At the same thickness, a material with lower conductivity gives a higher thermal resistance. Conductivity tells you nothing about a board’s compressive strength or reaction to fire. Distinguish the core properties from those of the finished product, especially for sandwich panels with metal facings.

Conditions in the building may differ from the reference conditions. The designer uses the applicable methods and material data to establish a value appropriate to the application. Temperature and moisture can matter. Any adjustment needs a documented basis and must reflect how the element will be used. A single percentage allowance applied indiscriminately to every material would obscure these differences.

Standards for the thermal and moisture properties of building materials describe methods for converting values between conditions. Check both the scope of the method and the range covered by its tabulated coefficients. The two ranges can differ. Extrapolating a graph to very high temperatures requires separate justification. A detailed calculation for a roof or a cold store requires data appropriate to that material and application.

On a roof, one side of the insulation can be much warmer than the other. Temperature varies through its thickness and over time. A reading on hot metal roofing describes the metal surface, not the entire board underneath. Bedroom air temperature is likewise insufficient for converting conductivity. Distinguish between ambient, surface and internal material temperatures.

Moisture can impair insulation performance. The effect depends on the product, how much water is present and how it reaches the material. A water absorption test cannot substitute for investigating a leak or condensation. If the design assumes a dry layer, detail and install the necessary water protection. Increasing the board thickness cannot correct a leaking roof or a faulty vapour control layer.

The properties of insulation foams can change over time. For a particular product, the declared value may already account for ageing under the applicable procedure. Check the documentation before treating it as the conductivity of freshly made foam. A trade name attached to a figure does not establish a separate scale for comparing quality.

Applying an ageing adjustment twice can distort the result. The facings on a PIR board form part of the product and affect its properties. A damaged board or a different facing needs its own assessment. A similar colour and rigid core are insufficient grounds for transferring data from one variant to another.

For a homogeneous layer, calculate thermal resistance R by dividing its thickness in metres by its conductivity. At 0.022 W/(m·K), a 10 cm layer gives about 4.55 m²K/W. At the same thickness and 0.031 W/(m·K), the result is about 3.23 m²K/W. Both figures describe the insulation layer alone. The U-value of a finished wall or roof needs a further calculation.

The U-value is expressed in W/(m²·K) and applies to a building element. Include the other layers, surface resistances and relevant corrections. Framing and fasteners can affect the result. At wall-to-roof junctions, assess thermal bridging. Compare a U-value requirement with the calculated U-value of the element. A result for a plain section does not cover the building as a whole.

Give the designer the product designation, thickness, facing type and current declaration of performance. Describe the room’s use and the expected moisture conditions. These differ between a cold store and a bedroom. If sales literature lists several conductivity values, check which products, thicknesses and conditions each covers before assuming an error.

With these details, the designer can select the layer and check the complete build-up. The low conductivity of PIR is useful where space is limited. Summer comfort also depends on glazing, shading, ventilation and thermal mass. An insulation datasheet alone cannot predict a fixed difference in room temperature.