ᐅ Exterior wall, interior insulation, followed by slate installation in new construction

Hello! Sorry if I’m bringing up a topic that has probably been discussed many times before. However, we have an additional issue: the later installation of slate cladding on the gables and the required substructure for it.

Background:
We are in the process of having a 150 sqm (1,615 sq ft) solid house built. It is intended to be a "standard house" that complies with the 2009 energy-saving regulation guidelines. It will be equipped with a gas condensing boiler combined with a solar system and underfloor heating. The house will have a white plastered finish; the gables are to be clad with slate.

Our builder has proposed exterior walls of 17.5 cm (7 inches) aerated concrete blocks ("Ytong") on the ground floor and upper floor. In addition, the interior walls on the ground floor will be made of 17.5 cm (7 inches) thick lime sand brick masonry as plan blocks. The interior walls in the upper floor will be built with 11.5 cm (4.5 inches) aerated concrete blocks. The facade is to receive a 140 mm (5.5 inches) external thermal insulation composite system (ETICS).

The slate facade cladding will also require an elaborate and costly thermal insulation system.

Now my question:
Is it possible to use a thicker exterior wall (for example 24 cm or 30 cm / 9.5 inches or 12 inches) with less insulation? Can the slate cladding be installed on counter battens with minimal insulation in between? Or could the upper floor perhaps be constructed with a timber frame system to avoid multiple layers of insulation and substructures?

We are complete beginners and would appreciate your suggestions and experiences.

Hello,

Franka schrieb:
We are about to have a 150 sqm (1,615 sq ft) solid house built. It should be a "normal house" that complies with the 2009 Energy Saving Ordinance standards. It will be equipped with a gas condensing boiler, solar panels, and underfloor heating.

So basically a KfW 85 efficiency house.

Franka schrieb:
Our builder proposed a 17.5 cm (7 inches) exterior wall (ground floor and upper floor) made of aerated concrete "Ytong". Additionally, the interior walls on the ground floor are planned as 17.5 cm (7 inches) thick sand-lime brick masonry blocks. The interior walls on the upper floor are to be made of 11.5 cm (4.5 inches) aerated concrete blocks.

What a, excuse me, nonsensical combination. Mixing these materials is already not ideal, but choosing sand-lime brick on the ground floor and aerated concrete on the upper floor makes no sense to me. If you pick these two materials, it would make more sense to have sand-lime brick upstairs and aerated concrete downstairs—since sound insulation is more important around the sleeping areas than on the ground floor. You could achieve the same result by building the entire exterior shell and ground floor walls from aerated concrete and constructing the upper floor interior walls as lightweight partitions. The latter also has the advantage of easier remodeling later on, for example, when the kids have moved out.

Franka schrieb:
Is it possible to use a thicker exterior wall (e.g. 24 cm or 30 cm) with less insulation?

If you use a 36.5 cm (14.5 inches) aerated concrete for the exterior shell, you don’t need any external thermal insulation composite system (ETICS).

Best regards

Bauexperte schrieb:
Hello,

So, a KfW 85 energy-efficient house.

What, excuse me, a ridiculous combination. Using a mixed material approach is not ideal in itself, but I don’t understand why you would use sand-lime brick on the ground floor and aerated concrete on the upper floor. If you choose these two materials, it would make more sense to have sand-lime brick on the upper floor and aerated concrete on the ground floor – sound insulation is more important around the bedrooms than on the ground floor. You can achieve the same result if you build the exterior walls and ground floor walls entirely out of aerated concrete and use lightweight construction techniques for the interior walls on the upper floor. The latter also has the advantage of making future remodeling easier, for example when the children move out.

If you use 365mm (14.4 inches) aerated concrete for the exterior walls, you don’t need any external thermal insulation composite system (ETICS).

Best regards

Hello! Thank you for your response! We have no knowledge ourselves, so we trust the builder. But these double insulation layers and then the insulation and substructure needed for the slate cladding seemed very strange to us. Especially with regard to the high costs.

Hello Franke:

1. I don’t understand why the interior walls on the ground floor are planned with sand-lime bricks and those in the upper floor with aerated concrete/cellular concrete—if anything, I would use sand-lime bricks for all interior walls. But it seems your builder might be having issues with the thin exterior wall anyway...
2. Which I would choose thicker anyway. Sure, 17.5cm (7 inches) plus insulation is cost-effective, but if anything, you should opt for 36.5cm (14 inches) aerated concrete load-bearing masonry.
3. Personally, I’m not a fan of drywall construction in general, but that’s a matter of preference.
4. "We have no idea and are therefore relying on the builder." Well, good luck with the construction—I hope you at least hire an independent construction supervisor.

Bauexperte schrieb:
....If you use a 36.5 cm (14.4 inches) aerated concrete block for the exterior wall, you don’t need any external thermal insulation composite system (ETICS)

Have you ever done an energy balance? Probably not! Modern building construction requires specialists both for structural load-bearing and thermal insulation to keep costs manageable.

@ Franka:
Make sure your trust is not being abused. External inspection should start with the planning documents!

Best regards

Hello Bauexperte,

Bauexperte schrieb:
....More than one are done by specialists – that’s why I made that statement

By "specialists" I actually meant the building construction itself, not the people.
However, it seems there are some deficiencies in your "specialists."
U-values of future-proof wall constructions should be ≤ 0.16 W/m²K; otherwise, one is not keeping up with the times.

Best regards