Moisture in the attic / vapor barrier

Dear forum!

This winter I noticed moisture on the roof (condensation). The house is about 30 years old, I hadn’t noticed this before, but I actually was hardly ever in the attic in winter. It is a mansard roof, meaning the actual attic is only a loft. As soon as it gets warmer, it dries out completely again. An expert said the vapor barrier or the adhesive has probably deteriorated over the years and now needs to be replaced. This means: remove existing GKF boards, remove vapor barrier, possibly also remove insulation (12cm glass wool), new substructure, increase insulation thickness, new vapor barrier, new GKF boards.

The drywall contractor said the 12cm insulation should be increased to 25cm. I don’t fully understand why this additional insulation is so important from a construction perspective since the moisture is on the attic (insulation/rafters) and not on the ceiling of the upper floor (meaning it is not too cold there). Basically, it makes sense to add more insulation there since the ceiling is already open. The question is whether 25cm is really necessary. The drywall contractor said fewer centimeters (even with efficient insulation material) doesn’t make sense because the insulation has to be thicker than the rafters to avoid thermal bridges. What do you think about this?

Would it also be an option to keep the defective vapor barrier and install insulation above the rafters on the roof (so that it doesn’t get so cold in the loft and the moisture should therefore be less of an issue)?

It is a house with a beam ceiling and a loft. The construction from the upper floor ceiling upward would therefore be: GKF 1.5cm, vapor barrier, wooden substructure for GKF, 12cm glass wool with rafters (on the slope) as well as wooden beams and cross beams (in the loft). Heraklit EPV boards 3.5cm on shuttering in the loft. The roof itself has only an underlay membrane, battens, and roof tiles = uninsulated roof over the loft, in the area of the wall slope it is of course insulated with the 12cm glass wool between the rafters.

I discovered the moisture along the entire north slope in winter, with focus above the bathroom roof window, but there were also moist spots in the middle of the loft (though weaker). That is why the expert said the vapor barrier probably has a problem in more than one place. The photos from construction in the 80s also show that apparently the vapor barrier/barrier was not attached to the plaster but directly to the wall (see 5th photo).

The drywall builder said the 12cm insulation should be increased to 25cm. I don't quite understand why this additional insulation is so important from a construction point of view, after all, the moisture is in the attic (insulation/rafters) and not on the ceiling of the upper floor (meaning it is not too cool). Basically, it makes sense to put more insulation there if the ceiling is already open. The question is whether 25cm is really necessary. The drywall builder said fewer cm (even with efficient insulation material) doesn't make sense because the insulation must be thicker than the rafters to avoid thermal bridges. What do you say about that?

You’re talking about insulation between the rafters, right? I would never make it thicker than the rafters! You need ventilation. If moisture ever gets in and you pressed the insulation tightly against it, good luck—the moisture can no longer escape! Better to have a cold roof with ventilated insulation.

Only partially. In the mansard area, that is approximately where it is circled in red, there is insulation between the rafters; in the attic, the rafters are not insulated but the collar beam ceiling is. In other words, the upper floor ceiling is insulated.