Is a sensible interior insulation feasible, if so how thick?
Yes, it is possible. I have studied a similar wall construction intensively but have not carried it out.
So I am purely sharing theoretical knowledge here:
The challenge:
An interior insulation shifts the dew point exactly between the insulation and the exterior wall.
This has two important consequences that need to be dealt with:
1.) Condensation from the indoor air occurs exactly there and the wall starts to rot from the inside.
- Solution A: Install a vapor barrier on the inside of the wall.
But this has proven to be completely unusable in old buildings because even the slightest leak immediately causes massive moisture damage.
- Solution B: Build the interior insulation "vapor diffusion open and capillary active".
This allows the condensed water to simply evaporate back inside without causing damage.
This is therefore
the only practicable solution in old buildings.
2.) The exterior wall is cold through and through and cannot sufficiently evaporate moisture penetrating from outside.
This also risks moisture damage, especially on weather-exposed sides.
Solution: The wall is protected against penetrating moisture by "hydrophobization".
This is a sort of coating/spray with a silicone impregnation so that rain beads off, like on shoes.
Certainly not expensive but it must be done.
Now to the insulation:
The thickness should not be sufficient to meet any energy saving ordinance or building energy law standard, because that would require at least 16 cm thickness and would leave almost no room inside (not to mention the costs).
The insulation is only supposed to make the space somewhat heatable.
I would therefore suggest 4 to 6 cm.
The construction must (very important!) be vapor diffusion open and capillary active so that moisture can be transported unhindered.
The following insulations are suitable for this:
- Heraklith wood wool boards "sauerkraut boards" (cheap and good, but not the best U-value).
- Calcium silicate boards (very good but also very expensive)
- Wood fiber boards (e.g. Steico Internal)
- Mineral fiber boards
Unfortunately, these insulation materials must be installed quite elaborately.. this is probably the biggest effort/cost factor:
- The boards must be fully glued to the exterior wall with a vapor diffusion open and capillary active material without air inclusions (e.g. applied serrated with clay base plaster and the correct technique).
- From the inside, of course, a correspondingly open finish must also be created.
This can be done with clay or lime plaster.
Cement-based plaster is of course not possible because it would block the moisture again.
Also, any interior paint must not hinder the moisture.
Maybe someone here still knows a simpler method?
Maybe a coating of the boards with clay paint is enough instead of plaster?
Regards!
Wolfgang