Vapor barrier vs vapor retarder

Hello,

there is really nothing against installing a vapor barrier on the inside of the rafters over a large area. This is practically feasible as long as it is accessible. However, the vapor barrier does not necessarily have to be glued directly under the rafters/insulation. It can also be part of the "wall construction" in the area of the sloping roof in the converted part of the attic.

Hello,

there is actually nothing really speaking against installing a vapor barrier extensively on the inside of the rafters. That is practically doable, as long as you can reach it. However, the vapor barrier does not necessarily have to be glued directly under the rafters/insulation. It can also be part of the "wall structure" in the area of the sloping ceilings in the finished part of the attic.

Thanks for the feedback.
Unfortunately, my question is still not answered.
It is said that the vapor barrier / vapor retarder is supposed to prevent moisture from below (living spaces) from reaching the thermal insulation.
Basically, though, even with a vapor barrier / vapor retarder, there is still moisture in the roof above. What happens to it?

What about moisture that comes from outside through the roof surface into the thermal insulation?
That is removed through "draft" in the insulation area.

Why then can’t you let the living space moisture reach the thermal insulation and let the insulation be ventilated?

Jochen

It is said that the vapor barrier / vapor retarder is supposed to prevent moisture from below (living spaces) from entering the thermal insulation. Basically, it is still the case that even with a vapor barrier / vapor retarder, there is moisture at the top in the roof.

No, that’s not really correct. I wrote that the insulation must be ventilated if humidity can enter from below (the living space). From above, that is from the roof, moisture must never reach the insulation. That is ensured by the underlay membrane. If you don’t have one, it will be difficult.

Why can’t the living space humidity be allowed to enter the thermal insulation and the thermal insulation be ventilated?

You can do that. But the ventilation must be "sufficient," and I believe that in your sketched design (installation at wall and floor with a 90° angle) this will not be possible; instead, you should rather insulate between the rafters.

No, that's actually not correct. I wrote that the insulation needs to be ventilated if moisture can penetrate from below (from the living space). From above, meaning from the roof, moisture must never get to the insulation. That is ensured by the underlay membrane. If you don't have one, it will be difficult.

Thanks for the clarification. I have now checked the roofer's offer. I see "laminated vapor-permeable underlay membrane" above the thermal insulation.
(maybe it is called "aluminum laminated"?

You can do that. But the ventilation must be "sufficient," and I believe that won't be possible with your outlined execution (laying on wall and floor at a 90° angle); instead, you should insulate between the rafters.

OK, understood. I will discuss this with the planner.

The crux is:
The house to be renovated, built in 1961, had no problems for about 45 years.
With unsuitable renovation from 2007 onwards, the problems started.

Jochen

There I now see a "laminated diffusion-open underlay membrane" above the thermal insulation. (it might also be called "aluminum laminated"?

As far as I know, there is no aluminum laminated diffusion-open underlay membrane, so I would assume the following: This underlay membrane is laminated onto the insulation.

Good. Then laminated. I could have thought of that myself since aluminum cannot be vapor-permeable.

Another question.

The two foils, vapor barrier below the thermal insulation and underlay membrane above the thermal insulation, must somehow be connected "airtight" to each other and to the environment.
How and with what is this done?

Jochen