One of the first things you'll hear from friends and family: "Don't do it, the house will shrink, you'll lose loads of square metres!" It's true that every material added to the interior side of the wall "steals" a little space from the room.
However, the perception of how much space we lose is usually far more exaggerated than reality. To decide whether the energy upgrade is worth the "sacrifice", you first need to understand the real numbers. Let's do the maths.
The most common misconception is that the room will shrink from all sides. This is simply not the case, and understanding why helps put the numbers in perspective.
Insulation is applied exclusively to walls in contact with outdoor air (or unheated spaces such as stairwells or pilotis). Interior partition walls that separate one heated room from another remain exactly as they were - they don't need insulation. In a typical bedroom, usually only one or two walls "thicken", not all four.
The centimetres lost depend directly on the method and the materials you choose. Here are the three main options:
The most complete and professional solution. A 5cm metal frame filled with rock wool, plus a single layer of plasterboard 1.25cm = 6.5–7cm total. If you opt for double plasterboard (for extra soundproofing or load-bearing capacity): ~8cm.
Ready-made sandwich panels with an EPS core bonded directly to the wall. No metal frame needed, so the total thickness is only 4–6cm. A good middle-ground option.
The ultimate DIY solution. Thin XPS tiles bonded directly and painted - no plasterboard at all. Total thickness: just 1–2.5cm. Minimal space loss but also the lowest thermal performance.
Our standard reference house: 10×10m = 40 running metres of external walls, total floor area 100 sq.m. Let's calculate the actual loss.
If we apply the "thickest" insulation (7.5cm frame + insulation) on all external walls: 40m × 0.075m = 3 sq.m. From 100 down to 97 sq.m. That's a loss of just 3% of the total floor area.
A typical bedroom with one external wall 3m long: 3m × 0.075m = 0.22 sq.m. The room goes from 12.00 to 11.78 sq.m. Practically, this is an imperceptible difference - you couldn't tell without measuring.
In a freezing, uninsulated bedroom, the space next to the external wall is essentially "dead" in winter. Nobody wants to place furniture or sleep against a wall that feels like a refrigerator door.
You lose 7cm on paper, but you gain back the full use of the entire room. The wall becomes "warm" to the touch, the bed next to it is no longer a freezing spot, thermal comfort soars, and the energy savings you enjoy year after year make those 3 sq.m. feel like the best investment you've ever made in your home.
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