Kingspan GreenGuard® Q350 Siding Underlayment comprises thick closed-cell extruded polystyrene (XPS) in fanfolded bundles. It is lightweight and easy to cut and install. It is designed to be used as a "leveling board," adding insulation and moisture protection in installation of new vinyl siding over old siding. Kingspan GreenGuard® FQ350 Siding Underlayment has the added feature of a reflective surface on one side for added energy efficiency.