High density polyurethane and polyisocyanurate construction boards and composite boards

A method of installing a roof system, the method comprising:

• a) applying a layer of insulation board to a roof deck, where the insulation board includes a polyisocyanurate core characterized by a density of from about 1.5 to about 2.0 pounds per cubic foot as determined by ASTM C303 and an iso index of at least 200, said insulation board further including first and second facers, where the facers include cellulosic fibers;
• b) applying over said insulation boards, a coverboard, where the coverboard includes a polyisocyanurate core characterized by a density of from about 3.5 to less than 6.0 pounds per cubic foot as determined by ASTM C303, said coverboard further including first and second facers, where the facers include glass fibers, said coverboard having a thickness of from 0.5 to 4.5 inches;
• and
• c) applying, over said coverboard, a membrane system.

A method of manufacturing a composite board, the method comprising:

• a) forming a high-density layer of polyisocyanurate on a glass-containing fabric;
• b) forming a low-density layer of polyisocyanurate on a cellulose-containing fabric;
• and
• c) adhering the high-density layer to the low-density layer via a hot-melt adhesive.