Insulate cold roof with clamp felt, price

....that the insulation of the roof space is visible, meaning there is no closed cavity between it and the room climate of the attic....

Greetings 11ant,

just to be sure again: By cavity you mean the case where there is no cladding with OSB, drywall, or similar. In such a case, the insulation would have to be as deep as the rafters to avoid cavities forming.
If we don't install cladding but only apply the vapor barrier foil, the cavity should not be a problem, right? :)

If we do not clad, but only install the vapor barrier foil, the cavity should not be a problem, right?

With a "self-supporting" ;-) stretched foil, you basically create the cavity that gives you work (???) - I put a question mark behind the meaning ...

Obviously, it depends on details that I do not yet fully understand 100%.

A small excerpt from "Bauwissenonline":

Moisture protection:
"Insulation materials must be especially protected from moisture, as it impairs the insulating effect and can lead to building damage (e.g., mold) in the long term. Protection against moisture from the outside is ensured by the roofing in combination with the underlay.
However, moisture can also occur in the roof structure from water vapor coming from the inside, which can subsequently lead to condensation. Flowing water vapor does not harm diffusion-open insulation materials as long as no condensation forms. To prevent this, a special diffusion-retarding layer (vapor barrier/vapor retarder) is required on the inside of almost every pitched roof structure, which must be tailored to the respective roof construction. In a diffusion-open roof structure, water vapor from the indoor air can diffuse outward through the insulation. With a vapor-tight roof skin without ventilation, a vapor barrier or a moisture-variable vapor retarder must be installed to prevent damage in the insulation layer."

Furthermore, regarding condensation caused by water vapor convection:
"To avoid internal condensation caused by water vapor convection, an intact airtight layer must be ensured. Especially small leaks in diffusion-tight membranes (vapor barriers) can cause severe moisture damage, as significantly more moisture can be transported by water vapor convection than by diffusion."

Ventilation:
Since previously mostly vapor-tight underlays were used, it was common to choose an insulation thickness less than the rafter thickness and thus create a ventilation layer directly above the insulation connected to the outside air. This is referred to as a ventilated roof. This allowed moisture accumulating under the vapor-tight underlay to ventilate out.

As far as I understand correctly, we are dealing with a "ventilated roof structure."
Therefore, a vapor RETARDER as a diffusion-retarding layer must be installed on the inside of the roof structure, or am I mixing something up?

You can certainly also "fold" the vapor barrier and stick it to the insulation. However, if you stretch it over the rafters and the insulation does not have the thickness filling the fields, you enclose a separate air volume between the insulation and the vapor barrier, which can no longer mix with the one in the attic. By doing this, you risk that different climates prevail in the air volumes on either side of the vapor barrier. In my view, this causes the vapor barrier to become wet with dew on the insulation side. With insulation as thick as the rafters, you avoid this problem (or else by "folding" the vapor barrier, which in my opinion is a bit of a tedious job). But beware - I am the expert here for draftswoman insults, windows, and a handful of other areas, but not for insulation stuff with all its incidental subplots!

But beware - I am the expert here for character insult, windows, and a handful of other areas, but not for insulation stuff with all its side issues!

I have already taken note of that and am considering it in my risk analysis ;) Thanks anyway!

Hmm, in that case, I probably would have done myself a favor with 24 cm insulation (rafter thickness 24 cm). It's unfortunate now that I quickly stocked up on the 18 cm one.

What would technically speak against installing the insulation flush on the inside now and sealing it with the vapor barrier foil, so that the cavity remains on the ventilated outer side? Nonsense or a possible alternative solution?

Stupid now that I quickly stocked up on the 18s.

From my point of view, that is not serious, just inconvenient: bending off at every field on the rafter, following the insulation and then re-aligning to the rafter level means 2x 6 cm more film width per field and quite a bit more effort, but I don’t see any real problems.

What would technically speak against applying the insulation flush on the inside now and finishing with the vapor barrier foil, so that the cavity remains on the ventilated outside? Nonsense or possible alternative solution?

I leave that to those who can contribute more than mostly opinion on that.