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 🙂
Finch039 schrieb:
It can't hurt to insulate the ceiling cavity.Yes, but mainly for soundproofing. And as I said, if the roof insulation is installed, you should also provide heating and ventilation there. Maybe less than if the space were regularly occupied, but moisture will still enter and may not be able to escape.I would probably avoid using a vapor barrier to allow moisture to escape, but at the same time install at least one hygrometer with a transmitter/receiver there and mount the display in a place I pass regularly.
But you’ve already written to the building inspector, so I’m curious to see what they say...
No, no... regarding heat... the gable wall is (still) not insulated, and he said that before the heat escapes through the attic, it's better to insulate the top living floor since the ceiling is open anyway.
You mean ventilation, because the roof will be sealed and the air can't escape? I understand...
But why heating? The room will initially only be used as storage or something similar...
You mean ventilation, because the roof will be sealed and the air can't escape? I understand...
But why heating? The room will initially only be used as storage or something similar...
If you don’t heat the room, it will naturally cool down despite the insulation. When it gets cooler, the air’s capacity to hold moisture decreases. This means the dew point moves closer to the room temperature. Eventually, moisture will condense and, due to the airtightness, won’t be able to dry out. Persistent dampness will cause mold problems over time.
Try searching for a dew point calculator; it’s quite straightforward.
For example, 50% humidity at 21°C (70°F) already corresponds to over 80% relative humidity at 12°C (54°F).
A smaller issue: If you don’t heat that space, you will be heating the roof from the upper floor as well (at least if you don’t insulate the floor now or only a little).
Try searching for a dew point calculator; it’s quite straightforward.
For example, 50% humidity at 21°C (70°F) already corresponds to over 80% relative humidity at 12°C (54°F).
A smaller issue: If you don’t heat that space, you will be heating the roof from the upper floor as well (at least if you don’t insulate the floor now or only a little).
Well, not necessarily. Even if you only go up there occasionally but regularly, you are still introducing moist air. If the surrounding structure is vapor-tight, the moisture cannot escape. From my point of view, this makes heating and ventilation even more important.
In the end, the energy consultant needs to decide, but I would guess: For ceiling insulation, focus more on soundproofing rather than maximum thermal insulation— for example, using wood fiber boards. The entire build-up should be as vapor-permeable as possible, combined with some form of minimal heating that activates once the temperature falls below 16°C (61°F) (maybe a split air conditioning system?). Alternatively, regular ventilation up there (a decentralized ventilation unit with heat recovery) would be good; ideally, both.
In the end, the energy consultant needs to decide, but I would guess: For ceiling insulation, focus more on soundproofing rather than maximum thermal insulation— for example, using wood fiber boards. The entire build-up should be as vapor-permeable as possible, combined with some form of minimal heating that activates once the temperature falls below 16°C (61°F) (maybe a split air conditioning system?). Alternatively, regular ventilation up there (a decentralized ventilation unit with heat recovery) would be good; ideally, both.
My main concern is to prevent moisture from penetrating and staying in the mineral wool insulation. If the moisture moves upward into the attic, that would be acceptable, since, in my understanding, regular ventilation would resolve the issue. That would be feasible (or alternatively, install a decentralized ventilation system, as you already mentioned).
With a vapor barrier, most of the moisture would remain inside the living area and would not reach the mineral wool at all – in that case, simply ventilating the living spaces (which is done anyway) would be sufficient and everything would be fine...
With a vapor barrier, most of the moisture would remain inside the living area and would not reach the mineral wool at all – in that case, simply ventilating the living spaces (which is done anyway) would be sufficient and everything would be fine...
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