How do you use the PIR calculator to choose a thickness for a specific wall?

Determine the insulation side separately. Adding PIR to the façade and from the inside can give a similar thermal resistance in the middle of the wall, but different moisture conditions. A tool that calculates only U should not be used as confirmation of the safety of both variants.

Enter the thickness according to the unit of the field. 80 mm is 0.08 m, not 0.8 m. Lambda has the unit W/(m·K), resistance R — m²K/W, and the U-value — W/(m²K). Converting these quantities can give a seemingly credible but completely wrong result.

The parameters should relate to the specific variant and thickness. For boards with a fleece, lambda may change with thickness. If the program assumes one fixed parameter for all PIR, it must be checked whether such a model actually fits the selected product.

You can calculate the thickness needed to achieve a given U-value or compare several available thicknesses. The target should follow from the design, requirements and pattern of use. For an office heated all day, a different system operating time matters than for a room used a few hours a month.

Don’t assume an old value from a guide is automatically a current legal requirement. The designer verifies the regulations applicable to the project. The calculator does not check all building-related conditions, even if it displays a green symbol next to the result.

Let us assume a nominal U-value target of 0.22 W/(m²K), a resistance of existing layers of 1.10 m²K/W and total surface resistances of 0.17. In this simplification, the required insulation resistance is 1/0.22 minus 1.27, i.e. approximately 3.28 m²K/W. The target is solely an example assumption.

For an assumed PIR lambda of 0.022, we obtain a thickness of approximately 72 mm. Then the actual available board is selected and the entire result is checked again. The calculation does not account for thermal bridges, corrections or moisture. It is therefore not a standalone selection for wall construction.

The result for the flat field does not describe the temperature at the reveal or at the ring beam. These locations may require a different analysis and a separate detail. The smaller the wall and the more openings, the more attention should be paid to its edges instead of looking only at the centre-of-assembly parameter.

With internal insulation, the airtightness of the perimeter and service penetrations is added. The calculator will not see a poorly sealed back box or an empty gap behind the board. The documentation must describe the method of execution so that the assumed properties have a chance of occurring in the actual room.

The comparison should be prepared for adjacent thicknesses available in the given system. Alongside the thermal result, record the total construction dimension, the finishing method and changes at the window. In a small retail salon, additional centimetres may affect the counter position or the passage width.

Add adhesive, sealants, fixings and the wearing layer to the budget. Do not attempt to calculate payback from the U-value difference alone. Floor area, climatic conditions, system efficiency and heating duration are needed. Without these assumptions, the payback period would be mere guesswork.

Keep the calculation date, the board variant name in the private documentation, the layer thicknesses and the assumed parameters. Mark uncertain data. The person checking can then more easily determine whether the difference in result arises from the material, rounding or a different model.

If the wall is damp, has timber elements or is to be insulated internally, a broader assessment than a simple U-value may be needed. Attach the inspection date and the units used in the form to the note. This allows a return to the same assumptions when the thickness, cladding or finishing build-up changes. A good calculator shortens the arithmetic work. It does not replace building survey, detail design or the decision on the approved layer build-up.