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

I want to build a KfW40+ prefabricated house with a basement, two full stories, measuring 12m by 8m (39ft by 26ft). The basement will house the technical equipment as well as a music room, both featuring 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 more cost-effective," so additional insulation would be applied between the basement ceiling and the ground floor, as well as insulation under the music room.

Provider 2 recommended calculating the basement within the thermal envelope.

I am leaning towards provider 2 because including the basement within the thermal envelope makes more sense to me. This also saves the extra insulation between the basement and the ground floor.

What do you think?

Thanks in advance.

If you want underfloor heating and mechanical ventilation in the basement, and the basement is also intended to be "habitable," you can only include the basement within the thermal envelope.

KalterKaffee schrieb:

I tend to prefer provider 2, since including the basement within the thermal envelope makes more sense to me. Also, this saves the extra insulation between the basement and the floor above.

However, additional costs arise for the insulation beneath the slab (or on the slab beneath the screed – but then a higher basement is necessary) and around the perimeter of the basement. This becomes more expensive than option 1.

What does the energy consultant say, the one who has to calculate KfW 40? Option 1 is simpler.

nordanney schrieb:

The additional costs for insulation under the ground slab (or on the slab beneath the screed – though this requires a higher ceiling in the basement) and around the basement still apply. This will be more expensive than option 1.

Provider 1 is quoting 8,000 euros for insulation between the basement ceiling and the ground floor, but also for insulation beneath the basement slab, about 18cm (7 inches) under the underfloor heating. Additionally, there is insulation on the interior door and under the basement stairs. This seems a bit elaborate to me.

nordanney schrieb:

What does the energy consultant, who has to calculate KfW 40 standards, say? Option 1 is simpler.

I don’t have an energy consultant; I rely on the provider, as they must do the calculations. At the moment, I’m only in talks with sales representatives to get an offer. But the KfW calculation will only happen after signing the contract.

The basement excavation will already cost quite a bit, so I’m not sure if it makes sense to save money by planning the basement outside the thermal envelope.

nordanney schrieb:

If you want underfloor heating and controlled ventilation in the basement, and you plan to use it as a living space, you must include the basement within the thermal envelope.

Provider 1 knows I’m planning a music room in the basement with underfloor heating and controlled ventilation. It’s not exactly a "living basement," but more like a "hobby basement."

I’m just a little worried that the energy consultant will say after contract signing that "this won’t work."

Thanks again.

The requirements for KfW40+ are very detailed. An energy consultant is mandatory; only with one is KfW40+ possible. If the basement is classified as "living space," it automatically becomes part of the thermal envelope. This eliminates Option 1. There you go, life can be that simple.

superzapp schrieb:

There must be an energy consultant; only with one can you do KfW40+.

I understand that, it was never in question 🙂 They only get involved after the contract is signed, just like the architect. This is about offers before signing the contract.

superzapp schrieb:

If the basement is classified as "living space," it automatically counts as part of the thermal envelope.

I never said the basement would be classified as "living space."

Provider 1 (seller) wants to classify the basement as a "hobby room" and therefore as a utility basement.
The basement is neither used for sleeping nor full-time living. The music room doesn’t even have windows and therefore can’t be classified as living space, right?

superzapp schrieb:

There you go, life can be that simple.

Sorry, but it’s not that simple.

The basement has no habitable rooms, the "hobby room" has no windows, but it does have underfloor heating and mechanical ventilation with heat recovery. That means it’s comfortable to spend time in the music room/studio, but it’s not a living area.

So both options seem “legitimate.” My question is what makes more sense for me.
I’m leaning towards provider 2 (inside the thermal envelope), but maybe I’m missing something.

KalterKaffee schrieb:

It’s clear to me as well, it wasn’t questioned 🙂 They only get involved later, after the contract is signed, just like the architect. This is about offers before the contract is signed.

But you can’t sign a contract if it’s not at all clear that your request can actually be realized. I also wouldn’t buy a car and then find out after the purchase that the all-wheel drive I wanted is not available in my model.