Calcium silicate brick interior/exterior insulation

Hello, we have recently become homeowners; it is a two-family house built in 1966, 17.5 cm sand-lime brick masonry, 150 sqm.

Currently, heating is done via a central oil heating system with small stoves.

The plan is to install a heating system with radiators (oil condensing boiler) and then insulate the house (including the roof and basement, the windows are new).

Initially, insulation of 14 cm with 0.35 was planned, but since this is difficult to implement due to the roof overhang and other structural conditions, I was advised to use 12 cm with 0.32.

The interior rooms are all covered with drywall and Styrofoam behind it, totaling 8 cm (walls and ceilings); I cannot yet say whether there is a vapor barrier or a cavity behind it, but during the heating works, the wall will have to be opened anyway.
This was already done during the construction phase, according to the seller!

Now my questions:

Does external insulation still make sense in this case?
If yes, is the proposed insulation OK?
How much savings can be expected from the external insulation?

Hello,

...It is planned to install a heating system with radiators (oil condensing boiler) and subsequently insulate the house (including roof and basement, windows are new).

Whether it is still advisable to choose oil as an energy source nowadays is very questionable.
Moreover, measures for building insulation reduce the heating load. Only when this is known can a heat generator be selected sensibly. Otherwise, frequent cycling is to be expected, which significantly reduces energy efficiency in part. In addition, the wear of certain components increases excessively.

...How much savings are to be expected from external insulation?

This can be calculated (or commissioned). Since the KFW again supports individual measures, professional supervision of the project is required anyway.

Best regards

The interiors are all paneled with gypsum plasterboards and Styrofoam behind them, a total of 8 cm (walls and ceilings)

The exterior walls too? If yes, a very strange type of building insulation. Sand-lime brick has hardly any insulating value and no 17.5 cm brick is really very thin, so insulation from the outside is absolutely necessary, but only with professional planning!

I better say nothing about the oil heating system!

Addendum:

...all covered with gypsum plasterboards and behind that styrofoam insulation totaling 8 cm (walls and ceilings)

I had overlooked that in my haste. Additional external insulation will probably cause a condensation problem, if it is not already present. Check first! Open 1.. 2 m² at a particularly vulnerable spot, e.g. the north wall, and determine the current condition.

Best regards

Hello Euro, unfortunately, in such a project the financial side must be considered and the switch to an oil condensing boiler was the most cost-effective solution here. Since new oil tanks are installed in the basement. Even if the long-term oil price development is not so positive. However, I can get firewood quite cheaply which I would like to use as another option. Hello blurboy, at the moment it is only insulated from the inside. The heating will only be installed once we know if and how insulation will be done and it will be adjusted accordingly. The thing with KfW is a bit of a matter, the initial consultant meeting is at least partially paid but if the consultation goes into more detail it costs our money and that is tight. The aspect of mold behind the interior insulation does concern me, I am thinking of a mold detection dog. Regards Klaus

Oh dear BJ.1996 and internal insulation, while almost every layperson knows that this is a no-go. So from my point of view, I advise you to tear out the entire internal insulation of the exterior walls and get a professional who insulates the house from the outside professionally or properly advises you for self-help, everything else is madness.