A fire-resistant thermal insulation product may be used in new home construction and/or remodeling projects of existing buildings. Individual beads or small clusters of polystyrene (PS) beads are obtained by processing gleaned waste polystyrene through a plastics granulator or other cutting or grinding device. The PS beads and/or small clusters are coated with liquid intumescent fire retardant (FR) and dried. A binding solution is added, and the resultant FR-coated PS plus binder is compacted into self-supporting blocks. Each block is then dried, and subsequently placed in a polyethylene bag and sealed. The blocks are appropriately sized to fill the cavity spaces between wood and metal studs in both exterior and interior walls of buildings, between the ceiling joists in attics, and floor joists in crawl spaces. The width, thickness, and length of the solid rectangular shapes are variable, for example, varied for 16-inch-on-center and 24-inch-on-center framing spacing, and for various cavity dimensions and desired R-values. These insulation products meet the required safety test criteria, such as ASTM E84 and UL 94, necessary to meet building code requirements and ICC-ES certification.