Can one leave polyurethane uncovered?

The National Building Code of Canada (NBCC), stipulates that the insulating plastic foam and combustible insulation should be protected by a thermal barrier coating or certain accepted materials for this purpose.

Coverings accepted for this purpose are gypsum, plywood, OSB (aspenite), and flame retardants.
One can refer to sub-sections 9.29.4 to 9.29.9 of the NBCC for technical specifications.