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 🙂
W
WilderSueden27 Feb 2023 14:04Tolentino schrieb:
That’s also why I suggested you should heat that area as well.Of course, that would make the insulation in the ceiling unnecessary again. For me, the gable wall is still an unknown. Do you plan to insulate it soon or sometime later? If it’s the latter, you will have a greater temperature difference and will need to be more careful.
I already mentioned this in my first reply. In the long run, it’s not completely unnecessary since it’s an expansion reserve and will eventually need sound insulation. But yes, the planning could have definitely been done better.
Important note about the wall! Once it’s undamped, you should definitely use a vapor barrier and ensure it is installed with a very tight seal.
Important note about the wall! Once it’s undamped, you should definitely use a vapor barrier and ensure it is installed with a very tight seal.
Tolentino schrieb:
I already mentioned this in my first reply. In the long run, it’s not entirely unnecessary since it’s basically extra space for future expansion and will need sound insulation eventually. But yes, it should have been planned better.
Important note regarding the wall! Once it’s left uninsulated, you definitely need to install a vapor barrier and ensure it is sealed extremely well. WilderSueden schrieb:
Although that would of course make the insulation in the ceiling redundant again.
For me, the gable wall is still a bit unknown. Are you planning to insulate it soon or sometime later? If the latter, you’ll have a bigger temperature difference to manage and will need to be more careful. To be honest? More like later. Right now, the priority is just to properly finish everything inside (neatly, correctly, and cleanly) and maybe move in by autumn. The baby is due in summer, so that will take precedence, and then the basement, garage, and garden need to be refreshed first. After that, I’ll have to see how the budget holds up (so far, it looks very good).
The original idea was not to heat the attic. In other words, the heat I supply to the living floor is supposed to stay there as much as possible. That’s why the insulation is planned. I didn’t focus on soundproofing there yet because mineral wool wouldn’t do much for that (I insulated the wood beam ceiling between the basement and ground floor with heavy loose fill, wood fiber boards, and dry screed panels).
This would all have been resolved if the attic were eventually to be converted into living space. Since this is not yet confirmed and is rather planned as a reserve (e.g., for a third child or similar), we found the insulation solution to be appropriate.
What I take away from this is that ventilation in the attic is important, preferably with a ventilation system, and since the attic will still cool down quite a bit despite the exterior roof insulation, it is all the more important to have the vapor barrier installed perfectly to prevent excessive moisture from rising upward.
What I take away from this is that ventilation in the attic is important, preferably with a ventilation system, and since the attic will still cool down quite a bit despite the exterior roof insulation, it is all the more important to have the vapor barrier installed perfectly to prevent excessive moisture from rising upward.
Last question on this topic: I have the wall connections here, which are quite full of holes at the top (there used to be a lattice battens underneath, which were torn out). Would you fill these with Rotband plaster / surface filler, smooth everything out, and then apply the adhesive at that spot, or would you start directly underneath? The grooves are about 3cm (1.2 inches) deep. The ceiling construction with wooden beams, battens, and drywall is about 5cm (2 inches) thick.
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