@fionax
Don't compare your house with your neighbour's house.
Suporex is a very warm building material and one of the best when it comes to the so-called microclimate.
Brick, on the other hand, depends on what kind.
Solid and hard bricks are a poor insulating material. But often German houses were built with porous bricks, which were fired, I think, with an admixture of sawdust. I myself owned a house built by the Germans with such bricks.
I did not insulate the house, and I also instructed a colleague who bought the house from me in 2012 not to insulate it either. The house is uninsulated, but it is very warm, precisely because of the brick.
The brick was porous and resembled suporex in texture and such a brick was very warm. If you have a house made of such bricks, it was a mistake to insulate it with 5cm thick Styrofoam, because you will only do yourself harm.
What I am about to write to you will certainly seem paradoxical.
Well, the thicker the wall and the warmer the wall material, the thicker the layer of polystyrene foam you should insulate it with.
It is not about thermal conductivity (heat loss), but about the conductivity of water vapour and moisture and the so-called dew point which will form inside the wall. Styrofoam is a material that is completely impermeable to water and water vapour.
From inside the house, moisture enters the wall, which normally has to be evacuated from the wall. Otherwise the wall will be all wet after some time. A wet wall will lose its thermal insulation. This water will accumulate inside the wall due to the so-called dew point. The dew point is the relationship between the level of absolute humidity and the temperature, which leads to saturation of the air with water vapour and condensation of water vapour in the form of dewdrops. Then the relative humidity inside the wall will reach 100%.
Such a drenched wall will conduct heat much better, i.e. heat loss will increase, and in addition, just under such thin polystyrene, this water will freeze. This will cause the bricks to lasso (crack and crumble). In addition, a fungus will develop under the polystyrene foam. After a while, the inside of the house will smell musty.
As far as fuel consumption is concerned, it depends on the heat loss of the building through the walls, but also through the windows and the ventilation. However, the greatest heat loss is through the highest ceiling or roof if it is a house with a usable attic.
A lot also depends on the preferred temperature in the house. Some people will say it's hot in the house if it's 18-19°C, while others if it's 24-25°C will say they're cold. I have a friend who has 28°C at home and sits in a thick jumper because he is cold. I, on the other hand, get tired at his place because I sweat sitting in a short-sleeved T-shirt.
Remember that maintaining a 1°C higher temperature in the house generates about 5-6% additional fuel consumption.
You yourself wrote that you have 18°C at home and it is warm.
I, for example, have to have 23-24°C and, of course, even if I lived in your house, I would have to burn about 30-40% more coal than you.
Now back to the wall and the dew point.
The wall is a partition between two media.
One is the warm and very humid interior and the other is the very cold and very low humidity exterior. Between these centres there is a transfer of heat energy from the warmer to the cooler medium. But in addition, there is also the penetration of a large amount of moisture into the interior of the porous wall. The closer to the outside of the wall, the lower the temperature of this wall and the more the relative humidity of the air in the wall increases.
Why do we need to give a thicker layer of insulation in the case of a warm wall?
The reason is to ensure that the entire wall is at such a temperature that the dew point does not occur inside the wall. If a wall is insulated with polystyrene foam, the moisture will not escape through the wall. This moisture must return to the inside. But in order for the wall to return this moisture to the inside then the whole wall must have a temperature above the dew point.
Below you have a table of dew points:
On the left you have the air temperature and at the top you have the relative humidity values at a certain temperature.
Take for example a temperature of 20°C and a relative humidity of 60%. These are the values that most closely correspond to real life in a normal home.
For this value the dew point will be reached when this air is reduced to a value of 12°C. If a wall has a high insulation value, heat penetrates this wall less easily. Then, underneath this 5cm polystyrene, the wall will reach e.g. 5°C. So the dew point will occur somewhere inside the wall and this water vapour will start to condense there. The wall will become wet. If the temperature outside drops even more, it turns out that the wall under the polystyrene foam may have e.g. not +5°C but -5°C, and then this water will freeze inside the wall.
What can be done to get rid of the dew point inside the wall?
1. Reduce the humidity by evaporating some of the moisture. Therefore, when using thin insulation, we should use insulation through which this moisture can be evaporated to the outside, e.g. mineral wool or open-cell (sprayed) PUR.
In winter, when frost comes, the air outside becomes very dry. This is why such a wall dries out quickly, because the water evaporates due to the very low air humidity.
2. Use vapour-impermeable insulation thick enough to ensure that the entire wall has a temperature above the dew point. Then the moisture will be given off to the interior when the relative humidity inside drops and the wall is saturated heavily with water vapour.
If the wall is very porous and thick, there is no point in adding additional insulation, because it does not provide much in the way of thermal insulation. But if you want to use polystyrene foam, it has to be thick enough, and this no longer makes economic sense.
Now, what do you use to heat the house?
Don't go into electric heating with an expensive induction boiler. That is simply throwing money down the drain.
Direct current heating is expensive and is the most expensive heating method available.
If you have the option of mains gas then don't even think about it, as it is currently the cheapest investment and heating costs are comparable to coal. Even cylinder gas (LPG) is only slightly more expensive if you buy your own cylinder. If you don't have the option of bringing in natural gas and setting up an LPG cylinder, then consider a heat pump. It is twice the investment of an electric induction boiler, but the heating costs are three times lower.
Or stay with your current boiler, but add a large heat buffer, e.g. 1000-1500 litres (the larger the better). Put additional heaters in it. You can buy such a set-up much cheaper than an induction boiler. Moreover, such a set is less prone to breakdown than induction or electrode boilers, and if a heater burns out, it is very cheap to repair. By having a buffer, you can arrange for tariff electricity and use electricity at a much lower tariff. This will reduce your heating costs by about 30-40% compared to if you bought an induction boiler without a buffer.
By having your current boiler connected to this buffer you will increase the comfort of your heating, increase the combustion efficiency and thus reduce the cost of heating with coal.
If you have to replace the boiler, replace it with a universal boiler in which you can burn coal and wood, but a boiler with bottom combustion and an obligatory buffer. The heaters will give you the option of heating the house when there is no one to burn in the boiler.
Induction boilers only make sense in houses with very low energy requirements. Then you may find that such a boiler will be more economical than a coal boiler. But this is not because heating with electricity is cheaper, but because in such houses, where the heat demand reaches 2-3kW at outside temperatures of -20°C, any coal-fired boiler will be heavily oversized, and an oversized boiler means low coal burning efficiency. That is, increased costs.
