walked on directly without another panel and used as storage surfaces. In the corners, use flexible wood fiber.

Alternatively, impact-resistant mineral wool panels with chipboard on top and flexible mineral wool in the corners.

As a top layer, I would recommend chipboard rather than OSB, as it is more vapor-permeable, so if room moisture enters the attic or insulation through leaks (stairs, drill holes for cables, ventilation), it can evaporate better. You don’t need dry screed from Fermacell or similar, since you won't be living up there. 20mm chipboard is sufficient, and with wood fiber panels you don't need any at all.

Always work in two layers with insulation material; with flexible material in corners, a single layer is also fine.

If there really is no insulation at all (are you sure there isn’t already screed plus a few centimeters of mineral wool?), I would install at least 200mm of WLG035, preferably 240mm. In unwalked areas with flexible material, don’t skimp and rather use 300mm WLG035, it’s very cheap. Wood fiber usually has WLG040, so add about 10% more here.

With concrete ceilings, insulating the hatch of the attic stairs is pointless because the surrounding concrete slab then acts as a prime thermal bridge. The staircase must be sealed airtight, and on top of this in the insulation plane, a further insulated hatch/insulation hood must be installed. For some stairs, this even has to be placed on the insulation plane. These are available ready-made or can be self-built from chipboard plus insulation.