Propink Fanfold Underlayment Board is perforated to permit water vapor to pass through the board to prevent the trapping of moisture within the structure. Propink Fanfold Underlayment Board is a thin extruded polystyrene fanfold board faced on both surfaces with a plastic film facer. Propink Fanfold Underlayment Board is engineered to provide resistance to air infiltration and liquid water penetration when used in residential and commercial exterior wall construction. Propink Fanfold Underlayment Board has an advanced cut folded hinge every 24 inches to provide ease of package assembly and ease of installation at the job site.