Does thicker insulation always cut heating bills?

The heat flux depends on the reciprocal of the total resistance. When there is already a great deal of insulation, a further portion of material reduces the U-value less than the first centimetres. This does not mean that thicker insulation is pointless. It only means that the gain must be calculated and set against the cost and the intended standard of the house.

Assume that all layers other than EPS and the surface resistances together give 1.00 m²·K/W. With 10 cm of EPS with a lambda of 0.040, the total resistance is 3.50, so U is approximately 0.286 W/(m²·K). With 20 cm we obtain U of approximately 0.167. With 30 cm the result falls to approximately 0.118 W/(m²·K).

The first increase of 10 cm lowers U by approximately 0.119, and the next by approximately 0.049 W/(m²·K). This is an illustration of the relationship under fixed assumptions. It is not a design, nor confirmation that the requirements for a particular house are met. A real calculation takes account of the properties of the masonry, corrections and junctions between building elements.

Wall insulation reduces losses through the walls. It does not directly change losses through old windows, a leaky roof or intensive air exchange. In a poorly insulated house the measure may be pronounced, but its share of the overall balance must be assessed. It is not possible honestly to attribute a fixed percentage of savings to each additional layer.

The bill is also affected by the heat source. Gas, electricity and district heating are billed differently. Some costs are fixed. After renovation, occupants may also raise the temperature, taking advantage of improved comfort. A smaller monetary saving then does not necessarily mean that the insulation performs worse than calculated.

If the insulation stops at the ring beam, plinth or reveal, the local heat flow may remain high. A thicker board in the middle of the façade will not automatically resolve such a thermal bridge. Before changing the thickness, it is therefore worth checking the continuity of the layer and its connection with the roof, floor and windows and doors. A plan for the whole building envelope is needed.

Air tightness and protection against water also matter. EPS boards are not a ready-made seal for all junctions. Draughts through an improperly executed detail require the cause to be eliminated. Moisture, in turn, can worsen the operating conditions of the building element. Additional centimetres will not replace repairing a leak.

Before renovation, an inspection and a thermal imaging survey carried out under suitable conditions can be helpful. The temperature distribution image must, however, be interpreted taking account of the weather, moisture and the method of heating. The colour alone in the image does not give the lambda of the material.

The cost of renovation includes scaffolding, substrate preparation, labour, the reinforcing layer and the finish. Some of these expenses remain similar at different thicknesses. Saving a few centimetres of material during a full renovation therefore does not always pay off. Adding insulation later may require repeating many costly works.

On the other hand, thickness affects fixings, window sills, flashings and the available space. Compare two complete options with an identical scope of works. Take account of the durability of the details and the risk of clashes. An exact payback period requires data on the building and the price of energy. A single general figure from an advertisement will not replace that calculation.

First establish the required thermal performance and the technical constraints. Then check how much the basic option costs and how much the additional thickness or lower lambda costs. Ensure both solutions are correctly installed. Comparing thick but poorly executed insulation with thinner and careful work will not allow the effect of the material itself to be assessed.

Various grades and thicknesses of EPS are offered by, among others, Swisspor, Arbet, Austrotherm and Styropmin. A manufacturer’s name does not define a single lambda or a single range of applications.

In practice, a good decision is the option that meets the design assumptions, maintains the continuity of the insulation and fits within a sensible budget for the whole investment. It is worth leaving a margin for difficult details. These often determine comfort at the window and a dry corner surface more than the last centimetre on a large, even wall.