A process for manufacturing an insulation board comprising a rigid polyisocyanurate foam core having two major surfaces and a facing material on at least one of the major surfaces, comprising (a) conveying a facing material along a production line for attachment to one major surface of the core; (b) applying a foam-forming mixture of polyisocyanurate to the facing material such that the mixture is applied along the entire width of the facing material; (c) optionally conveying a second facing material along the production line for attachment to the other major surface of the core; (d) conveying the facing material with applied foam-forming mixture into a laminator which comprises a gap for foam expansion and allowing the mixture to foam and expand to fill the gap within the laminator; and (e) curing the foam. The process provides excellent boardstock thickness control, minimal wasted densification at the foam/facer interface, greater compressive strength and high line speed.