ᐅ KfW40+ with a basement inside or outside the thermal envelope?

nordanney schrieb:

But you can’t sign a contract if it’s not clear at all that your request is feasible.

I assume that both options are feasible and each has its pros and cons.

Provider 1 suggests it is "cheaper" (and possibly easier for the builder) to design the basement outside the thermal envelope, insulating between the basement ceiling and the ground floor, which meets the KfW40 efficiency standard. However, additional insulation is still needed under the basement slab because a floor heating system will be installed for the music studio, occupying about 33m² (about one third) of the basement.

Provider 2 believes that if you already have floor heating and a controlled mechanical ventilation system in the basement, it makes more sense to insulate under the basement slab, thus including the basement within the thermal envelope. This is likely feasible, but the required insulation thickness depends on the energy calculation. The exact costs for this (like excavation etc.) will only become clear later.

The advantage of option #2 is that I could plan an additional basement room that could be declared as living space (with floor heating and a window), for example as a guest room or office.

That’s why I lean towards option #2, even though it involves a higher cost risk (for example, it might require more expensive insulation materials to include the basement in the thermal envelope), with more concrete costs only becoming clear after the energy calculation.

Just forget about KfW40+ and be done with it. It’s not cost-effective, especially with a basement.

KalterKaffee schrieb:

I want to build a KfW40+ prefabricated house with a basement, two full stories, measuring 12m by 8m (39 ft by 26 ft). The basement will house technical equipment as well as a music room, both with underfloor heating and connected to a controlled ventilation system. The plot is flat. Provider 1 suggested calculating the basement outside the thermal envelope as “better and cheaper,” which is why there will be insulation between the basement ceiling and the ground floor, as well as insulation beneath the music room.

In the scenario you described, I don’t see any room for choice (other than reconsidering the decision for KfW40+). Provider 1 has effectively disqualified themselves and made your final decision easy; the winner is Provider 2. Proceeding without consulting the energy consultant—who in my opinion is essential for KfW40+ projects—until after the fact would be a textbook recipe for disaster. Having a music room in a cold basement would be almost like planning a separate living unit for a piano tuner. If the plot is really as completely flat as you say—that the terrain does not even vary by the thickness of the basement ceiling—then by my basement calculation formula, you clearly don’t need a basement at all. The building coverage ratio would have to be extremely strict and limiting not to build the music room above ground.

---

https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

apokolok schrieb:

Just forget about KfW40+ and move on. It’s not cost-effective, especially with a basement.

As I have mentioned many times before: there are not only financial reasons to choose to build to the KfW40+ standard 😉

If commercial considerations are not the reason for choosing a house with a basement plus more than 40 square meters, then the implementation should not be decided commercially either, and no lazy compromises like "insulating one basement room and the ceiling" should be made.

superzapp schrieb:

As I have mentioned many times before: There are not only financial reasons to choose construction to the KfW40+ standard 😉

However, these are the only measurable and, for reasonable people, decisive reasons.
If someone wants to save the world and tell their neighbors how eco-friendly it is, they can do so, but for others, it holds no relevance.