Hope I’m not hijacking too much nor going too deep off the engineering deep end. I found this interesting in regard to my question to mr j.

The results of compression tests indicate that the foam of 16 kg/m3 density is not able to resist the mechanical force. The water absorption and reduction in buoyancy for this PU foam is high what indicates a significant number of open pores. The lighter foam shows various disadvantages: great discontinuities (cavities) at the border between portions of the foam, low compressive strength, high water absorption and loss in buoyancy caused by a number of open pores.

FWIW, 16kg/m^3 is basically 1lb/ft^3 density.

from: http://www.pg.gda.pl/mech/kim/AMS/022006/AMS02200605.pdf

This is specific to PU foam and a lot of the conclusion is specific to barge design but gets into similar ideas of what may cause a PU board to feel “dead” after a while.