Extruded polystyrene (sometimes called beadboard) is an economical choice for
larger areas, and it forms its own vapor retarding layer when properly installed and
sealed. High-density polystyrene and isocyanruate are denser insulation boards with
higher R-values. Isocyanurate usually has one or two foil faces. It is used to seal rim
joists but is a good choice for any basement wall location.
