Above-rafter insulation desired with existing photovoltaic system

But you probably don’t have planed rafters for an exposed roof truss, do you? How are you going to do that?

I had only researched this point very superficially so far and read that it can probably be done with a belt sander. But in general: It is not uncommon for an above-rafter insulation to be applied retrospectively during energy-efficient renovation measures. How is this problem usually solved?

I had only researched this point very superficially so far and read that it can probably be done with an orbital sander.

But you will probably still have to find a volunteer who does this by hand overhead, sanding or planing the inside of the roof.
In my opinion, this is almost impossible or hardly meaningful.
Either you manage the rafters and the sheathing nicely, which I doubt, or you install drywall, wood, OSB, etc. on the inside against it. The question is especially what you want to achieve regarding the insulation value or interior appearance and also what you can/want to invest. Usually, in renovations, you open one thing and face the next problem.
It also depends on the condition of the roof whether it needs to be renovated.
I also don't quite understand why good internal insulation shouldn't be enough to achieve a decent insulation value. What exactly is your goal with this, or the professional's?

We have completely redone our roof (inklusive Tausch von Schalung undSparren) and decided on blown-in insulation after consultation. This would better "keep heat out," dampen noise from the street, create a better indoor climate, and is ecologically sustainable in the long term.

Either you get the rafters and the sheathing done nicely, which I doubt, or you put drywall, wood, OSB, etc. on the inside against it.

How do others do it who apply external rafter insulation during energy-efficient renovation but have rough-sawn roof beams? Do they simply cover them up? Or do you do both external AND between-rafter insulation in such cases?

The question is particularly also what insulation value you want to achieve

I'll put it simply (that's what I told the building savings contract, too): If it is above 32°C during the day and about 20°C at night for weeks, I want to keep the temperature permanently below 25°C without air conditioning (so only by night ventilation). In my current attic apartment from the 70s, that's a catastrophe. If I close the windows and external blinds in the morning in midsummer, the temperature inside still rises faster than outside. Terrible insulation. ...and the roof tiles reach 70 - 80°C.

It also depends on the condition of the roof whether it needs to be renovated.

We have already talked about the lifespan of the roof tiles - but what damages are typically to be expected on other components after more than 40 years?

We completely redid our roof (including replacing sheathing and rafters) and decided on blown-in insulation after consultation.

Does blown-in insulation have a higher insulation value than between-rafter insulation with mineral insulating materials?

How do others handle this who install a roofing insulation above the rafters during energy refurbishment but have rough-sawn roof beams? Do they simply cover them up? Or do you do both above-rafter AND between-rafter insulation in such cases?

I'll put it simply (that's also what I told the building savings contract): If it's above 32°C during the day for weeks and about 20°C at night, then I want to keep the temperature permanently below 25°C without air conditioning (so only by night ventilation). In my current attic apartment from the 70s, it's a catastrophe. If I close the windows and exterior roller shutters in the morning in midsummer, the temperature inside still rises faster than outside. Poor insulation. ...and the roof tiles reach 70 - 80°C.

We've already talked about the lifespan of roof tiles – but what kinds of damage are typically to be expected on other components after about 40 years?

Does blown-in insulation have a higher insulation value than between-rafter insulation with mineral insulation materials?

Blown-in insulation is supposed to insulate heat better because it has more and denser mass. Therefore, its sound insulation properties are also better.

How do others do it when carrying out an energetic renovation with an exterior rafter insulation?

To be honest, I’m not exactly sure what "energetic" means in this context or what a non-energetic renovation would do differently in this regard.

Do they simply cover these? Or do they apply both exterior and intermediate rafter insulation in such cases?

As I said, it also has something to do with the overall condition of the roof truss and your planned investment framework. If these were actually older, rough-sawn rafters, I would probably choose intermediate rafter insulation and wouldn’t see why that shouldn’t be sufficient as insulation. By choosing the right insulation material, a good standard can be achieved. So once again the question: What standard does your advisor want to achieve?
Of course, you can also treat each rafter individually by planing, sanding, or covering it, but if a company is supposed to do this, you’d probably end up renewing the already older roof truss more quickly anyway.
Depending on the rafter thickness, you can install a lot of insulation with an intermediate rafter solution. If necessary, you can even double up inside or add a few centimeters of insulation on the roof. At my old house, we had 14 cm of glass wool, and I additionally clad the roof with 22 mm boards. The upstairs was also pleasantly cool in summer.
You should also not underestimate that the roof is ONLY one point to consider because of heat. My son had a small roof window without shading in his apartment. That little thing alone turned the place into a sauna. Since shading was put on it, it’s fine with the heat in summer.

Does blown-in insulation have a higher insulation value than intermediate rafter insulation with mineral insulation materials?

You can look at the respective WLG values or similar, whereby exterior rafter insulation like PUR etc., in my opinion, tends to protect against cold rather than heat.
If the roof outside is still okay and the framework as well, I would leave it as is and insulate from the inside; depending on the rafter thickness, you get enough insulation material in; if necessary, double up.
Currently, we live here in an old house with blown-in insulation, albeit not optimally installed. I still find the insulating effect pretty good.