Kingspan GreenGuard® XP38 Siding Underlayment comprises 3/8" thick closed-cell polystyrene (XPS) with perforated capsheets on both sides. It is configured in fanfolded bundles with cut-n-fold hinges. It is lightweight and easy to cut and install. It is designed to be used as a "leveling board", added insulation and added moisture protection in installation of new vinyl siding over old siding. It is available with a reflective surface on one side, Kingspan GreenGuard® XFP38 Siding Underlayment, for added energy efficiency.