How much real savings through better walls

Hello dear community,

as some of you already know, I am planning a new build according to KFW 55, whereby I set this as a minimum requirement and would like to have it even better if possible.

We want to build monolithically with Ytongg. We want the windows to be as good as possible anyway. Regarding floor and roof insulation, I am of the opinion that the jump from Kfw 55 to 40 does not make a big difference in terms of savings. I once saw a construction diary on the internet where the builder had a total surcharge of only about ~2500 € (+ 5000 € repayment subsidy) for improved roof and floor insulation. However, he already had a very good timber frame wall. With Ytongg, the prices vary quite a bit, and my question is of course which block is also cost-effective.

For example, for my house, a...

- 36.5 cm, Lambda 0.09, U-value 0.23 ~ 19,500 €
- 42.5 cm, Lambda 0.09, U-value 0.20 ~ 22,700 €
- 42.5 cm, Lambda 0.08, U-value 0.18 ~ 26,800 €
- 48.0 cm, Lambda 0.08, U-value 0.16 ~ 30,300 €
- 48.0 cm, Lambda 0.07, U-value 0.14 ~ 42,400 €

Prices are from price lists found on the internet.

I think the last block with a U-value of 0.14 and an additional cost of about ~12,000 € is already out the window.

I have two questions...

1. If we really stay with KfW 55, up to which block does the additional cost pay off? For the time span, I am being generous and assuming 30 years. By then, the house should be paid off. The goal is 25 years though.

2. Can Kfw 40 be achieved with the block with U-value 0.16?

A short note about the house.

- 180 m² living and usable area
- probably brine pump with trench collectors
- probably photovoltaic
- probably controlled residential ventilation with heat recovery
- at least KfW 55

I hope you can help me with this.

Best regards Judyyy

Search for "kfw 40 ytong". You will find a PDF from YTONG in which the company deals with the Energy Saving Ordinance 2014. There you will find configuration examples of how the corresponding KfW standards can be achieved with their materials. Accordingly, for KfW 40 the 48 cm block with U 0.14 is required if the monolithic construction method is to be maintained. Whether the additional cost is as drastic as you outlined will only be told to you by your potential building partner. We are planning on KfW 55 and will certainly also examine the extra cost for 40, especially since Qp is already given thanks to the planned brine-water heat pump, controlled residential ventilation with heat recovery, and photovoltaics, which probably already borders on passive house standard. However, I do not assume that it will be economical. If you want KfW 40, in my opinion you have reached the limit with monolithic construction. Gluing on thicker polystyrene is simply easier.

Yes, I know this PDF very well. :D

I once asked in another thread whether the minimum requirements are really binding because ultimately the annual primary energy demand is what matters, right?

Best regards
Judyyy

These are just examples. The actual calculation depends on the building, its shape, orientation, etc., and is carried out by the energy consultant. For orientation, one can use the values from the reference value calculation for [KfW 55], which ultimately represents a simplified procedure. The primary energy demand, essentially influenced by the system technology, as well as the transmission heat loss, influenced by the materials used, must fit.

That's why we actually brought an architect, who is also an energy consultant, on board.

Only he seems to be overwhelmed by the question of the cost difference between KfW 55 or 40. :rolleyes:
He says he'll do it the way I want. o_O

Architects always seem to be very quick to be satisfied with the explanation,...
KfW 40 is not worth it. :rolleyes:

Can this possibly be calculated like this?

If you now have heating costs of €800 with KfW 55 and a heat loss through the exterior walls of 30%, that is €240.

If you improve the wall from 0.23 to 0.16, that is an improvement of ~30%.
If I now subtract 30% from the €240, that is just €72 annually.
Then even after 50 years that is only a saving of €3600 but an additional cost of €10,000.
Can it be calculated like this or is this calculation complete nonsense???

Regards
Judyyy