Developer says: No more insulation!

This is a T12 brick.
A few things like the windows are also better, but mainly for environmental reasons, less to save money.

But objectively viewed, excessive insulation is a losing business.

With house building companies online (as far as their price lists are public) each KFW level costs around 10,000 euros.

With the first one (just barely meeting the Energy Saving Ordinance to KFW) the heating costs decrease from max. 750-800 euros to about 550 euros.
Let's say 250 euros saved per year.

10,000 euros put in the bank: 400 euros interest earned. And with each next level it gets even more unrealistic.

Since this topic is currently on my mind, I am bringing it back to life.

I am now facing the decision between double or triple glazing. The additional cost is €850.
What do you recommend?

....I am now facing the decision between double or triple glazing. The additional cost is €850. What do you recommend?

The additional cost is marginal! Only the guaranteed (certified) technical parameters matter, especially the U-value and the g-value. It is not uncommon that with a lower U-value, the g-value also decreases! Then you have not gained much, and the overall result can even be worse! Without a "real" consumption forecast, this cannot be assessed at all. Best regards.

I calculated the matter purely from an economic standpoint on the website "WWW.u-wert.net" and was somewhat surprised.

Wall construction 1 (standard property developer):
- 2cm lime-cement plaster
- 15 cm sand-lime brick
- 12 cm ETICS 035
- 2 cm lime-cement plaster
U-value of the wall 0.26
Heat loss: 18 kWh per m2 per heating period

Wall construction 2 (recommendation of most):
- 2cm lime-cement plaster
- 15 cm sand-lime brick
- 16 cm ETICS 035
- 2 cm lime-cement plaster
U-value of the wall = 0.20
Heat loss: 14 kWh per m2 per heating period

With an exterior wall area of 162 m2 and a saving of 4 kWh per year, I come to a saving of €45 per heating period at the current gas price. The 4% extra interest alone for my €2,500 investment amounts to €100 in interest per year, without having recouped a single cent of the investment costs.

I have now calculated the payback period for the investment of €2,200 and came to 44 years (!) assuming a gas price increase of at least 5% per year.

I came across this topic via Google, as I am currently dealing with it myself.
We currently have a 17.5 cm sand-lime brick with 12 cm 032 insulation planned. The consideration now is to increase to 16 cm.
The calculation here is similar to the quoted post, only I don’t know if I am also falling for an overly simplistic calculation or not.

Approx. 200 m² wall area, the surcharge from 12 to 16 cm is about €3-4/m² according to online construction shops.
According to the U-value calculator, it would be an improvement from 0.24 to 0.18 W/m²K and over the heating period a reduction in heat loss from 19 to 14 kWh per m².

Now the crucial question: are these saved kWh to be equated with the electricity consumption of the heat pump (ground source)?
Because then I would come to a completely different calculation.

Surcharge for the 200 m² at €3-4: €800
Saving: 5 kWh per m² x 200 m² would be 1,000 kWh saved per year.
At about 18 cents electricity cost per kWh, that would be almost €200 saved per year, and after 4 years it would have paid off.

So a clear recommendation to take 4 cm more or is it still the mentioned overly simplistic calculation?

...according to the U-value calculator, it would be an improvement from 0.24 to 0.18 W/m²K and, over the heating period, a reduction of heat loss from 19 to 14 kWh per m²

How were these numbers determined?

...now the crucial question, can these saved kWh be equated with the electricity consumption of the heat pump (brine)?

No, the demand for heating energy is indeed reduced by the decrease in transmission heat losses. Exactly how much should be determined precisely. The "saving effect" only applies to the electricity portion of the brine heat pump, but not to the much larger "energy portion from the ground." At the same time, the solar gains of the opaque components (AW) are reduced. It is possible that due to the reduction in heating load, a brine heat pump with one performance stage smaller can be chosen (investment savings, lower capital costs). On the other hand, it may happen that due to the improved insulation, the heat pump is significantly more oversized because of performance stages. The annual performance factor worsens. Then the intended effect may reverse or be ineffective altogether. Whether online building shops are fundamentally suitable for price formation should be verified.
Best regards.

How were these figures determined?

The U-value calculator "determined" this based on the monthly average temperatures from the German Weather Service.