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

Created on: 30 Dec 2020 11:05

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.

parcus31 Dec 2020 17:10

#31

Anbieter1 can also calculate based on unheated conditions. In residential construction, it is possible to use a two-zone model. However, this is rather uncommon and is usually applied in multi-family buildings and stairwells.

KalterKaffee31 Dec 2020 19:10

#32

Harakiri schrieb:

With waterproof concrete 28 cm (11 inches) and XPS insulation, it is only feasible with 180 mm (7 inches) or 200 mm (8 inches) perimeter insulation.

Okay, is there a big price difference between 200 mm (8 inches) and 140 mm (5.5 inches)? The current basement offer has waterproof concrete 24 cm (9.5 inches).

Harakiri schrieb:

What also works is core insulation made of PUR/PIR between precast wall elements plus XPS.

Ah, okay. I will discuss that with the basement builder. Thanks.

parcus schrieb:

The crucial point is how the thermal bridge surcharge is documented.
For this, the KfW provides three methods.
Only then can the concepts/specifications and costs be properly compared.

Hmm, sounds complicated 🙁

nordanney schrieb:

Correct, the heat pump MUST be inside the thermal envelope. So you still need a technical room above ground.

If that is the case, then offer 1 cannot be implemented. The heat pump is in the basement (air-to-water heat pump plus domestic hot water plus balanced ventilation combined unit). The technical room upstairs (ground floor) only has space for washing machine and dryer, nothing else.

KalterKaffee31 Dec 2020 19:25

#33

Thanks again to everyone for the advice and best wishes for a great start to the new year 🙂

Baubaubau31 Dec 2020 19:31

#34

Can't you simply have provider 2 include in the contract that you are building a KFW40+ house? Then it’s up to them to decide how thick the insulation needs to be afterwards. I wouldn't sign a contract otherwise, as everything is open to additional charges. What if they later tell you that the calculation requires 35cm (14 inches) of insulation and that it costs 275 euros per square meter? What then?

KalterKaffee31 Dec 2020 19:47

#35

Baubaubau schrieb:

Can’t you just have provider 2 include in the contract that you’re building a KFW40+ house?

That is in the contract, but “from the basement ceiling”… I can ask for a quote where provider 2 also supplies the basement, everything from one source.

Then I can also ask about the cost difference between KfW55 and KfW40+.

At first, I wasn’t sure if I could afford a basement. But after the soil survey, chemical analysis, quote from the excavation contractor for earthworks, and basement offer with 12cm (5 inches)/14cm (6 inches) perimeter insulation, I found the costs acceptable. That’s why I thought the prefabricated house manufacturer could then build the house on that basis. But it’s not that simple after all.

Now I know what to do, thanks again.

knalltüte31 Dec 2020 21:22

#36

nordanney schrieb:

Correct, the heat pump MUST be located within the thermal envelope. So you will also need an above-ground utility room.

This is not accurate (see our house construction thread with an external utility room where the heat pump is installed and which is not within the thermal envelope). This required additional phone calls between the energy consultant and the KfW to clarify. The relevant clause is apparently not phrased very clearly.

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

Reaktionen