Sandstone - KO criterion or isolatable?

Hello everyone,

not necessarily a construction topic, but I hope you experts will still share your expertise and experiences :)
We are currently looking for a house and now have a very interesting offer for a barn built in 1960, which was converted into a residential house in 2013 (230m² living space, 380m² usable area on which the energy certificate is based). Consumption was about 18,000kW over the last 3 years, probably so low mainly due to the well-insulated roof.

What we currently cannot assess well:
The walls are made of (natural) sandstone and are not insulated on the outside (only plastered). About 40cm thick.

Thermally, sandstone is supposed to be not ideal.

a) Are there any other disadvantages you see that I should know about?
b) Is it possible to insulate sandstone well? What would it approximately cost (12x12 floor plan, then 5m straight up and then another 7 extra meters sloping to the roof peak (triangle))
c) Is it a deal-breaker for you if one wants to keep the ancillary costs somewhat under control later, or can you already take the chance if the house is otherwise a good bargain?

Looking forward to the exchange!

My first idea, 18000 kWh with a heat pump require about 6000 kWh of electricity, photovoltaic with 15000 kWh produces 2.5 times that and should result in 0€ per month heating costs with self-consumption and feed-in tariff. I would have that calculated exactly, and a barn with photovoltaic looks super modern ;), PS the walls should not have any moisture spots

a) Are there any other disadvantages you see that I need to know?

Only general disadvantages of the house for you personally. You have to manage the heating costs yourself – moderate for the size.

b) Is it possible to insulate sandstone well? What would that cost approximately (12x12 floor plan, then 5m straight up and then another 7 extra meters sloping to the roof peak (triangle)

Sandstone is also "just" masonry. Everything can be insulated. Post some photos. But with basement ceiling insulation (if there is a basement), good windows, good roof insulation, and a well-planned heating system, in my opinion, further insulation is just a "nice to have" but not really economically sensible.

c) Is it a dealbreaker for you if one wants to keep ancillary costs relatively under control later, or can you already bite if the house is otherwise a good bargain?

No. But please free yourself from the idea of a "bargain." That doesn't exist today either. For some, e.g. DIYers, something like this can be a bargain. Others who see a heating renovation due to massively rising gas prices with your heating system (i.e. installing a heat pump, new underfloor heating, flooring, etc.) might even find the price excessive. After all, it's a huge shed that has to be maintained, heated, insured, cleaned, etc.

My first idea,
18000 kWh with a heat pump require about 6000 kWh of electricity,
Photovoltaics generating 15000 kWh produce 2.5 times that and should result in €0 per month heating costs with self-consumption and feed-in tariff. I would have that calculated exactly, and a barn with photovoltaics looks super modern ;),
PS the walls should have no moisture spots

Yes, that should indeed be calculated, because it is absolute nonsense. 15,000 kWh/year are achieved, according to a rule of thumb, with a 15 kWp system. But unfortunately, it produces 75% of the energy at times when it is not needed for the heat pump and feeds it into the grid for 8 or 9 cents. You generate just under 4,000 kWh of electricity yourself in winter.

On the other hand, besides the heat pump, there are investment costs of easily €15,000-20,000 for 15 kWp, financed currently at 5-6% private loan interest.

6,000 kWh of heating electricity at 25 cents/kWh costs €1,500. You can calculate for yourself when you will have recovered your photovoltaic investment, including interest. 15 years will easily pass.

Get well soon ;)

Yes, one should indeed calculate that, because it is absolute nonsense. According to the rule of thumb, 15,000 kWh/a is achieved with a 15kWp system.

That was the first idea, where is your second idea before you badmouth the first one. No proposals should be evaluated before 3-5 suggestions, but only then go through the advantages and disadvantages on the concrete object (logical problem solving).

Do you even need that much space? Sure, it might be "cheap" now, but renovation work strongly depends on the area. What you then pay per square meter of facade, for example, you have to find out yourself. It also strongly depends on the region and the specific insulation chosen. Also keep in mind that 40cm is already quite a substantial thickness, that's the new building standard. If you add insulation in front of that now and plaster it, the wall will become significantly thicker.