ᐅ Roof Insulation for New Hip Roof Construction to Meet KfW 40 Standard

Hello dear community,

we are currently planning our new KfW40+ single-family house with a prefabricated house provider. Although the contract is not yet signed, we are about 98.9% sure that this will be the one. The offer somewhat reveals the manufacturer’s construction method, which leads to my question.

I have absolutely no knowledge of house construction or anything similar, so here is the consideration of a complete layperson. The offer states the following:

• Nail plate roof truss, roof pitch => 23°

For an attic that is not intended for expansion, a nail plate roof truss structure is built in traditional craft-style according to static calculations. The attic space is not accessible with this roof structure.

Construction from inside to outside:
21 x 76 mm and 30 x 80 mm substructure
200.0 mm (8 inches) thermal insulation at ceiling level
solid nailed truss according to calculation
roof underlay membrane
counter battens 40 x 60 mm (1.5 x 2.5 inches)
roof battens 40 x 60 mm (1.5 x 2.5 inches) or 40 x 80 mm (1.5 x 3 inches)

• Increase of rafters and bottom chords in nailed plate roofs from 200 mm to 280 mm

Increase of rafters and insulation: 200 mm to 280 mm (8 to 11 inches) → U-value = 0.14
(for nailed plate roof bottom chord and insulation)

Does this mean that the “standard roof” insulation thickness is being changed from 200 mm to 280 mm?

Can you estimate if this is a reasonable and good level of insulation? Or could you go even further? What confuses me is that the insulation is not applied on the roof itself, but rather only in the uppermost floor ceiling. Does this mean that the roof is not insulated at all?

As someone who grew up in an attic, I am quite sensitive to this. It’s less about the thermal insulation for me and more about summer heat protection. I am approaching my forties and have always slept directly under the roof throughout my life. For the rest of my life, I would simply want to avoid spending summers “cooked” in my own heat.
Our bedroom is upstairs, and I am hoping that with KfW40, underfloor cooling, and of course good shading, my sleep quality will improve significantly in summer.

I am also a bit worried about the:

• ceiling retractable staircase as a space-saving folding ladder,

which is recessed into the attic when not in use

Isn’t this also a “gap” allowing heat to pass through?

I would greatly appreciate constructive feedback. Maybe my concerns are completely unfounded and everything is fine. I just don’t want to save in the wrong place when it comes to the biggest investment of my life.

Thank you very much and best regards
Dany