Is this somthing that is a weight issue?
Good question. I found another technical doc that clears this up. 90 is meant to be applied to both surfaces, 77 can be applied to one surface.
The figures show that the formulation of 90 has half the solids as 77 and half the coverage. If 90 is applied as recommended (spary both sides) then you end up with the same weight per sqft. If you were to apply to only one side ( not recommended) then it would be lighter.
It seems 77 is the better product to use for our purposes. 90 is stronger (230psi vs 77's 160psi) but it is irrelevant when taping cellophane to foam.