Insulation of the bathroom ceiling

Hello everyone,

we are currently renovating an old building (built in ’58) and it concerns the suspended ceiling in the bathroom.

The construction is a bit unusual, the bathroom is on the ground floor, but above it there is no room, only a rather shallow roof slope. We had actually only removed the existing ceiling in the hope of being able to suspend a new ceiling higher up. However, this hope was dashed by a massive concrete beam. So now a tension ceiling or drywall ceiling will be put back in the old place.

At least now we have the chance to revise the insulation. The old setup was straw ceiling -> thin glass wool -> thick glass wool -> roof. We don’t know exactly how the roof is insulated, but at least there is a foil visible on the underside of the roof.

My idea now was to clamp additional insulation between the rafters below the thick glass wool (there are wooden beams where that fits well) and then secure it with a foil. I’m just not quite sure whether I should install the foil as airtight as possible, i.e., as a vapor barrier, or rather not?

Since the previous setup was quite vapor-open and there were no problems in the past, I tend not to seal the foil airtight either.

What do you think? Or are there other ideas?

Best regards Tobias

I would tend to use a vapor barrier that is installed professionally. We have installed the isover Vario KM Duplex UV and have had no problems so far.

No vapor barrier but a vapor retarder as Schimi writes. That way you remain vapor permeable. Still, install everything properly = airtight. That’s how you regulate the moisture.

Another question: Is there anything between the roof and the glass wool? Or do the roof tiles come directly?