How much you save by insulating
Build the wall layer by layer, get its U-value and see what 6, 10 or 16 cm of insulation are worth on your heating bill.
Build the structure layer by layer, add the insulation you are considering and see the U-value before and after, the kWh and the money you stop throwing outside, and how many years it takes to earn the work back.
The order of the layers does not change the U-value, but it does change where condensation ends up, so entering them the right way round helps you read the result.
The lambda values listed are typical figures from the UNI 10351 and EN ISO 10456 tables. If you have the datasheet of the actual product, use its number through the custom material entry. An air cavity has no lambda: it counts as a resistance that depends on its thickness, following EN ISO 6946.
Leave the thickness at zero if you only want to know how much the structure loses as it is now. The target drives the reverse calculation, that is how many centimetres it takes to get there, and the legal limits that unlock grants change with the climate zone and with the rules in force.
Degree days here are on a 20 °C base, the one used across continental Europe, and the tables published by your energy agency or weather service give the value for your town. The cost is that of 1 kWh of heat actually delivered indoors, so already net of boiler efficiency: the suggested figures are indicative, and the heating cost comparison tool works it out from your own prices.
Splitting the two figures is what makes the thickness table honest, because going from 8 to 14 cm costs a few euro more per square metre while scaffolding, render and labour stay the same. Enter the tax relief you actually qualify for this year.
What a U-value is and how it is built
The U-value tells you how many watts cross one square metre of wall for every degree of difference between inside and outside. Lower is better. You get it by adding up the resistance of every layer, where each layer counts as thickness divided by lambda (in metres and W/mK), plus the two surface resistances that EN ISO 6946 sets at 0.13 inside and 0.04 outside for a wall, 0.10 and 0.04 for a roof, 0.17 for a floor with downward heat flow. Then U = 1 ÷ total R. A real example: the classic 1970s cavity wall, 1.5 cm plaster, 12 cm hollow blocks, 5 cm cavity, 8 cm hollow blocks, 2 cm render, reaches R = 0.89 and therefore U = 1.12 W/m²K. Add 10 cm of EPS at 0.036 and the resistance climbs to 3.67 while the U-value drops to 0.27, which is 76% less. Notice that an air cavity has no lambda: it counts as a tabulated resistance, at most 0.18 m²K/W, about what you get from seven millimetres of insulation. An empty cavity is not insulation.
From U to kWh, and why degree days are the missing number
The heat leaving a surface over a whole season is A × U × DD × 24 ÷ 1000 and comes out in kWh. Degree days are the sum, over every day of the heating season, of the positive difference between a 20 °C reference and the average outdoor temperature. The 24 turns degree days into degree hours and dividing by 1000 turns watts into kilowatts. Watch the base temperature: figures published at 15.5 °C in the United Kingdom or at 65 °F in the United States are considerably lower than base 20 °C ones, so never mix the two families of numbers in the same calculation. When the other side is not the open air but a cellar or a loft, the loss is scaled down by a correction factor, here 0.5 for cellars and garages and 0.9 for ventilated lofts.
Six centimetres or twelve? The answer is in the curve
Resistance grows in a straight line with thickness, but the U-value is its inverse, so the gain is all at the start. On the wall in the example, the first 6 cm of EPS already remove 65% of the losses, 10 cm reach 76% and 16 cm reach 83%: the six centimetres between the tenth and the sixteenth are worth 7 points against the 65 of the first six. That is why the thickness table shows the payback next to the saving. The best thickness is not the largest, it is the one where the extra cost of insulation stops paying for itself. The reverse calculation works from the other end, giving you the exact centimetres for a target U with thickness = lambda × (1 ÷ target U − current R).
What this calculation cannot see
Three things make it optimistic and two make it pessimistic, and both sides are worth knowing. It is optimistic because part of the heat you save was already being given to you by sunlight through the windows, by appliances and by the people in the house, because a real home is not held at a steady 20 °C all season, and because air leaks survive even behind perfect insulation. It is pessimistic because it only counts the surface you entered, while external insulation also cuts the thermal bridges at columns, beams and balconies, which matter a lot in an uninsulated building, and because the comfort of a wall that feels warm to the touch fits into no formula. Internal insulation of the same thickness gives the same U-value, but it leaves the thermal bridges untouched and pushes the dew point inwards, so it should be checked against a condensation calculation before it goes up.
The thermostat, one degree at a time
The second tab uses the same definition of degree days. Keeping one degree less all season removes one degree day from every heating day, so the saving is days × degrees ÷ degree days. The interesting result is that there is no universal percentage: with 183 days and 2,404 degree days one degree is worth 7.6%, while with 166 days and 1,415 degree days it is worth 11.7%. The familiar rule of 6 or 7% per degree comes from cold climates and understates the gain in mild ones. Read the figure as an upper limit, because on mild days there was no degree to give up in the first place. And keep in mind that one degree less is easy under a jumper but not for babies, elderly people or anyone unwell, where health comes before the bill. This tool gives an estimate and does not replace the opinion of a qualified professional or of your tax adviser.