ᐅ Sandstone – Dealbreaker or Suitable for Insulation?

Hello everyone,

Not necessarily a construction topic, but I hope you experts will still share your expertise and experience 🙂
We are currently looking for a house and have found a very interesting offer: a barn built in 1960, converted into a residential home in 2013 (230m² (2,476 sq ft) living space, 380m² (4,090 sq ft) total usable space, which the energy report is based on). Energy consumption was about 18,000 kWh over the last 3 years, likely due to the well-insulated roof.

What we currently find hard to assess:
The walls are made of (natural) sandstone and are not insulated on the outside (only plastered). About 40cm (16 inches) thick.

Thermally, sandstone is said not to be ideal.

a) Are there any other disadvantages you see that I should be aware of?
b) Is it possible to insulate sandstone walls effectively? What would the approximate cost be for a floor plan of 12 x 12 meters (39 x 39 feet), then 5 meters (16 feet) straight up, plus an additional 7 meters (23 feet) sloping to the roof peak (triangular shape)?
c) For you, is this a deal breaker if someone wants to keep running costs fairly manageable, or could this be a good bargain if the house is otherwise attractive?

Looking forward to your input!

My first idea:
18,000 kWh with a heat pump requires about 6,000 kWh of electricity.
A photovoltaic system producing 15,000 kWh generates 2.5 times that amount and, with self-consumption and feed-in tariffs, heating costs could be close to zero per month. I would have this calculated precisely. Also, a barn with photovoltaic panels looks very modern 😉
PS: The walls should have no damp spots.

Newbi23 schrieb:

a) Are there any other disadvantages you see that I should know about?

Only general disadvantages related to the house for you personally. You will have to manage the heating costs yourself – they are moderate for the size.

Newbi23 schrieb:

b) Is it possible to insulate sandstone well? What would the approximate cost be (12x12 floor plan, then 5 meters (16 feet) straight up, and then another 7 meters (23 feet) sloping to the roof ridge (triangle))?

Sandstone is just masonry like any other. You can insulate anything. Post some photos.
However, with insulation of the basement ceiling (if there is a basement), good windows, proper roof insulation, and a well-planned heating system, additional insulation in my opinion is only “nice to have” but not economically very effective.

Newbi23 schrieb:

c) Is it a dealbreaker for you if someone wants to keep their utility costs reasonably under control later, or can you go ahead if the house is otherwise a good bargain?

No. But please let go of the idea of a “bargain.” Such deals hardly exist nowadays. For some, such as DIY builders, this might be a bargain. Others who consider the cost of heating system upgrades due to sharply rising gas prices (e.g., installing a heat pump, new underfloor heating, flooring, etc.) might even find the price too high.
After all, it’s a huge structure that needs to be maintained, heated, insured, cleaned, and so on.

Nida35a schrieb:

My first thought:
18,000 kWh with a heat pump requires about 6,000 kWh of electricity.
A photovoltaic system producing 15,000 kWh generates 2.5 times that and should—with self-consumption and feed-in tariffs—result in zero heating costs per month. I would have that calculated precisely, and a barn with photovoltaic panels looks super modern anyway 😉,
PS the walls should have no moisture spots

Yes, that definitely needs to be calculated, because it is absolute nonsense. You reach 15,000 kWh per year roughly with a 15 kWp system. However, unfortunately, 75% of the energy is produced during times when the heat pump does not need it and it is sold to the grid for 8 to 9 cents per kWh. You only generate about 4,000 kWh of electricity for your own use in winter.

On the other hand, besides the heat pump, there are investment costs of easily 15,000 to 20,000 euros for a 15 kWp system, financed currently at 5 to 6% personal loan interest.

6,000 kWh of heating electricity at 25 cents per kWh costs 1,500 euros. You can calculate for yourself when you will have paid back your photovoltaic investment, including interest. Fifteen years is a reasonable estimate.

Get well soon 😉

xMisterDx schrieb:

Yes, that should definitely be calculated because it’s absolute nonsense. You can achieve 15,000 kWh/year roughly with a 15 kWp system according to the rule of thumb.

That was the first idea.
Where is your second idea before you criticize the first one?
You shouldn’t evaluate before having 3–5 suggestions.
Only then should you go through the pros and cons based on the specific property (logical problem solving).

Do you really need that much space? Sure, it might seem "affordable" now, but renovation costs heavily depend on the area. For example, you’ll need to find out yourself how much you pay per square meter (square yard) of facade. That also varies a lot depending on the region and the specific insulation chosen. Keep in mind that 40 cm (16 inches) is already quite a substantial thickness, typical for new-build standards. If you then add insulation on top and plaster it, the wall will become even thicker.