ᐅ KfW40 Wall Construction: Aerated Concrete, Insulation & Brick Cladding – Experiences and Tips

Hello everyone!

What are your wall constructions for a KfW40 house? I know that, in the end, an energy consultant calculates everything, but for now, it’s about simply comparing the wall builds from different home builders, without involving a KfW consultant.

Do you think it’s possible to achieve a KfW40-compliant wall using 175mm (7 inches) aerated concrete blocks, 140mm (5.5 inches) insulation with a thermal conductivity of 0.035 W/mK, and a brick veneer? The roof would be insulated with 260mm (10 inches), plus 8cm (3 inches) of insulation with a thermal conductivity of 0.040 W/mK under the slab. The windows have a U-value of 0.6 W/m²K and 82mm (3.2 inches) frames.

Mechanical ventilation with heat recovery, a heat pump, photovoltaics, and underfloor heating are, of course, included.

I’m looking forward to your experiences!

Buddy90 schrieb:

Do you think it’s possible to achieve a KfW40-compliant wall using 175mm (7 inches) aerated concrete blocks, 140mm (5.5 inches) insulation with a thermal conductivity of 0.035 W/(m·K), and brick cladding?

Try calculating the wall’s U-value online using Ubakus (spoiler: yes, the wall assembly just barely meets the standard, but in my opinion, it’s really borderline).
Whether you want to build with aerated concrete and brick is a different question altogether (I would skip the thick aerated concrete blocks and instead use 115mm (4.5 inches) calcium silicate bricks with thicker insulation—the wall will also be overall thinner while providing better and more reliable thermal performance).
Ultimately, the calculation for KfW40 must pass, and with the planned wall setup, you have practically no margin for error. I’d say it’s an inexpensive way to just meet the minimum target.

Is it only about KfW 40, or about subsidies? Then of course LCA is added as well.

Your windows (Uw!) certainly do not have 0.6, that refers to the glass. Depending on the size, it can quickly become 1.0...

Aerated concrete combined with brick veneer is currently the preferred choice of the house builder. However, they do not advertise KFW40 certification, only the specifications mentioned above and an energy demand of less than 50 kWh/m²a or final energy label A to A+.

My idea was to contact my independent KFW consultant to see if the house could still meet the requirements. I am interested in the KFW program 297. If the house basically meets this standard anyway, it would be a waste to miss out on the low interest rates...

That’s why I asked about your wall constructions. Maybe I can persuade the builder to install thicker or better insulation and thus definitely achieve the KFW40 standard.

Buddy90 schrieb:

Porous concrete combined with facing brick is currently the preferred method specified by the house builder.

Ok

Buddy90 schrieb:

This means installing thicker and/or better insulation to reliably meet the KFW40 standard.

Using more or better insulation is certainly possible. An energy consultant would need to calculate that. However, whether the house would still qualify for subsidies needs to be clarified, since it’s not only about the energy rating. There are many other factors involved.

nordanney schrieb:

Whether to build with aerated concrete and facing brick is another question (I would skip the thick aerated concrete block and rather use 11.5cm (4.5 inches) sand-lime brick with thicker insulation—the wall will then be overall thinner in addition to having better and more reliable insulation).

An interior wall with more insulation than the exterior wall—please tell me you’re not serious.

Buddy90 schrieb:

My thought was to contact my independent KfW consultant to see if he can still calculate the house’s certification. I’m referring to the KfW program 297. If the house basically already meets that standard, it would be a waste to pass up the favorable interest rates...

Sure, take advantage of it, but choosing or designing the house based on that afterwards is really putting the cart before the horse.

Buddy90 schrieb:

The currently preferred builder specifies aerated concrete combined with facing brick. However, he doesn’t advertise KfW 40, only the specifications mentioned above and an energy demand below 50 kWh/m²a, or a final energy rating of A to A+[...]
That’s why I asked about your wall constructions. Maybe I can get the builder to use thicker or better insulation and thus definitely achieve the KfW 40 standard.

What kind of builder is this who isn’t prepared off the shelf to deliver KfW 40/297 standards for customers? This is not an uncommon demand these days! (With some subsidy programs, I get the impression of government-driven market consolidation, because custom wall assemblies for owner-operated companies are hardly affordable.) Apart from that, I can only repeat my stone mantra: don’t try to convert the contractor into fulfilling a special wall construction request.

The insulation is applied to the load-bearing masonry shell. Changing the size of the insulation leads almost always to a shift in both wall layers. Either the load-bearing masonry shell moves inward (which disrupts the entire structural rhythm), and if it previously aligned in pattern, it now misaligns, causing the need to fill in patchy “repair pockets.” And/or the facing brick shell shifts outward, forcing irregular bonding patterns (requiring a higher-skilled workforce), which is practically guaranteed.

No matter how you turn it: your naive wish is a dangerous interference with the quality of your house. It is generally not advisable to treat exterior walls as mere styling options to be negotiated with the builder during the selection process. Exterior walls are among the most load-bearing elements of a house and must never be an afterthought in the planning stage. Therefore, you are being flagrantly irresponsible (/ foolish) if you sacrifice fundamental aspects of your house planning just for a subsidy. By the way, insulation material is available in different thermal conductivity classes (WLGs), so thickness alone is not the smartest lever to pull (if one even wants to be—something I clearly do not recommend—a gadget junkie). Dr. Frankenstein would be thrilled to see how perversely modern homeowners “tinker” with the building structure today.

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