"If joist and rafter insulation are present record joist only. If loft is fully boarded record insulation as 'unknown' unless householder has documentary evidence (maximum thickness is depth of joists) or is prepared to lift the boards. If the property has modern foil of or foam insulation at joists the depth of insulation should be entered as double its actual thickness. If varying levels of insulation an area weighted average should be entered, however if there is an area with no insulation the dwelling should be split to give different roof scenarios.