Excellent mold for mineral wool. Finnfoam and FF-PIR insulation products are used widely as vapor barrier in wall or roof structures that utilize mineral wool. They also improve the thermal insulation capacity of the structure, while reducing its overall thickness. Finnfoam and FF-PIR insulation panels also function as excellent molds for. 4' x 8' sheet. 1 side has a reflective foil face typically installed toward the warm air surface to reflect energy back toward the dwelling. Can be used in residential construction as insulated sheathing in above grade exterior walls and above and below grade interior walls crawl spaces. Highest R-value per inch of any rigid board insulation. Foamglas is more expensive than the other insulation materials we're used to using. The typical cost of Foamglas T4+ (the most common product for building insulation) is about $1.00 per board-foot, according to Rebel--roughly two-and-a-half times that of extruded polystyrene (XPS), which averages about $0.40 per board-foot.