ᐅ Perimeter insulation / slab insulation / brick width / window

I have received a proposal from a well-known developer company in a neighboring town. For budget reasons, I asked them to calculate the cost of the most affordable house that complies with the current energy saving regulations but does not have to meet KfW 55 standards or better. They prepared their calculation based on my specifications. I then gave the offer to an acquaintance who works in construction planning and project supervision and also collaborates with other building companies.

He also provided me with a cost estimate for handling the project himself.

His main criticisms:

- The developer uses only 30 cm (12 inches) Ytong blocks with a thermal conductivity (λ-value) of 0.09, whereas my acquaintance believes it should definitely be 36.5 cm (14 inches) Ytong blocks with a λ-value of 0.08.

- The developer included only 5 cm (2 inches) of perimeter insulation in the basement walls, but my acquaintance recommends at least 10 to 12 cm (4 to 5 inches).

- Basement windows have a U-value of 0.7, while he recommends 0.5.

- The foundation slab is insulated with only 50 mm (2 inches) of insulation, but it should be 100 mm (4 inches).

How would you assess this here?

I don’t quite understand. You wanted it to be cheap, and now they have offered a cheap option. Now your acquaintance is suggesting more insulation because it is better. Of course, more insulation is better. But that wasn’t your original goal.

The question is: Is it worth investing the additional cost of at least 10,000 in better insulation because the expenses will pay off within the next 5 to 10 years?

Whether it is sufficient is calculated on an individual basis; this calculation is part of the building permit / planning permission.
If the contractor calculates that this is adequate, then they have perfectly met your requirement. Where is the problem?

If the opinion of your acquaintance is the most important to you, then proceed with them. Otherwise, you will go crazy because there are simply too many ways to reach the goal in construction (and everyone naturally believes their way is the best).

No.
It’s not worthwhile.
If you save 500 per year on heating through better insulation — which would be a very high amount; in reality, the savings are much lower — you would have to heat the house for 20 years without interest or anything else just to break even on the cost of the insulation. Since we live in a house like the one you offered cheaply, I can tell you it is warm and comfortable to live in. My gas supplier estimates my annual cost at around 800. Karsten

Thank you for your assessments, that already helps me a lot. I do need to save quite a bit, but I was unsure whether the low insulation might cause problems for me later on.