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 🙂
Hmm, it seems he didn’t quite understand your problem.
When installing the vapor retarder, it’s important that it becomes more vapor-permeable towards the attic space (which is probably the case with the floorboards, unless they are varnished) and that you regularly ventilate up there.
When installing the vapor retarder, it’s important that it becomes more vapor-permeable towards the attic space (which is probably the case with the floorboards, unless they are varnished) and that you regularly ventilate up there.
Just received feedback from the energy consultant: a vapor barrier is absolutely necessary, especially since these are bedrooms and a bathroom. He would have chosen wood fiber insulation instead of mineral wool—but it’s too late, as the mineral wool is already installed under the ceiling. Well, then the vapor barrier it is. It should be done properly. I will now start plastering the walls. The walls are vapor permeable toward the attic. I’m thinking of using a decentralized ventilation system there.
Finch039 schrieb:
but used wood fiber material insteadGood man!Finch039 schrieb:
too late, unfortunately it’s already installed above the ceilingAh, I unfortunately missed that.Yes, mineral wool isn’t that much worse; it’s just not as ecological, nor does it insulate as well against sound and heat, and it’s not quite as vapor-permeable. On the other hand, it is completely non-combustible and cheaper…
Next time, definitely no more glass wool. That stuff is so irritating to work with, even with protective gear. Horrible.
The ecological aspect also plays a role – basically, it’s no longer up to date to use materials like that. I was a bit hasty with the order, unfortunately.
One more question about the vapor retarder. It really needs to be 100% airtight, as I’ve read. That makes sense.
Are the consequences really that severe with tiny holes or imperfect sealing? I mean, the goal is to do it as precisely as possible, and we are taking our time – but mistakes can always happen, even with professionals, if you are one. If there is a small opening where moisture can enter, but there is vapor permeability above, wouldn’t the moisture that got in also be able to escape or dry out again, or am I missing something?
The ecological aspect also plays a role – basically, it’s no longer up to date to use materials like that. I was a bit hasty with the order, unfortunately.
One more question about the vapor retarder. It really needs to be 100% airtight, as I’ve read. That makes sense.
Are the consequences really that severe with tiny holes or imperfect sealing? I mean, the goal is to do it as precisely as possible, and we are taking our time – but mistakes can always happen, even with professionals, if you are one. If there is a small opening where moisture can enter, but there is vapor permeability above, wouldn’t the moisture that got in also be able to escape or dry out again, or am I missing something?
I’m not entirely sure about that. Initially, I would have said that since the top is already insulated, the temperature difference isn’t that big and therefore, as you mentioned, it’s not as problematic. But because you’re now also insulating the warm side, the temperature difference will increase again.
That’s also why I suggested you should heat that area as well.
If you ventilate regularly, it’s probably less risky. But it still remains a potential weak point.
Alternatively, use a vapor retarder instead of a vapor barrier. That way, it is somewhat permeable everywhere and only slows down diffusion, so with controlled ventilation there is always enough time for drying.
For that, it’s best to install two decentralized fans on opposite exterior walls and wire them in alternating mode (one in, one out). This creates a proper airflow.
That’s also why I suggested you should heat that area as well.
If you ventilate regularly, it’s probably less risky. But it still remains a potential weak point.
Alternatively, use a vapor retarder instead of a vapor barrier. That way, it is somewhat permeable everywhere and only slows down diffusion, so with controlled ventilation there is always enough time for drying.
For that, it’s best to install two decentralized fans on opposite exterior walls and wire them in alternating mode (one in, one out). This creates a proper airflow.
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