THERMAL PERFORMANCE
The performance of PUR/PIR products can be explained by considering the four factors, which contribute to heat transfer:
Solid conduction – this factor is low in the PUR/PIR cellular structure as the solid phase typically accounts for about three to four per cent of the total volume of the low density insulation.
Gaseous conduction - the blowing agents used in PUR/PIR cellular structure, which become the gas phase, have very low thermal conductivity compared to other gases and to air.
Radiative transfer - due to their cellular structure PUR/PIR has comparatively low radiative heat transfer. Radiative heat transfer increases with increasing cell diameter.
Convective transfer - due to the fine closed cell structure of PUR/PIR insulation products, heat transfer through convection is insignificant and can be ignored.
A minimal contribution from each of these four heat transfer mechanisms results in an insulation product with excellent thermal performance.
All insulating products which are blown with an insulating gas can undergo changes in cell gas composition over time resulting in changes to thermal conductivity. Many products use impermeable facing materials such as aluminium foil and coated steel, which essentially eliminate thermal conductivity aging caused by migration of gases into or out of the insulation. In all cases PUR/PIR manufacturers subject their products to regulated testing regimes and quoted values not only take into account aging but also include safety increments to ensure that products deliver better than specified performance throughout their useful life.