ᐅ Is a vapor retarder necessary beneath the insulation of a ceiling between floors?
Created on: 25 Feb 2023 19:42
F
Finch039
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 🙂
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 🙂
Yes, it’s not quite easy to get decent photos. So, the light area at the top is the insulation, fitted between the wooden beams running horizontally across the image. At the top, near the wall, there is this groove where I’m still unsure which material would be best to fill it with.
Because right there, as high up as possible where the empty groove is now, I would like to connect and seal the vapor retarder airtight.
Then, the battens will be installed perpendicular to the beams.
Because right there, as high up as possible where the empty groove is now, I would like to connect and seal the vapor retarder airtight.
Then, the battens will be installed perpendicular to the beams.
