For the best fanfold siding underlayment available, choose the XP Series. The extruded polystyrene (XPS) foam offers superior stiffness, so your siding job has the smoothest finish possible. The underlayment is extra durable, thanks to flexible plastic facers on both sides. Plus it's perforated to allow moisture vapor to escape. The cut-and-fold hinge design ensures it lays flat. A perforated reflective facer that can enhance thermal performance is also available.