What data should you enter in a PIR thickness calculator?

Record whether insulation goes inside, outside or in an existing cavity. In a simple one-dimensional conduction calculation, changing layer order may leave the sum of resistances unchanged. It still matters for moisture. A calculator showing only U cannot establish whether the insulation is safely positioned.

List materials from inside to outside with their thicknesses, including masonry, plaster, existing insulation and the proposed new layer. Do not enter an enclosed air gap if the cavity will be ventilated on site. A layer absent from the actual building cannot be counted to improve the result.

For a composite board, find out whether the material library includes the whole product or only the core. Otherwise plasterboard can be counted twice or left out altogether. Flag uncertain material properties and decide how to verify them. In an older building, better inputs often matter more than extra decimal places.

PIR does not have a single conductivity. Foil-faced boards and render carrier boards may differ, and values may depend on the thickness range. Read the data for the exact product and confirm the unit is W/(mK). Do not copy the lowest number from a general description of PIR.

Check the thickness field: enter 120 mm as 0.12 if metres are required, or 12 if the unit is centimetres. Review the input summary afterwards. A large thickness error can produce an impressively low U-value unrelated to the board you intend to buy.

For a simple check, suppose the rest of the wall, including surface resistances, has R = 0.95 m²K/W. At 120 mm and λ = 0.022 W/(mK), PIR adds about 5.45 m²K/W. Total resistance is about 6.40 m²K/W and U about 0.156 W/(m²K). This checks arithmetic under the stated assumptions.

In the same model, 100 mm gives about 0.182 and 140 mm about 0.137 W/(m²K). If software gives very different values, check units, surface resistances and additional corrections. Small differences may reflect a different method; understand them before using the result in a design.

A calculation for the uninterrupted wall area does not cover all thermal bridges at windows, floors and corners. Fixings and non-uniform layers may need to be included too. Internal insulation requires a moisture assessment: a good U-value does not tell you whether old masonry can dry.

Check the facing’s intended use, fire requirements and fixing method separately. The calculator cannot confirm that an adhesive will hold a board on old plaster or select an entire façade system. Give the designer the outstanding checks with the calculations so they are resolved before installation.

Label each option with masonry type, PIR thickness, facing and target. Save inputs and results. You can then check whether a supplier’s offer matches what was calculated. Update the comparison if conductivity or available thickness changes.

Add the cost of the complete assembly and its finished thickness. Space for services may matter in a small room and sill depth on a façade. The final choice must combine thermal performance with buildable junctions. Give the installer more than the thickness returned by the form.

Also save the starting condition without new insulation. It shows the scale of improvement and helps spot errors, for example if the result worsens after resistance increases. Use filenames that distinguish preliminary calculations from the option adopted for the design.