ᐅ Is a vapor retarder necessary for the ceiling of the top floor?

Hello,

We have a continuous concrete slab above the upper floor, 20 cm (8 inches) thick. Now the attic space (cold loft) is to be insulated, meaning 20 cm (8 inches) EPS boards with a thermal conductivity of 0.035 will be installed on top. The insulation will, of course, be placed directly on the concrete slab. OSB boards will be laid above the insulation.

Is a vapor barrier or vapor retarder foil necessary?

Thanks and best regards

Christian

Directly place the OSB boards onto the Styrofoam?

Any type of insulation should be protected against moisture. This moisture also comes from the interior, from people. Therefore, a vapor barrier or membrane is needed where you have to protect the insulation from your breath.

If insulation is installed in the cold attic space along the sloped roof areas (that is, on the sides and upward), a vapor barrier should be placed between the room and the insulation. Plasterboard panels can then be installed between the vapor barrier and the room.

Sealing toward the underside is achieved using OSB panels with an airtight connection to the vapor barrier.

In another discussion, some opinions suggested that this approach might even be wrong because it does not provide proper protection. They also argue that if you cannot achieve a truly 100% airtight seal (meaning no holes even as small as 1 mm (0.04 inches)), it might be better to omit it entirely. In reality, however, this is very difficult to accomplish and to verify, especially when any cables or pipes pass through the insulation layer.

The architect, construction company, and building material supplier had a different opinion, stating that when OSB panels are used as flooring, there is no need for an additional vapor barrier below.

Kaspatoo schrieb:
Any type of insulation should be protected against moisture. This moisture also comes from the inside, from people. Therefore, you need a vapor barrier or membrane where you want to protect your insulation from your breath.

So, if insulation is installed on the sloped ceilings in an unheated space (i.e., on the sides and upwards), a vapor barrier should be installed between the room and the insulation. Between the vapor barrier and the room, you can then install drywall panels.

A seal at the bottom is achieved by OSB panels with an airtight connection to the vapor barrier.

In another discussion, some opinions suggested that this might even be wrong because it does not provide proper protection. They also said that if you cannot make it truly 100% airtight (i.e., no holes even as small as 1mm), it might be better not to use it at all. However, in reality, achieving this is very difficult, especially when any pipes or cables pass through the insulation layer.

The architect, construction company, and building materials supplier had a different opinion and said that when using OSB panels as a subfloor, a vapor barrier below is no longer necessary.

In my case, the insulation is directly on the concrete slab.... The sloped ceilings are not insulated. Is a vapor retarder or vapor barrier (or either) necessary here?

So the insulation is primarily meant to insulate the room below? Then you don’t need to insulate the sloped roof areas.

Additionally, the insulation should be protected from moisture coming from below. Depending on its thickness, concrete is more or less vapor-tight. However, since you don’t have standing water like you would with a basement wall or floor slab, I would assume that the concrete ceiling is sufficient and no additional vapor barrier is needed.

You can easily verify this by checking the surface of the concrete ceiling.