Hello everyone,
We purchased a partially renovated old building from 1928 last summer.
The roof, or rather the attic, was insulated by one of the previous owners with 120mm (4.7 inches) of mineral wool. I am not sure if a roofing underlay is present; the roof is quite old (according to the inspector, it is still in good condition and does not require immediate repairs), so I would lean towards no.
However, the vapor barrier consists only of a cracked aluminum foil layer, which is at best insufficient.
This has worked well for over 25 years, but we will be insulating the walls with about 7cm (2.8 inches) and the top floor ceiling (wooden beam ceiling) with about 12cm (4.7 inches) of blown-in insulation. This will likely shift the dew point, and I am not sure if I will be as lucky as the previous owner.
Therefore, I am considering installing a functional vapor retarder. However, I am wondering where it should be placed: directly under the attic insulation or under the top floor ceiling? In the latter case, there are also questions about the hatch in the roof, which is probably not airtight, as well as the skylight installed in the top floor ceiling to illuminate the hallway.
Where is the vapor barrier best positioned in this situation?
Thank you in advance
Tobias
We purchased a partially renovated old building from 1928 last summer.
The roof, or rather the attic, was insulated by one of the previous owners with 120mm (4.7 inches) of mineral wool. I am not sure if a roofing underlay is present; the roof is quite old (according to the inspector, it is still in good condition and does not require immediate repairs), so I would lean towards no.
However, the vapor barrier consists only of a cracked aluminum foil layer, which is at best insufficient.
This has worked well for over 25 years, but we will be insulating the walls with about 7cm (2.8 inches) and the top floor ceiling (wooden beam ceiling) with about 12cm (4.7 inches) of blown-in insulation. This will likely shift the dew point, and I am not sure if I will be as lucky as the previous owner.
Therefore, I am considering installing a functional vapor retarder. However, I am wondering where it should be placed: directly under the attic insulation or under the top floor ceiling? In the latter case, there are also questions about the hatch in the roof, which is probably not airtight, as well as the skylight installed in the top floor ceiling to illuminate the hallway.
Where is the vapor barrier best positioned in this situation?
Thank you in advance
Tobias
There are already several threads on this topic, so feel free to use the search function.
In principle, the insulation in the ceiling must be protected from moisture produced by humans, as insulation does not tolerate moisture well.
This means the vapor retarder (or consider a climate membrane, which is similar but, in my opinion, better) should be installed between the insulation and the interior space. The vapor retarder should be nearly 100% airtight. This means it must be sealed completely with tape and plastered over—no gaps, holes, cracks, or openings in the vapor retarder. Special adhesive tapes are available for this purpose.
In principle, the insulation in the ceiling must be protected from moisture produced by humans, as insulation does not tolerate moisture well.
This means the vapor retarder (or consider a climate membrane, which is similar but, in my opinion, better) should be installed between the insulation and the interior space. The vapor retarder should be nearly 100% airtight. This means it must be sealed completely with tape and plastered over—no gaps, holes, cracks, or openings in the vapor retarder. Special adhesive tapes are available for this purpose.
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