Window offers like day and night

Understood. So I need an expert who first calculates the current state for me and then has an idea of which measures would be best to achieve the 140%. I had already registered on the mentioned portal with the experts. Let's see if someone gets back to me. Otherwise, we won't really make progress.

Using an "energy efficiency expert" usually makes sense in existing buildings, mainly due to the funding opportunities provided by the KFW. However, you can also download corresponding software as a trial version and get a pretty good impression yourself with a few hours of effort about what is possible and which adjustment options affect what. And at least the HT value can be calculated very well by yourself simply by multiplying the individual component areas by the U-value (u-wert-Rechner (ubakus)), summing the results, dividing this sum by the total surface area of all components, and adding the thermal bridge surcharge.

Example (highly simplified): For a house with 200m² roof area, which has a U-value of 0.2, and 300m² wall area with a U-value of 0.4, at a standard thermal bridge surcharge for old buildings of 0.1, it would look something like this:

200 * 0.2 = 40 300 * 0.4 = 120 -------- Total surface area: 200 + 300 = 500m² HT sum: 40 + 120 = 160

HT building: 160/500 + 0.1 = 0.32 + 0.1 = 0.42

Here you can also clearly see how much detailed thermal bridge calculation can affect the result. If you only have to calculate with 0.07 instead of 0.1, for example, the HT improves to 0.39. This brings more than upgrading from simple triple-glazed windows to passive house windows.

By the way, I would actively contact the experts.

Roof? What roof?
(A little joke)

I had already started calculating myself. The Ubakus is a great tip! Thank you. I didn't know that yet.

We will definitely actively contact the experts. What causes me problems is finding the right one. A chimney sweep is of little use to us. We really need a professional and not a wannabe. Maybe ask the building authority?

What causes me problems is finding the right one

I would simply try searching for "KFW-Denkmal". Experts who can do that should generally also understand something about old (worthy-of-preservation) building fabric and be able to show appropriate options for treating it within the framework of a modernization.

But yes, in the end it unfortunately remains a leap into the cold water – as always :-(

At the moment, we are getting bogged down in the regulations. We found this on the internet:

Energy Saving Ordinance 2014: Component Requirements Clarified

A welcome clarification is provided by the Energy Saving Ordinance 2014 in § 9 (Alteration, Extension, and Conversion of Buildings).

The first paragraph of § 9 (Alteration, Extension, and Conversion of Buildings) in the Energy Saving Ordinance 2009 led to the most common misunderstanding among both building owners and professionals: Owners of old buildings who wanted to renovate a part of their facade, roof, or some windows mistakenly believed that they had to renovate the entire facade, roof, or all windows according to the Energy Saving Ordinance 2009.

While the Energy Saving Ordinance 2009 referred in the mentioned paragraph to the “affected external components” and the requirements for thermal insulation, the Energy Saving Ordinance 2014 formulates the text more clearly in § 9: Alterations must be carried out, if applicable, so that “the thermal transmittance coefficients of the affected areas do not exceed the maximum thermal transmittance coefficients set for such external components in Annex 3.”

In short: Only those areas of an external component that are actually "touched" or energetically changed must meet the Energy Saving Ordinance requirements.

Basically, the question arises whether a change of use requires drastic changes to the building at all. Again, from the internet:

... the Energy Saving Ordinance (Energy Saving Ordinance 2014) provides in § 10 (Retrofitting of Systems and Buildings) only the following retrofitting obligations, if applicable:

- Replace old boilers,
- insulate uninsulated heating pipes,
- insulate uninsulated hot water pipes,
- insulate topmost floor ceilings.

If that is the case, we would only be obliged to insulate the topmost floor ceiling, everything else is either already new or as required by the Energy Saving Ordinance.

The question that arises concerns the change of use. If I convert an industrial building into living space, which regulations then apply? After all, you can't just declare any garage a house. On the other hand, the house is not a garage – after all, it has 60 cm thick walls with gas, water, shit, etc.

I strongly suspect that we could indeed put apartments in there without having to carry out massive Energy Saving Ordinance renovation work. The only question is, where is that stated?

I sent you a link via PM, it is actually explained well there.

Edit: For the interested reader in brief:

1) Change of use without alterations -> No Energy Saving Ordinance
2) Change of use with structural alterations -> Components (e.g. windows) must comply with the Energy Saving Ordinance, or the entire building according to the 140% rule
3) Change of use with expansion/enlargement -> If new living spaces are created (e.g. turning a garage into a bedroom), the "touched" components must comply with the Energy Saving Ordinance (e.g. windows replacing the garage door).
If the expansion is more than 50m² and a new heat generator is installed at the same time, the expansion as a whole must comply with the Energy Saving Ordinance. That means also the exterior walls or the roof of the garage, even if nothing has actually been done to them.