Why does a PIR board’s resistance table give a slightly different conductivity from its label?

Start with the product identifier. A manufacturer may offer several boards in one range for different applications. Check that the installation guide table and the pack label refer to the same model before treating them as three figures for one product.

A manufacturer’s UK flooring guide gives λD = 0.022 W/(m·K) for one PIR board. Its 40 mm row lists a thermal resistance of 1.80 m²·K/W. Both figures belong to the board covered by that table. When comparing quotations, use the sheet for the model you intend to order.

Before dividing, convert the thickness to metres: 40 mm is 0.040 m. The 1.80 is the resistance of that thickness of insulation. If a spreadsheet uses 40 instead of 0.040, the result will be a thousand times too large even if the column heading looks correct.

Dividing 0.040 by 1.80 gives approximately 0.02222 W/(m·K). The trailing digits come from the calculator, not from an additional manufacturer’s measurement. They do not justify changing the declared 0.022 to 0.02222 in a product description.

Try the usual calculation: 0.040 / 0.022 gives approximately 1.818 m²·K/W. In the product comparison, record the published resistance for the board. Label 1.818 as your own cross-check so readers can see why there are two different numbers.

The same guide lists 3.40 m²·K/W for the 75 mm board. Reversing the calculation gives 0.075 / 3.40, approximately 0.02206 W/(m·K). Getting different back-calculated figures for 40 and 75 mm does not establish that the manufacturer declared two different conductivities.

This distinction also matters when importing product data. Where the document supplies a resistance, the catalogue should not silently replace it with a value derived from other fields. Keep the published value and the calculated check separate, each with its unit.

If the difference is large, check the column heading first: R and U are different quantities. Also verify that the resistance was not taken from an example for a complete floor while the thickness refers only to its insulation.

If in doubt, send the supplier the document reference, board designation and the two rows in question. Avoid inventing a percentage tolerance for accepting the figures. Documents can cover different product revisions, and the discrepancy needs resolving at source.

Keep the declared λD and the published resistance for the ordered thickness in the product fields. A field headed “design conductivity” serves another purpose: it records a value adopted for a particular use, with its basis, rather than a number reverse-engineered from a table.

Retain the table’s retrieval date and product code. If a later delivery carries a different identifier, you can recheck the figures without having to guess their origin. A change of that kind is particularly easy to miss when the trade name and thickness stay the same.