Construction of a multi-family house or 2 semi-detached houses according to KfW40+

Dear community,

The plots have just been purchased and the floor plans are also quite fixed.

Now we are at the very beginning of the remaining planning for this new building. I still have many questions about construction details and building technology that unfortunately have not yet been sufficiently clarified.
I would like to present the project as far as we have planned and what our ideas are. Due to the plot sizes and the development plan, there are unfortunately quite tight limits regarding the size (resulting, for example, in an external technical building, etc.).

The following key data are already fixed:
2 plots (2 owners = 1x my brother, 1x myself).
Boundary construction is mandatory, so 2 semi-detached houses will be created (one per plot with 2 residential units each). The building is two stories (eaves height unfortunately only max. 4.5m).
Gable roof with roof pitch 30-45°, max. height 9m (all max. heights measured from OKFB). No basement. Upper floors accessed via external stairs.

Left half 2x approx. 65m² each, right half 2x approx. 45m² each (each upper floor naturally about 4-5m² smaller than the ground floor).
Ridge runs from west to east, i.e. the roof area for photovoltaics is exactly south-facing.
Many windows/doors on the south side of the ground floor, one roof window per room on the upper floor. Attic insulated but only accessible via “emergency stairs”. Balconies for the upper floor facing west and east respectively.

3 residential units will be rented out, I will move into the small 45m² unit on the ground floor myself.

Separate approx. 8m² technical building separated from the house by approx. 1.5m distance (for heat pump, storage, water and electricity connections). Due to the required space for the technology and the boundary distances, hardly feasible otherwise.

Our wishes and details of the technology and construction method as far as implementable:

Solid wood construction. The wall and roof structure has already been "roughly" calculated by an energy consultant based on our first draft drawings and approved for KfW40 Plus. Some details on windows and exact thermal bridge calculations are still missing.
Wall construction from inside to outside (U-value: 0.11 W/m²K)
1.25 cm Gypsum board W/mK: 0.25
10.00 cm Solid wood wall W/mK: 0.13
26.55 cm Wood fiber insulation W/mK: 0.039
6.00 cm Softwood fiber insulation boards W/mK: 0.040 (plaster carrier)
1.20 cm Exterior plaster W/mK: 1.00
45.00 cm total

Roof construction from outside to inside (U-value: 0.13 W/m²K)
A Inhomogeneous layers: 10% structural wood
1.5 cm Gypsum board W/mK: 0.25
2.25 cm Air layer W/mK: 0.16
24.00 cm Structural wood W/mK: 0.13
8.00 cm Softwood fiber insulation boards W/mK: 0.042
B Inhomogeneous layers: 90% insulation between rafters
1.5 cm Gypsum board W/mK: 0.25
2.25 cm Air layer W/mK: 0.16
24.00 cm Structural wood W/mK: 0.13
8.00 cm Softwood fiber insulation boards W/mK: 0.042
35.75 cm total

Floor slab from inside to outside (U-value: 0.14 W/m²K)
7.0 cm Screed W/mK: 1.40
12.00 cm Impact sound insulation board W/mK: 0.032
24.00 cm Reinforced concrete with 1% steel W/mK: 2.30
10.00 cm EPS W/mK: 0.035
53.00 cm total
(The 10cm EPS will be replaced by glass foam gravel, thickness will be determined once windows are fixed and the building recalculated)

The wall construction may still be adjusted to achieve thinner walls by using energetically better materials (e.g. 0.031 mineral wool instead of wood fiber insulation boards with approx. 0.039). However, this only if a) the summer heat protection is also good and b) ecologically no too large compromises have to be made. Unfortunately, I still do not really know the pros and cons of mineral wool compared to wood fiber insulation boards?

The perimeter insulation of the floor slab should be made of approx. 40-60cm of layer-wise compacted glass foam gravel (the required insulation/thickness will still be calculated).

Overall, we want to build as ecologically as possible and achieve a high quality of living. Also, of course, because I want to spend the next 30-50 years here if all goes well. Our primary concern is not the cost of the building but what is energetically and ecologically sensible to implement. However, there is also a maximum budget. I believe economics and ecology can definitely be reconciled with at least a good compromise. We also hope not to exhaust this budget thanks to the currently quite high subsidies for KfW40 Plus buildings.

What we still wish for:

[*]KfW40 Plus
[*]Air-to-water heat pump (alternatively brine/water heat pump if significant advantages can be achieved thereby). There is not enough space for trench or surface collectors.
[*]Photovoltaics (= approx. 13-14KW peak possible), therefore possibly split into 2 systems
[*]Li-ion battery storage (approx. 13kWh possibly Tesla PowerWall or are there more ecological alternatives?)

