ᐅ Existing wall retrofitted with a double-leaf masonry construction using Poroton blocks

Hello everyone,

As part of a renovation, we built an internal wall with a 4cm (1.6 inch) air gap using Poroton blocks (11.5cm / 4.5 inches) inside our old building constructed with 24cm (9.5 inch) bricks. The goal was to improve thermal insulation and, above all, to create a smooth surface for plastering. The old masonry is partially out of plumb by 5cm (2 inches) and covered with clay plaster.

After some research, I am now concerned that the old exterior wall might become too cold due to this measure (frost resistance?) and that moisture could build up between the two walls.

Do I need to apply a membrane to the inside of the Poroton wall, similar to internal insulation? Currently, there is no plaster on the Poroton wall, so adjustments are still possible.

Could ventilation towards the interior be helpful? Ventilating to the exterior would be difficult to implement.

Thanks in advance for any advice on this topic.

Best regards,
Matthias

Well, what you did there is really quite a serious mistake.
But 11.5cm (4.5 inches) Poroton bricks provide so little thermal insulation that it probably won’t cause a dramatic impact.
A vapor barrier only helps if you can ensure the entire interior is airtight, which you cannot do, since you definitely have interior walls, ceilings, and a floor. In the end, this will create even more problems.

matze_2019 schrieb:
Old building (24cm (9.5 inches) brick) internally lined with 4cm (1.5 inches) air gap and Poroton (11.5cm (4.5 inches)) bricks. The goal was to improve thermal insulation and, above all, to create a smooth surface for plastering.

How did you come up with such a questionable idea? All interior insulation is applied directly onto the existing wall without an air gap. Either glued directly or installed with filling mortar. In principle, this can also be done with Poroton, but definitely not with an air gap.

matze_2019 schrieb:
After researching, I now fear that the old exterior wall will become too cold because of this measure (frost resistance?) and that moisture will develop between the two walls.

Frost damage is unlikely, as the insulation thickness is insufficient for that, but moisture problems will definitely occur. Maybe the moisture will be removed again, but this setup is problematic.

Sealing on the inside is difficult at junctions. Ventilation gaps are not effective from either side. If ventilated from the inside, you might as well remove the wall; ventilated from the outside, you gain nothing.

Rebuilding directly on the wall with filling mortar or using a different system altogether is better, or even better, insulating from the outside.

Unfortunately, we have now realized that it was a mistake. However, it wasn’t about insulation but about having a proper wall so that the plaster is even and cabinets can be firmly attached. Unfortunately, we didn’t use backing mortar. The gap with the air layer resulted from the uneven old wall.

Now, of course, we are worried about mold. In this case, would ventilating towards the inside make sense? For example, with air vents at the top and bottom of the Poroton wall?

There are two other walls we would like to straighten. I still have enough Poroton 11.5cm (4.5 inches) blocks. Can I masonry these using backing mortar, or is moisture a too great risk for my old building walls? We want to avoid up to 4cm (1.6 inches) of clay plaster because, on the one hand, it has already been covered with gypsum by the previous owner and, on the other hand, cabinets cannot really be mounted on it.

Adding to the previous post, the question is: How could I possibly backfill afterwards? We haven’t plastered yet. We would remove a few bricks from the top and use fairly fluid screed and a vibratory bottle to backfill, in order to somewhat salvage the whole thing. Would that be possible?

No, try looking for Isofloc or Unifloc; I would blow that in there.

It’s not as dramatic as you might think. After the war, brick walls were sometimes built with a cavity between the outer and inner walls. The important thing is that this air gap does not connect to either the outside or inside air. Filling it with loose cellulose prevents air circulation/convection within the cavity, which increases the insulation effect. The problem of dew point falling below the surface at very cold outside temperatures isn’t really different than before. The insulation performance of such a thin Poroton wall is limited anyway.

If you still have rooms left to do, maybe consider other options for interior insulation that provide better thermal performance with less loss of space.