A construction board includes a foam body having first and second planar surfaces, said foam body including a polyisocyanurate foam matrix defining a plurality of closed cells, said closed cells being at least substantially devoid of hydrocarbon blowing agents, and said foam body being characterized by a density, pursuant to ASTM C303, of at least 2.5 lbs/ft3; and a facer disposed on a planar surface of said foam body, said facer including a glass substrate having an internal planar surface proximate to said foam body and an external planar surface opposite said foam body, a first coating disposed on said external surface, and a second coating disposed on or proximate to said internal surface, where said first coating disposed on said external surface includes an inert filler, and where said second coating disposed on or proximate to said internal surface includes intumescent material.