How to choose the right insulation thickness

Why there is no single answer

Insulation thickness depends on three things: what the wall is made of, how cold your winters are and what you pay for heat. A 30 cm AAC wall and a 25 cm solid brick wall start from completely different places.

What is actually calculated

You start from the thermal resistance of the existing wall and add whatever is missing up to the value required for your climate zone. Around here that usually means 10–15 cm of expanded polystyrene or 10–12 cm of mineral wool.

• below 8 cm — wasted money: the savings never cover the labour
• 10 cm — the reasonable minimum for a brick wall
• 15 cm — the point where extra thickness stops paying for itself

Thermal bridges ruin the whole calculation

Fifteen centimetres on the wall field mean nothing if the reveals, ring beams and balconies stay bare. An untreated thermal bridge can cancel a quarter of the benefit of the whole system and it is exactly where condensation, and later mould, appears.

The practical conclusion

Ask for a calculation, not an opinion. A serious calculation also tells you the payback period — and if the answer is “20 years”, you learned something useful before spending the money.