How to use a PIR calculator to choose a practical insulation thickness

Check what the U-value represents: the panel alone, the entire wall or a result that already includes corrections. Check the units too. If the calculator expects metres, entering 100 instead of 0.10 changes the thickness a thousandfold. Without a record of the layers and assumptions, the result will be difficult to check later.

List the layers in order from the room to the outside. You will need the thickness and thermal properties of each one. If you only have a rough description of an old wall, run a conservative scenario and note what is uncertain. Selecting the best-performing material in every drop-down menu improves the figure on screen without changing the building.

Do not add the resistance of an air gap unless you know it exists. Damp or damaged insulation should not be entered as if it were a new layer in perfect condition. For a substantial renovation, it can be worth having the wall build-up investigated. Correcting a mistake in the thickness or junction details may cost more than the inspection.

Use the declared conductivity for the exact board, facing and thickness. For example, an aluminium-faced board might have a λ-value of 0.022 W/(m·K), while a fleece-faced façade board might have a value of 0.026 W/(m·K). Do not use the lower figure for a product that does not declare it. Even within a product range, conductivity can vary with thickness.

If the calculator accepts a declared thermal resistance R, do not count the same layer again by entering thickness divided by conductivity (d/λ). A hand calculation may differ slightly from the table because of how values are declared and rounded. Use the figures for the chosen product in the design.

As an illustration, take an existing wall with a U-value of 1.20 W/(m²·K) and a target of 0.20 W/(m²·K). The extra thermal resistance needed is 1/0.20 minus 1/1.20, or about 4.17 m²·K/W. Multiplying this by a conductivity of 0.022 W/(m·K) gives about 92 mm of insulation. At 0.026 W/(m·K), the same target calls for about 108 mm.

This estimate assumes that the existing U-value is correct and leaves out fixings and thermal bridges. It can reveal a unit error, but it is not a design specification. After choosing a thickness that is actually available, such as 100 or 120 mm for the appropriate board, recalculate the complete wall.

Check windows, lintels, ring beams and the wall-to-floor junction. A calculator that models a uniform patch of wall will not automatically cover each of these details. Mechanical fixings may also require corrections. If the displayed result only just meets the target, check these omissions before choosing a system with no margin.

Internal insulation also requires an assessment of heat, moisture and airtightness. A U-value will not tell you whether the masonry can dry or whether a corner will stay cold where the insulation ends. A narrow window reveal or an old beam needs its own detail, whatever thickness the calculator suggests.

Compare two or three realistic options. For each, record the total build-up thickness, the thermal result and any changes the work would entail. One option may need new sills; another may mean moving services. These differences can determine which option is practical to install and works best once the space is in use.

When asking for a design check, send the wall description along with the result. If opening up the wall reveals different layers, repeat the calculation. The final choice should account for the specific system, fixings, moisture conditions and project requirements. The calculator is useful when it makes those decisions easier to review.

Record the calculation date and the version of the product data. If a quotation prompts you to change the facing or thickness, update the result. Use file names that distinguish the approved option from earlier trials, especially when comparing several rooms.