Thermal Performance

Thermal Performance 

When calculating U–values to BS / I.S. EN ISO 6946: 2007, the type of mechanical fixing used may change the thickness of insulation required. The use of carbons steel fasteners of cross sectional area 4 mm2 has been assumed at a density of 16.7 per m2.

For the purposes of these calculations the standard of workmanship has been assumed good, and therefore the correction factor for air gaps has been ignored.

The figures quoted are for guidance only. A detailed U–value calculation and a condensation risk analysis should be completed for each project.


The U−values in the tables that follow have been calculated, under a management system certified to the BBA Scheme for Assessing the Competency of Persons to Undertake U-value and Condensation Risk Calculations, using the method detailed in BS / I.S. EN ISO 6946: 2007 (Building components and building elements. Thermal resistance and thermal transmittance. Calculation method), and using the conventions set out in BR443 (Conventions for U−value calculations). They are valid for the constructions shown in the details immediately above each table.

These examples are based on the use of 3 mm High Impact board bonded to 10mm,20mm PIR which is subsequently wrapped in a highly durable and robust vapour control layer


E-Therm Slim 3

  • 6mm Mineral Board
  • 50mm Ship lapped
  • Foiled Faced PIR
  • 6mm Mineral Board
U-Value Before Insulation  P/A  30mm 




0.36  0.2 0.23 0.20
0.42  0.25 0. 25 0.22
0.48  0.3 0.26 0.23
0.53  0.35 0.28 0.24
0.58  0.4 0.29 0.25
0.67  0.5 0.31 0.26
0.75  0.6 0.32 0.27
0.82  0.7 0.33 0.28