Flat roof insulation - 5 craftsmen / 5 opinions

The walls and bricks of our extension are finished.
7.5m wide and 5m long

Now we are looking for a roofer (far too late)
We have already had several... and everyone tells you something different about vapor barriers and insulation.
At first, I thought about tapered insulation, but every roofer told me it would be too expensive.

The roofing company I liked best wants to implement it as shown in the picture.
That way I can install the insulation myself.
Thanks to my government at home, the rafters have to remain visible.
With tapered insulation, the beams would be fully visible.
With the roofer's idea, 10cm would be visible. (I actually find the idea quite good)
Would such a setup be okay? The vapor barrier worries me a bit. Since I can only lay it between the rafters and then have to stick the foil to the rafters.

And what goes under the insulation? Or do you always want to look at the vapor barrier? Proper insulation looks different to me, but everyone has to decide for themselves. This way, a crutch remains with regard to the insulation and not much of the rafters remains visible. What speaks against the [Aufsparrendämmung]?

Allegedly, according to all roofers, that would be way too expensive.. As a roofing layman, I have to believe that for now...

Attached is our drawing. The engineer said he had sized the beams generously. 20x8cm The 5 meter sides are 40cm higher than the brick wall and 28cm in the front. The 20x8cm rafters should be on the wall. Then from the front start with 2% slope insulation 8cm up to the old building. The gutter should run along the entire front side.

If the beams are generously sized according to the engineer, rafters of 15x8cm could also be used. On top, a normal insulation over rafters 10cm PIR and then realize the 2% slope with battens.

Since our house is very old, 1938, the entire insulation (cavity wall insulation) is not as thick as today’s houses. That’s why I am/was of the opinion that starting with 8cm of slope insulation is completely sufficient.

Since our house is very old, from 1938, the entire insulation (cavity wall insulation) is not as thick as in today's houses. That is why I am/was of the opinion that starting with 8cm of tapered insulation is completely sufficient.

Well, just because the rest of the house is poorly insulated doesn't mean you have to do the same here. However, I would also have said that a sarking insulation is a simple, good, and cost-effective option. I don't see where the huge cost driver is supposed to be.

The company is coming by again today... Does anyone know about statics and can confirm that 15x8 rafters would be sufficient... 10cm PIR insulation... And the slope is realized through battens? The company said the first time that sloped insulation costs almost 2000 euros more because it has to be custom-made for the roof

Roof insulation and rigid foam need to be considered because of sound! Don’t forget the local conditions. PUR or PIR insulation hardly dampens any sound. I’m not talking about rain or the like, it’s more about traffic noise or other things. This doesn’t necessarily have to apply to you but it could. So it’s advisable to inform yourself in advance about what to expect. You don’t want to get into trouble later. I also can’t imagine that roof insulation will be much more expensive. The rafters will be planed anyway because they are visible, then you would only have to plane the sheathing and have a slightly different roof structure. But that won’t make that much of a difference. Regarding insulation with the vapor barrier. I have almost finished dealing with that topic myself. It can be done, but please have a professional briefly instruct you on how and what should be done. Otherwise, you might go crazy. :) For that reason alone, I would, for example, insulate everything possible from the outside. Simply less stressful.