I am currently renovating and modernizing my mother’s former apartment completely. At some point in the past, long before we bought the house, the heating recesses were bricked up—2 with pumice stone and 4 with aerated concrete blocks (Ytong). Naturally, there is an air gap between them.
The house dates from the 1930s and is built of rubble stone with what is probably clay. The plan is to insulate it from the inside with 6 or 8 cm (2.4 or 3.1 inches) of Multipor, since the rubble stone wall has an estimated U-value of 2.2, and with 8 cm (3.1 inches) of Multipor it will be around 0.4 to 0.45.
I have gutted everything so far, and new windows have already been ordered. This raises the question about the old heating recesses: should I leave them as air gaps or fill them with something? It is usually said to avoid air gaps, but in a rubble stone wall there is generally always some air anyway. These walls are never 100% solid rubble stone and clay.
Current wall structure from outside to inside:
Exterior render, possibly 1-2 cm (0.4-0.8 inches)
Rubble stone with clay, 20-25 cm (8-10 inches)
Some interior plaster, possibly lime-sand, 1-2 cm (0.4-0.8 inches)
Air gap 5-14 cm (2-5.5 inches)
Pumice or aerated concrete blocks 5-14 cm (2-5.5 inches) (yes, they just put in whatever fit, so the air gap varies accordingly)
Interior plaster, likely lime-sand first, then gypsum, about 2 cm (0.8 inches)
Now, on top of this, 6-8 cm (2.4-3.1 inches) of Multipor will be applied.
It is said that the gap should be closed with the same material as the rest of the wall—so rubble stone with clay. I would rather not introduce anything wet because of drying issues. Could I simply fill and compact it with dry sand or something similar?
The house dates from the 1930s and is built of rubble stone with what is probably clay. The plan is to insulate it from the inside with 6 or 8 cm (2.4 or 3.1 inches) of Multipor, since the rubble stone wall has an estimated U-value of 2.2, and with 8 cm (3.1 inches) of Multipor it will be around 0.4 to 0.45.
I have gutted everything so far, and new windows have already been ordered. This raises the question about the old heating recesses: should I leave them as air gaps or fill them with something? It is usually said to avoid air gaps, but in a rubble stone wall there is generally always some air anyway. These walls are never 100% solid rubble stone and clay.
Current wall structure from outside to inside:
Exterior render, possibly 1-2 cm (0.4-0.8 inches)
Rubble stone with clay, 20-25 cm (8-10 inches)
Some interior plaster, possibly lime-sand, 1-2 cm (0.4-0.8 inches)
Air gap 5-14 cm (2-5.5 inches)
Pumice or aerated concrete blocks 5-14 cm (2-5.5 inches) (yes, they just put in whatever fit, so the air gap varies accordingly)
Interior plaster, likely lime-sand first, then gypsum, about 2 cm (0.8 inches)
Now, on top of this, 6-8 cm (2.4-3.1 inches) of Multipor will be applied.
It is said that the gap should be closed with the same material as the rest of the wall—so rubble stone with clay. I would rather not introduce anything wet because of drying issues. Could I simply fill and compact it with dry sand or something similar?
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