PIR in numbers: do not confuse board conductivity with whole-element U-value

A useful quotation separates product identification, declared properties and the calculation for a particular building element. Calling a board “thermally efficient” does not replace any of those three items.

Thermal conductivity λ is expressed in W/(m·K). Under comparable conditions, a lower value means less heat conduction through the material. It does not tell you how many centimetres were installed in the roof. Check the full board designation and the document supplying the value instead of transferring a property from a similarly named product.

A layer’s thermal resistance R is expressed in m²·K/W. For a homogeneous layer, the simple calculation is R = d/λ, with thickness d in metres. A higher R means greater resistance to heat flow. That is the opposite direction of comparison from conductivity: “lower is always better” is not a valid rule for every number.

Before using the formula, convert 100 mm to 0.100 m. Entering 100 alongside conductivity expressed per metre produces an error by a factor of one thousand. Keep separate fields for quoted thickness and the value used in the calculation. This makes the mistake visible before options are compared.

Purely arithmetic example: with d = 0.100 m and an assumed λ = 0.025 W/(m·K), R is 4.00 m²·K/W. These numbers illustrate the formula, not the properties of any PIR board in the catalogue. An actual purchase requires data for the exact product and the appropriate calculation basis.

The reciprocal of that example resistance is 0.25 W/(m²·K). It must not automatically be labelled the finished roof’s U-value. That shortcut omits other layers and surface resistances, as well as the treatment of non-uniform construction. A single-layer result and a whole-element result need separate labels.

Rafters, fixings and junctions can make heat flow more complex than through a uniform section. Do not subtract an arbitrary “thermal bridge allowance” or add a percentage taken from another project. The calculation method must suit the actual assembly and project requirements.

Compare conductivity with conductivity, or resistance at a stated thickness with an equivalent resistance figure. If one quote gives only lambda and another a roof U-value, first obtain the missing information. Make a table of product, thickness, unit, parameter type and source before considering the price difference.

A declared product value and a project design input do not necessarily serve the same purpose. Retain the parameter designation as given in the source. If a declared-resistance table differs from your calculation using rounded conductivity, clarify the basis with the manufacturer or designer. Do not rewrite a declaration from one division calculation.

Thermal equivalence does not establish that two boards are interchangeable. Similar resistance does not confirm identical facings, load performance, permitted application or installation method. For a proposed substitute, record thermal suitability separately from other requirements. An overlooked criterion can matter more than a small numerical difference.

Do not translate an improved U-value for one element directly into the home’s annual bill. Other surfaces, air exchange, occupancy and heating equipment also enter the balance. Separate insulation-layer selection from assessment of the renovation’s energy outcome in your enquiry. Each needs a different set of inputs.

For final selection, provide the designer with the layer build-up, dimensional constraints, proposed board properties and the target for the building element. Record the approved product’s full designation and thickness in the order. Keeping the documentation version lets you check delivery against the design basis rather than reconstructing the decision from “PIR 100” alone.