Below DPC Insulation' thermal resistance of the build up should be at least 0.75 of that acheieved through the main wall. Following this, the thickness of insulation below DPC would need to be MIN 100mm (as insulation types used below DPC cannot offer as good a thermal conductivity value of the phenolic insulation which is 0.22mm). This detail would result in a step out / ledge at DPC level. Is there a best practice detail for 'Below DPC Insulation' when the main system is phenolic (with a thickness between 60-80mm)?