ᐅ Insulating an Extension with Exterior Walls Only 12 or 24 cm (Approx. 5 or 10 Inches) Thick

Hello community,

I just have a few questions and maybe someone has some answers for me.
So, no rush—I know the building’s condition isn’t ideal.

About the building,

it is a two-story structure measuring 5 x 3 meters (16.4 x 9.8 feet); one of the 5-meter (16.4 feet) sides is shared with the neighbor, who has the same building on their side. (The second floor has a sloped roof with knee walls.)
It has a half-gable roof with a new beaver tail tile covering and two Roto roof windows with blinds.

Regarding the walls: The ground floor has all four sides made of 24 cm (9.5 inch) masonry with older sand-lime bricks, definitely white 12 x 6 cm (4.7 x 2.4 inch) bricks.
The upper floor with the sloped roof, however, is built with reinforced corner pillars of 24 cm (9.5 inch) thickness supporting the roof, while the four walls in between are only 12 cm (4.7 inch) thick.

Now to my question/problem.

Basically, external insulation is not an option due to building constraints.

Is effective internal insulation feasible, and if so, what thickness would be appropriate? (I simply cannot figure out the calculations.)
I received an offer from a company suggesting attaching 80 mm (3.15 inch) calcium silicate boards on the 12 cm (4.7 inch) walls upstairs. Is this reasonable/sufficient?

What I would ideally prefer are Steico wood fiber boards.

If more information is needed, please let me know.
I just want to understand whether this is sensible/feasible.

Thank you very much,
Fabian

I don’t understand the whole house structure. 5x3m (16x10 feet) and 24cm (9.5 inches) exterior walls amount to only about 12sqm (130 sq ft). And there’s a staircase inside to get to the upper floor, which offers even less usable space?
And why does it need insulation? Is this a proper residential house with a bathroom, heating, etc.? With that size, I wouldn’t even consider taking away more interior space.

Fabian2024 schrieb:

I received an offer from a company that wants to install 80mm (3 inches) thick calcium silicate boards on top of the 12cm walls,

And on the bottom?

Fabian2024 schrieb:

Is that reasonable/sufficient?

Reasonable? I can say if I know what you plan to do with that little room. It’s definitely the most expensive solution you can find. It should work, though. Whether it is sufficient depends on what you need it for.

Fabian2024 schrieb:

What I might want are Steico wood fiber boards.

Inside? Why? That would be a completely different and more challenging wall construction because of moisture issues you can get very quickly, which could cause the wood fiber to rot right in front of you.

Fabian2024 schrieb:

I just want to understand if it’s reasonable/feasible.

I just want to understand the background of the small garden house and your insulation idea.

In order for anyone to help you, you need to describe the situation on-site much more clearly. It’s best to include one or two sketches. That way, it’s much easier to understand what you mean. You probably mean a space of 5x3 meters (16x10 feet), with a height of 3 meters (10 feet).

Also, you likely don’t intend to apply 80mm (3 inches) insulation to a 120mm (4.7 inches) wall, but rather mean centimeters and millimeters at different points. However, this is just speculation...

Fabian2024 schrieb:
Now to my question/problem.

Not yet, I need to read it four more times tomorrow first. Maybe you could take a picture; that might even resolve the communication difficulties. So far, I still can't follow your description even remotely :-(

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https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/

Also,@jens.knoedel

Yes, the size of the garden shed is probably accurate, but it is the only option I have to create additional living space.

The two rooms are intended to be used as storage upstairs and a play area for children downstairs. Yes, the footprint is about 12sqm (130 sq ft). The bathroom will be accessed from the main house through a door.

Downstairs, I’m not sure what kind of insulation the company intended; I only have the offer for the 12mm (0.5 inch) wall.

External insulation is a problem because these are all semi-detached houses, and the situation with the roofs and facades looks difficult. My neighbor is also quite particular about that.

The door shown here will be removed and replaced with a window. On the right, where the two doors are, the wall will be rebuilt and an entrance area with an entrance door will be installed. On the left is the bathroom with access.

I don’t want to discuss the usefulness or impracticality of the space — I know myself that it won’t be a large room. I want to discuss only the insulation aspect.

Thank you very much

Fabian2024 schrieb:

I want to focus solely on the insulation aspect.

1. The insulation aspect concerns the entire house, including the roof and ground floor.
2. If using interior insulation, then as suggested — whether it is sufficient (for whatever purpose) can be calculated or checked using tools like Uvalues (which will also show you that wood fiber boards simply glued to the walls will absorb moisture like rats on a sinking ship).
3. Exterior insulation is always the first choice, regardless of a neighbor’s personal opinion, especially when exterior insulation costs about one-third of interior insulation (rough estimate) and also provides more usable interior space. It is quite difficult to firmly attach anything to insulation boards.
4. If the attic is intended “only” for storage, I personally would not insulate the space for several thousand euros just to keep boxes warm. I would insulate the top floor ceiling from above in a walkable manner. You can even do this yourself on a Saturday for a three-digit amount. That way, you have enough budget left for a great renovation of the playroom below. You can start a new thread about its insulation and construction.