Outer wall, interior insulation, and then slate in the new building

Hello! Sorry if I’m bringing up a topic that has probably been discussed frequently. However, we have another problem: the later slate cladding of the gables and the required substructure.
Background:
We are in the process of having a 150 sqm solid house built. It is to be a "normal house" that complies with the Energy Saving Ordinance guidelines 2009. It is to be built with a gas condensing boiler with a solar system and underfloor heating. The house is to be plastered white; the gables are to be clad with slate.
Our builder has specified a 17.5 cm exterior wall (ground floor and attic) made of aerated concrete "Ytong". Additionally, the interior walls (ground floor) are to be made of calcium silicate masonry as plan blocks with a thickness of 17.5 cm. In the attic, the interior walls are to be made of 11.5 cm aerated concrete blocks.
The facade is to receive a 140 mm external thermal insulation composite system.
The slate facade cladding is also to receive an elaborate and costly thermal insulation system.
Now to my question:
Is it possible to use a thicker outer wall (e.g. 24 cm or 30 cm) with less insulation? Could the slate cladding not be installed on a counter-battening with minimal intermediate insulation?
Or could the attic possibly be constructed as a timber frame in order to avoid multiple layers of insulation and substructures?
We are complete laypersons and look forward to your suggestions and experiences.

Hello,

We are in the process of having a 150 sqm solid house built. It should be a "normal house" that complies with the Energy Saving Ordinance guidelines 2009. This will be built with a gas condensing boiler with solar system and underfloor heating.

So a Kfw 85 efficiency house.

Our builder proposed a 17.5 cm exterior wall (ground floor and upper floor) made of aerated concrete "Ytong". Additionally, the interior walls (ground floor) are made of calcium silicate masonry as plan blocks with a thickness of 17.5 cm. The interior walls in the upper floor are to be made of 11.5 cm aerated concrete blocks.

What a, excuse me, idiotic composition. A mix of materials is not the pinnacle of wisdom in itself, but why calcium silicate on the ground floor and aerated concrete on the upper floor baffles me completely. If you choose these two materials at all, it should be calcium silicate on the upper floor and aerated concrete on the ground floor => soundproofing around the sleeping areas is more important than on the ground floor. You achieve the same result if you build the exterior shell and the ground floor walls entirely with aerated concrete and construct the interior walls on the upper floor in lightweight construction. The latter also has the advantage that you can remodel more easily if necessary – for example, when the children leave the house.

[Quote="Franka, post: 22968"]Is it possible to use a thicker exterior wall (e.g. 24 cm or 30 cm) with less insulation?...[ /QUOTE]
If you use 36.5 cm aerated concrete for the exterior shell, you don't need any ETICS at all.

Kind regards

Hello,

So a KfW 85 efficiency house.

What an, excuse me, idiotic combination. Material mix as such is already not the ultimate solution, but why KS in the ground floor and aerated concrete in the attic completely escapes me. If you already choose these two materials, then surely KS in the attic and aerated concrete in the ground floor => around the bedrooms sound insulation is more important than in the ground floor. You achieve the same result if you build the exterior shell as well as the ground floor walls entirely from aerated concrete and the interior walls in the attic as lightweight construction. The latter also has the advantage that you can remodel more easily if necessary – for example, when the children move out.

If you use a 36.5 cm aerated concrete for the exterior shell, you do not need any external thermal insulation composite system at all

Kind regards

Hello! Thanks for the response! We have no idea and therefore trust the builder. But these double insulations and the upcoming insulation and substructure for the slate cladding seemed very strange to us. Especially because of the high costs.

Hello Franke:

1. Why the interior walls on the ground floor are supposed to be made of sand-lime bricks and on the upper floor of aerated concrete/porous concrete is completely beyond me -> if anything, at least all interior walls should be sand-lime bricks, but surely your builder apparently has problems with the thin exterior wall...
2. which I would choose thicker anyway, certainly 17.5 cm + insulation is cheap, but if then one should rather decide on 36.5 cm aerated concrete single-shell masonry.
3. I don't think much of drywall construction in general, but that's a matter of preference.
4. "We have no idea and therefore trust the builder."
Well then good luck with the construction, I hope you at least hire an independent construction supervisor

....If you use a 36.5 porosity aerated concrete for the exterior shell, you don't need any ETICS at all

Ever done an energy balance? Probably not! Modern building construction requires specialists both for load transfer and thermal insulation, then costs remain manageable

@ Franka:
Make sure your trust is not abused. External control already starts with the planning documents!

Best regards

Hello construction expert,

....More than one do let - there are specialists for that - otherwise I would not have made this statement

By "specialists" I actually meant the building construction, not people.
But with your "specialists" there seem to be some deficits.
U-values of future-proof wall constructions should already be <= 0.16 W/m²K, otherwise you have not recognized the signs of the times

best regards