A fire retardant article has a wood-based material with a polyisocyanurate foam article attached thereto, where the polyisocyanurate foam article covers 90% or more of the exposed portion of the wood-based material, has a thickness greater than 1.8 centimeters, a foam core having a density of 28-128 kg/m3, a trimer concentration characterized by at least one of (i) a trimer level in a range of 12-25 weight-percent and (ii) a molar ratio of trimer to sum of urethane and urea in a range of 1-8, a phosphorous concentration of at least 0.3 wt %, a combined concentration of phosphorous and bromine of at least 0.5 wt %, and a vapor permeable and non-combustible facer on at least one primary surface of the foam core.