🔋 The building's "battery"
Every cubic metre of concrete stores over 500 Wh/m³K. In a typical 100 m² home, the walls can "charge" enough energy to keep the space warm for hours after the heating switches off.
External vs Internal Insulation: The Battle of "Thermal Mass" and How It Affects Your Thermostat
You've decided to insulate. Will you "wrap" the house from the outside (External Insulation / ETICS) or install mineral wool and plasterboard from the inside? The position of the insulation radically changes the way you should set your thermostat. Let's meet "Thermal Mass."
1. What Is Thermal Mass?
Heavy materials (bricks, concrete, earth walls) have a special property: they absorb heat, store it and release it slowly. This property is called Thermal Mass. Think of your walls as giant "thermal batteries" that can charge - or remain empty.
🧱 High-mass materials
Concrete, fired bricks, natural stone, ceramic tiles - all have high specific heat capacity. By contrast, plasterboard, timber and lightweight materials store virtually zero energy.
📐 Insulation position is critical
Where the insulation is placed (inside or outside) determines whether the "battery" (the bricks) charges with the home's heat or remains permanently frozen - out of the game.
🌡️ Perceived thermal comfort
Thermal mass isn't theoretical. In a home with warm walls you feel deep, patriarchal warmth - steady, gentle, without drafts. It's the same cosiness stone mansions provide.
2. External Insulation (ETICS): The "Elephant"
With external insulation, we bond the insulating material (EPS, graphite polystyrene or mineral wool) to the outside wall. The bricks and concrete now sit inside the protected zone - the "thermal battery" charges normally.
🐘 High Thermal Mass
The house reacts like an elephant: slow to heat up (the energy first "charges" the heavy walls), but once warm, the walls become giant invisible radiators that emit heat for hours.
🌡️ Constant temperature 24/7
The ideal strategy: leave the thermostat at a steady 20-21°C without turning it on and off. Even if the boiler shuts down, the walls continue to warm the space for 4-6 hours.
💚 Perfect partner: HP + Underfloor
External insulation works perfectly with heat pumps (which love continuous, gentle operation) and with underfloor heating. Ideal for permanent residences.
🏗️ Eliminates thermal bridges
Because the insulation "hugs" the entire building (columns, beams, ring beams), thermal bridges disappear. The walls are protected from thermal expansion and contraction, extending the building's lifespan.
3. Internal Insulation: The "Sports Car"
Here, the insulating material (e.g. mineral wool) goes on the inside of the wall and is covered with plasterboard. The bricks remain "outside" - the thermal battery stays frozen all winter.
🏎️ Low Thermal Mass
The house reacts like a sports car: heats up instantly (energy hits the thin plasterboard and stays in the air), but cools down equally fast - in 30 minutes after the thermostat switches off.
⏱️ Intermittent operation (On/Off)
Here we use intermittent operation: turn the heating on only when you're in the room. Away at work? Turn it off entirely. The house will warm up in 10 minutes when you return.
🏖️ Ideal for holiday homes / Airbnb
Internal insulation is perfect for holiday homes, Airbnb properties, offices or bedrooms. It pairs perfectly with split air conditioners or fan coils, which heat the air "instantly."
⚠️ Limitations
Internal insulation does not eliminate thermal bridges, slightly reduces floor area, and can cause moisture problems behind the plasterboard if a vapour barrier isn't installed correctly.
4. Summary: Who Wins?
From a mechanical-engineering standpoint, external insulation is always the superior choice for a permanent residence. It eliminates thermal bridges, protects the structural frame, doesn't reduce floor area, and delivers unmatched, steady comfort 24/7.
🏠 Permanent home → External
Warm walls 24/7, steady thermostat, ideal with heat pump + underfloor. The "queen" of insulation solutions for permanent residences.
🏖️ Holiday / Office → Internal
Fast response, minimal floor area lost, ideal with split A/C units. The fallback solution when you can't modify the façade (e.g. listed building).
🌡️ Thermostat: 24/7 vs On/Off
External = 24/7 steady thermostat. Internal = On/Off only when you're home. Mixing up the strategy will waste energy and money.
🧮 Energy savings
Correctly setting the thermostat based on your insulation type reduces the bill by 15-25% without any additional investment - just knowledge of how your "battery" works.
🏡 The position of insulation isn't just aesthetic - it radically changes the heating "chemistry" of your house. External = Elephant (slow but enduring). Internal = Sports Car (fast but loses heat immediately). Choose wisely - and set your thermostat accordingly!
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