Y-Tong (aerated concrete) - quality building material? (sound insulation!)

Yes, that's true.

Then you need more insulation.
That is probably also good.

I think that the insulation will indeed be 2-4 cm thicker, but at the same time the 17.5 cm PB will be replaced by 15 cm KS.

The KS is similarly expensive as Ytong and the like. But since it consumes significantly more labor hours, it is ultimately uneconomical.

Is manageable.

My buddy is building a similar size simultaneously.
Additional costs about 2-3k Euros I think.

But I knowingly decided against it.

Der KS ist ähnlich teuer wie der Ytong und Konsorten. Aber da er deutlich mehr Lohnstunden raubt, unterm Strich unwirtschaftlich.

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The KS is similarly expensive as Ytong and the like. But since it takes significantly more labor hours, it is ultimately uneconomical.

The stone is cheaper.

For us, one floor of exterior masonry with two noses took about 2.5-3 days. I think you are overestimating the labor costs for stacking stones a bit. Especially since KS is also often built as a plan stone with placement aid. That's Lego for adults.

I have to bring it up again and I’m about to lose it. Now we got the number of a friendly expert from the architect (expert for sound and heat): "The Ytong is nothing more than foamed air. There is basically no sound insulation; you can’t argue that away from a building physics point of view," were his first words on the matter. I thought to myself: great.

He has now given us excellent advice and said that every stone has its justification, but it has to be well thought out. However, he would advise against the 36.5 Ytong. He said 17.5 or 20 cm sand-lime brick with a raw density of 1.8 and [WDVS] is totally fine – you would be at about 55 dB and that would already be good. Interior drywall is also okay.

We are now considering a 24 cm sand-lime brick + insulation. How thick should the [WDVS] be then for the Energy Saving Ordinance 2016?

Now again my question to you as independent technical advisors – is that so and is the price difference really that much compared to Ytong?

Building is exhausting.

And one more question: what is the Rw value of the Ytong 36.5 PP2 0.08 U-value and what is the one for the 24 cm KS? I am having trouble understanding all the technical data sheets.

Do I have to subtract the Rw value from the noise in order to get the noise level inside the room?

So, if the noise source is 5-6 m away and produces about 65-70 dB at its origin, do I have a maximum of about 15 dB inside with a 55 dB KS 24 cm? Is my calculation correct?