KFW 55 with thicker stone 42.5 cm aerated concrete?

Good day,

I am building a two-family house with 2 apartments each 118 sqm. I did not intend to apply for KFW funding, but since 01.07. there have been good subsidies. Facts: - Window planning U-value excellent. Triple glazing, U-value 0.90 // connection to geothermal system, district heating, all excellent for KFW 55 here as well. So primary energy values are great. Only the planned stone H H aerated concrete 36.5 (quality PP4 --- needed for statics/monolithic) has a value of U = 0.33 in the building envelope (Energy Saving Ordinance requires 0.24), so we won’t qualify for KFW 55 funding here. According to the architect and structural engineer, the stone would have to be 42.5 cm thick to qualify for KFW 55 funding. There is also the consideration to use PPW2 only on the upper floor, but then the sound insulation would be borderline.

Following questions: Does the thicker stone have disadvantages regarding comfort factor in the house? Would it be too stuffy? I do not plan a central ventilation system. Only decentralized ventilation via windows or pull/push valves on the upper floor. Is the stone maybe even the future? I would forgo the funding if the insulation then corresponds to the living quality. Unfortunately, I have many question marks here or am unsure. The apartments are to be rented out initially. We might move into the house later.

Is the building envelope value U = 0.33 really that bad? The energy certificate is in the middle B range (final energy demand 67 kwh/m2 a, primary energy demand 10 kWh). When viewed from a pragmatic perspective, the current plan and energy certificate are in a good range.

As the architect said yesterday: either sound insulation or thermal insulation, both are probably hard to reconcile.

According to the architect and structural engineer, the stone would have to be 42.5 cm thick to qualify for the KFW 55 subsidy. There is also the consideration to use the PPW2 only on the upper floor, but then the sound insulation becomes borderline. The following questions: Does the thicker stone have disadvantages regarding the comfort factor in the house? Will it be too stuffy? I am not planning a central ventilation system.

As mentioned, the same stone only thicker exclusively increases the duration of heat transmission. Only temperature passes through the wall, not CO2. On the topic of controlled residential ventilation, we just have a fresh neighbor thread here

At night you cannot increase the ventilation system because otherwise the noise level disturbs. During the day, the ventilation is audible but not particularly disturbing.

Regarding the mistrust in the tenants’ ventilation behavior, we also talked about controlled residential ventilation; during the phone call, I had a thinking error concerning the integration of the installation into the ceilings: in a two-family house, the central controlled residential ventilation is not floor-over-floor, so the ducts of the central controlled residential ventilation only run within the floor construction. Window rebate ventilations can be done but are basically partial detractions of the seals; I would rather consider MD instead of AD there. Essentially, the effect is like practicing half a centimeter of permanent tilt.

If you look at it entirely from pragmatic points of view, the current plan and energy certificate are in a good range.

That would be a sufficient reason for me not to undertake the redesign (tecture due to the wall thickness, recalculations of heating load and statics). Apart from that, I repeat here my astonishment and pose it as a question to the group: Has anyone here ever heard that aerated concrete at 36.5 thickness is only sufficient for KfW55 under the condition of limiting oneself to a low compressive strength variant? – As far as I know, Massivhaus Mittelrhein is currently also building KfW55 with aerated concrete 36.5 … A look into the funding conditions might be worthwhile; possibly it fits with controlled residential ventilation without wall thickness increase (?)

Has anyone here ever heard that aerated concrete with a thickness of 36.5 for KfW55 is only supposed to be sufficient under the condition of restricting to a low compressive strength variant?

That doesn't sound illogical to me, because lower compressive strength goes along with somewhat better insulation properties. So if the latter is just on the borderline with PP4, this small advantage with PP2 could then be sufficient. Just checked: The strongest PP2 differs from the weakest PP4 by 0.03 W/mK, the weakest PP2 from the strongest PP4 even by 0.08 W/mK (which is then twice as good here).

That doesn't sound illogical to me, since the lower compressive strength is accompanied by slightly better insulation properties.

I fully agree with that so far, only at

If the same [thing] with PP4 is already at the limit, this small advantage with PP2 might then be sufficient.

I then have to disagree: the provider of solid houses I mentioned is not limited to bungalows but builds many city villas and other two-story houses. The original poster wants to build a two-family house, so presumably also a two-story house. Therefore, I don’t believe that the statics should speak against using a commonly available standard 08/15 aerated concrete block, thus limiting the choice to a block requiring more wall thickness and having weaker insulation with the same thickness.

Well, to insulate better you either need a stone with lower thermal conductivity and the same thickness or with the same thermal conductivity and greater thickness. If the lower thermal conductivity is ruled out because of structural weakness, then only the greater wall thickness remains, or am I missing something?

I'm just briefly joining in here.

We also just built, previously the house was not supposed to be a KfW-55, with aerated concrete 36.5

Now we have built a KfW-55 house, also with aerated concrete blocks in 36.5, we only took one block here with a better U-value (additional cost for the entire house of about €2000), so it worked out.
So aerated concrete block is not the same as aerated concrete block ;)