How do you calculate the required thickness of PIR board for a wall?
PIR insulation
The required PIR thickness can be preliminarily calculated from the required thermal resistance of the wall. However, you need to know the existing layers and the parameters of the selected board. A calculator based only on the type of masonry gives too little information. The final selection also takes into account fixings, thermal bridges, moisture and the method of finishing. The calculation result must be translated into an available product suitable for the application. Record all wall layers, their thicknesses and thermal properties. Include existing insulation and voids, if they actually occur. For an old house, the documentation may need to be verified on site. Do not assume that every brick or aerated concrete wall has the same resistance.
What data should be prepared before calculation?
Record all wall layers, their thicknesses and thermal properties. Include existing insulation and voids if they actually occur. For an old house, the documentation may require an on-site check. Do not assume that every brick or aerated concrete wall has the same resistance.
The second piece of information is the target U-value specified for the building. The third is the data for the selected PIR product. A board with aluminium laminate and the variant for plaster may have a different lambda. Also record from which side the insulation is planned. This matters for moisture assessment and for details at windows and ceilings.
How to make a simple estimate?
For a homogeneous layer arrangement, the overall R can be taken as 1/U. The resistances of existing layers and surfaces are subtracted from this. The remaining resistance must be provided by the new insulation. Its thickness follows from the relationship d = R × λ. The thickness obtained in this calculation is given in metres.
Assume, purely as an example, U = 0.20 W/(m²·K) and a combined resistance of the existing wall and surfaces equal to 1.25 m²K/W. The required insulation resistance is 5.00 − 1.25 = 3.75. With λ = 0.022 we obtain 0.0825 m, i.e. 82.5 mm. This is not a ready recommendation for an 80 mm board. The example still omits corrections, and the commercial thickness must ensure the adopted target.
Why can a different finish change the result?
The properties of the board result from its entire construction. The documentation includes PIR variants with a lambda of 0.022, but also products with other values. For some boards with glass fleece, the lambda depends on thickness. Therefore one number cannot be used for all products.
If in the previous example the insulation suitable for the application has λ = 0.026, the required thickness is 97.5 mm. The difference is significant at the eaves or window sill. In a PIR composite with another material, the declared resistance for the finished board will be helpful. Calculating the entire thickness as pure PIR foam will overstate its capability. When ordering, use the documentation for exactly this variant.
What must be added to the result?
Fasteners, discontinuities and the way elements are joined can affect the actual heat flow. The appropriate corrections must be taken into account according to the calculation rules. Corners, plinths, ring beams and reveals require separate verification. Their influence is not replaced by simply thickening the insulation in the middle of the wall.
Also check for moisture. With internal insulation, the temperature of the wall and its drying behaviour change. A good U-value does not prove that the whole assembly is moisture-safe. If the wall is getting wet from the ground or from rain, the cause must be eliminated first. A gas-tight board is not a way of repairing such a defect.
Is it worth choosing a thicker board?
It is worth comparing adjacent available thicknesses with the same remaining layers. The price of the insulation alone is only part of the cost. Scaffolding, labour and finishing may remain similar. On the other hand, a thicker build-up may require new window sills or changes to flashings. The decision must be calculated for the whole façade.
When comparing price lists, check the unit, the coverage area and full packs. Do not mix the price of a roof board with the price of a façade variant. These are different products. A cheaper material with a good lambda will not become suitable under render without the correct system. Nor should a payback time be given without knowing the building’s losses, the heating and the energy cost.
Where space is limited, variants with a different lambda can also be compared, but only among products suitable for the application. A better thermal parameter of a roof board does not justify using it as a façade board. Sometimes changing a detail will be more beneficial than buying more expensive insulation. An example is a well-thought-out eaves termination or an adjustment to the setting of a window sill. Such decisions are best made before the new finish is applied.
What should a completed order include?
State the board type, thickness, required thermal resistance, format and edge type. Include the agreed method of installation and finishing. The area schedule should include reveals and the necessary cuts. For renovations, it is not worth ordering the full quantity before checking the condition of the substrate.
Keep the calculation result together with the assumptions made. If the contractor proposes an alternative, its parameters can then be compared and the change assessed. The words ‘PIR of the same thickness’ alone are not enough. The finished solution must meet the thermal objective and suit the wall, not merely fit within a thickness table.
Before covering the insulation, check the key connections. This is the last convenient moment to correct gaps and document the installation.
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