THIS LINK and also
THIS ONE shed some amazing light on real-world insulation.
Whether for a storage tank or a building envelope, the diminishing returns of extra insulation and the dramatic degradation in its effective R or U value with even tiny amounts of uninsulated/less insulated area are surprising. So much so, that improving more poorly insulated areas (by using low-e windows, fixing insulation gaps around obstacles, designing without thermal bridges, etc) is WAY more important than adding extra insulation past a certain point.
An uninsulated pulldown attic staircase (or gaps around insulation) of only 1% of the total area increases heat flow for an attic with R30 by around 25%, while adding insulation to R50 reduces heat flow by only about 5%!
I will be making sure there are no thermal bridges between the bottom of my tank and my basement slab....