ᐅ Is a vapor retarder necessary beneath the insulation of a ceiling between floors?

Hello everyone,

Starting situation: Between the upper living floor and the attic, there is a wooden beam ceiling. From below, we have “opened” it up, meaning we removed the old plaster and took out the lattice battens, exposing the bare beams.

After consulting with the roofer and energy consultant, we were told that insulating the ceiling is not absolutely necessary, but since it was already opened up, it would be sensible.

So today, I installed 100mm (4 inches) of 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 (6 inches) of exterior roof insulation with thermal conductivity class 023. The attic will remain an unheated attic. The floor here is covered with floorboards, so it is not airtight or vapor-tight towards the living area below.

The plan was to install a vapor retarder from below underneath the insulation. Upon further consideration, I realized today that this would be a major effort. The vapor retarder must be sealed airtight to the walls, which is not easy in an older building. For example, I would need extensive replastering at the top of the walls to create a smooth surface for sealing beads, and so on. It’s not that I’m avoiding the work, but if it’s that complicated, you start to seriously reconsider whether it’s really necessary.

So, I’m wondering whether a vapor retarder is actually needed below our insulated ceiling towards the attic. I don’t think it will get extremely cold up there with the exterior roof insulation, so the temperature differences might not be very large. Can anyone here assess this? I’m happy to provide more information if needed 🙂

Anyone have a brief assessment of this?

Detailed summary of the structure from top to bottom:

New roof with 160 mm (6.3 inches) exterior insulation
Unheated attic

Attic floor nailed wooden planks
Loose fill / clay screed between wooden beams
Upper wooden beams (towards the attic)
Wooden boards
Lower wooden beams (towards the living area)
Mineral wool insulation between the lower wooden beams, from below
(Vapor retarder???)
Battens / installation level
Drywall

Thanks a lot in advance! 🙂

In the basic construction, you will naturally have a temperature difference between the upper floor and the attic, and you insulate on the warmer side. Whether the difference is significant enough to require a vapor retarder is something the energy consultant can best advise you on.

It is somewhat questionable to insulate the roof structure (which basically means you also have to ensure airtightness in the attic space, otherwise insulating there doesn’t make much sense) and then still choose not to heat it. If you don’t want to heat the attic, it is generally more practical to only insulate the attic floor.

But if this is the planned approach, you have to work with it.

In my layman’s opinion, one option could be a kind of moisture- and temperature-controlled minimal heating system. In other words, installing not just a frost guard but a dew point monitor.

And/or installing ventilation with heat recovery in the attic space.

Otherwise, yes, a vapor barrier or vapor retarder should be applied on the warm side of the insulation.

In fact, the energy consultant (who I assume also planned the roof insulation) should be able to calculate whether you need an additional vapor retarder — that is, whether condensation will occur due to the temperature differences between the upper floor and the attic, and if so, at which outdoor temperature.

Thank you for your reply. I have also contacted a roofer and an energy consultant to get their opinions. However, it might take some time before I receive an answer, so I’m hoping for qualified responses here 🙂

Tolentino schrieb:

It’s a bit questionable to insulate the roof structure (which basically requires ensuring airtightness in the attic; otherwise, insulating there doesn’t make much sense) and then still not heat the attic. So if you don’t want to heat the attic, it’s actually more practical to insulate just the attic floor.
But if that’s the plan, you have to work with it.
In my layman’s opinion, one option might be some kind of moisture- and temperature-controlled minimal heating—more like a dew point sensor rather than just frost protection.
And/or install ventilation with heat recovery in the attic.
Otherwise, yes, a vapor barrier or vapor retarder should be installed on the warm side of the insulation.
Actually, your energy consultant (who I assume also planned the roof structure insulation) should be able to calculate whether you need an additional vapor retarder—that is, if condensation might occur due to temperature differences between the upper floor and attic, and if so, at which outdoor temperature.

To be honest, the roof insulation was kind of a given from the start, and at that time I didn’t really think much about insulating between the upper floor and attic floor...

The reason for the roof insulation was that we might want to keep the attic open as a potential living space in the future. Since it’s already quite small, we didn’t want to use insulation between the rafters or just on the attic floor (because that would reduce the ceiling height or eliminate the space), so we planned for roof structure insulation—which also qualified us for the KfW subsidy. And of course, the usual reasoning—roof insulation is the ideal solution; then you have no problems, and so on...

Airtightness in the attic should be ensured; the roofer said everything will be sealed and glued with 100% precision. He also mentioned that since the ceiling will be opened anyway, it wouldn’t hurt to insulate the attic floor. He also calculated and planned the insulation—160 mm (6.3 inches) is apparently standard there—I’ll check back with him.