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

Thank you first of all for your detailed explanation.

So, basically, only internal insulation with silicate boards would be possible?

I have also seen insulation wool used, for example, in stud wall construction with a vapor retarder/vapor barrier.

Fabian2024 schrieb:

I have also seen that insulation wool, for example in stud frame construction, was installed together with a vapor retarder/vapor barrier.

Yes, that is also possible. In that case, the work must be done with absolute precision. If possible, such methods should be avoided. Aside from that, you will need significantly more space compared to using insulation boards.

Fabian2024 schrieb:

Is a sensible interior insulation possible, and if so, how thick?

Yes, it is possible. I have studied a similar wall construction in detail but have not implemented it myself. So, I am sharing purely theoretical knowledge here:

The challenge: Interior insulation shifts the dew point exactly between the insulation and the exterior wall.

This has two important consequences that need to be addressed:

1.) Condensation from indoor air forms exactly there, causing the wall to start rotting from the inside.

- Solution A: Install a vapor barrier on the interior side of the wall.
However, this has proven completely unsuitable for older buildings because even the slightest leakage immediately leads to severe moisture damage.

- Solution B: Use interior insulation that is "vapor permeable and capillary active."
This allows condensation moisture to evaporate back inside without causing damage.
This is therefore the only practical solution for older buildings.

2.) The exterior wall remains cold through and through and cannot sufficiently dry out moisture entering from outside.
This also risks moisture damage, especially on weather-exposed sides.

Solution: Protect the wall by applying a "hydrophobic treatment."
This is a kind of coating or spray with a silicone-based impregnation to make rainwater bead off, similar to waterproofing shoes.
It’s likely inexpensive but essential.

Now about the insulation thickness:
It should not be thick enough to meet any energy-saving regulations or building energy codes, which would require at least 16cm (6 inches) thickness and would reduce interior space significantly (not to mention the costs).

The insulation is only meant to make the room somewhat heatable.
I would therefore suggest 4 to 6 cm (1.5 to 2.5 inches).

The assembly must be (very important!) vapor permeable and capillary active to allow free transport of moisture.

The following insulation materials are suitable:
- Heraklith wood wool boards ("sauerkraut boards") – cheap and decent but not the best U-value.
- Calcium silicate boards – very good but also quite expensive.
- Wood fiber boards (e.g., Steico Internal)
- Mineral wool boards

Unfortunately, these insulations have to be installed quite carefully, which is probably the main effort and cost factor:

- The boards must be fully and evenly bonded to the exterior wall without air pockets, using a vapor-permeable and capillary-active adhesive material (for example, a toothed application of clay-based undercoat plaster with the proper technique).

- On the interior side, an equally vapor-permeable finish must be applied.
This can be clay plaster or lime plaster.
Cement-based plaster is of course not suitable because it would restrict moisture movement again.
Also, any interior paint must not block moisture.
Maybe someone here knows a simpler method?
Would painting the boards with clay paint instead of plaster suffice?

Best regards!
Wolfgang

@Wolfgang, thank you very much for your post, really very interesting.

What if you protect the interior walls against penetrating moisture using this two-component waterproofing slurry? That should definitely achieve 100 percent watertightness, and then install studs with wood fiber or mineral wool insulation, followed by a vapor barrier?

Fabian2024 schrieb:

what if you protect the inside of the walls from moisture penetration using this two-component waterproof slurry

Hey Fabian!

In theory, many surface treatments can be completely waterproof. But what about connections at walls, corners, windows, doors, sockets, cables, pipes, ceilings, lamps, switches? These must be 100% sealed not only during construction but also remain so for many years. The smallest gap, damage, crack, detachment, drill hole, a driven nail, or damage from rodents or insects can cause hidden condensation and decay inside the wall. So I firmly stick to my recommendation that only a vapor-permeable insulation with active capillary action through all layers is practical.
Sealing the wall with this slurry (which I am not familiar with) would also be applied at the wrong level. Moisture should not penetrate that far into the wall if you want to achieve true vapor-tightness.

Yes, you're right, achieving 100% airtightness is probably just an illusion.

It's a really complex and extensive topic; I just watched the two videos you linked. I will look into vapor-permeable insulation options that might be suitable.

Did you have a favorite in your theory?