📉 The first 5cm
Deliver a gigantic drop in losses.
Changing Insulation Thickness from 5cm to 10cm: How Much Does the Gain Change?
One of the most classic questions when deciding to install external insulation or insulate the roof is: "Should I go with 5 centimetres, 8 or 10?" Human logic (and often the materials salesperson) tells us that if 5cm of insulation saves €1,000 a year, 10cm will save us €2,000. An absolutely logical thought, which in physics is however completely wrong.
In thermal insulation there is a relentless rule called the Law of Diminishing Returns. Let us see why the first centimetres do all the work, and why the last ones simply empty your wallet with no return.
1. The Law of Diminishing Returns
Imagine going out in the snow wearing only a t-shirt (Uninsulated wall). You are freezing. If you put on a thin but good jacket (5cm insulation), you feel enormous relief. The improvement in comfort is 80%.
If you then put a second, identical jacket on top (another 5cm, total 10cm), you will warm up a little more, but the difference will not be as dramatic as when you put on the first one. The extra improvement is perhaps 10-15%.
Exactly the same happens inside your walls:
📉 From 5 to 10cm
The gain is much smaller.
📉 From 10 to 15cm
The gain is almost negligible, while the cost of materials and special fixings skyrockets.
2. The 10x10 Model Experiment
Let us take the uninsulated wall of our digital house, which loses energy at a U-Value of 2.10 W/m²K, and see what happens as we add white EPS (polystyrene) in 5cm increments.
Step 1: From 0 to 5cm
We add 5cm EPS. The U-Value plummets from 2.10 to 0.55 W/m²K.
Result: We cut losses by 74%! The
house is "transformed".
Step 2: From 5 to 10cm
We double the thickness with another 5cm. The U-Value drops from
0.55 to 0.31 W/m²K.
Result: We cut an additional 11% compared
to the original state.
Step 3: From 10 to 15cm
We add 15cm. The U-Value goes to 0.21.
Result: We gained just an extra 4%.
3. The Economic "Sweet Spot"
When you go from 5 to 10 centimetres, you pay almost double for the insulation material, you use larger fixings, the window sills must be extended considerably, and there may be problems with balconies from lost space.
If all this extra hassle and cost only gives you 10% more oil savings, the payback (ROI) of the extra centimetres may take decades!
4. What Is the Ideal Thickness?
For Greece, it depends on the Climate Zone you are in:
🏖️ Islands & Southern Greece (Zone A & B)
5 to 7 centimetres is usually the ideal cost/performance sweet spot.
❄️ Northern Greece (Zone C & D)
Due to the heavier winter, the sweet spot shifts to 7 to 10 centimetres at most.
💡 Final Conclusion: Do not chase extreme insulation thicknesses (12-15cm) unless you are building a specialised "Passive House". The money you would spend going from 8 to 12cm of polystyrene will deliver infinitely better returns if you invest it in replacing your old windows with new, energy-efficient ones!
Related Articles
U-Value (Thermal Transmittance): What It Is & How It Is Calculated Uninsulated vs. Insulated House: How Much Money & kWh Do You Save? The 10x10 House: The Standard Model for Calculating Heat Losses Insulation investment Payback Period
Thermal Physics & Calculations: From Theory to kWh and Euros
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