ᐅ KfW 70 standard is not met, reference values for gas condensing boiler heating

Hi everyone,

I need some advice from you. Our new single-family home is currently being calculated, and the person doing the calculations called me to say that the house doesn’t meet KfW 70 standards, which shocked me because I actually thought we would almost reach KfW 55.

Here are the house details:
186m² (2000 sq ft) living area
Basement floor insulation: 8cm (3 inches) XPS, 0.04 WLG (thermal conductivity)
Exterior wall structure: 1.5cm (0.6 inches) gypsum plaster, 24cm (9.5 inches) Poroton T16 bricks, 18cm (7 inches) mineral wool insulation with 0.035 WLG, 2cm (0.8 inches) air gap, 11.5cm (4.5 inches) clinker brick facade
Windows: triple-glazed, Ug 0.7
Roof insulation: 6cm (2.4 inches) continuous insulation above rafters, 24cm (9.5 inches) mineral wool between rafters at 0.035 WLG, 6cm (2.4 inches) insulation below rafters also 0.035 WLG
Heating system: Buderus GBH172 T75S 14kW gas condensing boiler with a 750L (198 gallons) buffer storage, five SKS 4.0 solar collectors at a 45° roof pitch, south orientation, no shading
Underfloor heating on ground and upper floors

I hope I haven’t forgotten anything. Based on the transmission heat losses alone, this should almost be KfW 55, but the heating system is also taken into account. Although it’s a gas condensing boiler, it should still meet KfW 70.

Since I’ve collected quotes from several home builders and construction companies, I know what they typically do to reach KfW 70—and none of them even included a ventilation system, usually only gas condensing boilers with domestic hot water production. Regarding insulation, none of the houses even matched our planned specs.

My question to the experts: Is the person doing the calculations correct? That would basically mean almost all house providers are exaggerating their numbers... or did she make a calculation error? I understand that no opinion here replaces an official calculation, but a rough assessment should be possible.

Thanks in advance!

Good evening,

€uro will surely provide more details on this topic.

rebenstorf schrieb:

186m² (2003 sq ft) living area

Based on the structural calculation, quite a lot of concrete might need to be used in the exterior walls.

rebenstorf schrieb:

Roof with 6cm (2.4 inches) exterior insulation above rafters, 24cm (9.5 inches) insulation with a thermal conductivity of 0.035 W/(m·K) between rafters, and 6cm (2.4 inches) insulation with 0.035 W/(m·K) under rafters

Why have you invested so much effort in the roof? What is the intended benefit?

rebenstorf schrieb:

My question to the experts is whether my calculation person is correct, which would basically mean that almost all house providers are cheating.........

That depends on which providers you have asked.

Gas condensing boilers combined with solar are always a tricky subject; anyone claiming that KfW 70 standard can always be achieved with this combination should consider changing their profession. Whether your consultant is right or made a calculation error cannot be reliably answered here. Many factors—such as the concrete mentioned at the beginning—play a role, as well as the climate location.

Regards, Bauexperte

Hello,

rebenstorf schrieb:
.....and said that the house does not meet KfW 70 and I was shocked because I actually thought we would almost reach KfW 55....

What is the basis for assuming KfW 55?

rebenstorf schrieb:
...Now to the house data:....

Based purely on the initial data, it would formally qualify for KfW 70 (no guarantee without verification!!!)
Wherever people work, errors can happen. I am not error-free either! That is why I use plausibility checks in my calculations to prevent such potential issues.
The creator should be given the opportunity to review. Sometimes these are just formal minor issues or simply incorrect entries!
On the other hand, software errors can occur, which is absolutely not uncommon!!!
I am always surprised at how carelessly software is used. For liability reasons alone, every user should check results manually in example calculations, something that mere "button pushers" or "form fillers" rarely succeed in due to lack of professional expertise. Mainly just producing kilograms of paper!

rebenstorf schrieb:
.... So based on the transmission heat losses, it should be close to KfW 55, but the heating system is also taken into account....

