Using a PIR calculator to select wall-insulation thickness

You need the layer sequence, thicknesses, thermal properties and required wall performance. Record the fixing and finishing arrangements too. Mark unknown information as assumptions: the calculator generally cannot detect that a wall selected from its list differs from the actual one.

Establish whether the calculation concerns new construction or refurbishment. Internal insulation also needs a moisture assessment, which not every calculator provides. Read the tool’s scope and output description. The label “insulation calculator” does not mean it checks every requirement of the building element.

Do not select a generic “PIR” entry when the exact board’s properties are known. An illustrative comparison could use lambda 0.022 W/(m·K) for one version and 0.025 for another. Facings and thickness can affect declared values. For the real calculation, use the selected product’s data rather than a figure for the whole material family.

That difference affects selection. Check whether the program asks for declared conductivity, design conductivity or thermal resistance R; these inputs are not interchangeable. For plasterboard-faced products, establish whether the value describes the insulation or the complete composite. The technical sheet’s number and date help identify the data used.

For a homogeneous layer, R = d / lambda. Add the layer resistances and the appropriate surface resistances; the reciprocal of the total gives U. This describes the basic calculation only. Fixings, gaps or non-uniform layers may require corrections in a particular assembly, so do not omit them without checking.

As a calculation example unrelated to a particular product, 100 mm at lambda 0.025 gives R = 4.0 m²·K/W. If the rest of the assumed assembly contributes 1.0 m²·K/W, U is 0.20 W/(m²·K). This model does not automatically describe an existing wall. Different masonry or insulation data change the result.

If the calculator returns 106 mm, do not immediately order 100 mm. Check available thicknesses and recalculate. Conductivity may change in the adjacent thickness range, as it does for some boards with gas-permeable facings.

Watch the units: a field in metres requires 0.10 for ten centimetres. Entering 10 introduces a hundredfold error. Make a simple hand calculation as a check. If the result looks exceptionally favourable, verify the inputs first; a calculator can calculate a wrongly defined wall very accurately.

Find out whether the result includes fixings and non-uniform sections, and how the tool handles ring beams, lintels and reveals. It often gives only the U-value of a flat wall area. That is useful, but it is not a complete building heat-loss assessment.

Corners and junctions need separate details. Do not account for them by arbitrarily worsening the conductivity of all the masonry. Use a method appropriate to the problem. The final report should make clear what was calculated and what still needs assessment, avoiding false confidence in the result.

A low U-value does not guarantee freedom from condensation. Internal insulation changes temperatures within the existing masonry. Vapour properties and indoor conditions must be considered; a damp wall first needs investigation of the cause.

Simplified condensation methods have limits. They do not describe airflow through gaps or all liquid-water transport. If the calculator presents a single diagram, check its assumptions. The absence of a red warning does not establish that every installation method will last. Assess the wall’s condition independently.

The moisture report should identify assumed indoor and outdoor conditions. Without them, the result is hard to check. Changed ventilation or room use may require a new calculation. Retain the input file, not just a screenshot of the final result.

Give the supplier the full board name, thickness, format and application, together with the area and drawings of difficult junctions. Ask about the complete system’s components. A printout saying only “120 mm PIR required” is too vague for a reliable order.

Keep the dated report and all assumptions. Confirm the thermal target for the specific project and recalculate if the board changes. Similar thickness and facing colour do not guarantee equal performance. Calculators work best with clearly defined conditions; a printout without input data is difficult to verify.

The result alone does not replace design work, substrate assessment or junction details.

No. Thickness selection needs layer information and thermal properties. Area mainly determines the quantity of material.