A polyisocyanurate foam insulation product includes polyisocyanurate foam produced from reacting an isocyanate and a polyol blend having a functionality of at least 2.2. The isocyanate and the polyol blend are reacted so that the polyisocyanurate foam has an isocyanate index equivalent with or greater than 300. The polyisocyanurate foam includes a fire retardant and includes between 0.02 and 0.45 weight percent of a zinc salt compound. The foam insulation board exhibits a flame spread of no greater than 25 and a smoke index of no greater than 50 when exposed to flame conditions in accordance with an ASTM E-84 test.