Correct, this is deliberately required by the legislator because the efficiency of a system significantly influences the overall result.

rebenstorf schrieb:
.... My question to the experts is whether my calculating person is right, which would actually mean that almost all house suppliers are cheating.........

Here, one should carefully separate verification procedures from the reality one can actually expect.
Verification procedures only need to meet the "on paper" formal requirements, whereas real consumption costs are a completely different matter!

Best regards

Hello

thanks for the replies.
Now to the questions asked.

Why did you put so much effort into the roof? What is the point of that?

In my opinion, the roof is the most important component, and I simply believe it should be well insulated. Also, in my old house, I only had 18cm (7 inches) of insulation in the attic, and in winter I at least didn’t notice it getting cold, but in summer it got too warm too quickly. So I wanted significantly more insulation and more thermal mass, using stone wool instead of mineral wool, OSB boards instead of a vapor barrier, and insulation above the rafters.

What is the basis for assuming KfW 55?

I thought about KfW 55 because I know the U-values from the 2009 Energy Saving Ordinance, for example, exterior walls and ceiling between floors at 0.28 W/m²K, with 30% of that for transmission heat loss. For KfW 55 this would be 0.196 W/m²K my value according to the U-value calculator is 0.144 W/m²K. I applied values like this to all components, and then I thought it should come close to KfW 55 but definitely KfW 70. I understand that the U-value calculator only gives a rough estimate and does not consider thermal bridges and so forth, but even then there should still be enough margin upwards.

Here one should strictly separate between the verification method and the reality to be expected. Verification methods only need to comply formally on "paper," actual consumption costs are something completely different!

That is clear to me as well. In my case, it is about receiving the KfW funding program 153, with 2.1% interest, so for now the paperwork is what matters.

It seems to me as if the house is calculated according to the 2014 Energy Saving Ordinance, and then I could understand that KfW 70 is not reached because that now roughly corresponds to KfW 55. But from what I understand, the 2014 standard has only been valid for new builds since 2016.

Greetings from Lower Saxony

Hello,

rebenstorf schrieb:

In my opinion, the roof is the most important component, and I simply believe it should be well insulated. Also, in my old house, I had only 18cm (7 inches) of insulation, which was fine in winter—I at least didn’t notice any cold—but in summer, it got too warm too quickly, so I went with much more insulation and increased thermal mass, used stone wool instead of mineral wool, OSB boards instead of a vapor barrier, and installed exterior rafter insulation.

Basically, the insulating effect does not increase linearly with the thickness of the insulation; in my opinion, you could have saved that money.

**Regarding summer heat protection: the purpose of insulation materials is to reduce heat flow, not to store heat. A good solution for the roof area is what is called exterior rafter insulation, which is installed on the outside of the roof structure, fully wrapping the entire assembly. This protects the roof efficiently and economically from intense summer heat, because less thickness is required.

rebenstorf schrieb:

It seems to me as if the house was calculated according to the 2014 Energy Saving Ordinance, and then I could understand that KFW 70 is not achieved, since that roughly corresponds now to KFW 55. But according to my information, the 2014 regulations only apply to new buildings from 2016 onward.

The relevant date for the calculations is the date of the building permit or planning permission application. The 2014 Energy Saving Ordinance initially brings almost no changes for new builds; only from 2016 onwards must an additional 16% in energy savings be achieved. Therefore, it is very unlikely that your consultant made a calculation error.

** I borrowed this wording from RGZ; I couldn’t explain it better myself.

Regards, Bauexperte

Hello again,

I think I have found the mistake.
I reviewed the building application documents and noticed that my applicant (construction company) listed the solar yield as only 15%, which seems a bit low for a system with heating support. I then took the time to calculate the solar yield for my house and other specifications such as location using various online platforms. These calculations consistently showed a total coverage of 30%-35%, significantly higher—even though I understand these are approximate values, it is still about double the 15%.

Also, the application did not mention that a wood stove with a water jacket for heating support is being installed, which, from what I found, can also be credited with 15%. Together, that would be 50% (I know this is a bit optimistic) from renewable energy.

My question to you is: can it really be like that, or did I mix something up?
If not, achieving KfW 70 standard should not be a problem, right?

Best regards