There are at least 4 different kinds of plastic foam referred to here.
Icynene is the usual trade name for one type of Polyurethane (PU) foam that
is sprayed on the open framing of walls. The result is a somewhat pliable
insulation that is as much as R7 per inch. Another type of PU foam is
used a lot for roofs, and it is very rigid after it expands....you can walk on it
within seconds. Also about R7/inch. Both of these are 2-part foams and
pretty expensive.
The other major foam type refered to here is Polystyrene foam, which is
usually about R4-R4.5/inch. It is either expanded (EPS) or extruded (XPS)
and both are found in sheets, or molded to form packaging, or the carcasses
of Insulating Concrete Forms.
For comparison, fiberglass insulation is about R3.5/inch.
I once had a roof coated with PU foam, and it was priced similar to a
conventional re-roof job. I got a quotation for Icynene for the ceiling
joists of my garage (under-floor of the house) and it was the about the
same cost as roofing foam, so I went with the latter. No floor squeaks and
makes the floor seem stiffer. The additional structural support of this
kind of foam would be a great thing for an ag-style steel building. I have
also used lots of structural (hard) PU foam that is dispensed with guns
for sealing and gluing. This is a one part water-activated foam. Works great.
Gorilla Glue is a non-foamed water-activated polyurethane BTW.