Is a vapor barrier necessary under insulation on a floor slab?

Hello everyone,

Initial situation: Between the upper living floor and the attic, there is a wooden beam ceiling. From below, we have "opened" it, meaning we removed the old plaster and tore out the lattice battens, so the bare beams are visible.

Upon inquiry with the roofer and energy advisor, it was said that insulation of the ceiling is not necessarily required, but if it is already opened anyway, it makes sense.

So today, I installed 100mm mineral wool between the rafters. So far, so good.

Currently, the roof is still a cold roof, but it will be renovated according to KfW standards with 160mm exterior roof insulation WLG 023. The attic remains an unheated attic. The floor here is covered with floorboards, so it is not vapor-tight downwards towards the living floor.

The plan was a vapor barrier from below beneath the installed insulation. Upon further consideration, however, I realized today that this will be quite an effort. The vapor barrier must be airtight connected to the walls, which is not so easy in an old building, for example, I would have to carefully plaster around the top of the walls to create a smooth wall surface for the bead, etc. It's not that I shy away from the work, but when it gets that complicated, one thinks twice about whether the step is really necessary.

So I wonder whether a vapor barrier is necessary now under our insulated ceiling towards the attic at all. I don't believe it will get insanely cold with the exterior roof insulation in the attic, so the temperature differences won't be that huge. Can anyone here assess that? I am happy to provide further information :)

Someone with a very brief assessment of this?

Structure summarized in detail from top to bottom:

New roof with 160 mm external roof insulation
Unheated attic

Attic floor wooden planks nailed
Filling/clay screed between wooden beams
Wooden beams above (towards the attic)
Wooden boards
Wooden beams below (towards the living space)
Mineral wool between the wooden beams from below
(Vapor barrier???)
Battens / installation level
Drywall

A thousand thanks in advance! :)

From the basic construction, you naturally have a temperature difference between the upper floor and the attic and insulate on the warmer side. Whether the difference is large enough for you to need a vapor barrier is probably best advised by the energy consultant.

It is also a bit questionable to insulate the roof (i.e., actually inevitably also to ensure airtightness in the attic, otherwise there is no point in insulating there) and then still not heat it. So if you do not want to heat the attic, it is actually more sensible to insulate only the floor of the attic. But if it is planned that way, you have to deal with it. In my layman's opinion, a kind of moisture- and temperature-controlled minimal heating would be conceivable. So basically not a frost guard but a dew guard to be installed. And/or to install ventilation with heat recovery in the roof attic. Otherwise yes, vapor barrier/vapor retarder on the warmer side of the insulation. Actually, the energy consultant (who, I assume, also planned the roof insulation) should be able to calculate for you whether you need an additional vapor retarder, i.e., whether condensation can occur due to the temperature differences between the upper floor and the attic, and if so, from which outside temperature.

Thank you for your response. I have also contacted the roofer and energy consultant again, let's see what they think about it. However, it may take a while before I get an answer and I am hoping for qualified responses here :)

It is a bit questionable to insulate the roof (i.e. basically inevitably ensuring airtightness in the attic, otherwise there is no point in insulating there) and then still not heat it. So if you don’t want to heat the attic, it actually makes more sense to only insulate the attic floor. But if that’s how it’s planned, you have to deal with it. In my layman’s opinion, a kind of moisture- and temperature-controlled minimal heating would be conceivable. So basically not a frost guard, but a dew guard to be installed. And/or install ventilation with heat recovery in the roof attic. Otherwise yes, vapor barrier / vapor retarder on the warm side of the insulation. Actually, the energy consultant (who, I assume, also planned the roof insulation) should be able to calculate whether you need an additional vapor retarder, i.e. whether condensation can occur due to the temperature differences between the upper floor and attic, and if so, from which external temperature.

I have to be honest, the roof insulation was actually kind of a no-brainer and at the time I didn’t really think much about the intermediate insulation between the upper floor and attic...

The background for the roof insulation was that we wanted to keep the attic open as a potential reserve for living space. Since it is already relatively small, we didn’t want to rely on insulation between the rafters or insulation on the floor (then the room would be too low/no room) and planned with roof insulation – apart from the fact that the KfW subsidy was only available with that. And of course the usual – roof insulation is the ultimate solution, then you have peace of mind, etc...

Airtightness in the attic should be ensured, the roofer said it would all be sealed and glued 100% precisely. He also said that if the intermediate ceiling has already been opened once, it wouldn’t hurt to insulate it. He also calculated/planned the insulation, with 160 mm being the standard there – I will check with him again.