ᐅ Insulating an Old House with Thin Walls: Should You or Shouldn't You?

I am facing the decision of whether to add external thermal insulation with polystyrene to Grandma’s old house or not. The house is old, built in 1929. The interior is completely renovated, but the exterior is not, which is why I am considering whether to insulate it right away or just apply new plaster. In winter, the unheated rooms get very cold, and the exterior walls, including the plaster, are only 18cm (7 inches) thick—is that typical for a house from that time? Therefore, insulation might actually be a worthwhile investment, although I’m not sure if the difference with added thermal insulation would be noticeably significant. What do you think?
Not all rooms are heated around the clock in winter; when entering an unheated room, it feels very cold. So now we’re wondering how much difference it would make if the house had thermal insulation.

Susi1503 schrieb:
The house is old, built in 1929. Everything inside is new, but the exterior is not, so I’m considering whether to insulate it right away or just plaster it. In winter, the unheated rooms get very cold, and the exterior walls including plaster are only 18cm (7 inches) thick. Is that typical for the building period?

18cm (7 inches) with plaster is not typical for that construction year; it suggests a timber frame structure. For a masonry building from that era, I would expect a wall thickness of about 25cm (10 inches) (raw).

I would first clarify the wall construction. What about the windows? Also, the roof is probably not insulated either. When were the heating system and water pipes updated?

1929, to me, doesn’t immediately suggest that “if it’s insulated, it’s up to modern standards.” What exactly is “all new inside”?

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The windows were replaced with new plastic windows, and the roof was also redone, including insulation. Otherwise, everything inside is up to date, such as the boiler, water pipes and plumbing, radiators, etc.

The walls are made of stone—gray stones, but I don’t know the exact type. I’m not sure if there is any wood inside the walls. On the wall, there is a wooden structure, to which boards were attached that look like drywall sheets, and the total thickness (including plaster) is 18 cm (7 inches).

Could there be concerns about insulation with this construction method? Is it recommended at all?

Susi1503 schrieb:
the windows were replaced with new uPVC windows,

During installation, one should have been able to see how the walls are constructed.

Susi1503 schrieb:
The walls are made of stones, they are gray stones, I don’t know what they’re called. But whether there is wood in between, I don’t know. There is a wooden structure on the wall, on which panels were then attached that look like gypsum boards, and the total thickness (including plaster) is 18cm (7 inches).

Pumice stone is coarse-grained, ash gray in color, sometimes a bit earthier, and back then it came in small stone sizes. Aerated concrete (e.g., "Ytong" / Hebel, etc.) is light gray to cream white, often used in large formats and frequently applied in timber frame renovations — also because it can be easily cut into any shape and fit well. For timber-framed buildings — which at that time were very often fully plastered, so the timber frame was not exposed — it is common during renovations to straighten the walls using battens and plasterboard.

18 cm (7 inches) is a very unusual thickness for masonry walls. Stones back then were typically 25cm (10 inches) long and 12cm (5 inches) thick. Since exterior walls were never built solely with stretcher bond, the stone length generally determined the wall thickness. During the Adenauer era, the stone format changed to 24cm (9.5 inches) length and 11.5cm (4.5 inches) thickness. Only in the 1980s did very large stone formats (about half a meter long and a quarter meter high) come into use, including exterior walls built only in stretcher bond (i.e., without header layers), with stones produced to match the most common wall thicknesses.

In timber frame renovations, it was common to use aerated concrete blocks in thicknesses of 10, 12.5, 15, 20, or 25cm (4, 5, 6, 8, or 10 inches). Timber frame beam thicknesses in 1929 were likely mostly found in metric sizes (no longer inches), commonly 12, 14, or 16cm (5, 5.5, or 6 inches). Later, aerated concrete blocks were placed in the panels as replacements for clay-coated wattle infill. For exposed timber framing, the next thinner block thickness was used and then plastered over; for fully plastered walls, the closest thickness was selected and leveled using battens and plasterboard.

The total wall thickness you mentioned clearly argues against “masonry” and strongly suggests “timber frame.” Thermal imaging should make this quite visible.

Non-load-bearing interior walls were typically 12cm (5 inches) thick if the house was masonry. For interior partition walls, both in masonry and timber frame buildings, “rabbit” plaster mesh walls were also common; in masonry buildings, sometimes walls made with stones laid on their narrow side (about 6.5cm (2.5 inches) thick) were used.

Susi1503 schrieb:
are there concerns about insulation with this building method? Is it advisable at all?

What insulation materials to use and whether it should be applied on the outside or inside will need to be determined with specialists. Before that, it is crucial to clarify the wall construction. During window installation, one should have seen a lot to understand what kind of wall you are dealing with. Thermal imaging helps too: the pattern of the timber beams usually shows through clearly.

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