Is additional insulation of the attic sensible in new construction?

I don't have it in my head. How do you want to heat?

With an air source heat pump, a model that has a rather above-average annual performance factor for air source heat pumps.

So the attic should possibly be heated after all?

No, it should only not go over 35 and below 0 degrees year-round. Which insulation material would you recommend?

I have now made a rough calculation. According to the U-value calculator, the U-value of the roof is 0.20 W/m^2K without the additional insulation and 0.125 W/m^2K with the insulation. Heating the transmission losses of the house will cost me an estimated 500 EUR per year (losses from ventilation and heating costs for hot water are additional). The roof area accounts for 26% of the building envelope of the house, and I reduce the energy transmission of the roof by 38%. Therefore, I save about 10% of the transmission losses of the entire house through the additional insulation, resulting in a saving of about 50 EUR per year.

If the insulating mats for the area already cost 1500 euros, it will take 30 years to break even if I can finance at zero percent. So basically, the measure is silly.
However, I might do it anyway, also because of heat protection in the summer. And I would probably have put some kind of flooring in the attic anyway (chipboard or something), so insulation can equally be included in this flooring.

I would have to work myself into the calculation. I assume there is a flaw in the reasoning. But I have little time right now. Still, you are right for now, it will pay off very late.

However, the efficiency of the heat pump must not be neglected. Each degree higher supply temperature costs 2.5%. Through the indirect temperature control of the attic, higher costs will also arise here.

But I will stick to the tip for now: Try it for a year without it. I don't believe that energy costs will skyrocket.

You can also check whether the ceiling becomes a heat source in the summer.

Such a measure is really easy to do afterwards.

By the way, this is not my conviction. I believe the attic will only be used for stuff that is needed once or twice a year. You don’t want to use this staircase more often either. At least I don’t. These things are often insensitive to both cold and heat. If it were my house, I would omit insulation between the rafters and insulate the floor ceiling as much as possible.

But that is entirely your decision and has to be considered individually.

What would you suggest? Cheaper and somewhat more elaborate installation would also be okay. Rolling out mats and putting stepping plates over them? Other materials? It shouldn’t be much thicker than 10-15 cm, otherwise I can no longer bend over in the attic. We don’t have a basement, so the attic serves as a quite important storage area. Also, the temperature range in the attic should not deviate too much from the house in the deepest winter and in the height of summer, so should the insulation of the ceiling beams possibly be a bit weaker than the insulation between the rafters?

The ceiling insulation is the real deal for the heated area—it should be good. The insulation clamped between the rafters above is what keeps your substitute basement frost-free; that is sufficient.

I'll stick with the tip for now: try it without for a year

Good tip, but I know myself. Either I do it right away or realistically I'll never do it... If you find a calculation error, let me know. I've implicitly already factored in the 23-degree pitch of the gable roof in the numbers (makes only about a 10% difference).

The ceiling insulation is the real D. for the heated area - it should be good. The rafter insulation above is the one that keeps your replacement cellar frost-free, it is sufficient in a satisfactory way.

Did you insulate both, as floor and between rafters? What material did you choose for the walkable floor insulation, and how expensive was it approximately? Any problems with humidity in the attic?