(Problem with the electricity: what do we do with the excess electricity that is not “used up” by the heat pump and cannot be used as common electricity? Tenant electricity is apparently difficult/complex?!)

[*]Controlled residential ventilation
[*]Additionally, we wish for cooling if possible. Various options are available, all with different pros and cons:
[*]Cooling via underfloor heating (then done by the heat pump) with low efficiency
[*]Cooling via separate ceiling/wall cooling (cost/benefit?)
[*]Ceiling heating/cooling alternative to underfloor heating
[*]Cooling via the ventilation system with downstream cooling coils per residential unit (disadvantage: condensate not/easily controllable??)
[*]Air conditioning unit(s)
[*]Idea: lay a kind of earth collector to supply permanently cool air into the ventilation. Operated by bypass circuit when cooling is required.

So, that would be the first batch. I look forward to plenty of feedback.
We will be at the company we want to build with during the next week and have our plans checked/improved where necessary. Only then can we get cost estimates for the building envelope.

Can you rent out the apartment economically?
Is an external technical room possible? It already gets more expensive if the rooms are located in the garage.
Why KfW 40 Plus and not simply Passive House?

Nabu provides an overview of ecological insulation materials
Google Nabu Dämmstoffe

WDVS Fraunhofer
There is research involving mealworms and a facility (to my knowledge not yet operational) for recycling. If both work on a large scale, they should not be underestimated.

Solid wood and ecology.
There is not enough wood to meet the construction boom without other materials. The question is where your wood comes from, not where it is ordered from.
Otherwise, I find solid wood good and would use it again. Only regarding ecology, I have my doubts.
Unfortunately, there is no breakdown of which building materials have environmental impacts from production to disposal including transport (water pollution, CO2, other emissions, feeding the world population, wildlife, consequences of deforestation, consequences of monocultures).

Since you are building KfW40, read up on the systems from Stiebel LWZ604 air and Tecalor 2.5. These are ventilation heating systems with inverter air-to-water pumps. As a result, they are more efficient than pure air heat pumps.

Use photovoltaic electricity for your own consumption and feed in the rest.

We don’t have cooling in summer and haven’t missed it after 2 summers. (Passive house made of solid wood, LWZ 604 Air) Shading is important, otherwise you will get steamed.
I could imagine that small apartments heat up faster. More heat is produced inside the apartment than in a large single-family house. More heat is produced.
Some are in favor of air conditioning, others are satisfied with heat pump cooling. It probably depends on how warm you like it.

Interesting, I would be much more interested in why you are planning a mega-expensive giant property to then live in 45sqm? I assume you live alone and your brother does too? How old are you both? Will it be a shared retirement home? Will you show us some sketches? :)

no no no... All just wrong assumptions. I already have an (old) house which is now completely used as an office, and the rest (one floor) is rented out. I used to live there with my family (with a small office), later, when the children were out of the house, only with my wife. But living in the office permanently is not healthy... Now I still live in a "too big" apartment, but I am moving into the house or rather into my apartment alone (I am now separated). I rent out one floor, my brother rents out both apartments. He "builds just for fun" with me. He already has several apartments, plots, houses, etc. But I might have different preferences than others here in the forum. Big is not always better. I once lived testwise in a TinyHouse, that was a bit too small. I am now over 50, so I don’t want that much stuff in my apartment anymore. There is still enough space for children, grandchildren, and other visitors. I also want to design a nice large outdoor area that I use in summer. Even though I no longer live in the office, I am regularly there for 12 hours. So why 100m²? Just for sleeping and showering? Actually, it is not supposed to be about the circumstances why we plan such a property as we do. But how we can implement our wishes well ... :cool:

Can you rent out the apartment economically? Is an external technical room possible? ...

Fortunately, renting out economically is not necessarily required. It should only contribute partially. There are also social aspects here! The planned total scope (budget) is supposed to be paid off within 12-13 years (excluding the land).

An external technical room is permitted as a boundary construction (since it has no fireplace = meaning no gas/oil, etc.).

If your brother builds for fun, I ask myself, is there really a shortage of affordable nice 2 to 3-room apartments or is there rather a shortage of affordable housing for 4-5 person families, so that one creates from the semi-detached house with 2x around 65 sqm rather 1x family house? Here where I live, that would be more adequate rental space if you don’t need absolute returns. It also has the advantage that tenant turnover is probably minimized. Just an idea.