From the 2007 thesis "What are the Financial- and Carbon-optimal Points for Return on Investment inInsulation?" by Jamie Bull, University of East London.
The work "examines whether it is worthwhile to continue lowering the elemental U-values demanded by Building Regulations in light of the diminishing returns found when increasing insulation thickness.... Key results are that the carbon-optimal point is far beyond the financial-optimal point for all materials assessed. The carbon-optimal point is also beyond the requirements of Building Regulations. However the financially-optimal point (before accounting for co-benefits) is below Building Regulations. Therefore it is found that, at the margins, super-insulation is an expensive way of reducing lifetime CO2 emissions."